EP2142839A1

EP2142839A1 - Offloading pipeline

Info

Publication number: EP2142839A1
Application number: EP08737236A
Authority: EP
Inventors: Peter Roberts
Original assignee: Verderg Ltd
Current assignee: Verderg Ltd
Priority date: 2007-05-03
Filing date: 2008-04-30
Publication date: 2010-01-13
Also published as: JP2010526256A; CN101680593A; GB2448916A9; CA2684925A1; KR20100016156A; GB2448916A; GB0708560D0; AU2008247154A1; WO2008135780A1; BRPI0810999A2

Abstract

A system for transporting low temperature fluids comprising; a carrier pipe (2); and at least one inner product flow pipe (1) located within and thermally isolated from the carrier pipe (2); wherein the product pipe (1) has a greater length than the carrier pipe (2) and is incorporated into the entire length of the carrier pipe (2) by following a non-linear path, such that thermal contraction of the product pipe (1) due to a change in temperature of the product pipe (1) is accommodated by elastic geometric distortion of the product pipe (1).

Description

Offloading Pipeline

Technical field

[0001] This invention relates to a pipeline system for transferring low temperature fluids, such as liquefied natural gas (LNG). In particular for transporting fluid along the seabed between an offshore terminal and an onshore facility.

Background art

[0002] Where large volumes of natural gas are produced, a common technique for transporting the gas to distant markets involves liquefying the gas at a coastal location in the vicinity of the gas source and then transporting the liquefied natural gas (LNG) in specially designed storage tanks aboard a sea-going vessel. To form the LNG, natural gas is compressed and cooled to cryogenic temperatures, i.e. -16O⁰C, to increase the amount that can be carried in the storage tanks. Once the vessel reaches its destination, the LNG is off-loaded into on-shore storage tanks from which the LNG can then be revaporised as needed and transported on to end users through pipelines or the like. It is desirable to locate LNG import or loading terminals a long way offshore when it is necessary to get clearance under the keel of a large LNG tanker if the sea bed shoals gently away from the coast and to gain health, safety and/or environmental advantages by siting the loading/offloading facility well away from the coastal environment.

[0003] Presently known cryogenic fluid transfer systems include systems installed on causeways or trestle jetties specially built between the onshore tanks and the offshore receiving/loading facility. Supporting an LNG loading line on such a long causeway or jetty is expensive. The trestle must be sufficiently high above the water surface to preclude exposure of the transfer line to the water and in some cases avoid interfering with existing marine traffic on the surface. The costs associated with construction demand that the off shore facility is located close to the shore, which in turn may require the dredging of new access channels and the building of additional harbour facilities to insure safe passage for the deep draft LNG transport vessels. Also the return line to circulate LNG during idle periods may be made up and insulated separately from the main transfer line of the flow system, thereby significantly adding to the costs, complexity, and the safety concerns of the transfer system. A cryogenic fluid transport system submerged in the water as a pipeline supported by the sea bed, however, allows the receiving/loading station to be located at greater distances from the shore and eliminates the need for the causeway or trestle jetty and the need to dredge access channels and build expensive port facilities in areas where they would otherwise not be required to handle deep draft LNG vessels.

[0004] A problem with the transport of cryogenic transport of fluids in pipelines is that as the low temperature fluids enter the pipeline it leads to substantial shrinkage of most pipe materials, thereby generating significant thermal stress.

[0005] For fluid transfer systems placed on the sea bed it is preferred to have an insulated "pipe-in-pipe" configuration for LNG loading/offloading to preserve the cryogenic temperature of the LNG that is being transported where the annulus space between the product pipe or pipes and the outer carrier pipe is a vacuum. The thermal contraction of the product pipe can be accommodated through multiple and/or large expansion loops on a trestle jetty or causeway supported pipeline system. These pipeline systems having expansion loops are still substantially linear, having right angles to incorporate the bends into the linear product pipe. Having predominately linear pipelines in a system there is still only essentially a change in length of the product pipe, when a change in temperature occurs, and the thermal contraction can produce localised stresses due to the bends positioned along the product pipe to incorporate the expansion loops. Additionally the performance of such expansion loops in a subsea "pipe-in pipe" configuration laid on the sea bed may be compromised by silting, and such large expansion loops in a sub sea "pipe-in-pipe" pipeline are difficult to fabricate and install. Alternatively a material with a very low thermal expansion co-efficient, such as INVAR™ (36% Nickel Steel), can be used for producing the fluid carrying inner pipe, however this is relatively expensive. [0006] It is an object of the invention to accommodate the thermal shrinkage of the product pipe for subsea fluid transport systems. In particular the invention provides a fluid transport system for transferring cryogenic fluids between offshore and on shore facilities by introducing an additional length of product pipe into the carrier pipe by forming the product pipe into a nonlinear path so that when thermal contraction occurs there is a change in geometry and length along substantially the whole length of the product pipe.

Disclosure of the invention

[0007] A first aspect of the invention comprise a system for transporting low temperature fluids comprising; a carrier pipe; and at least one inner product flow pipe located within and thermally isolated from the carrier pipe; wherein the product pipe has a greater developed length than the carrier pipe and is incorporated into the entire length of the carrier pipe in a non-linear path such that thermal contraction of the product pipe due to a change in temperature of the product pipe is accommodated by elastic geometric distortion of the product pipe.

[0008] Particularly preferred low temperature fluids that the system can transport are cryogenic fluids, for example fluids having a temperature around - 16O⁰C, such as LNG.

[0009] The extra length of product pipe compared to the carrier pipe is accommodated by the different path the product pipe follows through the carrier pipe. The product pipe has a greater length than the carrier pipe at substantially equal temperatures, such as before cryogenic fluid flow through the pipe, and the product pipe is never shorter than the carrier pipe when there is a change in temperature. The shape of the product pipe differs from the shape of the carrier pipe before the flow of cryogenic fluid through the product pipe, so that when the product pipe is cooled by the introduction of cryogenic fluids, the thermal contraction is accommodated primarily by the change in geometry of the product pipe, and therefore its path through the carrier pipe, rather than solely by a decrease in the length of the product pipe which occurs in substantially linear pipeline systems. [0010] Preferably the product pipe is insulated by a vacuum throughout the carrier pipe. The vacuum is formed in the annular space between the carrier pipe and product pipe.

[0011] Preferably the non-linear path is a substantially continuously curved path. The product pipe can follow a curved path substantially along its whole length having substantially no straight sections.

[0012] A substantially non-linear product pipe will result in a change in length and a change in geometry of the product pipe to accommodate thermal contraction, rather than just the change in length that is found in substantially linear pipeline systems which undergo thermal contraction due to a change in temperature. Having a continuously curved product pipe will distribute the stresses across the whole product pipe during thermal contraction compared to predominately linear pipeline systems incorporating expansion loops where thermal contraction can produce localized stresses at the right angle bends present to include the loops into the product pipe and also at intermediate bulkheads where these may be fitted.

[0013] Preferably the product pipe has a regular shape, such as a sinusoidal shape or steep angled helical shape before the product pipe is exposed to a change in temperature. This change is temperature can be caused by the flow of cryogenic fluid through the product pipe.

[0014] Preferably the system comprises two or more product pipes located inside the carrier pipe. This allows for greater transported volume of fluid through the system.

[0015] The system can include spacers for supporting the product pipe. They can be located such that they prevent the product pipe from becoming truly straight within the carrier pipe. Preferably the spacers are located at about every half a wavelength of the product pipe where it is configured in a planar wave shape.

[0016] Bulkheads can be attached to the ends of the carrier pipe and product pipe. These can be located at either end of the pipes to form an insulated seal. [0017] The system can further comprise valves for regulating the flow through the product pipes, such that re-circulation of product fluid can be obtained, for example to maintain cryogenic temperature inside the casing pipe when no LNG delivery is taking place. [0018] The system can further comprise a tapping through the carrier pipe to permit a vacuum to be drawn inside the carrier pipe. This allows a vacuum to be formed around the product pipe or pipes to prevent heat flow into the product causing its temperature to rise. Brief description of the drawings [0019] Figure 1 shows a sinusoidal shaped product pipe inside the carrier pipe at ambient temperature;

Figure 2 shows a single product pipe formed into a helix shape inside the carrier pipe at ambient temperature;

Figure 3 shows a plan view of a fluid transport system having four helix shaped product pipes inside the carrier pipe;

Figure 4 shows the onshore termination end of the fluid transport system;

Figure 5 shows the offshore termination end of the fluid transport system; and

Figure 6 shows a sectioned view of the fluid pipeline.

Mode(s) for carrying out the invention

[0020] Preferred embodiments of the invention are now described with reference to the figures. A fluid transfer system for transporting fluid, in particular cryogenic fluid such as LNG, has a "pipe-in-pipe" configuration comprising a product pipe 1 located inside a carrier pipe 2. At either end of the pipe- in-pipe configuration a bulkhead 6 arrangement forms an insulated air-tight seal about the carrier pipe. Figure 1 shows one embodiment of the invention. The product pipe 1 of Figure 1 has a sinusoidal shape however the product may comprise other similar regularly repeating shapes. The sinusoidal shape allows the extra length of product pipe to be accommodated into the shorter carrier pipe. The pipe 1 can be preformed into the sinusoidal shape by axial bucking or plastic deformation, such that the product pipe has a sinusoidal shape inside the carrier when the product pipe is at an ambient temperature, i.e. before the flow of cryogenic fluid through the fluid transport system, when the product pipe and carrier pipe are at substantially equals temperatures. When cryogenic fluid, such as LNG, flows through the product pipe 1 the LNG cools down the product pipe causing it to thermally shrink. The reduction in length of the product pipe 1 as it cools is accommodated primarily by the change in geometry of the product pipe through the straightening out of the flexure in the pipe, causing the path of the product pipe through the carrier pipe to change rather then by thermally induced material axial strain. The product pipe will straighten out within the elastic limits of the pipe. If the product pipe were initially straight the reduction in developed length would be accommodated by plastic axial strain. In addition to the change in geometry of the product pipe, the product pipe may also have a change in length, however the length of the product pipe will never become shorter than the length of the carrier pipe. The straightening out of the product pipe 1 as it cools is helped by slots present in the supporting spacers 11. The spacers and slots are positioned so that the do not permit the product pipe to become truly straight when at cryogenic temperatures. It is useful for the product pipe to retain sufficient out-of-true straightness to allow the product pipe 1 to return back to its original configuration when the product pipe returns to an ambient temperature, such as when the cryogenic fluid is no longer flowing through the pipe.

[0021] Spacers are provided to keep the product pipe from touching the inner surface of the carrier thereby preventing a thermal short circuit by-passing the vacuum insulation, to precipitate managed product pipe buckling to the chosen wavelength under modest axial compression, and to prevent the line becoming truly straight at low temperatures such that any required managed buckling or shape recovery back to the initial configuration occurs when the product line temperature returns to ambient.

[0022] The spacers can be made from any suitable material, such as nylon, and may be attached to the product pipe or to the carrier pipe for fabrication purposes. Preferably the pipes are assembled by slipping the product pipe complete with spacers attached to the product pipe, up inside the carrier. The axial spacing of the spacers may be at every half wave length or at any such other spacing required to achieve the required shape of the product.

[0023] The distorted product pipe shape can be achieved either by buckling the product pipe inside the carrier pipe under an externally imposed axial loading during fabrication of pipeline system, by plastic pre-forming or a combination of the two. An imposed axial compressive force load on the product pipe at ambient temperature is reacted by a corresponding tension in the carrier, and is transmitted by the end bulkheads. This axial load is much less than would have been imposed by axial thermal contraction of the product pipe had the product pipe been in a conventional straight configuration.

[0024] The wavelength of the distorted shape will depend on the internal diameter of the carrier. The product pipe has sufficient developed length to accommodate the thermal shrinkage of the product pipe relative to the carrier without it overstressing at cryogenic temperatures, so that there is sufficient residual distortion in the cryogenic configuration of the product pipe for it to return to its original shape when it undergoes repeated thermal cycling between cryogenic and ambient temperatures.

[0025] The wavelength of the sinusoidal shaped product pipe is also selected such that it keeps the product pipe within the radius of curvature permitted such that any pre-load required at room temperature to induce the product pipe to buckle inside the carrier pipe is achieved and only induces moderates stress and/or strains.

[0026] The shape for the product pipe is such that when cooled it will accommodate thermal shrinkage through a change in shape rather than by a thermally induced material axial strain, allowing materials such as Stainless 308 or 9% Nickel steel to be used for the fabrication of the product pipe, in subsea LNG pipelines.

[0027] The carrier pipe 2 is vacuumed-filled to provide insulation and is sufficiently strong to additionally accommodate the external pressure differential of the hydrostatic pressure. The carrier pipe 2 may itself be placed in a further pipe. This further outer pipe can provide additional insulation, act as a back-up layer of insulation, and provide a damage protection layer.

[0028] The carrier pipe may not be straight, due for example to the sea bed undulation where it is to lie. Such out-of-straightness of the carrier pipe will influence how much eccentricity is needed to achieve the desired initial buckling of the product pipe during ambient temperature fabrication conditions, and to achieve the recovery of the buckled shape as the product line returns from cryogenic temperature to ambient conditions.

[0029] Any sinusoidal "waviness" of the product line may be in the horizontal or vertical plane or at any angle in between. The orientation is preferably selected to accommodate the carrier pipe out-of-straightness depending on whether it deviates more in plan or elevation.

[0030] In an alternative embodiment of the invention the product pipe 1 can have a pre-formed helical configuration at ambient temperature, as shown in Figure 2. The fluid transfer system contains a product pipe 1 pre-formed into a steep angle helix contained within a vacuum inside the carrier pipe 2. The helical product pipe 1 can be precompressed as a spring before being secured to the carrier pipe 2 by end bulkheads 6, so that reduction in the developed length of the product pipe 1 as it cools to cryogenic temperatures is accommodated by elastic stretching of the product pipe helix, as for a conventional spring.

[0031] The carrier pipe can carry more than one helical shaped product pipe. The Multiple product pipes can be interleaved as in a multi-start screw thread. Figure 3 shows one configuration where four product pipes 1 are situated in the carrier pipe 2. Spacers 11 help keep each of the product pipes from contacting the inner surface of the carrier pipe and to help prevent the pipes becoming truly straight at low temperatures. While the multiple product pipe system is exemplified with four product pipes located in the carrier pipe, any number of suitable pipes can be used.

[0032] The helical product pipe can fabricated by seam welding in a conventional way but additionally pulling an offset parallel to and between the mating edges of the plate forming the seam, thus inducing a permanent steep- angle helix in the product pipe. An alternative method for fabricating the helical product pipes inside the carrier is to insert them as a pre-fabricated bundle into the carrier from the remote end, straight and over length compared to the carrier, with the spacer rings already secured to the product pipes and then secure the remote end of the product pipe encastred to the carrier with bulkhead assembly. At the near end of the carrier pipe the free ends of each product pipe are twisted, in a circle having a diameter smaller than the carrier diameter, at the same time as each product pipe is allowed to rotate about its own axis such that the product pipes are wound up into interleaved spirals. Once formed into a spiral shape the product pipes can be secured to the near-end bulkhead which prevents the product pipes from unwinding about their local individual axial centreline. The product pipes are then precompressed by pushing the bulkhead towards its mating flange on the carrier to provide additional thermal contraction capacity before also securing the bulkhead to its mating flange on the carrier with appropriate insulation. The carrier then reacts the axial pre-compression load in tension and the axial rotational moment in torsion of the product pipe trying to unwind about the carrier centre line. As the product pipes are filled with LNG, the axial pre-compression load will turn into tension as the product pipelines contract placing the carrier into compression. It is anticipated that as the product pipes are twisted around and compressed into the configuration of interleaved springs in this way, frictional forces will accumulate between the spacers and the inside of the carrier pipe. These friction forces will be additive such that a situation may occur where the imposed torque and compression at the near-end bulkhead are inducing maximum permissible stresses in the product pipe immediately behind the near-end bulkhead, but that the relative movement induced between the product pipe and the carrier pipe decays down the length of the carrier pipe until an anchor point is reached beyond which there is no relative movement axially between the product pipes and the carrier and another anchor point beyond which there is no movement of the product pipes circumferentially relative to the inside of the carrier pipe. This occurrence of "locked-up stresses" will temporarily preclude further winding up and compression of the product pipes into a spring. However where the entire LNG pipeline is fabricated in multi-kilometre lengths, and supported on an on-shore roller path permitting the entire length to be towed into the sea, that this roller path will also permit the entire LNG pipeline to be rotated about its long axis. The process of rolling the pipe over whilst twisting and compressing the product pipes will "unlock" the friction anchor points sequentially as they form and permit the helical shape being induced into the product pipes at the near end to safely migrate down the entire length of the carrier to the remote end. Where installation of the LNG pipeline from a lay barge onto the sea bed occurs, preventing the pipeline from being rolled over, formation of frictional anchor points can be mitigated by allowing the carrier pipe to flood with sea water, such that appropriately designed product pipes become neutrally buoyant inside the casing pipe, thus preventing formation of any friction forces opposing product pipe rotation and compression. Suitable facilities to subsequently de-water and dry the casing pipe will be needed in this eventuality.

[0034] While the flow system is particularly exemplified wherein the product pipe has a sinusoidal or helical shape, other shapes for the product pipe can be used wherein the length of the product pipe at ambient temperatures is greater than the length of the carrier pipe, to allow for thermal shrinkage when the product pipe cools down to cryogenic temperatures.

[0035] Each end of the carrier pipe and product pipes are secured to a bulkhead arrangement. Figures 4 and 5 describe bulkhead arrangements for the onshore and offshore ends in more detail.

[0036] Figure 4 shows an onshore termination arrangement for the fluid transfer system. The product pipes 1 located in the carrier pipe 2 terminate at a bulkhead 6 which forms an insulated air-tight seal with the carrier pipe 2. A flange 3 welded to the carrier pipe 2 is attached to an onshore end terminal flange 5. This can be by any conventional attachment means, such as a nut and bolt arrangement 4. A suitable insulating material 7, 8 is used to prevent heat flow from the carrier pipe 2, flanges 3, 5 and bolt 4 into the bulkhead 6 and product pipe 1 , and fluid inside it, which are at cryogenic temperature. The air tight seal is provided to maintain a vacuum inside the annulus 9 of the carrier pipe 2. Preferably the seal is also watertight to prevent water from entering the carrier pipe. A tapping arrangement 10 into the carrier pipe 2 can be provided to give access to the annulus 9 to allow the creation of a vacuum inside the carrier pipe 2 using an external vacuum pump. Alternatively the tapping arrangement may be provided into the bulkhead. The product pipes 1 are welded to the bulkhead 6 to form a structural and air tight seal. Figure 4 shows only one product pipe in the carrier for clarity, however multiple product lines can be used which will all terminate at the bulkhead. Further inshore from the bulkhead the fluid, such as LNG, is transported to onshore storage facilities using conventional arrangements and kept at cryogenic temperatures using conventional insulating materials 13.

[0037] Figure 5 shows an embodiment of the offshore termination of the system. A further bulkhead 6 forms an insulated airtight seal with the carrier pipe 2 at the offshore end of the pipes as described above for the onshore termination of the flow line system. Insulating material 16, 12 prevents heat transfer from the carrier pipe 2 to the bulkhead 6 and product pipes 23, 24, 25, 26. The bulk head 6 can be attached to an insulated cryogenic tanker coupling terminal 14. Further offshore from the bulkhead the LNG is kept at cryogenic temperatures inside the LNG tanker and offloading gantries using conventional arrangements.

[0038] When there are multiple product pipes inside a single carrier pipe and during periods of time when LNG is not actually being offloaded or onloaded through the product pipes, such as at times between LNG tanker vessels arrivals, cryogenic temperatures can be maintained inside the vacuum-insulated annulus of the carrier by continuously circulating LNG down one or more product pipes and then back through the other product pipes, by the use of valve arrangements at the offshore and onshore terminations of the fluid transport system. An example of a valve arrangement for carrier pipe comprising four product pipes at the offshore termination of the LNG pipeline is shown in Figure 5. A single flow loop can be established to have LNG from the onshore facility flow out through product pipe 23 and then returned back through product pipeline 26, by having valves 17 and 19 closed and diversion valve 21 open to direct the LNG from product 23 to product pipe 26. A second LNG loop can be established out from the inshore terminal through product line 24 and back through product line 25 where valves 18 and 20 are closed and diversion valve 22 open. However once a tanker arrives all the product pipes can be used for rapid offloading of the fluid to give a short tanker turn around by closing diversion valves 21 , 22 and opening valves 17, 18, 19, 20.

[0039] When LNG is retained at the tanker end offshore, flow loops can be established for circulating the LNG out and back through the product pipelines using an arrangement of valves to divert the flow of fluid from one product pipeline to another at the onshore termination of the pipelines, similar to the valve arrangement described for the offshore end.

[0040] Figure 6 shows a sectioned view of a complete LNG pipeline. Each of the helical shaped product pipes 1 located in the carrier pipe 2 are supported by spacers, such as spacing rings 11 attached to it at suitable locations along the length of the pipe 1. The distance between the spacers 11 will depend on the conditions where the fluid transport system is being placed, such that they support the product lines inside the carrier, prevent thermal short circuiting across the vacuum insulation between the product pipes and carrier, and facilitate any axial movement of the product pipe inside the carrier as they cool down to cryogenic temperature and undergo thermal shrinkage when LNG is introduced into the pipes. A concrete weight coating 15 can be applied to the complete pipeline to provide stability for the pipeline on the sea bed, under ambient seawater current flows and mechanical protection. Trenching along all or some of the length of the pipeline can also provide protection for the pipeline assembly.

[0041] The completed LNG pipeline can be supported down a ramp onto the seabed floor from the tanker terminal offshore and in a gradually sloping trench down the shore from the onshore terminal onto the seabed. The fluid transport system can be used to offload fluid from a tanker into an onshore terminal or can be used to transport fluid from an onshore facility to onload the fluid onto a tanker. [0042] The LNG pipeline can be made on lay barge or fabricated completely onshore and then towed into place using conventional seabed pipeline towed installation techniques. Alternatively the LNG Pipeline is fabricated in sections, each one which is towed into place and joined together to form a complete line.

[0043] While the invention is described with reference to the transfer of cryogenic fluids, in particular LNG, the system is suitable for transporting other fluids, which cause a change in temperature of the product pipe.

Claims

1. A system for transporting low temperature fluids comprising; a carrier pipe; and at least one inner product flow pipe located within and thermally isolated from the carrier pipe; wherein the product pipe has a greater length than the carrier pipe and is incorporated into the entire length of the carrier pipe by following a non-linear path, such that thermal contraction of the product pipe due to a change in temperature of the product pipe is accommodated be elastic geometric distortion of the product pipe.

2. A system according to claim 1 wherein the product pipe is insulated by a vacuum throughout the carrier pipe.

3. A system according to claim 2 wherein the non-linear path is a substantially continuously curved path.

4. A system according to any of claims 1-3 wherein product pipe has a sinusoidal shape before the product pipe is exposed to a change in temperature.

5. A system according to any of claims 1-3 wherein the product pipe has a helical shape before the product pipe is exposed to a change in temperature.

6. A according to any of claims 1-5 comprising two or more product pipes located inside the carrier pipe.

7. A system according to any of claims 1-6 wherein the carrier pipe comprises at least one spacer for supporting the product pipe.

8. A system according to claim 7 wherein spacers are located at about every half a wavelength of the product pipe.

9. A system according to any of claims 1-8 comprising a bulkhead attached to each end of the carrier pipe and product pipe.

10. A system according to any of claims 1-9 further comprising at least one valve for regulating the flow through the product pipe.

11. A system according to any of claims 1-10 further comprising a tapping through the carrier pipe to permit a vacuum to be formed within the carrier pipe.