GB2315835A - Method for producing pipeline by assembly at sea of successive pipes, and pipes for the implementation of this method - Google Patents

Abstract

The pipeline is constructed by welding adjacent inner pipes (11, 21) end-to-end with spaced outer tubes (12, 22) and then locating a sleeve (6) over the junction zone (5) and securing the sleeve (6) to the outer tubes with a hardening bonding agent. Each outer tube (12, 22), at its downstream end, has a stop (13) to locate the sleeve (6) in position over the junction zone and to ensure a secure bond between the sleeve and the outer pipes, by reducing torsional stress in the welded joint and the bonding regions between the sleeve and the outer pipes.

Description

METHOD FOR PRODUCING PIPEWORK BY ASSEMBLY AT SEA OF SUCCESSIVE PIPES, AND PIPES FOR THE IMPLEMENTATION OF THIS METHOD The present invention concerns the design and manufacture of pipework, and in particular reference is made to a preferred application in which pipework systems known as pipelines are installed at sea from a barge, for the purpose of conveying fluids composed in particular of petroleum products in the form of gases or liquid hydrocarbons.

This pipework is traditionally assembled from a series of identical pipes, manufactured in the factory, which are transported to the site at sea and once there are assembled end-to-end, connecting the previous downstream pipe already integrated into the pipework to successive upstream sections, and are allowed to descend into the sea after assembly of the pipes.

Generally, the pipes are each composed of coaxial tubes which form, in the running section of the pipe, a thermally insulating envelope around an inner tube which delimits the conduit in which the fluids to be conveyed circulate. In order to permit without excessive complication the end-to-end welding of two inner tubes belonging respectively to two successive pipes to be joined, an advantage is provided by ensuring that each inner tube extends beyond the length of the other components at each extremity of the pipe, leading to the definition of a junction zone which extends over the end portions of the inner tubes projecting beyond the extremities of the associated outer tubes.

Following a known practice as described in the application for a French patent lodged by the present applicant under the number 2 721 681 for pipes composed of two coaxial steel tubes surrounding a thermal insulation envelope, the assembly joints between two successive pipes involve a coupling sleeve which is positioned to cover the junction zone and which is bonded at each end of the zone to the external peripheral surfaces of the adjacent end sections of the outer tubes. The junction zone between two successive pipes consisting of the end portions of two inner tubes welded end-to-end, is thus enclosed and protected and is continuous with the running section of the pipes.

The same patent document describes how such a bond is achieved by means of a hardening bonding agent injected into any free space remaining between the sleeve and the terminal sections of the outer tubes which it covers and between the same sleeve and a filling material which can be added in the junction zone around the end portions of the inner tubes which extend beyond the thermal isolation envelope, this being closed and watertight at each end of the terminal sections of the outer tubes.

It is known that assembly operations carried out on barges at sea involve at least three successive separate work positions. At the first of the positions, the two pipes to be joined are held in alignment with their inner tubes in contact, and the end surfaces of the inner tubes of the upstream and downstream pipes are welded together. At a second position, the sleeve is positioned to cover the junction zone, then at the third position a hardening bonding agent is injected through the wall of the sleeve.

These three stages take place at fixed positions on the barge and involve a relative movement of the pipes which travel sequentially between the stages in passing from one position to the next. At each movement, the barge is moved forward by a corresponding distance while at the stern of the barge the assembled pipeline progresses towards the exterior of the barge, where it is allowed to descend into the sea. Throughout assembly of the pipeline, the pipeline is more or less retained by a tensioner located at the stern of the barge. The tensioner operates by means of wheels which press against the external peripheral wall of the outer tubes. Two assembled pipes are thus made to descend into the sea while a new assembly phase is being performed for the most recent assembled downstream pipe and the next upstream pipe.

The present invention brings a .-2finement to such a technique by equipping each pipe, at the periphery of the end section of its outer tube, with a retaining stop which acts together with the coupling sleeve while passing the tensioner, such that the stop mechanically maintains the sleeve in its longitudinal position relative to the end sections of the outer tubes.

The principal purpose of this retaining stop is to prevent the joint of hardening bonding agent, advantageously composed of an organic resin which hardens by polymerisation and is applied at a low thickness, from being subjected to shear stresses which could result in its damage or the rupture of the seal which it provides, when the tensioner, acting more upon the sleeve than on the outer tubes themselves, tends to induce a differential longitudinal displacement between the sleeve and the outer tubes which it surrounds.

This stop is used in an orientation towards the downstream end face of the sleeve i.e. towards the junction zone between the upstream and downstream tubes, on the downstream end section of the outer tube of each pipe.

The subject of the present invention is therefore pipes intended for the construction of pipelines at sea by longitudinally assembling, end-toend, successive pipes which are characterised by the fact that they incorporate, in the running section of each pipe, an outer tube forming and/or making a thermal insulation envelope around an internal pipe which extends in length beyond the ends of the outer tube in a junction zone between two successive pipes which are connected by welding the end faces of the inner tubes, and by the fact that a portion of the downstream end of each outer tube possesses on its external periphery a stop to retain on one upstream end face a coupling sleeve with which it is associated and which links the downstream end portion to an upstream end portion of the outer tube of the preceding pipe, covering the junction zone.

It should be understood that that which is referred to as the thermal insulation envelope may take numerous forms which differ in their methods of fabrication. In particular, the thermal insulation envelope may be composed of a thermal lagging material interposed between two metal tubes or can be obtained by creating a vacuum or by filling the enclosed chamber formed by such pipes with a suitable gas. The thermal insulation envelope may also be materially distinct from the outer tube.

Furthermore, it is possible for the thermal insulation function of the envelope to be secondary in relation to other functions which determine the choice of the form which the envelope takes. As an example, the invention applies in certain of its variants to the fabrication of pipelines made from pipes in which an inner tube carrying hydrocarbons is surrounded, in the running section of each pipe, by a concrete tube which is sufficiently thick to constitute an envelope providing principally a ballast function at the same time as a thermal insulation function.

Another aspect of the invention is a method for installing pipelines at sea by the end-to-end assembly of successive pipes, with which method the pipes thus defined and the associated coupling sleeves are assembled, and a permanent retaining joint is provided by watertight bonding between each sleeve and the end sections of the outer tubes which it covers on either side of the junction zone between two successive pipes whose inner tubes are end-to-end welded during installation.

The invention thus proposes a procedure for constructing pipelines by assembling successive pipes on a barge at sea, for petroleum products in particular, according to which prefabricated pipes are successively connected with each pipe comprised of coaxial tubes forming or making, in the running section of the pipe a thermal insulation envelope around an inner tube, and with the inner tube extending lengthways beyond each end of the pipe to describe a junction zone where the inner tube of a pipe is welded to the inner tube of a preceding pipe.

Following this method, after having fitted on the downstream tube a coupling sleeve having a free sliding clearance on the outer tubes of the pipes, the end faces of the inner tubes of a downstream and an upstream pipe are welded end-to-end and the sleeve is caused to slide until it covers the junction zone; the sleeve is then immobilised by bearing against a peripheral stop fitted to the downstream end section of the outer tube of the upstream pipe; a hardening binding agent is then injected through the wall of the sleeve, the agent being selected, if not to truly bond, at least to permanently maintain the sleeve in a fixed relative position on the end sections of the outer tubes; and the two pipes thus assembled are made to move towards the exterior of the barge by means of a tensioner which has wheels which press successively against the external wall of the downstream outer tube and the sleeve, passing over the said stop.

When assembling the successive pipes, one thus ensures that any coupling sleeve between a downstream tube and an upstream tube is automatically immobilised against potential relative movement of the outer tubes of the two connected pipes. Since this immobilisation is of a mechanical nature, it avoids excessive loads being applied to the binding agent joint before the agent has hardened. The same advantage is found if the binding agent is not injected at that stage, which is the case in installations where the injection station is placed between two tensioners.

It is thus possible to benefit through greater freedom in the choice of the binding agent which may be used and its hardening conditions, depending on the permanent role which it is required to perform in the finished pipeline.

In general it is a requirement that mechanical continuity of the pipeline is always secured during its utilisation, by transmitting traction and bending forces from one pipe to another via a sleeve placed between them. Frequently it is also a requirement that a watertight seal is provided to prevent water penetration into the junction zone covered by the sleeve.

During the operation of positioning the pipeline by progressive movement of the pipeline into the sea, the coupling sleeve between two pipes connected end-to-end passes through the tensioner. In practice, there is a period during which the connected pipes are essentially retained only by the sleeve. This can give rise to shear loads the effect of which the stop in accordance with the invention is precisely intended to cancel out.

In effect, any differential tractive force between the sleeve and the pipes which it connects, under the action of the tensioner combined with that of gravity, must be transmitted by the joint of binding agent. These loads increase with decreasing thickness of the joint. The joint is preferentially made from a material with a high power of adhesion to the surface of the two surrounding tubes, in the frequent situation where this is desirable with a view to ensuring good water tightness. If the shear loads are too high in reiation to the power of adhesion, the sleeve may tend to slide on the outer tubes with a resulting degradation of the seal formed by the binding agent thereby destroying the linkage between the sleeve and the pipes, both in terms of mechanical bonding and water tightness.

The provision of a stop on the downstream end of the upstream outer tube enables such undesirable effects to be eliminated, as the tractive forces which act in opposite directions on the sleeve and on the inner tubes are transmitted by the stop on the sleeve to the outer tube.

Thus the shear loads which could occur within the binding agent due to differential driving of the sleeve by the tensioner can be eliminated.

According to another feature of the invention, it is desirable for the peripheral sleeve retaining stop on the end section of the outer tube of the upstream pipe to be continuous around the whole of the outer tube. As a variant, the stop may be discontinuous if its revolution period is suitable for the practical conditions for connection of the pipes.

A further advantage of the stop according to the present invention lies in the fact that it allows accurate positioning of the sleeve to cover the junction zone, during the fitting stage of the sleeve.

It is preferable for the hardening binding agent to be composed essentially of a basic organic material which hardens by polymerisation, in particular unsaturated polyester resins or polyurethane resins. In general this composition can be selected from among the commercially available single- or two-component adhesives which provide rapid hardening, for example from 2 to 10 minutes.

According to another feature of the invention, the stop is factoryfitted to the outer tube. The prefabricated pipes brought to the barge are therefore equipped with this stop, with the result that the procedure according to the invention requires no more time than known procedures.

According to a particularly advantageous mode of implementation of the invention, the stop comprises a continuous flange formed around the outer tube of each pipe. This allows optimum compensation through the sleeve of the shear loads, with no requirement for the pipes to be orientated in a particular manner.

Such a peripheral stop is advantageously formed by a separate piece fixed to the wall by welding. According to one of the preferred variants of the invention, it may also be constituted by an appropriatelymachined welded bead, in particular in order to offer a bevelled shape on the running section side of the outer tube.

Under the practical conditions which usually apply, each pipe being made from two coaxial tubes enclosing an envelope for thermal insulation and/or ballast, the length of the sleeve is of the order of two to five times the outside diameter of the pipe (i.e. the external pipe), the latter being commonly from 100 to 800 mm and the length of the junction zone is commonly of the order of 200 mm. This provides good transmission of the bending moment between two successive pipes without concentrating the bending moment on the weld made between the inner tubes.

Furthermore, the free sliding clearance mentioned above ranges advantageously from 2 to 10 mm between the external diameter of the pipes and the internal diameter of the sleeve. Each of the two mating surfaces is generally smooth i.e. their condition on leaving the workshops where the pipes and sleeves are fabricated. In practice, though, the manufacturing tolerances of the tubes always leaves a superficial roughness which facilitates adhesion of a resin intended to provide watertight bonding, and the manufacturing tolerances in terms of roundness ensure more effective immobilisation by the stop, while still allowing the sleeve to slide.

A particular form of implementation of the invention will now be described in more detail which will assist in an understanding of the essential features and advantages, it being clear however that this form of implementation is chosen as an example and is in no way restrictive. Its description is illustrated by the attached drawings, in which: - Figure 1 shows a longitudinal section of two pipes assembled according to the invention; - Figures 2 to 4 show the three stages of assembly; - Figure 5 is a schematic representation of a barge during the laying of a pipeline; - Figure 6 shows the tensioner during the passage of the sleeve; and - Figure 7 is a cross section showing an advantageous profile of the stop.

For reasons of clarity, the same elements are generally designated by the same reference numbers and the representations in the drawings are schematic without necessarily respecting the same scale in dimensions.

In accordance with the present invention in the particular mode of implementation described and illustrated by the Figures, a pipeline for petroleum products, in particular for gaseous or liquid hydrocarbons, is constructed on site on a barge at sea from a series of pipes, all identical to each other, by joining successive pipes end-to-end which are progressively added to the pipeline during its laying on the sea bed.

In Figure 1, we see two pipes, a downstream pipe 1 which is connected to the pipeline already constructed and an upstream pipe 2 which is assembled end-to-end to the downstream pipe 1. The arrangement illustrated by the Figures assumes that the assembled pipes leave the barge by the stern i.e. at the left of the Figure.

Each of the pipes 1 and 2 comprises two coaxial, cylindrical steel tubes i.e. an inner carrier tube 11 or 2; and an outer protective tube 12 or 22. The outer tube has an anticorrosive protective coating. The annular space between the inner and outer tubes may be fiiled with materials providing various functions. In the situation illustrated, the insulating capability is obtained by creating a vacuum in the annular space between the two tubes of the running sections, spacers 3 being placed between the two tubes.

As shown in Figure 1, the outer tube 12 or 22 is shorter than the inner tube 11 or 21. It extends over most of the length of the inner tube and over the whole of its median section, constituting the running portion of the pipe, but leaves at the opposite ends of the inner tube 11 or 21 two extending portions where the inner tube extends beyond the outer tube and has no envelope.

This determines a junction zone 5 which is delimited by the facing ends of two outer tubes 12 and 22 and which extends along the extended sections of the end-to-end inner tubes. This junction zone may be covered with a filling material during the assembly operations. At its extremities i.e.

at each of the longitudinally opposed ends of each tube, a welded flange 10 is fitted as an intermediate part linking the outer tube to the inner tube so as to provide watertight closure of the envelope space of the running section.

A coupling sleeve 6 is placed around the outer tubes 12 and 22 such that it covers the junction zone 5 where there is no outer tube and extends over the respective end sections of the two facing outer tubes.

The dimensions involved may imply for example, for pipes with running lengths of 12 metres or 24 metres, inside diameters of 150 mm to 600 mm for the inner tubes and 220 mm to 700 mm for the outer tubes, with the inner tubes extending longitudinally 100 mm beyond the outer tubes at each end, bearing in mind the accessibility requirements for the operation of welding the inner tubes together.

The sleeve 6 has an annular cylindrical shape, with an inside diameter which leaves around the outer tubes 12 or 22 of the pipes 1 and 2 a sliding clearance sufficient to be able to absorb their possible deformation and in particular the effects of alignment tolerances and any shrinkage which occurs during welding operations.

The inner surface of the sleeve is smooth, as is the external surface of the outer tubes. In other words no particular machining is needed on a continuous cylindrical tube arriving from the factory. The sleeve has a thickness at least equal to, and preferably slightly greater than, that of the outer tube.

The sleeve 6 is immobilised in a fixed relative position on the outer tubes by means of a rapid-hardening bonding agent.

In the example described, this bonding agent is composed of a rapid-hardening organic adhesive resin and more particularly an adhesive based on polyurethane with a hardening time of approximately 5 minutes.

The sliding clearance is of the order of 3 to 5 mm, which represents a satisfactory compromise between a low thickness of the layer of bonding agent 7 and good mechanical strength of the assembly.

The sleeve is advantageously positioned longitudinally such that it then extends symmetrically over the end sections of the outer tubes 12 and 22. Its length is chosen in the range of 2 to 5 times the outside diameter of the pipe, for example of the order of 1 to 2.5 metres.

In accordance with the invention, the downstream end of the outer tube 22 of the upstream pipe 2 is fitted with a peripheral stop 13, in this case executed in the form of a continuous flange around the outer pipe. It is fixed by welding in the factory. The prefabricated pipes delivered to the barge are thus fitted with this stop.

The stop 13 is shown in detail in Figure 7 in a form which renders it easy to produce directly, in advance or on site, by welding and machining a bead on to the outer tube. The face 41 of the stop 13 oriented towards the central section of the pipe is bevelled and its opposite face oriented towards the downstream end of the pipe has an annular groove 42 in its interior part in contact with the outer tube 22. A peripheral outer lip 43 is cut which forms on the upstream side a clear radial surface for flush peripheral contact with the sleeve 6.

Such a configuration allows the quantity of material necessary for the manufacture of the flange on the hollow side to be limited and simultaneously avoids the presence of sharp connecting edges which could engender excessive stresses at the limits between the flange and the outer tube. At the same time, its oblique shape contributes to providing it with adequate mechanical strength with respect to its function of retention and of maintaining the fixed position of the sleeve relative to the corresponding outer tube in the longitudinal direction.

Figures 2 to 4 illustrate the three stages of assembly of the two pipes 1 and 2. The operations are carried out on a barge as shown in Figure 5. This barge moves forward in a sequential manner in the direction of the arrow 33. A tensioner 31 is installed at the stern, shown in detail in Figure 6. The tensioner 31 comprises a type of double caterpillar with wheels 37 of a flexible material such as rubber, which grip the outer tubes by applying pressure.

This tensioner maintains the pipeline 32 under tension as the pipeline is progressively laid on the sea bed. The tensioner holds the pipeline, whose weight tends to drag it out of the barge. It allows the pipeline to progress at the desired rate by the movement of the chains which carry the wheels.

For each cycle of assembly of two pipes, the barge moves forward sequentially so as to present the junction zone 5 at the successive work positions 34 to 36. It will be understood however that in practice there could be more than one of each station, particularly the welding station.

At the first position 34, as illustrated in Figure 2, end-to-end welding is performed on the two free sections of the inner tubes 11 and 21, which are kept in contact at their end faces. Rather than an external cradle, it is preferable to use for this purpose an hydraulic gripping device (not shown) engaged in the interior of the tubes. We also see from the Figure that the sleeve 6 has previously been fitted onto the outer tube 12 of the downstream pipe 1.

At the next position 35, the sleeve 6 is made to slide on the outer tube 12 then onto the outer tube 22, so as to bring it into contact with the stop 13 while covering the junction zone 5. The junction zone may previously have been filled with a filling material as shown in Figure 2.

At position 35, the hardening bonding agent 7 is introduced under pressure through injection holes drilled for that purpose through the wall of the sleeve 6 in order to fill the space remaining around the extensions of the inner tubes in the junction zone between the linking parts or flanges 10 and the sleeve 6.

It will be observed that for reasons of convenience, when injection occurs the circular orientation of the sleeve 6 is such that the injection holes 8 are positioned in the lower part of the pipes arranged horizontally during the assembly operations. In a manner which in itself is traditional, vent holes (not shown) are provided opposite the holes where the bonding agent is injected i.e. in the upper part of the sleeve. While it can be accepted that the bonding agent may not reach the extremities of the sleeve, without compromising the desired efficiency of immobilisation or of watertight bonding between the sleeve and the outer tubes, it will generally be more convenient, in accordance with the present invention, to operate under pipe assembly conditions which mean that the bonding agent while still fluid may overflow from the zone of contact between the parts when injection takes place.

In order avoid such an overflow, a temporary ring is used in association with the essential components of the invention. The ring is fitted to the outer tube in order to take up approximately the outside diameter of the sleeve at its extremity, together with an inflatable seal held in a rigid annular ring, also used temporarily until the bonding agent has hardened sufficiently to no longer flow freely.

In this way the invention provides mechanical strength which is sufficient for the mechanical loads which arise during assembly and when the conduit is in use, in a uniform manner over its whole length. At the time of connection and during the life of the finished pipeline, the welded inner tubes are sufficient to withstand the basic traction forces, while the sleeve, immobilised over its length, efficiently absorbs the bending forces to which the welds in the junction zone might be subjected. In conclusion, from one running section of the pipe to another there is mechanical continuity at the level of the junction zone, in particular from the point of view of rigidity (product of moment of inertia times Young's modulus) along any axis normal to the axis of the pipe.

When the above three stages are complete, the barge moves forward and the pipeline leaves the barge by the stern while continuing to be retained by the tensioner. It therefore progresses towards the seabed under the effect of its weight. This movement stops when the position for the next assembly cycle is reached.

Because of the stop 13 which is situated upstream from the sleeve, the traction forces exerted on the sleeve are transmitted to the outer tube 22 and thence to the inner tube. This avoids the occurrence of undesirable shear stresses on the layer of bonding agent 7. As a result of this the drive action of the tensioner on the sleeve does not produce, on the link between the sleeve and the outer tubes, any deleterious effect on the strength and life expectancy of the pipeline once completed.

Claims (12)

1. Method of assembling pipelines, in particular for petroleum products, to be constructed at sea on a barge, according to which prefabricated pipes are progressively joined, the pipes each being composed in the running section of the pipe of an outer tube forming and/or providing a thermal insulation envelope around an inner tube delimiting a duct for the flow of a fluid to be conveyed which extends beyond the length of the outer tube at each end of the pipe over a junction zone between two successive pipes where the end face of the inner tube of a pipe is welded to the end face of the inner tube of a previous pipe, this method being characterised by the fact that a coupling sleeve is fitted on the downstream pipe, providing a free sliding clearance on the pipes, the inner tubes of a downstream pipe and an upstream pipe are welded together end-to-end, the sleeve is then made to slide until it covers the junction zone, immobilised against a peripheral stop located on an outer downstream end of the upstream pipe, a hardening bonding agent is injected through the wall of the sleeve in order to maintain the sleeve in a fixed relative position on the outer end sections of the pipes, and the pipes so assembled are made to move towards the exterior of the barge by means of a tensioner fitted with wheels which press against the outer walls of the pipes and the sleeve.

2. Method according to claim 1, characterised by the fact that the hardening bonding agent is an adhesive based on organic resins.

3. Method according to claim 2, characterised by the fact that each pipe has two coaxial steel tubes delimiting between them the envelope in the running section of each pipe.

4. Method according to any of claims 1 to 3, characterised by the fact that the stop is factory-fitted onto the outer tube.

5. A pipe for connection to form a pipeline by implementing the method according to claim 1, the pipe having an outer tube forming and/or making a thermal insulation envelope around an inner tube delimiting a conduit for the flow of a fluid to be conveyed which extends beyond the length of the outer tube at each end of the pipe, characterised by the fact that the pipe has a peripheral stop fixed to a section of the downstream end of the pipe around its external surface.

6. A pipe according to claim 5, characterised by the fact that the stop is formed by a distinct part which is fixed by welding to an outer tube for limiting the envelope.

7. A pipe according to claim 5 or 6, characterised by the fact that the stop comprises a continuous flange around the outer tube.

8. A pipe according to any of claims 5 to 7, characterised by the fact that the stop is composed of a welded bead machined in order to form a bevelled edge on the running section side of the outer tube and an internal annular groove on its face oriented towards the end of the outer tube.

9. A pipe according to any of claims 5 to 8, characterised by including a sleeve having a length which ranges between two and five times the outside diameter of the pipe.

10. A pipe according to claim 9, characterised by the fact that the external wall of an outer tube limiting the envelope and the internal wall of the sleeve provide a free sliding clearance between the outer tube and the sleeve which ranges from 1 to 10 mm.

11. A method of assembling a pipeline substantially as hereinbefore described with reference to the accompanying Figures.

12. A pipe for use in a pipeline substantially as hereinbefore described with reference to and as shown in the accompanying Figures.