AU2004202939B2 - Method for installing a pipeline - Google Patents

Description

00451310,1 2 METHOD FOR INSTALLING A PIPELINE FIELD OF THE INVENTION The invention relates to pipelines and, more specifically, to pipelines that carry products to/from an offshore location from/to an onshore location.

Particularly, the invention relates to a method of installing a subsea pipeline that traverses a shoreline.

BACKGROUND OF THE INVENTION A conventional method for installing large diameter long distance subsea pipelines, is termed the 'S-Lay method'. This method involves welding joints of pipe together on a construction barge or a self propelled construction ship.

In this method, a construction barge is anchored to the seabed and is moved across the seabed by progressively relocating the anchors using anchor handling ships. In the case of a construction ship, the ship's position is controlled using a Dynamic Positioning (DP) system incorporating bow and stern thrusters linked to a computerised positioning system so that no anchors are required (Figure 1).

An offshore pipeline laid from one offshore location to another using the S- Lay method involves setting a "dead man anchor" on the seabed behind the construction barge or ship with a cable attached to the pipeline end on the barge.

As the joints are welded together on the barge to construct the pipeline, the barge is moved forward so the pipeline is pulled off the back of the barge into the water, under controlled tension. The controlled tension is important to ensure that the formed pipeline leaves the barge or ship with a curvature no less than a minimum curvature, otherwise bending of the pipeline beyond a permissible degree will cause damage to the pipeline. As shown in Figure 1, the pipeline forms a curve as it goes over a curved launch ramp at the stern of the barge and spanning in a catenary configuration to the seabed.

To prevent the pipeline from buckling between the construction vessel and the seabed, adequate tension is maintained by clamping the pipeline in tensioning 004513101 3 devices that control the tension in the pipeline as the vessel is moved forward. The pipeline moves through these tensioners in a controlled operation that feeds the pipeline off the back of the vessel.

The pipeline welding operation continues until the laydown location for the pipeline is reached, at which point a laydown cable is attached to the end of the pipeline and the pipeline is lowered, under controlled tension, to the seabed.

For an offshore pipeline laid from shore to an offshore location using the S- Lay method, as an alternative to the dead man anchor, the pipeline end may be connected to a line from an onshore winch that pulls the end of the pipeline ashore from the construction vessel (Figure The pipeline construction then continues on the barge or ship in the manner described above. By running the winch and pipeline across the shoreline aboveground or in an open trench, there is significant visual impact and construction work which may be undesirable in coastal locations. A variation on this shore crossing pipeline installation method involves forming a horizontally directionally drilled (HDD) hole from an inland location to a subsea location, and hauling the pipeline ashore through the HDD hole (Figure 3).

The remainder of the pipeline is then constructed according to the S-Lay method described above.

A variation on his method is to pre-install a liner pipe in the HDD hole and then pre-install a section of pipeline through the lined HDD hole with the offshore end of the pipeline section some distance beyond the subsea exit point from the HDD hole (Figure 4a). The offshore end of this pipeline section is then recovered to the laybarge using a cable (Figure 4b). The laybarge is positioned at a safe distance offshore and may pull the pipeline further offshore through the hole to the laybarge location whereon the conventional ship or barge based pipeline construction methods are employed to complete installation of the pipeline (Figure 4c). Although overcoming the disadvantages of visual impact and construction work associated with running a pipeline aboveground across a shoreline, this method involves the expense of having two pipeline construction stations; one onshore to construct and feed the initial section of pipeline through the HDD hole and another on a construction vessel to continue offshore construction of the pipeline.

00451310, 4 Another method of installing subsea pipelines involves constructing the pipeline onshore to avoid the high costs associated with mobilisation and use of a construction barge. The pipeline is constructed onshore close to the water, usually perpendicular to the shoreline and then launched from the shoreline for towing to the offshore location for the pipeline. The technique requires careful selection of the shoreline launch site, primarily, to ensure adequate accessibility for the pipeline construction equipment, the pipeline is not damaged by wave or current action during launching from the shoreline or during towing and minimal onshore excavation and backfilling work to provide a suitably levelled site from which to launch into the sea. Conventional pipeline assembly techniques are used to assemble a bundle for offshore tow. The pipeline strings may be positioned on rollers or rail bogeys to facilitate towing the assembled pipeline string across the beach and into the water. Once in the water the pipeline must be towed across the shore approach zone where wave action and cross shore currents may act to destabilise the pipeline, resulting in lateral and vertical instability.

The lateral instability problem can be magnified by hydrodynamic loads on buoyancy modules that may be required at regular intervals along the length of the bundle to facilitate towing.

It is an object of the invention to alleviate, at least in part, one or more of the problems mentioned above.

While the invention will be described in relation to subsea pipelines, it should be understood that the method can be used to install pipelines in relation to lakes and rivers.

The term 'pipeline' used throughout the specification should be taken to include a pipeline, a pipeline bundle that incorporates secondary pipeline and umbilicals piggy-backed on a primary pipeline, umbilical, power cable or telecommunications cable either separately or in a bundled configuration.

SUMMARY OF THE INVENTION The invention provides in a first aspect a method of installing a pipeline to an offshore location, the method including the steps of: forming a lined HDD hole that traverses a shoreline and that has onshore and subsea openings; 004513101 constructing the pipeline onshore in proximity to the onshore opening of the HDD hole; inserting the pipeline through the HDD hole for retrieval upon exiting from the subsea opening; and towing the retrieved pipeline to a designated offshore location such that the pipeline extends from the designated offshore location.

It is preferable that the pipeline includes one or more lengths. In one preferred embodiment, the towed one or more lengths are connected after completion of towing to extend the pipeline from the offshore location to an onshore location through the HDD hole.

In another preferred embodiment, the one or more lengths are connected after completion of towing to extend the pipeline between two remote offshore locations.

It will be appreciated that by inserting the pipeline through the HDD hole for towing to the designated location avoids the necessity for a construction barge or ship on which the pipeline is constructed and launched under strictly controlled tension. This method allows most pipeline construction work to be performed onshore rather than constructing the pipeline onshore and offshore. It will also be appreciated that launching the pipeline from a subsea opening may avoid problems associated with launching pipelines from a shoreline exposed to wave and current action.

The step of forming an HDD hole includes lining the hole with a liner.

Preferably, the liner is a metal tube. The liner tube has been adopted to prevent the HDD hole from collapsing and to provide a smooth pathway to enable insertion and sliding of the pipeline through the HDD hole.

To further reduce friction, the pipeline may be constructed with a plurality of low friction collars spaced apart along the length of the pipeline such that, in use, the collars space the pipeline from the metal tube and co-operate with the metal tube to reduce the friction of the pipeline being inserted through the HDD hole.

The collars may be removed after exit from the HDD hole to avoid dragging along seabed or to avoid impacting any pipeline supports that may be located beyond the subsea opening of the HDD hole.

004513101 6 Insertion of the pipeline through the HDD hole may include pushing the pipeline into the HDD hole from the onshore opening via the use of a crane and/or a winch. To assist with insertion, the retrieved pipeline may be pulled through the HDD hole by ocean-going craft, such as tug boats, barges or ships.

Buoyancy means may be incorporated onto the pipeline to assist the ocean-going craft with towing of the pipeline to the designated offshore location.

The buoyancy means may be incorporated during the step of constructing the pipeline or the methods according to the first and second aspect may include a further step of incorporating buoyancy means as the pipeline exits or after the pipeline has exited the subsea opening of the HDD hole.

The buoyancy means may be inflatable units attached to the pipeline prior to insertion of the pipeline into the HDD hole, or attached as or after the pipeline exits the subsea opening, and inflated after the pipeline has exited the subsea opening. Alternative buoyancy means include solid buoyancy units attached to the pipeline as the pipeline exits or after the pipeline has exited the subsea opening and filling the pipeline with air or a suitable gas to improve the buoyancy of the pipeline.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic side view of commencement of pipelay from one offshore location to another with a typical offshore pipeline construction technique according to the S-Lay method.

Figure 2 is a schematic side view of commencement of pipelay from shore to offshore with a typical pipeline construction technique according to the S-Lay method using a shore based winch to pull the pipeline ashore.

Figure 3 is a schematic side view of commencement of pipelay from shore to offshore with a pipeline construction technique according to the S-Lay method, using a shore based winch to pull the pipeline through a HDD hole.

Figure 4 is a schematic side view of a pipeline construction technique from shore to offshore according to the S-Lay method, with the pipeline construction commencing onshore and the HDD section of the pipeline pulled offshore through the HDD hole for recovery by the construction vessel.

004513101 7.

Figure 5 is a schematic side view of a preferred embodiment of a pipeline installation technique according to the present invention. A towed pipeline launched through an HDD shore crossing.

Figure 6 is a cut away perspective view of a pipeline that is inserted through an HDD hole according to the installation technique in Figure 5, and views of a typical low friction collar that may be strapped around the pipeline.

Figures 7a and 7b are a schematic side view and a schematic plan view of a subsea ramp structure that may be used to control the curvature of a towed pipeline after it exits a lined HDD hole.

Figures 8a and 8b illustrate use of the known bottom tow technique for tow of a pipeline through an HDD hole and for offshore tow of a pipeline after the trailing end of the pipeline has exited from the HDD hole.

Figures 9a and 9b illustrate use of the known off-bottom tow technique for tow of a pipeline through an HDD hole and for offshore tow of a pipeline after the trailing end of the pipeline has exited from the HDD hole.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to a preferred embodiment of the first aspect of the present invention, an installed pipeline 10 traverses a shoreline 12 and extends between an onshore location 14 and a body of water 16. The body of water may be coastal sea or an inland lake or sea or even a river.

Referring to Figure 5, the pipeline 10 is installed by forming a horizontally directionally drilled (HDD) hole to have an onshore opening 22 on the onshore location and a subsea opening 24 formed in the sea floor. The subsea opening 24 is formed below the surface of the body of water 16 such that the pipeline 10 that exits the subsea opening 24 is not unacceptably affected by wave action or currents in the body of water 16. The HDD hole 20 is fitted with a liner 21 formed preferably of a metal in order to stabilise the HDD hole against collapse and to provide a smooth pathway to enable insertion and sliding of the pipeline through the HDD hole.

The pipeline 10 is constructed in a parallelity of sections aligned parallel and in proximity to the onshore opening 22 of the HDD hole. The sections are fed 004513101.

8 consecutively into the HDD hole 20 with the following sections being joined to the leading section already inserted into the HDD hole.

The pipeline 10 is constructed (Figure 6) with a primary pipeline 32 surrounded by a coating or coatings 34, which may include an outer concrete weight coating, and includes a piggy back bundle 36 held to the coating 34 by straps 38. The bundle includes moulded spacers 36 and secondary pipelines and an umbilical line 42 for transmitting control signals and or electrical or hydraulic power. Typically, the straps sit in recessed grooves (not shown) formed in the surface of the outer concrete weight coating to ensure that the straps 38 do not catch when or are not damaged by the pipeline 10 being dragged across the sea bed during bottom towing. The pipeline 10 may further include buoyancy units which are deployable after the pipeline exits the subsea opening 24 from the HDD hole. To reduce the friction of the pipeline 10 within the HDD hole, collars 44 formed of a low friction material are spaced at intervals along the pipeline 10. An example of a suitable low friction material from which the collars 44 may be formed is ultra high molecular weight polyethylene (UHMW-PE). By decreasing the friction of the pipeline 10 within the HDD hole 20, the pipeline 10 can be inserted through the HDD hole with less force being applied. The collars may be mounted directly over moulded spacers 36, or elsewhere along the pipeline, and fastened with straps to provide adequate strength. These collars 44 may be removed after the pipeline exits the HDD hole 20 and may only be required along a limited length of the trailing end of the pipeline to reduce the maximum required towing force. During the initial stages of towing, back tension may have to be applied to the onshore end of the pipeline 10 to aid in controlling its movement through the HDD hole The pipeline 10 is run over supports 50, which may be a fixed construction or two or more side boom cranes, that is configured to ensure that the pipeline enters the onshore opening 22 at an angle which substantially conforms to the drilling angle of the HDD hole 20 without exceeding the allowable curvature of the pipeline. This angle typically is in the range of 9 to 12 degrees from the horizontal.

The pipeline 10 is run over supports 52 (which may be required subject to the physical constraints of the project in question), which are installed subsea and may be structurally connected to the HDD hole liner 21. The supports 52 are 004513101-, 9 configured to ensure that the pipeline 10 exits the subsea opening 24 at an angle which substantially conforms with the drilling angle of the HDD hole 20, which is typically in the range of 5 to 10 degrees from the horizontal, and then is directed through a vertical curve to align with the direction of the towing force, without exceeding the allowable curvature of the pipeline.

According to a preferred embodiment (may not be required) (Figures 7a and 7b), the supports 52 are rollers. The rollers are components of a subsea ramp structure constructed as a composite structure comprising rollers 52, beams 54 and clamps 56.

The subsea ramp structure 52, 54, 56 may be installed subsea by lowering by crane from a construction vessel or by towing from shore. Final alignment relative to the subsea opening of the HDD hole 24 may be achieved with the assistance of divers, buoyancy units, temporary supports and rigging provided from a diving support vessel 62. Local excavation of the seabed 58 may be required to gain access to the liner 21. When aligned, the clamps 56 are bolted around the liner 21.

The configuration of the ramp structure 52, 54, 56 is designed with clamp orientation, roller heights and structural strength to fix the position of the rollers 52 relative to the position of the subsea opening 24 during offshore towing of the pipeline. The configuration of the rollers 52 is designed to ensure that the allowable curvature of the pipeline 10 is not exceeded during towing. The ramp structure 52, 54, 56 may rest on the seabed 64.

Collars 44, when used, are removed after exit from the subsea opening 24 and before each collar 44 reaches the first of the supports 52.

The pipeline 10 may be inserted through the HDD hole 20 from the onshore opening 22 by pushing the pipeline 10 into the opening with a winch or mobile crane.

Once a leading end of the pipeline 10 has exited the subsea opening 24 into the body of water 16 and is beyond the supports 52, divers attach a towing head (not shown) onto the leading end of the pipeline 10 such that the pipeline can be towed to a designated location by ocean going craft, such as tug boats, ships or barges 66, or by a winch at a fixed onshore or offshore location. The 004513101% towing head includes a connection point for the tow cable and typically is about 6 metres long with an internal space defining a pig receiving space. Typically pigs are run through the pipeline 10 to clean the pipeline, check its internal diameter to confirm that the pipeline 10 has not been damaged) and push water into or out of the pipeline prior to the commissioning of the pipeline 10 for transport of products such as gas or oil.

Prior to connection of the towing head, the pipeline 10 is prevented from flooding by a pressurised internal bladder or a similar sealing device located in the leading portion of the pipeline 10. The internal bladder or alternative proprietary sealing device resists the hydrostatic pressure as the pipeline exits the subsea opening 24 and hence prevents pipeline flooding prior to subsea connection of the tow head by divers. The pressurised internal bladders or alternative proprietary sealing devices are removed from the pipeline 10 by internal pipeline pressure pushing the bladder into the pig receiving space in the tow head after completion of towing operations.

Inflatable buoyancy units or solid buoyancy units may be attached to the pipeline 10 as or after it exits the subsea opening 24.

Any of the conventional pipeline towing techniques may be utilised for towing the pipeline 10 to the location where the pipeline is to be set in position, i.e.

the designated location. The most likely techniques to be adopted are either the bottom tow technique or the off-bottom tow technique (Figures 8 and These techniques have a long history of use in offshore pipeline installation.

The off-bottom tow technique differs from the bottom tow technique because more buoyancy is required and in addition to buoyancy units, lengths of weight chain are attached to the pipeline at regular intervals. This system is designed to lift the pipeline off the seabed while trailing the ends of the chains along the seabed to allow the pipeline and buoyancy combination to render the pipeline neutrally buoyant at a fixed distance off the seabed. The higher the pipeline floats, the more chain is lifted until neutral buoyancy is achieved.

It should be appreciated that installing pipeline 10 according to this method enables the pipeline 10 to be constructed in a single location on shore. Moreover, as the pipeline 10 is inserted through the HDD hole 20 the installed pipeline 00451310t 11 may traverse a shoreline 12 to link an offshore location, such as an offshore well, to an onshore location, such as a processing plant. Moreover, the pipeline 10 may be launched through the HDD hole so that one or more lengths of pipeline 10 exit the subsea opening 24 and so the respective ends of the pipeline 10 can be joined together to link respective remote offshore locations. In this case a trailing head would also be installed on the trailing pipeline end after it exits the HDD hole.

Pressurised internal bladders or alternative proprietary sealing devices would be used to prevent water from flooding the pipeline as described above.

The method of the invention may also include the step of installing any pipeline shaped object, such as a pipeline shaped length that is used to improve the lateral stability of a pipeline length against hydrodynamic forces. A length or lengths of a pipeline shaped object could be pulled into the water and towed to a required location either before or after connection to pipeline length or lengths.

Thus, pipeline lengths may be towed into the water and located adjacent to these pipeline shaped lengths or vice versa.

After completion of towing, the pipeline shaped lengths may be filled with grout or some other material to increase their submerged weights per unit length.

The pipeline lengths may be strapped or connected in some way to the pipeline shaped lengths to increase the lateral stability of the pipeline lengths.

Guidance of pipeline lengths into parallel and adjacent locations relative to pipeline shaped lengths would have to be engineered using subsea installation techniques. The pipeline shaped lengths may be some distance apart from the pipeline lengths upon completion of towing and may have to be hauled towards the pipeline lengths using winches or similar devices prior to connection to pipeline lengths.

Claims (4)

1. 004513101. 12 THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A method of installing a pipeline to an offshore location, the method including the steps of: forming a lined HDD hole that traverses a shoreline and that has onshore and subsea openings; constructing the pipeline onshore in proximity to the onshore opening of the HDD hole; inserting the pipeline through the HDD hole for retrieval upon exiting from the subsea opening; and towing the retrieved pipeline to a designated offshore location such that the pipeline extends from the designated offshore location.
2. 2. The method of claim 1, wherein the pipeline includes one or more lengths, the towed one or more lengths being connected after completion of towing to extend the pipeline from the offshore location to an onshore location through the HDD hole.
3. 3. The method of claim 1, wherein the pipeline includes one or more lengths, the one or more lengths being connected after completion of towing to extend the pipeline between two remote offshore locations.
4. 4. The method of any one of claims 1-3, wherein the step of constructing the pipeline includes the step of fitting low friction collars to the pipeline to reduce the coefficient of friction between the lined hole and the pipeline inserted through the HDD hole. The method of any one of the previous claims, wherein a buoyancy means is incorporated onto the pipeline after exit from the subsea opening of the HDD hole to assist towing of the pipeline to the designated offshore location. 14 July 2004 By Freehills Carter Smith Beadle Registered Patent Attorneys for the Applicant Michael John Stenford Warwick William Stenford