GB2547688A

GB2547688A - A method for one-shot solid-state welding of pipelines

Info

Publication number: GB2547688A
Application number: GB1603346.6A
Authority: GB
Inventors: Zarisfi Kasra
Original assignee: Impressive Eng Ltd
Current assignee: Impressive Eng Ltd
Priority date: 2016-02-26
Filing date: 2016-02-26
Publication date: 2017-08-30
Also published as: GB201603346D0

Abstract

A Pressure Containing Coupler (PCC) comprises a closed shell 1 around its outer surface containing fluid 6. The fluid 6 is pressurised to expand the PCC and its inner hole. Two stationary pipes 3 are inserted tightly into the pressurised PCC sockets making a joint assembly. The PCC may be rotated and compressed between the two pipes 3, such that the touching faces between the PCC and the pipes 3 are heated up by friction to a suitable temperature below the melting point for forge welding. The fluid 6 is then rapidly depressurised so that the PCC suddenly stops and contracts over the pipes to metallurgically bond the heated faces of the joint assembly in a solid state weld. This generates two girth welds simultaneously between the pipes and the coupler in one attempt in a matter of a few seconds. Alternatively, the joint may be heated by friction from another member, electrically or by any other means. A mechanical joint may also be formed. Also disclosed are methods of testing the joint by placing the assembly in tension (figure 74) and for pressure testing a void (3, figure 78) between two parallel welds (4,5, figure 78).

Description

Description: A Method for One-Shot Solid-State Welding of Pipelines 1. Introduction

Pipeline systems are an important part of modern infrastructure. They have a wide range of applications and have been used over a long time for such things as water supply and sewage transport. Pipelines are also important assets in the oil and gas industry for both offshore and onshore use. Joining the pipe elements (line-pipes) has been always a key task in pipeline construction, and welding is the most common means of achieving this, especially for metal or plastic pipelines.

Pipelines in oil and gas industry must be reliable with a high degree of integrity, and the industry has always aimed to improve existing pipeline welding techniques and to develop new ones. Although high quality welds are achievable with current techniques, welding still is a time consuming activity which requires skilled labour. This makes it expensive. In onshore pipeline construction projects, alignment, welding and Non Destructive Testing (NDT) activities typically represent around 20% of the project cost. In offshore construction projects, these activities become even more costly, as construction time is limited by weather and lay barges are expensive to hire.

Over the past 30 years, much effort has been expended in reducing welding time. Major gains have been made by automating previously manual welding techniques, but few gains have come from changing the technology applied. One shot welding is a fast welding technique that has been an attractive solution for many years, but it has never become commonplace in pipeline construction, mainly because of the difficulty in controlling the weld forming between the pipes, thereby preventing an homogeneous high quality weld. It also requires expensive equipment. Since weld quality is the primary requirement, this cannot be sacrificed in order to gain the time and cost savings offered by a quicker welding technique.

Long-distance pipelines, with lengths in excess of 1,000 km, are now common parts of oil and gas infrastructure. Recently, the industry has also favoured more efficient pipelines (with larger diameters and higher operating pressures) to increase their transmission capacity. This necessitates the use of materials with higher strength (X100 and above) than normal (e.g. X65, X70). While such high strength materials are readily available, the weld-ability of these materials is a challenge for current fusion welding techniques. A solid-state welding procedure could be a possible solution for joining ultra-high strength line-pipes (X100 and above).

Considering the development history of pipeline welding techniques, some welding engineers and specialist believe that further improvements in speed will be only achieved by advanced automation and computerisation of existing welding techniques, rather than the introduction of new or radical methods. Whilst appreciating the use of automated systems, the author (who has extensive experience in pipeline design and engineering) disagrees with this discarding of a possible revolutionary solution for current pipeline welding limitations.

This paper presents a new one shot welding technique, called Radial Pressure Welding (RPW), which has the same fundamental features as solid-state welding (or forging). It is believed that this method's primary advantage is to improve the quality of welds used to join pipelines made of high strength material and also to significantly reduce the welding time which results in a significant cost saving for pipeline construction projects. 2. Current Pipeline Welding Technology

Current modern steel pipe welding techniques can be traced back to the early 1800s. This technology has improved over the time, with a significant step forward being made by the development of the Gas Metal Arc Welding (GMAW) technique in 1969.

Today's automated GMAW techniques achieve high quality welds using mechanised welding bugs with a computerised control system. GMAW currently is one of the best and most common technologies for pipeline construction. Although using welding machines has increased the speed of GMAW compared to manual welding, it is still a time consuming pipeline construction activity and therefore one-shot welding techniques have always been attractive as a possibility for increasing productivity (see figures 1 and 2).

Flash-butt welding is a one-shot fusion based welding technique which has been used more than other variants. Although frequently used in projects in Russia and the Ukraine, it has not been widely used elsewhere mainly because of the mechanical properties of welds obtained by this method: a homogenised fusion is difficult to achieve with flash-butt welding (see figure 4).

Reducing the time is not the only goal for improvements in pipeline welding technology. Probably more important is the need to be able to weld engineered materials such as ultra-high strength steel (X100 and above) and complex alloys. Using ultra-high strength steel in pipeline projects, especially in those with long overall length, significantly reduces the project cost.

As GMAW is a fusion based welding technique, this presents a limiting factor for welding ultra-high strength heat-treated steels. This is mainly because of the mechanical characteristic of the joint's Heat Affected Zone (HAZ). The heat of the fusion results in grain growth in the microstructure of the base (pipe) material in the vicinity of the weld. The mechanical properties of the HAZ are typically poorer than the base material and the weld. As the strength of the steel increases, the weld-ability of the material decreases and consequently fusion welding becomes more challenging. The heat of the fusion also affects alloy-based pipe materials, resulting in a requirement for post weld heat treatment, which is time consuming and expensive (see figure 3).

Although X70 and X80 steel grade is now commonly used in onshore projects, X65 is still the dominant grade for offshore applications. There is much research interest in this area and it is expected that further use of X100 will provide greater cost reduction. A solid-state welding procedure can be a future solution for joining ultra high strength and complex alloy pipelines. Using this method, the base material is not melted and therefore its properties are not altered by the intense heat used in fusion welding. 3. Solid State, Friction and Exploding Welding

Solid state welding, or forge welding is a method which does not involve the melting of the materials being joined or the addition of filler metal. The welding process is performed below the melting point of the base materials. Pressure may be used to compress the joint.

Friction welding relies on friction to cause the heating that is needed to produce a weld. The friction is created by a compressive force which contacts the work pieces, which are moving relative to one another either linearly or in rotation. The mechanical energy is converted to heat at the joint, due to the friction. The weld between the work pieces faces is generated by a final compressive force, called an Upset Load. The Upset load is typically a sudden compressive force and one which has a higher magnitude than the compressive force used to generate the friction and therefore heat. During the welding process, depending on the method being used, small pieces of the material will be forced out of the working mass, and this is called flash. It is believed that the flash carries away debris and therefore friction welding has a self cleaning mechanism. If it is at the outer surface of the joint and it is accessible, the flash is normally machined and removed (see figures 5 and 6).

Explosion welding involves the joining of materials by pushing them together under extremely high pressure. The energy from the impact plasticises the materials, forming a weld, even though only a limited amount of heat is generated. The process is commonly used for welding sheets of dissimilar material, such as the welding of aluminium with steel (see figure 7).

Friction and explosion welding are both one-shot welding techniques. Neither of these methods are commonly used for welding pipelines.

The advantages of friction welding are summarised below: • Fast joining time: the one shot welding process takes a few seconds; • High quality weld, improving the mechanical property of the joint because of the mechanical work; • Melt-free, minimising the HAZ and avoiding grain growth in engineered materials such as high-strength heat-treated steels; • Ability to weld ultra-high strength steels, materials with tensile strength of above 700 MPa and complex alloys; • Ability to weld dissimilar materials; • No additional filler material required; • Inert gas not necessarily required; • Self-cleaning joint surfaces (less preparation required). A friction-based girth butt weld may be achievable for short length pipes (see figure 8). However it is impractical to achieve between two longer pipes because of the difficulty in rotating, for example, a 12m line-pipe.

To overcome this, a method has been tested to perform friction welding of two stationary line-pipes. This involves spinning a disk (or flat washer) between the two line-pipes to generate the friction. This method inevitably creates an outgrowth inside the pipe after welding. The outgrowth can be the weld flash or the disk itself and must be machined down through the welded pipe. Although high quality welding between the two pipes and the disk has been achieved by this technique, the post weld machining requirements of the joint makes this method unfavourable (see figure 9).

Radial Friction Welding (RFW) is a one-shot forge welding process which was developed by The Welding Institute (TWI). In this method, a bevelled ring is rotated in a V-shaped groove between the ends of two stationary pipes. The ring is rotated under compression, and generates heat to make a weld between the ring and the two pipes, thus joining the assembly together. An internal support mandrel is provided at the weld location to prevent pipe collapse (see figures 10 and 11).

This technology has been available for around twenty years but is not commonly used in pipeline construction due to the high cost of equipment. RFW is, however, used for joining advanced alloys (such as titanium alloy) in risers used by the offshore oil and gas industry. RFW is the closest relative to the Radial Pressure Welding concept presented in this paper. Although the RPW and RFW techniques are different, the experience gained from RFW confirms the technical feasibility of RPW for making high quality welds.

The advantages of the existing RFW technology are summarised below. • No rotation of pipes required; • Solid phase process; • Highly reproducible weld quality and properties; • High production rates achievable; • Dissimilar materials can be welded; • No additional filler material required.

The main disadvantage is that the requirement for expensive equipment makes it commercially unrealistic. 4. A Ring Shape Pressure Vessel

Imagine a flat disk with a central hole, rather like a flat washer, made of steel. When the washer is heated, it expands. A common misconception is that the hole shrinks because the washer width (the distance between inner and outer diameter) increases due to the expansion. Actually, the hole expands due to the relative expansion. This is also true of a torus shape container. When a car or a bicycle inner tube (or a swimming ring) is inflated, it may seem that the hole (inner diameter) shrinks. Actually, the inner diameter increases. Although the tube thickness (torus cross section diameter) increases due to the pressure, the tube inner hole expands because of the tube's overall expansion: the tube hole expansion is a relative expansion (see figures 12 and 13).

Given the above, now imagine a pipe and a washer where the pipe diameter is slightly bigger than the washer hole. If the washer is heated, it will expand so that the pipe can be inserted into the washer hole. Cooling the washer back to ambient temperature causes it to contract and grab the pipe. Again, extending the above example to a pipe and a ring shape pressure vessel, imagine a torus shaped pressure vessel made of rigid material like steel and a pipe with a diameter slightly bigger than the torus hole (inner diameter). If the vessel is filled with a fluid which is then pressurised, the vessel will expand and the pipe can be inserted into the torus hole. Depressurising the vessel causes it to contract and grab the pipe. The touching faces between the pipe and the vessel are compressed by a uniform radial force (see figures 14 and 15).

In both the disc and torus examples given above, a radial compressive force is generated. However the torus shape pressure vessel can create a sudden radial force (because it can be depressurised in a fraction of second) while the disc can only generate a gradual force since it needs time to cool down.

The ring shape container (or pressure vessel) has a hole for accepting a component (e.g. a pipe or a bar) and behave like a coupler. Therefore the ring shaped container is called Pressure Containing Coupler (PCC).

The expansion behaviour of a ring shaped container in response to an internal pressure is the basis for the novel pipe joining technique presented in this paper.

The radial compressive force generated by depressurising the vessel over a fitted component can be used to join the PCC to the pipe or any other cylindrical components thus inserted. This offers an excellent opportunity to generate a force (and therefore friction) for use in the friction welding of pipelines. A PCC can be rotated about two stationary pipes so that the friction between the PCC and the pipes heats the touching faces to a temperature suitable to achieve forge welding. If the PCC is then rapidly depressurised, this results in a sudden radial compression force which acts as an upset load to bond the hot surfaces and weld the PCC to the pipes.

The term "cross section" as used in this paper refers to the intersection of the component body with a plane passing through the longitudinal axis of the component, or to the intersection of a plane passing through the axis of rotation of the component's surface of revolution.

The ring shaped pressure vessel can have various cross sectional shapes (e.g. a torus shaped vessel has a circular cross section). 5. Pressure Containing Coupler Design for Pipelines PCCs can be specifically designed for connecting line-pipes. In such a case, an assembly consisting of two line-pipes which are connected by a PCC positioned between them can be conceived. In this paper, this joint assembly is called a PCC-Pipes assembly (see figures 18 and 20).

The PCC designed for joining line-pipes has a shell which has a semi-torus shape. The PCC geometry can be visualised as a surface of revolution generated by revolving a semi-circle in three-dimensional space about an axis coplanar with the circle and where the straight edge of semi circle is parallel with the axis of rotation.

In the other words, the PCC is made of two components: a cylindrical coupler which makes the PCC inner hole and a semi-torus shell which makes the outer surface of PCC and covers the coupler. The space between the coupler and the shell is closed and can contain a fluid (e.g. water) (see figure 16).

The coupler inner hole expands when the stored water is pressurised. Equal pressure distribution within the fluid space means that the contact surface becomes curved. Therefore the inner hole expands most at the coupler ends and the least in the middle of coupler and the cylindrical geometry of the coupler is changed to a hyperboloid geometry. Therefore for joining or welding the PCC and the pipes, it is important that the PCC sockets are located at the ends of the coupler where the hole expansion is at a maximum. If exaggerated, the coupler will have a kind of saddle shape cross section when pressurised, however in reality this deformation is small and may not be visible to a naked eye (see figure 17).

The coupler's inner diameter is designed to be slightly smaller than the joining pipes' outer diameter. The inner surfaces of the coupler at both ends are tapered to have a semi-conical (truncated cone) or chamfered shape. The outer surfaces of the pipes are also tapered at both ends to have a semi-conical or chamfered shape. The chamfered section of the pipe is inserted into the chamfered socket of the coupler (see figure 18).

Although the chamfered sections can have varying angles and designs, they should be machined at the outer face of the pipe and the inner face of the coupler and not at the edge of the pipe or coupler. The chamfered sections are not extended through the whole wall thickness of the pipe or the coupler and therefore the pipe and the coupler have part-plain and part-sloping ends. The pipe and the coupler are typically tapered to a small angle, for example less than 10 degrees relative to the pipe or coupler longitudinal axis. A gently tapered slope at the ends increases the chamfered section width, so that more length of pipe can be inserted into the coupler, therefore increasing the contact surface between the pipe and the coupler.

In this paper the "start edge" of the coupler chamfered section means the edge inside the coupler and the "end edge" of the coupler chamfered section means the edge at the end of coupler (see figure 21).

The chamfer angle of the PCC changes when it is pressurised. Design of the pipe chamfer should take this variation into account, especially for friction welding applications. The pipe chamfer angle should be matched with the PCC chamfer angle when pressurised so that the chamfer surface of the pipe almost perfectly fits with the chamfer surface of PCC when the pipe is inserted into a pressurised PCC.

Achieving this match between the PCC and pipe chamfer surfaces requires tight dimensional tolerances and machining the chamfered sections requires dimensional accuracy. These requirements do not compromise typical line-pipe manufacturing tolerances. A line-pipe has an acceptable wall thickness tolerance (typically ± 12.5%) and an acceptable ovality or out of roundness tolerance (typically 0.01 diameter). Machining the outer surface of the pipe corrects the pipe manufacturing tolerances at the pipe-end chamfered sections. For example after machining, the pipe ends can have chamfered sections with varying sloping width and plain sections with varying thickness around the pipe circumference. However the machined chamfered surface of the pipe can be almost perfectly fits into the coupler sockets (see figures 23 and 24).

The chamfer axis is not necessarily projected on the pipe axis. Although it is preferable to machine the pipe such that the chamfer axis becomes parallel with the pipe axis, this is not an essential requirement and some tolerance is acceptable. The pipe may be intentionally chamfered so that the chamfer axis has a specific angle relative to the pipe axis, for example to create a gentle mitre bend, which can be used instead of pipe cold bending for onshore pipeline construction. It should be noted that whilst mitre bends are not usually acceptable for pipeline girth butt welds, they may be acceptable in a PCC-pipes assembly (see figures 25 and 26). A line-pipe with a positive wall thickness tolerance should be machined more than a line-pipe with a negative wall thickness tolerance, so that both pipes have a similar chamfered (semi-conical) shape. The chamfer shape, size and dimensions should be standardised for different applications, but the standardisation of the chamfered sections of pipe or coupler is not within the scope of this paper.

It should be noted that line-pipe ends are also machined and bevelled at the edge for a typical common welding procedure like GMAW. The plain ends of the pipes are usually bevelled with a sharp angle (e.g. 60 degrees) relative to the pipe axis, thus creating a "V" shape groove between the two pipes for butt welding. However, this machining does not help with correcting the manufacturing tolerances at the butt weld joint because the bevel extends through the whole wall thickness of the pipe. The bevelled ends of two line-pipes cannot be perfectly matched due to the manufacturing tolerances. In rare cases, the mismatch between the pipes results in a failure late in the life of the pipeline, because of localised stress concentration. This is despite the butt weld being qualified and the pipeline passing its hydrotest during construction.

The PCC coupler material and mechanical design should be chosen to satisfy the overall pipeline design. However, not all of the coupler's mechanical design characteristics may be determined by the pipeline design. The joining force and radial compressive force between the pipes and PCC will depend on the coupler's mechanical characteristic. For example, to achieve an increased joining force for welding, a coupler may require a thicker wall than the line-pipe (see figure 19).

The design of the PCC shell is mainly based on the pressure rating of PCC and the pressure required to expand the PCC, regardless of the line-pipe design. The shell wall thickness is expected to be thinner than the coupler since the shell is seeing almost pure tensile stresses when the PCC is pressurised, while the coupler sees bending stresses. The shell and coupler can be made from similar or different materials. They can be made in one cast or one forge or they can made separately and welded together. The PCC should be standardized and could be made on a factory assembly line, to allow mass production to reduce the unit cost. However, the method of PCC production is not within the scope of this paper.

The shell may be designed with a different cross sectional curvature (rather than a half circle as described in this paper) and the coupler may be designed with a different cross sectional curvature (rather than a straight line as described in this paper) however PCC base design is a cylindrical coupler with a semi-torus shell.

The PCC shell can be machined and removed after the pipes are jointed or welded to PCC's coupler. This may be required due to pipeline trenching, free expansion, inspection, etc. 6. Radial Pressure Welding

Radial Pressure Welding (RPW) is a solid state friction welding technique which uses a PCC for joining two pipes. RPW can be used for joining steel, alloy, metal, plastic or polymeric material pipes. Using this method, a pressurised PCC is rotated about two stationary pipes which have been chamfered and inserted into the PCC as described above. The friction between the chamfered sections generates heat and raises the temperature of the touching faces. A force may be applied to push the pipes into the PCC while it is rotating to increase the friction and generate more heat. This continues until the touching surfaces between the pipes and the PCC reach a suitable temperature for forge welding. The PCC is then suddenly depressurised, such that the PCC's coupler contracts and the heated surfaces of the pipes and the coupler are pressed together to become metallurgically bonded. The two girth (circumferential) welds generated by this method are called "RPW welds". The sudden radial force generated by the coupler contraction compresses the chamfered sections of the coupler over the chamfered sections of the pipes. The chamfered sections of the PCC-Pipes assembly are sufficiently heated by friction so that this compressive force welds the components together (see figure 19 and 22).

It should be noted that other sources of heat commonly used in welding (e.g. induction, electrical arc, direct flame, ultrasonic) may be used instead of friction to heat up the chamfered faces of the pipes and coupler and make an RPW weld (see figures 98 and 99). 7. RPW Weld Design

It is accepted that butt welding is best practice for joining pipe sections (line-pipes). A butt weld is a fully penetrated weld and forms a continuous pipeline body when joining the line-pipes. Therefore, from design point of view, a girth butt weld behaves like the body of the pipeline. The stress and strain in the pipeline due to external and operating loads (such as internal pressure, expansion etc.) are similar in the girth butt weld section to those in any other section of the pipeline (see figure 30).

However, the welds generated between the coupler and the pipes by the RPW technique behave differently to a typical girth butt weld. Using a PCC for joining line-pipes does not form a continuous body for the pipeline. Hence, the joint responds differently to the external and operating loads than the rest of the pipeline (see figures 22 and 29).

Longitudinal (or axial) external load on the pipeline results in shear stress in the RPW weld. A bending load on the pipeline also results in shear stress in the RPW weld. In general, the shear strength of a ductile material is less than its tensile strength and vice versa for a brittle material.

Pipeline materials must be ductile and therefore they have lower shearing strength than tensile or compressive strength. For example, the shear yield stress of steel is slightly above half of its tensile yield stress.

Considering the above, an RPW weld width should be greater than the pipe wall thickness, in order to provide an equal axial strength in the joint as in the body of the pipeline. As an example, the RPW weld width may be twice the pipe wall thickness. In this case, for a specific axial load, the shear stress in the RPW weld is half of the tensile stress in the pipe body. So in this design, the shear strength of the weld is higher than the tensile strength of the pipe and therefore if an extreme longitudinal or bending force is applied to the pipeline, the pipe will fail before the RPW weld is sheared (see figure31). A shear-based weld design (for example a coupler-pipe assembly with two typical lap-joint girth welds) is not economical for common welding techniques like GMAW, because such a weld would need almost four times as many weld passes compared to a standard girth butt weld. Using the RPW technique, the two welds between the pipes and the coupler are generated simultaneously in one shot, regardless of the required width of the weld. Therefore RPW welding can have economic advantages over other techniques (see figure 34).

An RPW weld between the pipes and the coupler is a shear-based design weld. Unlike a typical girth butt weld, an RPW weld does not contribute to the pipe hoop (circumferential) strength against internal pressure. The hoop stress in the pipe and the coupler due to the internal pressure is in a different plane to the RPW weld. Looking at a two dimensional cross section of the PCC-Pipes assembly (ignoring the Poisson effect for simplicity) the PCC-Pipes assembly reacts similarly to internal pressure with or without an RPW weld. This characteristic of an RPW weld is important and has many advantages as listed below (see also figures 27 and 28). • The pipeline never bursts due to an RPW weld failure, since the weld does not contribute to the pipe pressure resistance capacity. Therefore an RPW weld has better integrity than the standard girth butt weld. • A corollary of the above is that any defects or unbound surfaces in an RPW weld can only result in a leak and not in a burst failure. • A corollary of the above is that any growing crack in an RPW weld can only result in a leak and not a burst failure. • A corollary of the above is that an RPW weld has a much longer fatigue life than a girth butt weld and can resist cyclic loads much better than a girth butt weld. • Because an RPW weld does not contribute in the pipe pressure resistance, and the line-pipes and PCCs are pressure tested in the factory (a mill strength test) the welded joints and the pipeline do not need a pressure test (strength test) after fabrication. This saves time and reduces the overall cost of construction, especially for offshore pipelines. • A defect in a girth butt weld directly reduces its strength against internal pressure, because a girth butt weld acts like a tensioned girth (or circular) chain. If a chain link fails, then the whole chain will fail and become useless. Hence, a defect in the weld can result in a pipe rupture. However, an RPW weld acts like many short tensioned chains, arranged around the pipe circumference parallel to the pipe axis. If a chain link fails, other chains carry the load of the broken chain. This indicates that an RPW weld can have a higher acceptable defect threshold compared to a girth butt weld. This simplifies inspection and reduces nondestructive testing (see figures 29, 30, 31 and 32). • Residual stress has a smaller distribution in an RPW weld compared to a butt weld. This is because the RPW weld can be generated quickly (e.g. in less than 20 seconds). Therefore, the touching surfaces of the PCC-Pipes assembly are heated up to a high temperature but only locally, as there is not enough time for the deeper sections of the joint wall to reach a high temperature. More importantly, the residual stress in an RPW weld is a radial stress. Radial stresses in pipes (especially thin walled pipes) are small or negligible. Therefore the combination of the RPW weld radial residual stress and the radial stresses from operating forces should be well within allowable limits (see figure 33). In contrast, the residual stress in a butt weld is mainly axial which is directly combined with the operating stresses. This is important in fatigue design and for stress corrosion cracking. The magnitude of RPW residual stress is unknown at this stage, but can be as high as the material yield point, similar to fusion butt welding.

It should be noted that shear-based joint design is well known in the pipeline industry and can be seen, for example, in pipeline repair clamps, split tees for-hot tap applications, smart flanges, threaded couplers and plain couplers with adhesives.

For offshore pipelines, the coupler in the PCC also acts as a buckling arrestor. This is a design advantage which prevents propagation buckling due to external hydrostatic pressure. A pipeline which is constructed with PCCs does not have a continuously uniform interior surface as the couplers are creating steps inside the pipe. These steps can tear the seals of a pipeline pig or other tools. Therefore the inner edge of the pipe should be rounded and any sharp edges removed.

This joint step can also generate turbulence in the pipeline fluid, which can have a negative or positive effect. In some cases turbulence is unfavourable and can increase the pressure drop in the pipe and increase pipeline erosion.

To mitigate these issues, the inner surface of the pipe can be tapered at both ends to make a smooth transition between the inner surface of the pipe and the inner surface of the coupler. Another option is to insert a bushing into the coupler's hole. The bushing can be a short length of pipe which is extended between the "start edges" of the coupler chamfered sections in such a way that, after welding, the bushing stays flush with the inner surface of the pipe. The coupler bushing should have the same internal diameter as that of the pipeline and should be bonded to the inner surface of the coupler. The bushing could be made from the same material as the coupler or of a different material, for example, a steel coupler could have a bushing made of High Density Polyethylene (HDPE). The bushing could also be formed from the coupler body, for example by machining (see figures 35, 36, 37 and 38). 8. RPW and Pipeline Materials RPW can be used for welding a variety of pipe materials including metals such as steel and a variety of alloys, plastics and non-elastomeric polymers pipes (including those listed below), composites and many others.

Common plastic pipe materials that are suitable for RPW include: ABS (Acrylonitrile Butadiene Styrene), PVC (Polyvinyl Chloride), UPVC (Unplasticized Polyvinyl Chloride), CPVC (Post Chlorinated Polyvinyl Chloride), PB (Polybutylene), PP (Polypropylene), PE (Polyethylene), LDPE (Low Density Polyethylene), HDPE (High Density Polyethylene), PVDF (Polyvinylidene Fluoride) and PERT (Polyethylene of Raised Temperature Resistance).

Since RPW is a solid state welding technique, it can be used for joining high and ultra-high strength carbon steel pipes, including pipes made of steel grade X100 and above. In the melt-free RPW procedure HAZ is minimised and grain growth in the material microstructure is avoided. This feature, which may be unique to RPW, is important since using high strength material in a pipeline project significantly reduces the cost. For example if the carbon steel grade X65 (commonly used for offshore pipelines) is replaced by grade X120, the wall thickness and the weight of the pipeline can be decreased by about 40%, while the additional material required to manufacture the PCCs only amounts to about 1% to 2% of the overall pipeline weight. An additional benefit is that reducing the weight of the pipe sections also decreases the transportation cost for both onshore and offshore projects (see figure 33). RPW may simplify the welding procedures for advanced alloy materials and solve the welding challenges in welding these alloys. RPW may be remove the need for Post Weld Heat Treatment (PWHT) for pipes made of thick C-Mn steel, Corrosion Resistant Alloys (CRA) such as SS316, SS304, Duplex, Super duplex, Inconel 625 and 825 or other alloys. One of the aims of PWHT is to reduce the residual stresses in the weld. This reduces the risk of Hydrogen Induced Cracking and Stress Corrosion Cracking. Residual stress in an RPW weld is a radial stress with a small distribution, and the stress level is lower at the joint section compared to the pipe body (e.g. the hoop stress at the joint section is less than half of the hoop stress at the pipe body). This is simply because the wall thickness of the pipe and the coupler are added together at the welded section. Given the above, costly and time consuming PWHT operations may not be necessary for an RPW weld. In general PWHT of welds is time consuming and costly.

The inner surface of a Carbon Steel PCC can be clad with a CRA material to become corrosion resistant. The RPW capability to weld different materials makes it ideal for welding cladded pipes. The touching surface of the pipes and the coupler (the chamfered sections) can also be clad to become corrosion resistant, which helps to prevent build-up of iron oxide or other debris on the chamfered faces prior to welding.

In a similar fashion, the inner surfaces of the coupler and the chamfered sections of pipes and couplers can be alloy plated for corrosion protection and weld cleanliness. The plated alloy may be specifically selected to improve the weld quality and mechanical properties. 9. RPW Equipment and Tools RPW equipment is not complex or expensive. It has two main components: a stationary system for aligning and fitting the PCC-Pipes assembly and a revolving system for spinning the PCC, enabling friction welding.

These components can sit inside or outside of the pipe or PCC-Pipes assembly. RPW tool design will vary depending on the application and the size of the pipeline. While there are many design options for RPW equipment, and detailed design of RPW equipment is not within of the scope of this paper, one is expanded here by way of example (see also figure 39).

External Supports: Like any typical pipeline construction project, the pipeline which is welded by RPW should be supported above the ground so as to be accessible while it is being constructed. The constructed section (welded section) of the pipeline should sit on fixed supports. These can be as simple as a block of concrete or a timber. They can have an adjustable height mechanism (like a car jack) or they can be simple wedge shapes made of wood or other material. The line-pipe which is to be connected (welded) to the constructed section of pipeline should sit on aligning supports. These supports should have rollers to hold the pipe, to move the line-pipe along it length and to rotate the line-pipe about it longitudinal axis. The rollers should be able to move both vertically and horizontally so that they can vary the height and orientation of the line-pipe to align it with the constructed section of pipeline. The aligning supports could be motorised and could use a control system. They could also be equipped with laser beam measuring system and be computerised so as to be able to align the line-pipe automatically and with high accuracy. Tools with the functionality described above are commonly used on boats for offshore pipeline construction. The number of supports and their arrangement can vary according to requirements, however a few supports may be required close to the joint at each side (see figure 39).

Internal line-up clamp: A line-up clamp is used to provide more accurate alignment. The clamp supports and fixes the joint before welding. Line-up clamps are well developed tools in the pipeline industry. The clamp can be placed internally for large size pipes and externally for small size pipes. A line-up clamp for RPW will be similar to a typical clamp, but it needs handle 3 pieces (2 pipes and a PCC) where the middle piece (the PCC) needs to rotate. The RPW clamp may also incorporate an additional mechanism to pull the line-pipe toward and away from the constructed section of the pipeline. This pulling mechanism would push the pipe ends in and out of the coupler and can be powered by hydraulic, magnetic or mechanical means. Like existing line-up clamps, an RPW clamp can use hydraulic, magnetic or mechanical tightening and releasing mechanisms. The RPW clamp may be equipped with load sensors, laser measuring tools and have a computerised control system. Among other things, this will allow a clamp to communicate with the supports in order to place the line-pipe such that it minimises the weight of pipe on the joint before welding (see figure 40).

Revolving system: The revolving system can be internal or external. An internal revolving system is a part of the function of an RPW line-up clamp. The clamp would have a motor at the centre which rotates the PCC (see figure 40). In order to avoid uncontrollable vibration during the RPW procedure due to an imbalanced weight in the PCC, the PCC should be balanced at the factory prior to welding. This balancing process is similar to that used for balancing a turbine shaft or a car tyre. The revolving system may, however, incorporate an additional balancing system to adjust any minor imbalance in the rotating assembly and minimise vibration. The rotational frequency of PCC should be different to the natural frequency of the pipeline to avoid any resonance effects. The external supports may also incorporate dampers to absorb vibration. The revolving system could also use a mass dynamic vibration absorber to reduce the vibration. Such a tool could be installed outside of PCC-Pipes assembly on a stationary pipe close to the joint. It could also be part of the external supports.

The PCC shell could also incorporate internal baffle plates. The baffles would rotate the water along with the PCC body, and since the mass of stored water is considerable, the water would obtain a large amount of rotational kinetic energy. This would give the PCC a reasonable rotational inertia which would allow the PCC to continue rotating if the motor is switched off. The rotational inertia in the water would also help to optimise the motor power output (see figures 54 and 55).

Bursting mechanism: As described in the foregoing, RPW welding requires that the PCC is depressurised as quickly as possible. This speed is increased if the PCC's shell can be machined to burst while it is still rotating. To achieve this, a bursting mechanism is envisaged which is very similar in function to a lathe. The body of the bursting mechanism is fixed either by having its own support fixed to the ground or by connection to stationary pipes or by connection to an external pipe support. The mechanism has a cross slide, a swivelling top slide and a tool clamp (like a lathe) with a cutting tool installed in the clamp. While the PCC is rotating, the cutting tool moves forward to the shell and starts cutting, thus creating a groove on the shell. This process is continued, making the groove deeper until the remaining wall thickness of shell reaches a critical point and the shell bursts. The rotational inertia of the PCC helps to keep it rotating while its shell is being machined (see figures 39, 48 and 49).

The shell may be cut in a number of different places, but best practice would be that the tool cuts the shell in the middle (i.e. at the place of largest circular cross section) in such a way that the shell is cut symmetrically into two equal sections. The cutting tool is best located at the bottom of PCC-Pipes assembly so that the burst opening starts at the bottom and the stored water drains from the bottom of the PCC-Pipes assembly. A plastic ring or similar protection should be used to cover the outer edge of the weld if the pipe and coupler are not coated. This is because splashing water on the weld can result in sudden cooling of the weld edge, which may affect the mechanical properties of the material at this location.

The bursting mechanism should be fitted with a protection cover for safety purposes. During the RPW procedure, the joint assembly should remain fully covered until the shell burst and the weld is completed. The shell bursts in a controlled manner. A baffle internally covers the machined groove and the burst area. This baffle prevents the release of large amount of fluid after bursting. When the shell bursts and a section of the machined groove is opened the stored fluid in the PCC cannot be released freely since it must pass through the baffle's holes. The baffle behaves like a damper and controls the energy release of the burst.

As an alternative, the PCC could have a valve to enable pressurising and depressurising. This valve should be a control on/off valve so it can be opened quickly to release the pressure. This valve should be installed on PCC's shell.

Computerised controlling and recording system: rpw is a quick welding procedure and therefore many actions need to be arranged and performed, in a specific sequence, in a short time. A computerised control system is therefore recommended. This system should also measure and record all relevant data during the welding procedure. This data includes, but is not limited to: PCC pressure; PCC expansion and deformation due to pressure; PCC rotational speed before welding; PCC weight; alignment tolerance; pipe chamfer and coupler chamfer match tolerance; contact surface temperature gradient; heat distribution; shell burst time; length of time to heat up; frictional force and line up clamp pulling force. 10. RPW Procedure

The RPW welding procedure may vary depending on the application (e.g. offshore or onshore use). As an example, one possible onshore procedure is detailed here, based on using the tools described above. Offshore considerations are also described.

Onshore Procedure

The steps listed below describe a typical onshore RPW welding procedure, with 4 main stages (preparation, alignment and assembly of joint, rotating the PCC and welding). 1. Preparation a) Alignment supports are replaced by fixed supports for the newly welded pipe section. b) Alignment supports are put in place along the constructed section of pipe in order to support a new line-pipe. A line-pipe is lifted and laid on the alignment supports. c) A PCC is fully filled with water. Trapped air should be avoided since it makes the PCC imbalanced. Large amounts of trapped air are also dangerous in the bursting procedure, since the pressurised air absorbs much more potential energy than the water. To minimise trapped air in the PCC, the shell could have a small filling hole, fitted with a check valve through which to fill the shell and also a needle-size hole nearby, to allow air to escape. The check valve functions in the same way as a car tyre valve, and the valve body should sit inside the PCC. When the PCC is full of water, a needle size pin may be inserted into the air escape hole and this hole closed by a spot weld or solder (see figure 52 and 53). It is recommended that a small mobile workshop (e.g. in the back of a truck) be available for PCC preparation. d) More water is pumped into the PCC to pressurise the stored water. To provide additional safety, the PCC should be covered while it is being pressurised. The PCC final pressure must be clearly defined in the procedure. The pressure and time shall be accurately measured and recorded. After the PCC is pressurised, the valve hole is spot welded or soldered. This minimises the risk of leakage during the welding procedure (see figure 53). The expansion and deflection of the PCC may also be measured and recorded.

An alternative is to fill and pressurise the PCCs with water at the factory, thereby making them ready to use at the construction site. However this may be ruled out by safety concerns since in general, transporting pressurised equipment is not recommended. 2. Aligning the pipe and assembling the joint a) The RPW tools are disconnected from the previous weld. b) The inner line-up clamp is released from previous weld joint and is moved through the pipeline to reach the open end (see figure 42 and 47). c) The welding tool's battery or nitrogen bottle are charged or replaced with new ones if necessary (see figure 41). d) A pressurised PCC is placed on the inner clamp revolving machine and is fixed there (see figure 43). e) The new line-pipe is pulled into the PCC and a preliminary alignment is performed with the pipeline by using the alignment supports (see figure 43). f) All RPW tools are put into place. g) The protection covers on chamfered sections of the clamp and pipes are removed. It is believed that the flash in a typical friction welding operation takes the weld debris away with it, thereby providing a self-cleaning mechanism. This is not the case with RPW welding, since the plastic deformation is not big enough to create a flash. Therefore, in RPW operations, the touching surfaces must be as clean as possible without any debris or dirt prior to welding. The cleaning can be performed on site, but in order to save time and cost, it is preferable that the chamfered ends of pipes and PCC are cleaned and covered at the factory after being machined. A polymeric resin can be used to cover the chamfered sections to protect their surfaces from corrosion and any possible dirt. The polymeric cover should not be bonded to the steel and should be easily removable. The chamfered faces of the coupler and pipes may also be alloy plated in order to avoid corrosion and to keep them clean (see figure 93).

The preparation and set up time for the pipes and tools for RPW welding is expected to be similar to typical automated welding techniques like GMAW.

The following stages would be performed automatically by the RPW tools. h) An inert gas starts blowing. An inert gas is not typically used for friction based welding. The necessity of an inert gas for RPW is not clear at this stage. If required, the gas can be released from the inner clamp in such a way that the gas flows from inside the PCC-Pipes assembly to the atmosphere via the gaps between the PCC and the pipes. Purified Argon and Nitrogen are most commonly used as an inert gas or a shielding gas. i) The new line-pipe is pushed into the PCC so that the PCC is firmly compressed between the two pipes and the chamfered sections of the coupler sit on the chamfered sections of the pipes. j) The inner line-up clamp is locked and any final alignment is performed by the clamp and the aligning supports (see figure 44). A laser dimensional measurement can be made on the PCC-Pipes assembly to ensure that the chamfered sections fit perfectly.

3. Rotating the PCC a) The line-pipe and the PCC are slightly pulled pack by the line-up clamp to free up the PCC for rotation (see figure 45). b) The revolving machine is powered and PCC starts rotating. The motor remains on until the PCC reaches the specified rotational speed (see figure 45). It should be noted that the energy consumed by RPW welding (i.e. the energy required for rotating the PCC) is much less than the energy consumed by conventional fusion based welding techniques like GMAW. c) System vibration should be kept under control. The rotational speed should be set so that it is not close to the pipeline system's natural frequency. Dynamic vibration absorbers should be adjusted to minimise any minor vibration. 4. Welding a) The new line-pipe is pulled in by the line-up clamp and the PCC is firmly compressed between the two pipes while it is rotating. The friction between the coupler and the pipes heats the touching surfaces to the temperature required for forge welding. In this step of the procedure, the temperature and speed should be measured and recorded accurately (see figure 46). The faces of the pipe and coupler chamfered sections can be knurled. This may help with better heat distribution and may also provide room for plastic deformation. Various knurled patterns can be envisaged, for example a pattern with series of straight ridges, or a helix of straight ridges, or a typical criss-cross pattern. The behaviour and advantages of a knurled chamfer face are unknown at this stage.

If the line-up clamp does not have a mechanism to pull in the new line-pipe and compress the PCC-Pipes assembly, a factory-made micro hole can be drilled into the PCC shell, so that the water leaks out gradually after the PCC is pressurised. In this way, the PCC contracts gradually while it is rotating, thereby compressing the touching surfaces, increasing the friction and generating heat. Although this method could be a viable option for achieving a compressive force to increase the friction, it would appear difficult to control the rate of the leak and consequently to control the load and the heat generated. b) The bursting machine is activated. A cutting tool is moved forward to the PCC shell, which makes a girth (circumferential) groove on the shell. Cutting is continued and the groove gets deeper until the shell bursts and the required upset load for friction welding is released in a fraction of a second. The rotational motor should be powered off before the PCC bursts. After the shell bursts the PCC stops rotating immediately, because it becomes welded to the stationary pipes. Two RPW girth welds are generated between the pipes and the coupler. The welding stage is performed in a few seconds.

Offshore Considerations

The procedure described above is intended for onshore RPW welding. An offshore procedure would be different in stages 1 (preparation) and 2 (Aligning the Pipe and Assembly of the Joint). However it would be quite similar in stages 3 (Rotating) and 4 (Welding).

To speed up the process of offshore construction, two line-pipes are often welded together to make one longer segment. For example two 12m line pipes are welded together in advance to make one 24m pipe segment. The RPW procedure is well suited to this arrangement. Since the pipe-lay boat moves relative to the sea bed, alignment supports used in offshore operations can be fixed to the boat and can be arranged at both sides of the PCC-Pipes assembly, where they perform the same function as the onshore alignment supports described above. In offshore construction, the onshore fixed supports described above are replaced by typical offshore pipeline tensioners, which are commonly used for paying off the pipeline.

Unlike pipeline reel-lay boats which can continually pay out pipeline and lay it on the seabed, S-lay or J-lay vessels make intermittent movements with many stops, to allow the line-pipes to be welded to the pipeline and allow the field joints to be coated. This reduces the speed of laying. A typical speed of reel-lay is about 14 km/day (or 9.7m/minute) while typical speeds for S-lay and J-lay are respectively about 5 km/day (or 3.5m/minute) and 1.5 km/day (or lm/minute). It is estimated that the overall time to perform the RPW weld procedure is about a minute. This means that RPW welding can be executed at around 24m/minute (or 34.5km/day). This is higher than the reel-lay speed. Therefore, using RPW for offshore pipeline construction should significantly increase the speed of S-lay and J-lay vessels, above even that of reel-lay boats. This reduces vessel time and consequently decreases project construction costs.

The S-lay or J-lay boats may have sliding decks or sliding tables, so as to be able to pay out the pipeline continually and avoid intermittent movement of the vessel. Using RPW welding, all alignment supports and RPW welding tools are fixed to the sliding table, so that when the table slides, the tools and supports also move relative to the boat. Whilst detail design of this mechanism is not within the scope of this paper, an example is provided here by way of illustration (see figure 59).

Imagine a sliding table which can move along the length of a boat, where the boat pays out the pipeline from the stern and has typical pipeline tensioners installed at the stern. The table moves toward the bow of the boat, where a new line-pipe is loaded on the table and it is connected (e.g. by a clamp) to the end section of the constructed pipe. As the pipeline is being paid out, the table moves toward the boat stern and slides with the pipeline. The newly-loaded line-pipe is aligned and welded to the constructed pipeline, while the line-pipe, welding tools and the supports are all moving with the table towards the stern of the boat. A new section (for example a 24m section) has now been added to the pipeline. The table is then disconnected from the pipeline, moved towards the bow of the boat and the procedure is repeated (see figures 56, 57 and 58). A boat can easily have 2, 3 or more welding tables on its deck, so that it can weld and lay several pipes of varying diameters and specifications simultaneously. Having several fabrication units on the boat increases its productivity and significantly reduces vessel time. 11. RPW Weld Finishing

The PCC shell must be removed after the welding procedure is complete. On occasions, the shell is left on the pipeline. Cutting the PCC shell is a basic task and can be done manually without need for skilled workers. Automated cutting tools (for example various saws, milling machines and cutting tools) can be used to cut the shell. Press cutting tools can also be used to cut the shell and press forming tools can be used to flip the edges of the shell that has been cut (see figure 22).

The shell can be discarded as a waste material or it can be used as a support for the pipeline, especially for large diameter pipelines laid above ground. For this, the shell can be cut in two along its diameter to make two half rings. This half ring can be installed on the ground in varying arrangements such that the pipeline sits on the curved back of the ring (see figures 60 and 61). A further option for removing the shell is to machine the shell while it is still rotating. In this method, the shell is machined using the method described above to achieve bursting, but at three points: one in the middle of the shell for bursting and one at each side of the shell, where the coupler and shell are connected. Three circumferential grooves are cut simultaneously into the shell surface, and the machining is controlled such that the middle groove reaches the critical burst depth sooner than the two side grooves and the shell bursts at the middle groove. This method assists with removing the shell, since part of the shell's wall where it joins the coupler is machined during the welding operation, and only the remaining part of the wall needs to be removed after the welding is complete in order to separate the shell from the coupler (see figures 50 and 51). A refinement of the above is to fit the PCC with internal baffles. The baffles are distributed along the circumference of the PCC, positioned radially and only connected to the shell. The baffles catch the stored water and rotate it with PCC, whereupon the water mass attains high rotational inertia. When the shell bursts, the pressure is released immediately and the PCC stops suddenly because it has become welded to the stationary pipes. However the water trapped between the baffles has inertia and still wants to move, so it pushes the baffles, making the shell rotate whilst the coupler has stopped. This results in a strong shear force between the shell and the coupler. If this shear force is designed to be higher than the shearing capacity of the remaining shell walls (once the side grooves have been cut) then the shell will be sheared off and will separate from the coupler. Further, the side grooves may be heated (using friction) to reduce the shear yield point of the material in the vicinity of the shell-coupler joint. It is possible for these methods to be used to reduce energy consumption and reduce labour time in removing shells (see figures 54 and 55).

Almost all steel and alloy pipelines need an anti-corrosion coating. Many pipelines have an insulation coating to improve thermal performance or have a concrete coating for protection and stability. To allow welding, the line-pipe ends cannot be coated. Furthermore, the heat from fusion welding damages the anti-corrosion, insulation or concrete coatings. Hence, there needs to be a bare pipe section at both ends of the line-pipe for welding. Additionally, the equipment required by welding machines (e.g. the rails for welding bugs) may require that end sections of pipe are left bare. This means that after welding, these bare sections must be coated on site, referred to as field joint coating. Field joint coatings are time consuming and costly and typically the field joint coating doesn't give the same performance as the main body coating material.

One important advantage of RPW is that it minimises the field joint coating requirements or removes them altogether. The heat generated by RPW welding is localised and does not transfer far from the weld. Therefore, line-pipes can be fully coated along their whole length. To cater for chamfering requirements, the coating thickness can be increased at the plain ends of the line-pipe. The steel and coating are machined together such that the coating is tapered along, and flush with, the steel chamfered section.

Similarly, two polymeric or plastic rings can be bonded to the end faces of a plain end coupler. When the coupler is chamfered for RPW, the plastic rings and the coupler are machined together such that the plastic rings are tapered along, and flush with, the coupler chamfered section.

Using this method in a PCC-Pipes assembly, the steel chamfered sections of coupler face the steel chamfered sections of the pipe, and the plastic (polymer) chamfered sections of the coupler face the coated chamfered sections of the pipe. While the steel sections are heating up to become welded, the polymeric sections are also heating up and become welded. Therefore, welds are performed simultaneously between the steel sections and between the polymeric/coated sections of both the pipes and the coupler (see figures 62 and 63).

The outer surfaces of the coupler (i.e. the internal surfaces of PCC) can be factory coated so that after removing the shell (described above) the exposed section of coupler is found to be already coated. In this case, the only uncoated surfaces that remain after welding are the cross sections of the PCC shell after it has been cut. These areas are not sensitive, so they can be left exposed, but they can be easily and quickly covered by a basic coating if necessary.

Thermal insulation coatings are usually thick coatings. It is possible that the plain end of this coating can be welded to the PCC shell while the pipe is being welded. The coating can be finished adjacent to the pipe chamfered section. The plain end of coating can be lightly machined and chamfered with a negative angle, so that the outer face of coating comes forward. During the weld, the outer edge of the coating will touch the shell and be heated up due to the friction generated, and so melt and become bonded to the shell. When the shell is cut, the cut is made above the insulation coating, so that a section of shell is left between the pipe insulation and the coupler's factory coated insulation. This left-over section of the shell has only a minor effect on the thermal performance of the insulation (see figures 64 and 65).

Having a concrete coated pipeline in offshore applications, and using the productive RPW-based S-lay and J-lay boats (described above) removes the need for trenching, backfilling and rock dumping. This reduces offshore construction costs (see figures 70 and 71). Concrete coatings mainly provide protection and stability for pipelines and therefore concrete coating does not need to be continuous along the pipe. In these cases, the shell may be left on the joint to act as a protective shield and a polymeric band can be rolled over the shell to cover the sharp edges of the burst section. If the shell has to be removed the coupler shall have a suitable factory made coating applied for protection. Also the PCC can be configured with circular wings to sit over any unprotected section of the pipe. These wings are made of two short length pipes (cylinders) which are connected symmetrically to either side of PCC shell. The wing inner diameter would generally be bigger than the outer diameter of either the coupler or the pipe. The wings could be made of steel or a polymer. RPW can also be used to join steel pipe-in-pipe systems. Using this method, the PCC performs 4 welds simultaneously in one shot. In a pipe-in-pipe system, the inner pipe is the main operational pipe, and the outer pipe is a shield for protecting the inner pipe and its thermal insulation. The outer pipe does not directly contribute to taking any operating loads, and therefore the outer pipe welds are secondary welds, used to seal the outer pipe, while the inner pipe welds are the main welds which carry the loads.

The PCC for a pipe-in-pipe joint could have circular wings connected to its shell, in such a way that while the coupler sits on the inner pipe, the wings sit on the outer pipe. The wings and the outer pipes have chamfered sections, and are welded together in the same way as normal RPW (see figures 66 and 67).

An alternative design is for the outer pipes to have plain or formed ends, which are flush with the PCC shell in the joint assembly. In this arrangement, the end faces of the outer pipes touch the PCC shell, are heated during the welding procedure and are friction welded to the shell (see figures 68 and 69).

Since the outer pipe welds are not structurally critical, resins or polymer materials can be used to bond the outer pipes to the PCC shell for sealing. A PCC can be used to weld plastic-lined pipes. A typical method for making such a pipe is as follows. Fabricate a long section of steel pipe (e.g. 500m or 1000m) and then pull a plastic liner through the fabricated steel pipe. These long sections have weld-links at their ends so that they can be welded together to make a long pipeline. The weld links usually have an alloy cladding. There is no plastic weld along the plastic liner and therefore theses long sections can only be installed by reeling and not by S-lay or J-lay methods.

For welding plastic-lined pipes, the PCC bushing can be made from the same plastic (polymer) as the plastic liner of the pipe. In the PCC-Pipes assembly, the plain-end faces of the bushing touch the plain-end faces of the plastic liner. The touching faces of the bushing and the plastic liner are heated up by the friction and become melted and welded together whilst the coupler is being welded to the pipe. The bushing and the plastic liner ends may have tapered sections for improving the welding of the plastic. The procedure for the plastic weld between the bushing and the plastic liner is similar to existing typical plastic pipe welding, but in this case, the heat is provided by friction rather than induction. Alternatively, a polymeric resin can also be used to bond the coupler bushing to the line-pipe plastic liner (see figures 72 and 73). 12. RPW and Pipeline Testing and Commissioning

Tension Testing:

As previously described, pressure strength testing is not required for pipelines which are constructed using RPW. This is because both the coupler and the line-pipes are pressure tested at the factory. Flowever the shear strength of an RPW weld may need to be confirmed by a tension test. A tension test tool is made of two split clamps which are connected by hydraulic cylinders. The clamps form the end sections of the tool, and the hydraulic cylinders form the middle section of the tool. The clamp has hinged sections (e.g. 3 semi-cylindrical sections connected by hinges) so that it can sit around the pipe (see figure 74).

The tension tool is installed on a welded line-pipe, between two couplers, in such a way that the tool clamps sit on the ends of the line-pipe. The clamps have circular jaws which sit adjacent to the edges of the couplers, touching the plain end of the couplers. After the tool is installed, the hydraulic cylinders are activated to expand and to push the clamps on the couplers. The hydraulic force generated tries to pull the coupler out of the pipe. This results in a tensile stress in the pipe, a compressive stress (bearing stress) on the coupler edge (plain end) and a shear stress on the RPW weld. In a typical design, the bearing strength of the coupler and the shearing strength of the weld should be greater than the tensile strength of the pipe. Thus, the hydraulic force shall be set against the pipe tensile strength. As an example, the pipe can be stretched to 90% of its tensile yield stress, but the weld may be designed to have smaller shear strength than the pipeline tensile strength. In this case, the hydraulic force should be set to create a precise shear stress in the weld, as specified in the test procedure.

Where the coupler edges are covered (for example by an insulation coating) or where they are not accessible (for example because of concrete coating) then PCC shell can be stiffened using stiffeners between the coupler and the shell so that the clamps of the tension tool can sit behind the PCC shell instead of the coupler (see figures 75 and 76).

Leak Test:

The PCC coupler may be designed such that it has a small groove on each of its chamfered faces. The groove is machined closer to the "start edge" of the chamfer (the terms "start edge" and "end edge" have been defined previously). With such as design, following completion of the RPW weld, a small ring-shaped hole will be formed which splits the RPW welds into two sections. The wider RPW weld section at the end edge of the chamfer is the main weld (which provides the joint strength) and the narrower section weld section at the start edge of the chamfer is the sealing weld (which holds the pipe content) (see figures 77 and 78).

The ring shape hole between the two sections of the RPW weld can be filled with fluid, which is then pressurised to provide leak testing of the RPW weld to a given pressure. For example the fluid can be pressurised to the pipeline design pressure times a safety factor. The pressurised fluid contained in the ring shape hole can confirm that both sections of RPW welds are sealed and watertight. This localised leak testing can be performed instead of an overall leak test of the pipeline. This would save time and significantly reduce the cost of pipeline commissioning, especially for offshore pipelines (see figure 80).

In order to fill the ring shaped hole with fluid and pressurise it, an access hole (or holes) can be drilled into the coupler after the completion of the weld. This should be drilled at the top of the pipe. After the leak test the access hole covered by a spot weld or solder (see figures 79 and 81).

The pressurised liquid in the ring shaped hole can also prove the quality of the welds. Although standard and typical weld radiography is applicable to RPW, using a radio contrast agent (typically iodine or barium compounds) can help to provide a better picture of the weld. A radio contrast agent can be injected into the ring shaped hole and then pressurised. The agent will penetrate into any cracks or unbound areas of the weld and will help to identify weld defects.

The fluid in the ring shape hole can be permanently stored, at pressure, over the whole operating life of the pipeline. The pressure can then be regularly monitored as a measure of the health of pipeline and confirmation of the integrity of the weld and the pipeline. If a crack should grow along the weld or across the wall, the stored fluid will leak into the pipe or out to the environment and a drop in pressure would act as a warning to the operator. Keeping the fluid stored in the ring shaped hole at a higher pressure than the pipeline content fluid would help to ensure that a crack is detected before the pipe content leaks out (see figure 81).

In order to have a fluid permanently stored at pressure, after the ring shape hole is filled, a special check valve can be inserted into the access hole. More fluid is pumped into the ring shape hole through the valve to pressurise the fluid. Once at the required pressure, hot melted soldering material can be injected into the valve to block the valve and trap the fluid. The valve body may be also sealed using a modified pipe compression fitting (lock bush fitting). In this case, a pin (for example a copper pin) is pushed into the valve body to plastically deform and block the valve. The protruding section of the valve body is then removed and the access hole covered by a spot weld or solder (see figures 80 and 81).

It is expected that the sealing section of an RPW weld will be more susceptible to corrosion than any other interior surface of the pipeline. This is because it is at the corner of the joint. Hence, the sealing section of the weld behaves as a corrosion coupon. If the pipeline is not properly managed (e.g. the correct quantity of corrosion inhibitor is not used during the operation) the sealing section of the weld will corrode, the fluid stored in the ring shaped hole will leak and the operator will be warned.

The pressure of the stored liquid in the ring shape hole can be measured by using various techniques. For example, a Piezoelectric sensor can be installed in the coupler groove before welding, so that after the fluid in the ring shaped hole is pressurised, the embedded sensor will measure the pressure. A Bourdon tube mechanism can also be used. A thin ring-shaped Bourdon tube can be installed in the coupler groove before welding. The tube forms almost a full circle, but with a small gap in its circumference. When the ring shape hole is filled with a fluid and pressurised, the external pressure applied by the fluid to the Bourdon tube closes the gap and a full circle is formed. This could act as a switch which activates a transducer. A long life battery and a transducer chip can be installed in the tube so that while the liquid is pressurised and the Bourdon tube gap is closed, the transducer sends a signal (for example through the pipe wall) showing that the weld and the pipe are in good condition (see figure 79).

The seal section of RPW weld can be of a different material from the main weld section. For example the pipes and the coupler can be made such that the seal weld section is made of CRA material while the main weld section is made of carbon steel (see figures 82 and 83).

Nitrogen Injection:

Oil and gas pipelines are commissioned before operation and the air or water is removed from the pipeline before introducing hydrocarbons. Offshore pipeline commissioning is usually a costly task. It is asserted above that RPW speeds up the welding phase. With RPW welding, the pipeline can be filled with nitrogen or other gases while it is being welded so that the pipeline is already filled with nitrogen and ready to accept hydrocarbons at the completion of construction. This considerably reduces the cost of an offshore pipeline project.

One way of achieving this is as follows. The line-up clamp used for RPW welding could have a nitrogen bottle installed at its head. The bottle on the tool shall be easily replicable with a new fully filled nitrogen bottle or shall have a quick disconnect system to be charged and refilled via a hose in a matter of a few seconds. The welding tool can also have a battery at the head to supply power for its systems. The battery shall also be easily replicable with a new fully charged battery in about a few seconds. This can be facilitated by ensuring that when the welding tool moves toward the end of the pipeline and the battery and the nitrogen bottle are the first parts which emerge (see figure 41).

An intelligent pig can be located downstream of the pipeline when the pipe is laid on the seabed and connected to the welding tools by an umbilical. The umbilical contains several cores, including a hose for nitrogen injection, a power cable, a communication/data cable and a puling rope. The pig is motorised and moves with the welding tool inside the pipe while the pig is on the seabed and the welding tool is on the boat. Nitrogen is injected by the welding tools into the downstream end of the pipeline via the pig, so that the pipe volume behind the pig is being filled with nitrogen while the pipeline is being constructed. The intelligent pig can provide all of the necessary information regarding the as-laid condition of pipeline and can even transfer it to the welding tools for storage/recording. Should the pig break down, it can be pulled from the pipe by the rope which is connected between the pig and the welding tool (see figure 59).

Weld Non Destructive Testing:

Most of Non Destructive Test (NDT) methods that are used for inspecting a butt weld can be also used for inspecting an RPW weld. The radiography can be made more accurate by use of radio contrast agents. Magnetic Flux Leakage (MFL) can also be a more accurate way of inspecting an RPW weld. Although MLF cannot define the depth of a defect in a typical butt weld, this is not a concern for RPW welding, since the defect can only be at the interface of the pipes and the coupler, and the depth of the defect is already known. Ultrasonic methods are also suitable for inspecting RPW welds. Intelligent pigs equipped with ultrasonic or MFL tools can be used during operation for in-line inspection of the pipeline and RPW welds.

13. Advantages and Disadvantages of RPW RPW has almost all of the advantages of solid state and friction welding. The advantages and disadvantages of RPW can be summarised as follow: RPW Advantages: 1. It is a fast welding procedure. Reducing construction time is critical to reducing the cost of pipeline projects, especially for large diameter pipeline and offshore pipeline projects. It is estimated that the overall time to complete an RPW weld procedure (including preparation and alignment) is one or two minutes, while the weld itself is generated in a few seconds. 2. It has the ability to weld high- and ultra-high strength steels (for example steel grades X100, X120 and above) and materials with tensile strength of above 700 MPa. This significantly reduces the material weight and cost. 3. It has higher weld integrity compared with a butt weld. Since an RPW weld does not affect hoop strength, an RPW weld failure can only result in a leak and not a burst. 4. It has the ability to weld dissimilar materials, for example steel to polymers. 5. It has the ability to weld complex alloys, dissimilar alloys and steels with different tensile strengths. 6. It is a melt-free procedure, which minimises the HAZ and avoids grain growth in engineered materials such as high-strength heat-treated steels. 7. It removes the need for hydro test and leak test procedures. These tests are costly, especially in large diameter pipelines and offshore pipeline projects. 8. It minimises or removes the need for field joint coatings. This increases productivity and quality and reduces the cost. 9. It requires less weld repair when compared to butt welds, since RPW weld can have a higher acceptable defect threshold compared to a girth butt weld . 10. It provides a high quality weld, improving the mechanical properties of the weld due to the mechanical work. 11. It removes the requirement for PWHT in thick-walled steel pipes and alloy pipes because the residual stress in RPW welds is a radial stress. 12. No additional filler material is required. 13. RPW welding can accept higher acceptable manufacturing tolerances threshold compared to a girth butt weld. 14. It removes the requirement for a field joint coating procedure. 15. It can be used for welding pipe-in-pipe systems and also for welding plastic-lined pipes. 16. PCCs offer possible factory mass production to reduce their unit cost. 17. RPW tools and machinery is neither complex nor expensive when compared to RFW. 18. It does not involve rotation of pipes, in contrast to a typical friction welding method. 19. No mandrel of internal support is required, in contrast to an RFW method. 20. RPW welding can be performed underwater. 21. The PCC can be used for other joining applications not involving an RPW procedure. RPW Disadvantages: 1. Cleaning is required before welding. In contrast with a typical friction weld, RPW does not produce flash and therefore does not have a self-cleaning mechanism. 2. It has no track record. Like any other new technology it will take time for RPW become a well-known welding technique. 3. It may not be economical for small diameter pipes. 4. It may not be economical for low grade steel pipes. 5. It requires an additional component (the PCC) for joining line pipes. 6. It may need an inert gas. 7. A manual welding variant of this procedure is not feasible: it requires RPW welding tools. 8. Removing the PCC shells produces waste material. 9. Water is required to pressurise the PCC, so a water supply is required at site. 10. A localised weld repair is not available. The whole pipe needs to be cut and welded again. 14. Radial Impact Welding (RIW)

Radial Impact Welding (RIW) is a modified RPW technique which is especially applicable to welding large diameter pipes. Almost all of the RPW characteristics explained above are applicable to RIW. The term "large diameter pipe" is used as a general term here, as the optimum range of pipe diameters is not yet known.

It is expected that RPW can be used for welding large diameter pipelines. However the force needed to generate a large friction weld would need a large and heavy PCC, and handling and rotating the PCC would become a challenge.

Experience gained from explosion welding is used here to optimise the use of the stored energy in the PCC. In explosion welding, a gap must be provided between the two metal plates which are to be welded. In this technique, some of the energy released from the explosion is transferred into kinetic energy, which moves the top plate downward. The top plate hits the bottom plate with a certain velocity, so that the kinetic energy is converted to the material's plastic strain energy and the impact pressure (or dynamic pressure) welds the two plates together. If there is no gap between the two plates, the explosion still presses the top plate to the bottom plate (with static pressure), but the plates are not welded together, since the magnitude of the static pressure is smaller than that of the impact pressure.

The expansion in a large PCC due to applied pressure can be considerable. For example a PCC with an inner diameter of 1000mm can be expanded to 1002mm, within the elastic strain region. This means 1mm gap can be set between a pressurised PCC and the pipes prior to welding. The PCC can be designed to accept plastic strain (deformation) while it is pressurised and depressurised. In this way, the gap between the pressurised PCC and the pipes can be increased. As an example design, the coupler could have many longitudinal groves about its outer circumference. When a PCC with such a coupler design is pressurised, the continuous area below the groves plastically deforms due to the hoop strain while the area between the grooves stays in the elastic region (see figures 88, 89 and 90).

If a big enough gap can be achieved between the coupler and the pipes, then RPW acts in the same way as explosion welding. This technique is called Radial Impact Welding (RIW). In this method, when the PCC bursts, the released potential energy converts to kinetic energy, moving the coupler body toward the centre. The coupler hits the pipe with a certain velocity, the kinetic energy is converted to plastic strain energy and the impact pressure welds the coupler to the pipes. The impact pressure (dynamic pressure) in the RIW technique is greater than the static pressure in the RPW technique for the same sized coupler. Therefore, for large diameter pipes where a gap is achievable, using RIW can reduce the size of the PCC.

In the RIW technique, the pipes and the PCC can be stationary. Using this method, a thin plate of metal (e.g. Titanium or Titanium alloy) or other material can be rotated inside the gap in such a way that it touches the chamfered surfaces of the pipes and the coupler while it is rotating. The friction between the plate and the CCP-Pipes assembly generates heat and raises the chamfered surface temperature to the required level. The thin plate is removed from the gap and the PCC shell bursts immediately so that the coupler hits the pipe and becomes welded (see figure 84, 85, 86 and 87).

In the RIW technique, the heat required to raise the temperature of the pipes and coupler chamfered sections may be provided by sources other than friction. For example, the heat may be provided by induction, electrical arc, direct flame, ultrasonic or other common sources of heat used in welding.

Where a stationary PCC is used, the bursting system should be rotary. The bursting system would rotate a cutting tool around the PCC's shell and the cutting tool would create a girth groove on the surface of the shell. The cutting tool would continue to deepen the groove in the shell wall until the shell bursts. 15. Pressure Containing Coupler General Specifications

Although PCCs can be made in various shapes and can be used for various applications, all have the following unique characteristics. • A PCC is a chamber which can hold a fluid (e.g. water). • A PCC has a hole (or socket) on its body like any other coupler. • The fluid stored in a PCC can be pressurised to expand the PCC hole. • An object can be inserted tightly into the expanded PCC hole. • The object can be clutched by PCC when it is depressurised and its hole contracts. • The object and the PCC can be rotated relative to each other in such a way that the friction between the touching faces generates heat and raises the temperature to that suitable for welding. Then PCC can be suddenly depressurised, thus contracting and causing a sudden radial compression force that welds the two components.

The above represents a universal PCC specification for general joining applications. A PCC can be used to connect various components, for example, two pipes, two bars, or a pipe to a bar. It can also be used as a reducer to connect two pipes or bars of different diameters and also as a transition piece to connect a pipe or a bar to other shape profiles, such as a square profile (see figures 96 and 97).

Although a PCC can have different shapes (e.g. the object and the PCC socket can be cylindrical or conical) and can be used for different applications, the main focus of this paper is to introduce the PCC in combination with a friction welding technique as a novel solution for joining pipes. This has been explained further in the previous sections. 16. Other Applications

Subsea Applications: RPW can be used for underwater and subsea applications. Although limited information about subsea friction welding is available, it is known that a friction welding procedure can be performed underwater. Unlike other welding techniques, the source of heat used in friction welding is not affected by the surrounding environment. As there is almost no space between the two touching faces, water cannot intervene in the weld procedure. At the microscopic level, any water trapped between the weld faces will be vaporised by the heat from the friction and will escape from the joint. Above about friction welding is applicable to RPW as well. A subsea radial pressure welding procedure is very similar to an onshore RPW welding procedure. Almost all of the foregoing discussion about RPW is also relevant to subsea RPW (see figure 95).

Mechanical joints, such as flanges, are common in subsea applications. The diving activities involved in bolting up and closing a subsea flange pair are time consuming and therefore costly. Subsea flanges can also be a source of leaks. Subsea tie-in flanges can be replaced by RPW subsea welds and RPW subsea welds can be performed much more quickly than installing subsea flanges.

In deep water applications, diver-less mechanical joints (such as diver-less couplings and clamps) are complex and expensive. Installing this equipment is also time consuming and expensive. Replacing these components with RPW subsea welds could be very cost effective. ROVs (Remotely Operated Vehicles) could be used to perform RPW subsea welds. RPW could also be used instead of costly hyperbaric welding for joining pipes and cylindrical components.

Non-welding applications: PCCs can be used to join pipes or other cylindrical elements without an RPW weld. A PCC can be designed to apply a considerable bearing load (or grabbing force) on a pipe after it becomes depressurised and the friction between the coupler and the pipe due to the bearing load can be enough to resist the operating loads of the pipeline. The load from the coupler could be high enough to plastically deform the pipe and a polymeric resin or other material could be used as an adhesive between the coupler and pipes to seal the joint and make it watertight. The basis for a weld free and friction based mechanical joint for pipelines has been already confirmed in the use of Zaplock joints.

Weld free PCCs would be stationary and would therefore need less tooling. They can be installed manually and can be depressurised gradually via a valve. The weld free PCCs would require a stronger and heavier structure in comparison to the RPW PCCs, and this could limit their applications, especially for large diameter pipelines, and make them expensive. In order to gain enough friction, the coupler and the pipes in a weld free PCC joint would require much wider chamfered sections when compared to an RPW weld (see figures 91 and 92).

To some extent, the stages 1 (preparation) and 2 (Aligning the pipe and assembling of the joint) as explained above are also applicable to weld free PCC joining, and it is expected that a weld free PCC would have a simpler aligning and preparation procedure. RPW for reeling applications:

It is not anticipated that RPW would be used in reeling applications, as this welding method has a higher speed when used with an S-lay or J-lay application. However, if RPW were to be used for reeling applications, the PCC can be designed with a bend stiffener. The bending stiffness of a pipeline constructed using RPW is intermittent, and it suddenly increases at the joint. This results in an uneven curvature and un-uniform plastic deformation of the pipe during reeling. For example, the coupler is bent less than the average, while the pipe section within a diameter distance of the coupler is bent more than the average. This can buckle and bulge the pipe. To address this problem, the coupler could have girth grooves circumferentially made along its chamfered sections. These grooves would be at the outer side of the coupler and not on the chamfered faces and they would get deeper towards the end of the chamfered section. In this design the bending stiffness is increased gradually from the pipe to the coupler, thereby avoiding localised buckling or bulging of the pipe (see figure 94).

Applications different from joining:

Since the PCC is a factory made product, additional equipment can be fitted on to it for purposes other than joining and welding.

Sacrificial (or galvanic) anodes can be attached to the coupler at the factory. This can be helpful in offshore applications as they can be used instead of typical pipeline bracelet anodes. An added advantage is that the cathodic protection system is put in place during pipeline construction (see figures 64 and 65). A tube could be coiled around the PCC. A hot liquid could then be circulated in the coil to warm the pipeline content. A PCC could also have an electrical heating system installed (e.g. a heating element or an electromagnet for induction heating) to warm the pipeline content.

Description of Drawings:

Figure 1 shows a mechanised and automated GMAW welding system with two heads or two welding bugs. 1- An active welding bugs which is welding and travelling down 2- Second welding bugs (inactive) 3- Welding torch 4- Electrical arc as the source of heat for fusion 5- Rail for the welding bugs

Figure 2 shows a GMAW weld area and a GMAW torch. 1- Welding torch 2- Contact tube 3- Electrode 4- Shielding gas 5- Work piece 6- Molten weld metal 7- Solidified weld metal

Figure 3 shows a microstructure of a fusion based weld. 1- Weld metal 2- Heat Affected Zone (HAZ) where the material grains are coarsened 3- Base material with fine grain

Figure 4 shows a Flash-butt welding. This technique is a one-shot fusion based welding technique. Sparks can be seen in the picture. 1- Constructed section of pipeline 2- New line pipe is being welded 3- Flash-butt welding tool Figure 5 shows a typical friction welding machine. 1-Welding machine revolving section 2-Chuck 3-Rotary work piece 4-Stationary work piece 5- Heated area between the touching faces of the work pieces 6- Weld flash

Figure 6 shows a friction welding procedure. 1- Work pieces 2- The revolving machine spins the rotary work pieces 3- The stationary work piece is pushed on the rotary piece to increase the friction and generates heat 4- Upset load or final compressive force 5- Revolving machine is stopped 6- A friction weld is generated between the touching faces of the work pieces

Figure 7 shows an explosion welding procedure. 1- Explosive material 2- Top plate or clad plate 3- Gap between the two plates. The gap is necessary so the top plate hits the bottom plate at certain velocity 4- Bottom plate or main plate 5- Upper plate hits the bottom plate due to the explosion 6- The welded section between the two plates

Figure 8 shows two pipes were butt welded by a friction welding method. One pipe was stationary and the other one was rotating during the welding procedure. Rotating a long pipe piece may not be practical. Flash shall be machined for pigging. The inner flash may not be accessible for machining where a long line-pipe (12m) is welded. Any flash at outer surface of pipe is accessible and can be machined. 1- Pipes 2- Flash at inner surface of the pipes 3- Flash at outer surface of the pipes

Figure 9 shows two pipes and a disc (flat washer) were butt welded by a friction welding method. Both pipes were stationary and the ring was rotating during the welding procedure. The disc outgrowth shall be machined for pigging. The inner outgrowth of the disc may not be accessible for machining where a long line-pipe (12m) is welded. 1-Pipes 2-The middle disc 3-Flash

Figure 10 shows two pipes and a chamfered ring were butt welded by a Radial Friction Welding (RFW) method. Both pipes were stationary and the solid chamfered ring was rotating during the welding procedure. 1-Pipes 2-Solid chamfered ring 3-No Inner flash

Figure 11 shows Radial Friction Welding procedure. A solid chamfered ring is radially compressed while is rotating about the two stationary pipes.

Two pipes and a chamfered ring were butt welded by a Radial Friction Welding (RFW) method. Both pipes were stationary and the solid chamfered ring was rotating during the welding procedure. 1- Pipes 2- Mandrel, an internal support to avoid pipe collapse 3- Solid chamfered ring

Figure 12 shows a disc or a flat washer expansion due to a thermal load. The thermal expansion results in an overall increase in the size of washer where the hot washer diameter "A" is bigger than the cold washer diameter "a", the hot washer width "B" is bigger than the cold washer width "b", the hot washer hole diameter "C" is bigger than the cold washer hole diameter "c" and the hot washer thickness "D" is bigger than the cold washer thickness "d". Expansions are shown exaggeratedly. 1-A cold disc or flat washer 2-The cold washer hole 3-A hot disc or flat washer 4-The hot washer hole

Figure 13 shows a torus shape pressure vessel which can contain a fluid. The vessel expansion due to an internal pressure is described here. Pressurising the stored fluid results in an overall increase in the size of the torus shape vessel where the pressurised vessel diameter "A" is bigger than the unpressurised vessel diameter "a", the pressurised vessel width "B" is bigger than the unpressurised vessel width "b" and the pressurised vessel hole diameter "C" is bigger than the unpressurised vessel hole diameter "c". Expansions are shown exaggeratedly. 1- An empty torus shape vessel 2- The empty torus shape vessel inner hole 3- A pressurised torus shape vessel 4- The pressurised torus shape vessel inner hole 5- The stored fluid in the torus shape vessel

Figure 14 shows a pressurised torus shape pressure vessel where a pipe tightly inserted into the vessel inner hole. The pipe outer surface is flush with the vessel inner hole surface. 1- A pressurised torus shape pressure vessel 2- A pipe inserted into the vessel inner hole 3- The pressure load from the pressurised fluid stored in the vessel.

Figure 15 shows a depressurised torus shape pressure vessel which contracts over a pipe where it was previously (figure 14) inserted into the vessel inner hole. A radial compressive force due to the vessel contraction and the reaction force from the pipe, compress the touching faces of the vessel and the pipe and lock the vessel over the pipe. Deformations and contractions are shown exaggerated. 1- The depressurised torus shape pressure vessel 2- The pipe which was inserted into the vessel before depressurisation of the vessel 3- The radial compressive force due to the vessel contraction 4- The reaction compressive force from the pipe 5- The touching faces of the vessel and the pipe

Figure 16 shows an empty Pressure Containing Coupler (PCC) with a semi torus shape. PCC has two main components, a coupler and a shell. 1- A shell with a half circle cross section 2- A cylindrical shape coupler 3- The PCC's (coupler's) inner hole 4- The space between the shell and the coupler which acts as a container for storing a fluid

Figure 17 shows a pressurised PCC with a semi torus shape. The PCC overall expansion increases the inner hole diameter. The coupler becomes hyperboloid in geometry since it is bent and curved due to the pressure. The coupler inner hole expands the most at the coupler ends. Deformations are shown exaggerated. 1- A shell with a half circle cross section 2- A hyperboloid shape coupler 3- The PCC's (coupler's) inner hole 4- The space between the shell and the coupler which acts as a container for storing a fluid 5- The pressure load from the pressurised fluid stored in the vessel 6- Maximum inner hole expansion at the coupler ends

Figure 18 shows a joint assembly prior to welding which includes a pressurised PCC at the middle where two line-pipes inserted into the PCC's sockets. The joint assembly before welding called PCC-Pipes assembly. 1- Expanded PCC's shell due to the pressure 2- Deformed PCC's coupler due to the pressure 3- Pipes inserted into the PCC's sockets 4- The fluid like water stored in the container 5- The PCC's (coupler) sockets 6- The pressure load from the pressurised fluid stored in the vessel

Figure 19 shows a welded PCC-Pipes joint welding which includes two line-pipes which are connected by a depressurised PCC at the middle. 1- PCC's shell 2- PCC's coupler contracts over the pipe after depressurisation and get straightened 3- Pipes are welded and connected by PCC's sockets 4- depressurised fluid stored in the container like water 5- The welded section of pipe and coupler, A RPW weld 6- The radial compressive force due to the coupler contraction and straightening 7- The reaction compressive force from the pipe

Figure 20 shows a joint assembly where a PCC rotates about two stationary line-pipes. 1- Two stationary pipes 2- A PCC rotates about the pipes

Figure 21 shows a PCC-Pipes assembly prior to welding.

1-Pipes which are going to be jointed 2-A PCC 3- PCC's coupler 4- PCC's Shell 5-End edge of the coupler chamfered section (coupler socket) 6- Start edge of the coupler chamfered section (coupler socket) 7- End edge of the pipe chamfered section 8- Start edge of the pipe chamfered section 9- Touching faces between the pipes and the coupler 10- The fluid like water stored in PCC

Figure 22 shows a welded PCC-Pipes joint which is generated by Radial Friction Welding (RPW) technique. The PCC's shell is removed after welding and the PCC's coupler remain connected between the two pipes. 1- The jointed pipes 2- The coupler after shell is removed 3- RPW welds between the pipes and the coupler 4-Plain end of the pipes 5-Plain end of the coupler

Figure 23 shows a PCC which is used to connect a thin wall pipe to a thick wall pipe. Line pipes wall thickness fabrication tolerances can be modified by the chamfered section at the pipe end. The thick wall pipe is machined more to be matched with the thin wall pipe. 1- The thin wall pipe 2- The thick wall pipe 3- The coupler 4- The thick wall pipe chamfered end

Figure 24 shows a PCC which is used to connect a standard pipe to an oval pipe. Line pipes out of roundness fabrication tolerances can be modified by the chamfered section at the pipe end. The oval pipe is machined more to be matched with the standard pipe. 1- The standard wall pipe 2- The oval wall pipe 3- The coupler 4- The oval wall pipe chamfered end

Figure 25 shows a PCC connecting two pipes while they are eccentric. The pipes and the coupler longitudinal axes are parallel but they are not co-axial. This provides some degree of acceptance for chamfering tolerances. An eccentric chamfered section of pipe can be acceptable if it is within the allowable tolerances. 1- The first pipe 2- The second pipe 3- The coupler 4- The pipe chamfered end 5- The first pipe longitudinal axis 6- The coupler longitudinal axis 7- The second pipe longitudinal axis

Figure 26 shows a PCC connecting two pipes while their axes are not parallel. This provide some degree of acceptance for angular chamfering tolerances. The pipe end can be chamfered with a specific angle to create a mitre bend. 1- The first pipe 2- The second pipe 3- The coupler 4- The first pipe longitudinal axis 5- The coupler longitudinal axis 6- The second pipe longitudinal axis

Figure 27 and 28 show the pressure load from the pipeline content on an un-welded and a welded coupler-pipes joint. The pipes and the coupler are simplified as flat plates in a 2D free body diagram. The pressure direction is almost perpendicular to the interface of the coupler and the pipes. Therefore the internal pressure load transfers similarly from the pipe to the coupler with or without a weld. The welds between the pipes and the coupler do not contribute in the joint hoop strength. Figure 27 shows an un-welded and figure 28 shows a welded coupler-pipes joint. 1-The pipes 2-The coupler 3-Internal Pressure 4-Resultant reaction from the pipe and the coupler

Figure 29 shows two pipes joined by a coupler with two RPW welds. The top figure shows a 3D view of the coupler-pipes joint, the figure at the middle shows a cross section of the coupler-pipes joint and the figure at the bottom shows a chain analogy arrangement representing the RPW welds strength behaviour. The shear based RPW welds act like many short tensioned chains which have arranged around the pipe circumference parallel to the pipe axis. 1-The first pipe 2-The coupler 3-The second pipe 4-Short length chains representing RPW welds

Figure 30 shows two pipes joined by a typical girth butt weld. The top figure shows a 3D view of the welded pipes, the figure at the middle shows a cross section of the welded pipes and the figure at the bottom shows a chain analogy arrangement representing the girth butt weld strength behaviour. The girth butt welds act like a single tensioned girth (circular) chain. 1- The first pipe 2- The girth butt weld 3- The second pipe 4- A single circular chain representing the girth butt weld

Figure 31 shows two pipes joined by a coupler with two RPW welds. The top figure shows a 3D view of the coupler-pipes joint. The figure at the middle shows the welded area and a defect on the first pipe. The welding area shall be almost double of a typical butt weld area (see figure 32) to satisfy a shear based design. The figure at the bottom shows the defect as a missing link in a chain. Although one of the chain link is failed other chains carry the load. 1- The first pipe 2- The coupler 3- The second pipe 4- The RPW weld surface for a shear based design 5- A defect on the weld surface 6- Short length chains representing RPW welds 7- A missing link in a chain representing a defect

Figure 32 shows two pipes joined by a typical girth butt weld. The top figure shows a 3D view of the welded pipes. The figure at the middle shows the welded area and a defect on the first pipe. The welding area is the same as the pipe cross section area. The weld behaves the same as the body of the pipe. The figure at the bottom shows the defect as a missing link in a chain. Failure of a link results in the failure of the whole chain. 1-The first pipe 2-The girth butt weld 3-The second pipe 4-The surface of the girth butt weld 5- A defect on the weld surface 6- A single circular chain representing the girth butt weld 7- A missing link in a chain representing a defect

Figure 33 shows residual stresses and HAZ in RPW welds. The residual stresses in the weld are radial stresses. The HAZ is a small area because the heat for welding is distribution locally. 1- Pipes 2- Coupler 3- Radial residual stresses 4- HAZ shown schematically

Figure 34 shows two pipes joined by a coupler with a conventional welding technique. The number of required weld passes are almost 4 times more than a standard girth butt weld. 1-Pipes 2-Coupler 3-welds by a conventional welding technique

Figure 35 shows a PCC-Pipes assembly which has a bushing for smoothing the pipeline internal surface and for avoiding steps and sharp edges inside the pipeline. The plain ends of bushing face the plain ends of the pipes. PCC shell is omitted for simplicity 1- Pipes 2- Coupler 3- Bushing with the same internal diameter as the pipes

Figure 36 shows a welded PCC-Pipes joint which has a bushing for smoothing the pipeline internal surface. 1-Pipes 2-Coupler 3-Bushing flush with the inner surface of the pipes

Figure 37 shows a PCC-Pipes assembly where the internal face of the pipes at the ends are tapered to avoid steps and sharp edges inside the pipeline. PCC shell is omitted for simplicity 1-Pipes 2-Coupler 3-Pipe ends tapered from inside

Figure 38 shows a welded PCC-Pipes joint where the internal face of the pipes at the ends are tapered to avoid steps and sharp edges inside the pipeline. 1-Pipes 2-Coupler 3-Pipe ends tapered from inside

Figure 39 shows the RPW tools schematically for an onshore construction. 1- Ground 2- Constructed section of pipe 3- welded Coupler 4- External supports - Fix supports 5- New PCC in place for welding 6- Protection cover 7- Bursting mechanism 8- Cutting tool 9- New line-pipe 10- External supports - Aligning supports 11- Aligning supports wheels 12- A section of line-up clamp

Figure 40 shows an internal line-up clamp tool schematically . 1- Pipes 2- PCC 3- Revolving system 4- Revolving system's clamp 5- Pulling mechanism hydraulic cylinder 6- Pulling mechanism spring 7- line-up tightening and releasing mechanism 8- Line-up mechanism clamps 9- The tool wheels

Figure 41 shows an internal line-up clamp which is pulled out from a constructed section of pipeline to start the welding procedure. The battery and nitrogen bottle (pressure vessel) are installed at the head of the tool so they can be replaced with a fully charged battery and a fully filled nitrogen bottle respectively if necessary. 1- Constructed pipeline 2- line-up tightening and releasing mechanism 3- Line-up mechanism clamps in unlocked position 4- Pulling mechanism spring in relax condition 5- Pulling mechanism hydraulic cylinder inactive 6- Revolving system inactive 7- Revolving system's clamp unlocked 8- The tool wheels 9- Battery 10- Nitrogen or inert gas bottle

Figure 42 to 47 show a line-up clamp function during RPW procedure. The line-up clamp battery and nitrogen bottle are omitted in these figures for simplicity.

Figure 42 shows an internal line-up clamp which is pulled out from a constructed section of pipeline to start the welding procedure. First a PCC is pulled over the tool head to be fitted on the revolving system and after that a new line pipe is pulled over the tool head. The springs are relaxed and the hydraulic cylinders are inactive. The clamp inside the constructed section of pipe can be locked temporarily to provide better stability however all other clamps are unlocked. 1- Constructed pipeline 2- line-up tightening and releasing mechanism at the end of the tool 3- Line-up mechanism clamps in locked position 4- The tool wheels 5- Pulling mechanism hydraulic cylinder inactive 6- Pulling mechanism springs are in relax condition 7- Revolving system inactive 8- Revolving system's clamp unlocked 9- line-up tightening and releasing mechanism at the head of the tool 10- Line-up mechanism clamps in unlocked position

Figure 43 shows an internal line-up clamp inside a PCC-Pipes assembly. The hydraulic cylinders are activated to stretch the springs. All clamps are unlocked and the tool is standing on its wheels inside the pipe. The new line-pipe is aligned with the constructed section of pipe and is manually pushed into the PCC so the chamfered sections of the pipes are tightly fitted in the coupler's sockets. 1- Constructed pipeline 2- PCC 3- New line-pipe 4- The tool wheels 5- Line-up mechanism clamps in unlocked position 6- Revolving system inactive 7- Pulling mechanism hydraulic cylinder is activated 8- Pulling mechanism springs are stretched by the hydraulic cylinders

Figure 44 shows an internal line-up clamp inside a PCC-Pipes assembly. All the clamps are locked. The joint assembly is fully aligned. The coupler is fitted on the revolving system clamps. The hydraulic cylinders are activated to stretch the springs. 1- Constructed pipeline 2- PCC 3- New line-pipe 4- The tool wheels 5- Line-up mechanism clamps in locked position 6- Revolving system inactive 7- Revolving system's clamp locked to the PCC 8- Pulling mechanism hydraulic cylinder is activated 9- Pulling mechanism springs are stretched by the hydraulic cylinders

Figure 45 shows an internal line-up clamp inside a PCC-Pipes assembly. The hydraulic cylinders push out the pipes from PCC to make it free for rotation. The revolving system is activated and spins the PCC. All the clamps are locked and the springs are stretched further. 1- Constructed pipeline is slightly pulled out from PCC socket 2- PCC is rotating about the pipes 3- New line-pipe is slightly pulled out from PCC socket 4- The tool wheels 5- Line-up mechanism clamps in locked position 6- Revolving system activated 7- Revolving system's clamp locked to the PCC 8- Pulling mechanism hydraulic cylinder is activated 9- Pulling mechanism springs are stretched further by the hydraulic cylinders

Figure 46 shows an internal line-up clamp inside a PCC-Pipes assembly. The hydraulic cylinders are relaxed so the springs pull the pipes into the PCC's sockets while it is rotating. This generates friction and heat for welding. The PCC shell bursts and the coupler is welded to the pipes. The revolving system motor is deactivated before the shell bursts. All the clamps are locked. 1- Constructed pipeline is pulled into the PCC socket 2- PCC is rotating about the pipes 3- New line-pipe is pulled into the PCC socket 4- The tool wheels 5- Line-up mechanism clamps in locked position 6- Revolving system activated 7- Revolving system's clamp locked to the PCC 8- Pulling mechanism hydraulic cylinder is relaxed 9- Pulling mechanism springs are pulling the pipes into the PCC's sockets

Figure 47 shows an internal line-up clamp which moves toward the new end of the constructed pipe. The springs are relaxed and the hydraulic cylinders are inactive all clamps are unlocked and the revolving system is inactive. The line-up clamp moves inside the pipe on its wheels. 1- Constructed pipeline 2- Welded PCC with shell removed 3- Line-up mechanism clamps in unlocked position 4- The tool wheels 5- Pulling mechanism hydraulic cylinder inactive 6- Pulling mechanism springs are in relax condition 7- Revolving system inactive 8- Revolving system's clamp unlocked

Figure 48 shows a PCC-Pipes assembly with a bursting mechanism. A cutting tool moves forward the PCC's shell. The tool penetrates into the shell and creates a groove about the shell circumference. This continuous until the shell bursts 1-Pipeline 2-PCC is rotating about the pipes 3-PCC's shell is machined 4-A groove on the shell circumference 5- A cutting tool

Figure 49 shows a welded PCC-Pipes joint after the PCC shell bursts. The cutting tool is located at the bottom of the system to avoid the water splashes on welds 1-Pipes 2-Welded coupler 3-PCC's shell 4-A groove on the shell circumference 5-The burst section of the shell 6- A cutting tool

Figure 50 shows a PCC-Pipes assembly where the shell is machined to burst. It is also machined to be removed from the coupler. The shell is machined at 3 points simultaneously. 3 grooves are generated on the shell. 1- Pipes 2- PCC is rotating about the pipes 3- PCC's shell is machined 4- A groove at the middle of the shell for bursting 5- Two grooves at the side of the shell for removing the shell from the coupler 6- A cutting tool for bursting 7- Two cutting tools for removing the shell

Figure 51 shows a welded PCC-Pipes joint after the PCC shell bursts. The shell wall is already machined to be separated from the coupler. Remaining wall of the shell can be cut later manually. Remaining wall of the shell can be sheared due to the rotational inertia of the shell and the stored water. 1- Pipes 2- Welded coupler 3- PCC's shell 4- A groove at the middle of the shell 5- Two grooves at the side of the shell for removing the shell from the coupler 6- Shell remaining wall 7-The burst section of the shell 8-A cutting tool for bursting 9-Two cutting tools for removing the shell

Figure 52 shows a PCC filling and pressurising procedure. The shell has a hole with a check valve for filing the container with water and a needle size hole for air to scape. When the container is fully filled with water the needle hole is blocked so the PCC can be pressurised via the check valve. The valve body stay inside the container (shell). 1- PCC's shell 2- PCC's coupler 3- Stored fluid or water 4- Shell hole for filling the container 5- Needle hole, air is escaping out 6- Check valve installed in the shell hole 7- A hose from a pump for filling and pressurising the container, water is being injected into the container

Figure 53 shows a PCC's shell holes after pressurisation. The holes are spot welded or soldered to avoid any possible leakage during welding procedure. 1- PCC's shell 2- remain body of valve in the hole 3- Spot weld on the shell filling hole 4- Spot weld on the shell needle hole 5- Check valve

Figure 54 shows a PCC's shell which is equipped with baffles and stiffeners. The stiffener is a pipe which is welded to sides of the shell close to the shell edges. None of stiffeners or baffles is connected to the coupler. The baffles and stiffeners have holes for continuity and having a constant water pressure in the vessel. In this design the stored water or fluid rotates with PCC. The baffle which internally covers the machined groove shown in figures 50 and 51, is designed to prevent the release of large amount of fluid after bursting. The stored fluid in the shell must pass through this baffle's holes to be released. 1- PCC's shell 2- PCC's coupler 3- Radial baffle for spinning the water 4- Pipe shape stiffener 5- A ring shape baffle covers the bursting area 6- The radial baffle is cut to show the holes on the ring shape baffle 7- The ring shape baffle's holes

Figure 55 shows a lateral cross section of a PCC's shell which is equipped with baffles and stiffeners. The baffles positioned radially inside the shell spin the stored fluid with PCC. None of stiffeners or baffles is connected to the coupler. 1- PCC's shell 2- PCC's coupler 3- Radial baffle for spinning the water 4- Pipe shape stiffener 5- A ring shape baffle covers the bursting area 6- The pipe shape stiffener hole 7- The ring shape baffle's holes

Figure 56 to 58 show a RPW procedure on a boat for offshore construction. The boat and the surrounding environment are omitted in these figures for simplicity.

Figure 56 shows a sliding table at the very end of the constructed section of the pipeline for example at the boat bow, ready to start the welding procedure. All welding tools are installed on the table and move with the table. 1- Boat deck 2- Sliding table, is not connected and is stationary 3- Aligning supports 4-Bursting mechanism and the cutting tool 5- Table clamp for connecting the table to the pipeline 6- Constructed section of pipeline which is being paid out and is being laid on the seabed 7- A welded coupler 8- Boat pipe tensioners

Figure 57 shows a new line pipe is loaded on the table. After that the table is connected to the constructed section of the pipeline with a clamp and starts moving with the pipeline. PCC-Pipes assembly is made and welding procedure is started. The new line-pipe is welded while the table is moving. 1- Boat deck 2- Sliding table is connected and moving with the pipeline 3- Aligning supports 4- Bursting mechanism and the cutting tool 5- Table clamp connects the table to the pipeline 6- Constructed section of pipeline which is being paid out and is being laid on the seabed 7- A welded coupler 8- Boat pipe tensioners 9- New line-pipe loaded on the table

10- A section of inner line-up clamp 11- PCC

Figure 58 shows the new line-pipe is welded and added to the pipeline. The table is disconnected from the pipeline and moves toward a new end of pipeline. The procedure is repeated for continuous production. 1- Boat deck 2- Sliding table is connected and moving with the pipeline 3- Aligning supports 4- Bursting mechanism and the cutting tool 5- Table clamp connects the table to the pipeline 6- Constructed section of pipeline which is being paid out and is being laid on the seabed 7- A welded coupler 8- Boat pipe tensioners

Figure 59 shows a pipeline lay boat using RPW technique for construction of an offshore pipeline. An intelligent pig downstream of the pipeline is connected to the RPW line-up clamp. The pig is motorised and moves inside the pipe. Nitrogen is injected into the pipe from the line-up clamp via a hose connected to the pig. The pipeline is getting nitrogen filled while it is being constructed. 1- Seabed 2- Sea water level 3- Lay barge (boat) 4- RPW welding tools and line-up clamp 5- Boat pipe tensioners 6- Constructed section of pipeline which is being paid out and is being laid on the seabed 7- A connection line between the line-up clamp and the pig including: a hose for nitrogen injection, a power cable, a communication and data cable, a pulling rope 8- an intelligent pig 9-Nitrogen filled section of pipeline

Figure 60 shows the use of PCC shells as pipe supports. After the welding the shell shall be removed from the coupler. The shell ring is cut in half and each half shell is installed on the ground as a pipe support. In this arrangement the half shells are installed diagonally perpendicular to the pipe axis.. 1- Pipeline 2- Half shell support

Figure 61 shows the use of PCC shells as pipe supports. After the welding the shell shall be removed from the coupler. The shell ring is cut in half and each half shell is installed on the ground as a pipe support. In this arrangement the half shells are installed at the sides of the pipeline parallel with the pipe axis. 1- Pipeline 2- Half shell support

Figure 62 shows a PCC-Pipes assembly for welding a steel or metal pipe with corrosion coating. The pipe coating thickness is increased at the end of the pipe and is tapered along with the steel chamfered section. The PCC has a polymer ring which is tapered along the coupler chamfered sockets. In the PCC-Pipes assembly pipes polymeric sections face the coupler polymeric sections and the pipes steel sections face the coupler steel sections. 1- Pipes 2- PCC 3- PCC's coupler 4- PCC's shell 5- Pipes corrosion coating 6-Coupler factory made corrosion coating 7-PCC polymeric rings bonded to the shell and the coupler 8-Thicker corrosion coating at the pipes ends 9- The PCC polymer rings touch the pipe coating 10- The coupler sockets metal section touch the pipes chamfers metal section

Figure 63 shows a welded PCC-Pipes joint of a steel or metal pipe with corrosion coating. The PCC polymer rings is bonded to the pipe coating by a friction based plastic welding. The plastic welds are performed simultaneously with the main welds between the pipes and the coupler. Alternatively a polymeric resin or adhesive can be used for bonding the coupler to all type of pipe coatings shown in figures 62 to 73. 1- Pipes 2- Coupler 3- Main welds between the pipes and the coupler 4- Pipes corrosion coating 5-Coupler factory made corrosion coating 6- The plastic weld between the polymer rings and the pipe coating 7- Remaining uncoated section of shell

Figure 64 shows a PCC-Pipes assembly for welding a steel or metal pipe with thick insulation coating. The pipe coating thickness is tapered with a negative angle so the coating edge touches the PCC's shell. The coupler can have a factory made insulation coating. The coupler can have a factory made anode for the pipeline cathodic protection. 1- Pipes 2- PCC 3- PCC's coupler 4- PCC's shell 5- Pipes insulation coating 6-Coupler factory made insulation coating 7- Coupler factory made anode 8- The pipe insulation coating tapered edge touches the shell 9- Void between the shell and the coating

Figure 65 shows a welded PCC-Pipes joint of a steel or metal pipe with thick insulation coating. The pipe insulation coating is bonded to the PSS's shell by a friction based plastic welding. Remaining uncoated section of shell has negligible effect on the pipeline thermal performance. 1- Pipes 2- Coupler 3- Pipes insulation coating 4-Coupler factory made insulation coating 5- Coupler factory made anode 6- Pipe insulation coating is bonded to the PCC shell and no void left 7- Remaining uncoated section of shell

Figure 66 shows a PCC-Pipes assembly for welding a pipe in pipe system. The PCC has two ring shape wings which are connected to the shell. The wings sit on the outer pipe. The wings and the outer pipes ends are chamfered with the same methodology for the main pipes (inner pipes) and the coupler. 1-Inner Pipes (main pipes) 2-PCC 3-PCC's coupler 4-PCC's shell 5-PCC's ring shape wings 6- Outer pipe 7- Insulation coating 8- Coupler factory made insulation coating 9- The PCC wings touch the outer pipe chamfered section 10- Void between the shell and the coating

Figure 67 shows a welded PCC-Pipes joint of a pipe in pipe system. Four girth welds are generated simultaneously by RPW method between the PCC and the pipes. 1- Inner Pipes (main pipes) 2- Coupler 3- Insulation coating 4- Coupler factory made insulation coating 5-Outer pipe 6-PCC's ring shape wings 7-The weld between PCC wings and the outer pipe 8- Void between the shell and the coating

Figure 68 shows a PCC-Pipes assembly for welding a pipe in pipe system. The outer pipe ends are tapered with a negative angle so the pipe ends sit on the PCC's shell. 1- Inner Pipes (main pipes) 2- PCC 3- PCC's coupler 4- PCC's shell 5- Outer pipe 6- Insulation coating 7- Coupler factory made insulation coating 8- The outer pipe ends touch the PCC's shell 9- Void between the shell and the coating

Figure 69 shows a welded PCC-Pipes joint of a pipe in pipe system. The outer pipe ends is welded to the PCC shell by a friction based welding. Alternatively a polymeric resin or adhesive can be used for bonding the outer pipe to the PCC's shell. 1- Inner Pipes (main pipes) 2- Coupler 3- Insulation coating 4- Coupler factory made insulation coating 5- Outer pipe 6- The weld between PCC's shell and the outer pipe 7- Void between the shell and the coating

Figure 70 shows a PCC-Pipes assembly for welding a pipeline with concrete coating. The PCC has two ring shape wings which are connected to the shell. The wings cover the pipe concrete coating ends. 1- Pipes 2- PCC 3- PCC's coupler 4- PCC's shell 5- PCC's ring shape wings 6- Concrete coating 7- Coupler factory made concrete coating or protection coating 8- The PCC wings covers the pipes concrete coating ends

Figure 71 shows a welded PCC-Pipes joint of a pipeline with concrete coating. The PCC's wings protect and covers any on coated section of the joint between the pipes and the coupler. 1- Pipes 2- Coupler 3- Concrete coating 4- Coupler factory made concrete coating or protection coating 5- PCC's ring shape wings

Figure 72 shows a PCC-Pipes assembly for welding a plastically lined pipeline. PCC has a plastic bushing with similar material as the plastic liner of pipeline. The plain ends of bushing face the plain ends of the pipes and plastic liner. PCC shell is omitted for simplicity 1- Pipes 2- Coupler 3- Pipe plastic liner 4- Plastic bushing with the same internal diameter as the pipes plastic liner

Figure 73 shows a welded PCC-Pipes joint of a plastically lined pipeline. The PCC bushing is bonded to the pipe plastic liner by a friction based plastic welding. The plastic welds are performed simultaneously with the main welds between the pipes and the coupler. Alternatively a polymeric resin or adhesive can be used for bonding the pipe plastic liner to the coupler bushing. 1- Pipes 2- Coupler 3- Pipe plastic liner 4- Bushing flush with the inner surface of the pipes plastic liner 5- Plastic weld between the coupler bushing and the pipe plastic liner

Figure 74 shows a tension test tool. The tool is made of split clamps and hydraulic cylinders. The clamps has hinges so they can be opened and closed to be fitted around the pipe. The clamp jaws sit behind the coupler plain ends. The hydraulic from cylinders is transferred to the clamps jaws. The tension test tool tries to pull out the couplers from the ends of the pipe. 1-Pipes 2-Welded couplers 3-Tension test tool 4-Hydraulic cylinders 5-Split clamps 6-Coupler plain ends 7-Clamp Jaws

Figure 75 shows a tension test tool for a pipeline with a thick coating. Since the couplers plain ends are embedded in the coating, the remaining body of shell is supported by stiffeners so the tension tool can apply a force to the stiffened shell. The PCC has stiffeners along its length which are arranged about the outer surface of the coupler. The pipe coating is tapered at the end to provide room for the clamps jaws to sit behind the remaining section of the shell. In this figure a cross section of pipeline is shown for clarity. 1- Pipes 2- Welded couplers 3- Thick coating 4- Coupler factory made coating 5- PCC's stiffeners 6- Remaining body of the shell 7- Tension test tool 8- Split clamps 9-Hydraulic cylinders 10-Clamp Jaws 11-Tapered section of coating

Figure 76 shows a lateral cross section of a PCC with stiffeners. Stiffeners are arranged about the outer surface of the coupler. 1- Coupler 2- Stiffeners 3- Coupler factory made coating

Figure 77 shows a PCC-Pipes assembly where the PCC's coupler has a circumferential machined groove on its chamfered sockets. The groove separates the sockets chamfered section into two sections. A groove can also be machined on the pipes chamfered ends. PCC shell is omitted for simplicity 1- Pipes 2- Coupler 3- Grooves on the couplers chamfered sockets

Figure 78 shows a welded PCC-Pipes joint where a ring shape hole is generated in the body of the joint. The groove in figure 77 becomes a closed ring shape hole after welding. The RPW weld is separated onto two sections. The wider weld section is the main weld which provides the joint strength and the narrower section is the seal weld which holds the pipe content. 1- Pipes 2- Coupler 3- Ring shape hole 4- Seal weld 5- Main weld

Figure 79 shows a zoomed cross section of a welded PCC-Pipes joint with an internal ring shape hole. An access hole is drilled on the coupler to be able to fill the ring shape hole with a fluid. A ring shape Bourdon tube is installed in the coupler groove before welding for measuring the fluid pressure in the ring shape hole. The fluid is atmospheric and there is gap between the ends of the Bourdon tube. 1- Coupler 2- Pipe 3- Ring shape hole filled with a fluid like water 4-Acsses hole 5- Bourdon tube 6- The gap between the bourdon tube ends

Figure 80 shows a zoomed cross section of a welded PCC-Pipes joint with an internal ring shape hole. For leak test only scenario, a check valve is inserted into the access hole and a fluid is pumped into the ring shape hole and get pressurised. After the test the valve is taken out and the hole if is filled with a resin or get spot welded or get soldered.

For the fluid to stay pressurised over the life time of the pipe, a tee shape valve is inserted into the access hole. A fluid is pumped into the hole via a check valve to get pressurised. After pressurisation a solder or other material is injected in to the valve body to block the valve hole. 1- Coupler 2- Pipe 3- Ring shape hole filled with pressurised fluid 4-Acsses hole 5- Tee shape valve 6- Check valve 7- A piston for injecting solder or other material 8-Connection hose from pump

Figure 81 shows a zoomed cross section of a welded PCC-Pipes joint with an internal ring shape hole. The fluid is pressurised and trapped in the ring shape hole over the life time of the pipe. The access hole is covered by a spot weld or solder. The Bourdon tube gap is closed due to fluid pressure. 1-Coupler 2-Pipe 3-Ring shape hole filled with pressurised fluid 4-remaining body of valve and spot weld or solder blocking the access hole 5- Bourdon tube 6- The gap between the bourdon tube ends is closed

Figure 82 shows a PCC-Pipes assembly for welding a mechanically lined pipeline or an alloy clad pipeline. The PCC's coupler may have a circumferential machined groove on its chamfered sockets to separate the carbon steel section from the clad section. In the PCC-Pipes assembly the pipes alloy clad sections face the coupler alloy sections and the pipes steel sections face the coupler steel sections. PCC shell is omitted for simplicity 1- Pipes 2- Coupler 3- Grooves on the couplers chamfered sockets 4- An alloy liner mechanically bonded to the pipe 5- Pipe end alloy clad section 6- Coupler alloy cladding 7- Weld between the alloy liner and the clad section of pipe

Figure 83 shows a welded PCC-Pipes joint of a mechanically lined pipeline or an alloy clad pipeline. The main weld which provides the joint strength is a steel weld and the seal weld which holds the pipe content is an alloy weld. 1-Pipes 2-Coupler 3-Ring shape hole 4-An alloy liner mechanically bonded to the pipe 5- Pipe end alloy clad section 6- Coupler alloy cladding 7- Weld between the alloy liner and the clad section of pipe 8-steel main weld 9-Alloy seal weld

Figure 84 shows a PCC-Pipes assembly for Radial Impact Welding (RIW). There is gap between the chamfered section of pipes and the chamfered sockets of the coupler. The PCC can be stationary. Thin plates are rotate between the pipes and the coupler. The friction between the touching faces produces the required heat for welding. The gap increases along the chamfered sections length and the plates have a wedge shape. This provides the room for the plates to be inserted into the gap or be pulled out from the gap. 1- Pipes 2- PCC 3- PCC's coupler 4- PCC's shell 5- Wedge shape plates 6- RIW friction tool is rotating between the pipes and the coupler

Figure 85 shows a RIW friction tools. The tool has a ring where wedge shape plates are installed. The tool's ring is connected to a motor and rotates. The tool's wedge shape plates are inserted into the gap between the pipes and the coupler. 1- The RIW friction tool ring 2- The RIW friction tool wedge shape plates

Figure 86 shows a PCC-Pipes assembly for Radial Impact Welding (RIW) immediately after the PCC bursts. The pipes and the coupler chamfered faces have been heated up to a suitable temperature for welding and the friction tool plates have been pulled out from the gap. The PCC shell bursts and the coupler is deforming and moving toward the centre to hit the pipe at a certain velocity. 1- Pipes 2- PCC 3- PCC's coupler 4- PCC's shell 5- Friction tool and wedge shape plates 6- Coupler deformation and movement speed toward the centre

Figure 87 shows a welded PCC-Pipes joint which is generated by Radial Impact Welding (RIW) technique. The PCC's shell is removed after welding and the PCC's coupler remain connected between the two pipes. 1- The jointed pipes 2- The coupler after shell is removed 3- RIW welds between the pipes and the coupler

Figure 88 shows a PCC specially designed for RIW application. The coupler has factory made longitudinal grooves on its outer surface to allows the inner surface of the coupler to expand and deform plastically. The coupler's sockets expansion can be increased by a controllable plastic deformation. The required gap for RIW can be achieved by increasing the sockets expansion. 1- PCC's coupler 2- PCC's shell 3- Longitudinal grooves at the coupler outer surface 4- Couplers sockets

Figure 89 shows a lateral cross section of a PCC specially designed for RIW application. The factory made longitudinal grooves are distributed about the circumference of the coupler. 1- PCC's coupler 2- PCC's shell

Figure 90 shows a zoomed lateral cross section of a PCC specially designed for RIW application. The areas between the grooves are elastically deformed and stay in the elastic region however the continuous area at the bottom is plastically deformed. When the coupler is depressurised the areas where elastically deformed find their original shapes. This results in the coupler contraction. 1- Coupler 2- Longitudinal grooves 3- Elastically deformed areas 3- Plastically deformed area

Figure 91 shows a PCC-Pipes assembly for none welding application. PCC can be used for joining pipes without RPW procedure. A polymeric resin or an adhesive is placed between the chamfered section of the pipes and coupler to seal the joint 1- Pipes 2- Pressurised and expanded PCC 3- PCC's coupler 4- PCC's shell 5- A polymeric resin or an adhesive

Figure 92 shows a bonded PCC-Pipes joint without RPW. The joint shear or tensile strength is provided by the friction between the pipes and the coupler chamfered faces due to the coupler contraction. The resin or the adhesive seals the joint and may also increase the strength capacity of the joint. 1- Pipes 2- Contracted coupler 3- A polymeric resin or an adhesive is set between the pipes and the coupler

Figure 93 shows factory made protection covers of the chamfered section of the pipes and the coupler. The chamfered sections are machined and cleaned in the factory to be ready for welding. The protection covers which can be a polymeric resin keep the faces of the chamfered sections clean from any dirt and corrosion. The protection covers shall be removed before welding and therefore the polymeric cover shall not be bonded to the steel and should be easily removable. 1- Pipes 2- PCC 3- Pipes' chamfered end protection cover 4- PCC's chamfered sockets protection cover

Figure 94 shows a bonded PCC-Pipes joint for an offshore pipeline which is going to be installed by reeling method. The grooves gradually increase the bending stiffness of the joint from the pipes toward the coupler to avoid bulging in the pipe during the reeling procedure. 1-Pipes 2-Coupler 3-Circumferential grooves on the coupler

Figure 95 shows a schematic RPW under water application. The PCC-Pipes assembly and the welding tools are submerged in water. The surrounding water does not affect the RPW procedure. 1-Pipes 2- PCC 3-Welding tools 4-Water

Figure 96 shows a PCC joint assembly for connecting varying component with varying size. The PCC and RPW application is not limited to joining similar size pipes. PCC can be a reducer or transition coupler. 1- Pipe 2- Bar (solid cylinder) 3- PCC 4- PCC's coupler with conical shape or reducer coupler

Figure 97 shows a welded joint of two different components with different diameters. As an example a smaller diameter bar (cylinder) is welded to a larger diameter pipe. 1-Pipe 2-Bar (solid cylinder) 3-Reducer coupler

Figure 98 shows a PCC and pipes arrangement for a none friction based RPW. The PCC can be stationary and the pipes ends can stay out of the coupler's sockets. The chamfered sections of the pipes and the coupler are heated by other sources rather than friction so their surfaces reach to a suitable temperature for forge welding. 1- Pipes 2- PCC 3- PCC's coupler 4- The chamfered sockets of coupler are receiving thermal energy 5- The chamfered section of pipe ends are receiving thermal energy

Figure 99 shows a welded PCC-Pipes joint with none friction based RPW. After the chamfered sections are heated up, the pipes are inserted and pushed into the coupler sockets and after that the PCC is depressurised so the radial compressible force due to the coupler contraction welds the hot chamfered faces of the pipes and the coupler. 1- Pipes 2- PCC 3- PCC's coupler 4- The heated pipes are pushed into the coupler 5- radial compressible force due to the coupler contraction 6- An RPW between the pipes and the coupler without friction heating

Claims

Claims:

1. A coupler, also known as coupling, for connecting two pipes wherein the coupler has a closed shell around its outer surface and the coupler is able to contain and store a fluid in the space between the coupler and the shell; wherein the coupler is a ring shaped pressure vessel so the stored fluid is pressurised above the ambient pressure to expand the coupler and consequently expand the coupler inner hole ends where the coupler's sockets are; wherein two pipes are inserted into the coupler's sockets while the liquid is pressurised and the sockets are expanded to make a joint assembly before connection; in such a way that after the above assembling procedure, the coupler rotates while the pipes stay stationary and the rotating coupler is compressed between the two stationary pipes wherein the inner surfaces of the coupler's sockets touch the outer surfaces of the inserted sections of the pipes; wherein the friction between the moving faces of the coupler and the stationary faces of the pipes heats up the touching faces to a temperature suitable for welding; in such a way that after the above heating procedure the fluid is depressurised so the coupler stops and contracts over the pipes to weld the heated faces of the joint assembly wherein two circumferential welds also known as girth welds are generated simultaneously between the two pipes and the coupler.

2. A system according to claim 1, wherein the pipes and the coupler are made from steels or alloys so the joint assembly is welded based on solid state welding also known as forge welding where in this method the temperature at the interface between the clamps and the pipes is raised below the melting point of the pipes and the coupler materials; in such a way that at a suitable temperature for welding, the compressive radial force due to the coupler contraction over the pipes creates metallurgical bonds between the coupler sockets' inner faces and the outer surfaces of the inserted sections of the pipes.

3. A system according to claim 1, wherein the pipes and the coupler are made from similar or different materials including metals, steels, alloys, plastics, thermoplastics, ABS (Acrylonitrile Butadiene Styrene), PVC (Polyvinyl Chloride), PB (Polybutylene), PP (Polypropylene), PE (Polyethylene), PVDF (Polyvinylidene Fluoride).

4. A system according to claim 1, that the coupler is referred as a PCC (Pressure Containing Coupler) wherein the PCC has a semi torus shape where the straight edge of the half circle at the cross section is parallel with the axis of rotation so the PCC is made of two components, a cylindrical coupler which makes the PCC inner hole and a half torus shell which makes the outer surface of the PCC and covers the PCC's coupler; wherein when PCC is pressurised, while it wholly expands the coupler is bent and the cylindrical geometry of the coupler is changed to a hyperboloid geometry with the maximum expansion at the ends of the PCC's inner hole in such a way that this changes and the overall expansion may not be visible to a naked eye.

5. A system according to claim 1 wherein the inner surface of the coupler at its both ends is tapered to have a chamfered socket with a semi-conical (truncated cone) shape also the outer surface of the pipes is tapered at both ends so the pipes have chamfered ends with a semi-conical (truncated cone) shape in such a way that the chamfered sections are not extended through the whole wall thickness of the pipes and the coupler and therefore the pipes and the coupler have part plain and part chamfered ends; wherein the chamfered section of the pipe can be inserted into the chamfered section (socket) of a coupler in such a way that the chamfered faces of the pipe touch the chamfered faces of the coupler.

6. A system according to claim 1 wherein the coupler shell bursts to suddenly depressurise the coupler and releases the stored potential energy wherein the shell is machined while it is rotating till it bursts in such a way that a cutting tool moves forward to the shell and cuts the shell and creates a groove on the shell wherein this process continues making the groove deeper until the remaining wall thickness of shell reaches a critical point and the shell bursts.

7. A system according to claim 1 wherein an internal line-up clamp is used to align the coupler and the two pipes together wherein the line-up clamp has a mechanism to close and open the joint assembly in such a way that the line-up clamp is able to push the pipes ends into and pull them out of the coupler's sockets and therefore is able to sandwich the coupler between the two pipes while it is rotating wherein this mechanism can be a hydraulic, magnetic or mechanical mechanism; and the line-up clamp has two clamps at the ends which can lock to the pipes and a revolving system with a motor at the middle which can hold and rotate the coupler wherein the coupler is balanced for rotation in the factory prior the welding procedure and a mass dynamic vibration absorber is used to reduce the revolving system vibration.

8. A system according to claim 1 wherein the coupler's shell has internal baffles distributed along the shell circumference and positioned radially so the baffles rotate the stored water with the coupler body to increase the system kinetic energy and rotational inertia.

9. A system according to claim 1 wherein a tension test tool is made of two split clamps which are connected by a few hydraulic cylinders is used to test the shear strength of the girth welds in such a way that the tension tool is installed on a welded pipe segment between two welded couplers wherein the hydraulic cylinders push the end clamps at the back of the couplers and try to pull out the pipe from the couplers.

10. A system according to claim 1 wherein each coupler socket has a groove on its inner face so that after welding a ring shape hole is created between the coupler and the pipe in each girth weld wherein the ring shape hole is filled with a fluid and pressurised to enable leak testing of the girth weld and confirm that the weld is sealed and watertight.

11. A system according to claims 1 and 10 wherein a ring shape Bourdon tube is installed in the coupler's socket groove before welding wherein the tube is almost a full circle with a gap in its circumference in such a way that pressurising a fluid in the ring shape hole applies external pressure on the Bourdon tube and closes the gap to make a full circle tube wherein this function is used as a switch to show and confirm the pressure of the injected fluid in the ring shape hole.

12. A system according to claim 1 wherein the pipes and the coupler are made of steels or alloys with polymeric coatings in such a way that in the joint assembly the steel sections of the coupler face the pipes' steel sections and the polymer sections of the coupler face the polymer sections of the pipes wherein while the steel sections are heated up to become welded and the polymeric sections are heated up to become welded also and therefore the welds are performed simultaneously between the steel sections and between the polymeric sections of the pipes and the coupler.

13. A system according to claim 1 wherein the pipes and the coupler are made of steels or alloys with polymeric coatings in such a way that in the joint assembly the pipe coating touches the coupler shell and becomes welded to the shell while the pipes are welded to the coupler.

14. A system according to claim 1 wherein the coupler is used to weld two pipe-in-pipe systems in such a way that in the joint assembly the outer pipes sit on and touch the coupler's shell or a part which is connected to the coupler's shell wherein this method four girth welds are generated simultaneously between the coupler and the two pipe-in-pipe systems.

15. A system according to claim 1 wherein the welding technique is used for offshore pipeline construction using S-lay or J-lay vessels in such a way that the vessel is able to pay out the pipeline continually to avoid intermittent movement by having a table or tables which can slide and move on the deck wherein this method all aligning supports and welding tools are installed and fixed to the sliding table and move with the table relative to the boat wherein the table is connected to the end of the constructed pipe and starts moving with the pipeline when after that a new pipe segment is loaded on the table, is aligned and is welded to the constructed pipeline while the pipeline is being paid out so after welding the new pipe segment becomes the end of the constructed pipe so the table is disconnected from the pipeline and moves towards the new end and the procedure is repeated.

16. A system according to claim 1 wherein a gap is considered between the coupler and the pipes in the joint assembly in such a way that the shell bursts and the released potential energy converts to kinetic energy moving the coupler body toward the centre wherein the coupler hits the pipes at a velocity and the kinetic energy converts to strain energy and the impact load welds the coupler to the pipes.

17. A system according to claim 1 and 16 wherein the coupler and pipes stay stationary and a few plates made from titanium, titanium alloy or other material are rotated inside the gap in such a way that the plates are touching the surfaces of the pipes and the coupler while they are rotating so the friction between the touching faces heats the pipes and the coupler wherein the plates are removed from the gap and immediately after that the coupler shell bursts so that the coupler hits the pipe and becomes welded.

18. A system according to claim 1 wherein the coupler has a bushing inserted into the coupler hole which the bushing is a short length pipe located between the coupler sockets in such a way that after welding, the bushing stays flush with the inner surface of the pipe and therefore the coupler bushing has the same internal diameter as the pipeline internal diameter wherein the bushing is bonded to the inner surface of the coupler.

19. A system according to claim 1 wherein the coupler shell is removed after completion of the weld and is used as a support for the pipeline wherein the shell is partially cut by cutting tools for removal while the coupler is rotating.

20. A system according to claim 1 wherein the welding technique is used for under water and subsea applications and be used instead of current available subsea mechanical joints and hyperbaric welding for both diver based and diver less underwater and subsea joining.

21. A system according to claim 1 wherein the coupler and the pipes are heated using techniques other than friction for generating heat such as electrical induction heat, electrical arc, direct flame, ultrasonic heat.

22. A system according to claim 1 wherein the coupler connects the two pipes without welding in such a way that after the joint assembly is made the coupler which is stationary contracts over the pipes so the joint stays connected based on the static friction between the coupler and the pipes due to the bearing load from contraction wherein a polymeric resin or adhesive is used between the coupler and pipe touching faces to seal and make watertight the joint.

23. A system according to claim 1 wherein the coupler is used to connect different components such as connecting two bars or a pipe to a bar wherein the coupler is used as a reducer to connect two pipes or two bars with different diameter wherein the coupler is used as a transition to connect a pipe or a bar to other shaped components like a square profile.

24. A system according to claim 1 wherein a pipeline pig is installed downstream of the pipeline and is connected to the welding tool by an umbilical contains several cores, including a hose for nitrogen injection, a power cable, a communication/data cable and a puling rope wherein the line-up clamp has a replicable and rechargeable pressure vessel for storing nitrogen in such a way that nitrogen is injected by the welding tool into the downstream end of the pipeline via the pig, so that the pipe volume behind the pig is being filled with nitrogen while the pipeline is being constructed.