Abstract:
The invention is an underwater LNG pipeline system in modular sections wherein multiple LNG pipelines utilizing expansion joints to compensate for contraction are connected together by braces to form an integral frame, and including pressure vessels enclosing the expansion joints to permit access to the expansion joints for inspection or maintenance. In a second embodiment, the frame is a separate elongate space frame connected to the pipeline and the pressure vessels. In a third embodiment, the pipelines are INVAR steel and no expansion joints or pressure vessels are included.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     This invention relates to underwater cryogenic pipeline systems. In particular, the invention relates to underwater liquified natural pipeline gas systems that are designed for use in ice infested waters and are adapted to be readily maintained or repaired. 
     2. Description of related art 
     The state of the art of underwater liquified natural gas (LNG) pipeline systems is outlined in substantial detail in U.S. Pat. No. 4,718,459, titled &#34;Underwater Cryogenic Pipeline System&#34;, filed Feb. 13, 1986 and having Ser. No. 829,054, which is hereby incorporated by reference. 
     Several substantial design considerations for LNG pipeline systems are discussed in my copending application identified above. They include: material selection for durability and toughness; compensation for contraction of the pipeline; insulation to reduce heat loss; and anchoring. Another consideration in LNG pipeline design is maintenance and repair of the pipeline. Cryogenic pipeline systems, except those using high nickel content steel, and in particular 36% nickel (or &#34;Invar&#34;) steel, rely on expansion joints, such as bellows or pipe loops, to compensate for thremal contraction in the system. These expansion joints are where much of the axial movement in the LNG pipeline system occurs. In a metal bellows expansion joint the integrity of the bellows is important because the metal bellows is thinner than the inner pipe wall and is subject to stresses from both pressure loadings and from movement of inner pipe. Thus, it may be desirable to provide some way to access the expansion joints after the pipeline system is installed so that the expansion joints may be inspected or, if necessary, repaired or replaced. 
     Two approaches to LNG pipeline system maintenance and repair are briefly discussed below. In U.S. Pat. No. 3,379,027 to Mowell, the pipeline may be disconnected at its offshore end and withdrawn from the protective casing if repairs are necessary. In underwater tunnel LNG pipeline systems, the pipelines are in a watertight tunnel that may be entered for repairs if needed. An example of such a system is the Cove Point, Md. LNG receiving terminal underwater tunnel system that is described in several publications. 
     SUMMARY OF THE INVENTION 
     The invention solves the problems in the prior art with underwater LNG pipelines. In particular, the invention permits maintaining or repairing the expansion joints in an underwater LNG pipeline system in a relatively inexpensive and expedient manner. The invention is a frame supported underwater LNG pipeline system having integral pressure vessel enclosures around the expansion joints. The pipeline system of the invention comprises a plurality of insulated LNG pipelines. Each pipeline utilizes conventional metal bellows expansion joints at regular intervals to compensate for contraction. A steel watertight jacket surrounds each of the insulated LNG pipelines to prevent water from entering and damaging the pipeline or the insulation around the pipeline. The metal jacket is preferably carbon steel, although it could be 9 percent nickel steel to provide a cryogenic jacket material in case a LNG leak develops inside the pipeline. The jackets of the pipelines are interconnected by a plurality of braces to form a strong truss structure, or frame. Connecting the pipelinse together makes the frame a strong structure, and obviously much stronger than the individual pipelines. Pressure vessels are attached to the frame and to the outer jackets of the pipelines and enclose the expansion joints. The pressure vessels include a hatch adapted to be connected to a diving bell or submersible vehicle so that if maintenance or repair of the expansion joints is required, divers may enter the pressure vessel through the hatch to gain access to the expansion joints. 
     The pressure vessel is preferably a steel vessel with internal dimensions sufficient to enclose the expansion joints and permit working room for inspecting and repair, if necessary. The vessels in the preferred embodiment are sized to contain four pairs of expansion joints and should be designed to withstand the external water pressure at the design depth, loadings on the pressure vessel resulting from burial in the sea bed and loading imposed by integration into the frame. In addition, the pressure vessel should be designed to withstand full pipeline design pressure, including surge, or transient pressures, in the event a leak into the pressure vessel develops in the pipeline. A safety relief valve may be provided in the pressure vessel to vent excess pressure. 
     The configuration of the expansion joints of the pipeline is preferably the same as disclosed in my copending patent application identified above. The expansion joints are in closely adjacent pairs spaced about 73 m. (240 ft.) apart. An intermediate pipe anchor is provided between the two expansion joints in each pair to anchor the inner pipe to the frame inside the pressure vessel. Approximately midway between successive pairs of expansion joints, the inner pipe of each of the pipelines is attached to its surrounding outer jacket, and thus the frame, by a second type of intermediate pipe anchor. One expansion joint is therefore located between successive intermediate pipe anchors. As a result, the inner pipe is firmly anchored about every 36.6 m. (120 ft.). This distance between anchors is within the capabilities of presently commercially available expansion joints. 
     The LNG pipeline system described above is preferably prefabricated in modular sections and transported to and connected on-site. The pipeline system may be laid on or under the sea bed using conventional methods. In areas where there is a possibility of ice &#34;gouging&#34;, the pipeline is preferably buried under the sea bed to reduce any possibility of damage. 
     Since the inner pipes of the pipelines are attached to the frame, heavy anchoring is not needed to transmit forces in the inner pipes to the ground. Most of the forces are taken by the truss system. Since the pipeline system will also be subject to transient forces, such as hammer forces, for example when a valve in the system is closed suddenly, some method is needed to transmit such transient forces to the ground. Since the transient forces will be concentrated at the ends of the pipeline, ground anchors 11 may be provided at the ends of the pipeline. When the pipeline system is buried in the sea bed, the friction from the surrounding soil acts as additional anchoring, called &#34;virtual anchoring.&#34; 
     In another embodiment of the invention, the inner pipe is 36 percent nickel steel called &#34;Invar&#34; steel. Due to the low coefficient of thermal expansion of Invar steel, no expansion joints are required. Thus, the other features of the construction of this embodiment will be the same as the first embodiment, however, no expansion joints are needed and no pressure vessels are utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The several embodiments of the invention are shown in the drawings, in which like reference numbers indicate like parts. A description of each drawing follows: 
     FIG. 1 is a plan view of an embodiment of the invention for transporting LNG from a shore-based storage facility to an offshore loading facility. 
     FIG. 2 is a perspective view of a section of the preferred embodiment of the invention including a cutaway view of the interior of the pressure vessel of the invention. 
     FIG. 3 is a partial cutaway side view of a pressure vessel module of the preferred embodiment of the invention. 
     FIG. 4 is a cross sectional view through the pressure vessel of FIG. 3. 
     FIG. 5 is a partial cutaway side view of a straight module of the preferred embodiment of the invention. 
     FIG. 6 is a side view of a primary intermediate pipe anchor of the invention. 
     FIG. 7 is a cross sectional view of the pipe anchor of FIG. 6. 
     FIG. 8 is a side view of a main guide support of the invention. 
     FIG. 9 is a cross sectional view of the main guide support of FIG. 8. 
     FIG. 10 is a side cross sectional view of a secondary intermediate pipe anchor. 
     FIG. 11 is a cross sectional view of the intermediate pipe anchor of FIG. 10. 
     FIG. 12 is side view of a sliding guide support. 
     FIG. 13 is a cross sectional view of the sliding guide support shown in FIG. 12. 
     FIG. 14 is a partial cutaway side view of a corner module of the preferred embodiment of the invention. 
     FIG. 15 is a partial cutaway side view of a riser module of the preferred embodiment of the invention. 
     FIG. 16 is a cross sectional side view of an insulation hanger support of the invention. 
     FIG. 17 is a side view of a second embodiment of the invention. 
     FIG. 18 is a cross sectional view at lines A-A of the embodiment of the invention of FIG. 17. 
     FIG. 19 is a cross sectional view at lines B-B of the embodiment of the invention of FIG. 17. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description covers loading a LNG tanker. The invention and the description are equally applicable to all other cryogenic products and all similar pipeline applications. 
     FIG. 1 shows the invention. The underwater LNG pipeline system is indicated generally at 1. The pipeline extends from a shore-based LNG storage tank 2 to an offshore loading facility 3. LNG from production facilities is supplied to the storage tank 2 through a supply pipeline 4. An outlet valve manifold 5 controls flow of the LNG from the storage tank into the onshore end of the pipeline system. The offshore end of the pipeline system is connected to conventional LNG loading arms 6 that are connected to the LNG tanker 7. A LNG loading valve manifold 8 may be included in the system to control flow of LNG during loading operations. The pipeline includes a plurality of pressure vessels 9 that surround the expansion joints in the system. The pressure vessel include hatches 10 that permit divers to enter the pressure vessels to inspect or repair the expansion joints. The pipeline system is shown buried in the sea bed. If buried, the lines should be laid about 2 m. (6.5 ft.) under the sea bed. 
     FIG. 2 shows a section of the preferred embodiment of the invention comprising a four pipeline frame supported system buried in the sea bed. The soil is cut away to show the details of the system. The system consists of four steel jacketed LNG pipelines 14. The pipelines are connected together by braces 16 to form a strong truss structure. A pressure vessel 9 is connected to the outer jackets 17 of the pipelines in a leak tight manner. The pressure vessel also includes a hatch 10 adapted to permit diver access to the interior of the pressure vessel. A concrete coating 18 applied by conventional methods may be needed to counteract the buoyancy of the system so that it will stay submerged and buried in the sea bed. 
     FIG. 2 also illustrates the interior details of the pressure vessel 9. These details are described below. 
     The preferred embodiment of the invention is preferably a modular system, consisting of interconnected pressure vessel modules of FIGS. 3 and 4, straight modules of FIG. 5, corner modules of FIG. 14 and riser modules of FIG. 15. The system is preferably buried in the sea bed and anchored at its offshore end by attachment to the offshore loading facility 3. The components of the system are discussed in detail below. 
     FIG. 3 shows the details of a pressure vessel module. The pressure vessel module of the pipeline system includes four pipelines comprising two LNG loading or unloading lines, one vapor return line and one common spare line for LNG or vapor. All the pipelines are identical so that if any one of the lines is damaged, the common spare will be used in its place. Each of the pipelines consist of an inner pipe 25 that is constructed out of a cryogenic material, preferably 304L stainless steel. Insulation 26 surrounds the inner pipe to reduce heat loss. Concrete coating is not shown in FIG. 3 allthough it may be required as mentioned above. The outer jacket 17 of each of the 4 pipelines is firmly attached to the pressure vessel 9, such as by welding. The connection must be water and gas tight. The outer jackets 15 thus serve as integral structural members of the truss structure formed by the outer jackets 17 and the interconnecting braces 16. Since the pressure vessel is a structural element of the frame, it therefore needs to be designed accordingly so it will withstand forces imposed during installation and during operation of the pipeline system. 
     A plurality of crossmembers 27 extend laterally across and are attached to the inside of the pressure vessel to anchor and support the inner pipe and insulation. Four main guide supports 28 attached to the crossmembers are mounted around the insulation to support the weight of the pipe and insulation. Primary intermediate pipe anchors 29 between the main guide supports are also attached to crossmembers 27 and anchor the inner pipe to the frame through the pressure vessel. Each of the pipelines includes a pair of expansion joints 30. The primary intermediate pipe anchor is preferably located between the two expansion joints in each pair. Insulation 26 is applied in a thick layer around the expansion joints 30 to reduce heat leakage. A glass reinforced epoxy cover, described in more detail below, also preferably surrounds the insulation at this point. 
     The pressure vessel 9 includes an access opening covered by a hatch 10. The exact design of the hatch 10 is not part of this invention. The purpose of the hatch is to permit access to the interior of the pressure vessel 9 by divers or personnel transported to the pressure vessel by a submersible. The pressure vessel preferably includes a pressure relief valve 31 mounted through the pressure vessel wall and a purge valve 32 through the wall of the pressure vessel at the bottom of the vessel. The pressure relief valve 31 prevents damage to the pressure vessel by overpressure in the event of a pipeline leak. The purge valve permits the interior of the tank to be purged prior to entry into the vessel by personnel. 
     The pipeline system also preferably includes a plurality of sliding guide supports 35 to support the weight of the insulation on the inside of the outer jacket. The sliding guide supports are preferably provided at intervals of about 4.5 m (15 ft.). The sliding guide support is a steel saddle with sliding blocks attached to is outer surface that will permit relative movement between the inner pipe and insulation and upper jacket. The construction of the sliding guide supports is detailed below. 
     The modules are preferably 36.6 m. (120 ft.) in length. This is the preferred length based on the contraction compensating abilities of commercially available metal bellows expansion joints for a pipeline of 26 inches diameter. Naturally, with design changes it may be possible to utilize modules having any preselected length. 
     In FIG. 5, a straight module of the preferred embodiment of the invention is illustrated. The construction of the straight module is similar to the construction of the straight sections of the pressure vessel module. The straight module includes a plurality of sliding guide supports 35. In addition, each pipeline in the straight module includes a secondary intermediate pipe anchor 36. The location of the intermediate anchors is selected so that when the straight modules are connected to the pressure vessel modules the distance between the primary and secondary intermediate pipe anchors (29, 36) is not in excess of the pipe length that can be handled by the expansion joint 30. The construction of the primary intermediate anchors and the inner guide supports is detailed below. 
     The primary intermediate pipe anchor design is illustrated in detail in FIGS. 6 and 7. The anchor consists of at least two spaced apart rings 40 fixed to the inner pipe 25 between the expansion joints 30. Strong vertical support beams 41 are attached to the lateral crossmember 27 inside the pressure vessel by bolting or welding. The support beams 41 extend upwardly on each side of the inner pipe between the annular rings 40. Four insulating blocks 42 are located between the support beams 41 and the annular rings 40. The annular rings 40 are then attached to support beams 41 by nuts and bolts 43 extending through holes drilled through the insulating blocks 42. The insulating blocks are preferably made of material that is both strong in compression and has a relatively low coefficient of thermal conductivity. A suitable material is a resin impregnated wood, such as &#34;Permali,&#34; a product manufactured by Permali, Inc., Mt. Pleasant, Pa. Insulation 26 surrounds the expansion joints and anchor assembly to reduce heat loss. 
     The details of construction of the main guide supports 28 are shown in FIGS. 8 and 9. The main guide supports are not fixed to the pipeline. Instead, they fit around the GRE covering around the insulation with sufficient clearance to allow axial movement of the insulation, if necessary. However, the clearance needs to be sufficiently small that the expansion joint 30 is protected from lateral forces and movements resulting from the weight of the inner pipe and from bowing of the inner pipe during installation, start up, and shut down of the pipeline system. The main guide support consists of an uppe saddle 45 and a lower saddle 46. The saddles are preferably relatively wide because the weight of the inner pipe 25 and the lateral forces at the saddles are transmitted through the insulation surrounding the inner pipe. The saddle is wide to transfer these forces over a large area of the insulation. The upper and lower saddles are connected at flanges 47 and 48 by any convenient means, such as nuts and bolts. The lower saddle 46 is attached to two upright support beams 49 that are fixed to a crossmember 27 inside the pressure vessel. The thermal insulation 26 surrounding the inner pipe 25 under the main guide supports is preferably a strong, high density polyurethane foam in view of the loads that will be resisted by the main guide supports. This is discussed in greater detail below. 
     The insulation 26 is preferably polyurethane foam with a glass fiber reinforced epoxy, or GRE, covering. The insulation may be sprayed onto the inner pipe 25 before the assembly of the inner pipes into the modules or it may be prefabricated in sleeves. The GRE covering should reduce the possibility the deterioration of the insulation and provide for a long life. The insulation is preferably sprayed on in several, preferably three, individual layers with a crack arresting glass cloth (not shown) molded between each of the layers. The glass cloth should be bonded to the insulation, for example with epoxy or polyurethane foam, so that each layer of insulation is surrounded by a watertight covering. Except for Invar inner pipes, it is necessary to have a radial gap between the outside of the inner pipe and the inside of the insulation so that contraction of the inner pipe will not crack the insulation. Any of the known methods of obtaining the radial gap may be utilized, such as use of a cloth or other material applied around the inner pipe before insulation is sprayed. Thus, if the insulation is sprayed around dummy pipes to produce a hollow sleeve of sprayed insulation material, the dummy pipe is sized so that the radial gap is provided when the dummy pipe is removed and the insulation sleeve is fitted over the inner pipe during construction of the modules. 
     Preferably, the insulation around the inner pipe under the main guide supports 28 will be a high density polyurethane foam thermal insulation with a greater compressive strength than the lower density foam insulation used elsewhere in the pipeline system. For example, a polyurethane foam having a density of 30 kg/m 3  is considered a high density polyurethane foam. A high density foam is preferred because the insulation at this point is a bearing member that supports the weight of the inner pipe and all loadings caused by bowing. A stronger insulation material is preferred at this point because the main guide supports will resist buckling of the inner pipe and will secure the expansion joints against unexpected rotation, or bending. 
     The secondary intermediate pipe anchor 36 is shown in detail in FIGS. 10 and 11. The secondary intermediate pipe anchors consist of an inner spool 55 and an outer housing 56 adapted to be welded to the inner pipe and outer jacket, respectively. Two spaced apart thrust rings 57 are fixed to the outside of the inner spool at about its midpoint. A plurality of ribs 58 are attached between the two rings for reinforcement. Four annular thrust rings are attached to the inside of the outer housing 56. Two of the thrust rings 59 are located adjacent to and on either side of the two thrust rings 57 on the inner spool. Two back-up rings 60 are spaced further away from the first two rings 59. A plurality of reinforcing plates 61 are connected between the two sets of the thrust rings on the outer housing 38. Forces are transmitted from the inner pipe of the pipeline to the inner spool 55 of the intermediate anchor. These forces are transmitted from the inner spool to the outer housing and outer jacket through two insulating rings 62 between the thrust rings 57 on the inner spool and the thrust rings 59, 60 on the outer housing. The insulating rings are held in position by a plurality of bolts through the thrust rings and insulating rings. The insulating rings are preferably made of a material that is strong in compression and has a low coefficient of thermal conductivity, such as Permali. The inside of the secondary intermediate anchor is filled with insulating material to reduce heat loss. 
     Because of the relatively complicated design of all of the anchors, it is preferable they be shop fabricated as units ready to be interconnected with the other elements of the pipeline system when the modules are constructed. 
     FIGS. 12 and 13 are views of the sliding guide supports of the invention. The sliding guide support 35 is strapped around the insulation 26 on the inner pipe 25 inside the outer jacket. The sliding guide support comprises a metal saddle 40 that fits around the lower portion of the insulation. A number of steel straps 66 are fitted around the top of the insulation 26 and attached to lugs 67 on the saddle 65 by bolts or screws. Angle brackets 68 are attached to the outside of the saddle by any suitable means, for example rivets, and sliding blocks 69 are attached between the angle brackets. The sliding block is made of a relatively hard material that will slide on the inner surface of the outer jacket 15. A suitable material would be a polyamide plastic block. 
     A corner module for the preferred embodiment invention is illustrated in FIG. 14. The corner module is designed to be connected to the end of a pressure vessel module. The inner pipes and outer jackets of the corner module are bent to define a corner. As in the other modules, the corner module includes a plurality of sliding guide supports 36 on the horizontal sections of the inner pipe. Each pipeline in the corner module also includes two main anchor supports 70. A first main anchor support is located on the horizontal section of the corner module and a second main anchor support 70 is located on the vertical portion of the corner. The main anchor support is described in detail below. No expansion joints are included between the two main anchor supports on each pipeline because for a large radius corner like the one shown in FIG. 14, for example, about five times diameter for a 26-inch pipe corner module in which the horizontal length of the inner pipe and the vertical height of the inner pipe between the anchors is about exceed 12.3 m. (40.4 ft.), expansion joints are not needed. The construction of the main anchor supports 70 is essentially the same as the design of the secondary intermediate pipe anchors 36. However, the main anchor supports 70 are constructed out of heavier materials and preferably include more and heavier strengthening ribs and plates than the secondary intermediate pipe anchors 36, since the main anchor supports must be designed to withstand at least the normal axial loadings due to temperature, as well as the full transient loadings on the inner pipe. For a 26 inch pipeline system, the loadings resisted by the main anchor supports may be in excess of about six to seven times the design load for the intermediate pipe anchors 36. Once the main anchor support loadings are known, a calculation of the necessary sizes of the components of the main anchor support is relatively simple. 
     Each of the pipelines in the corner module also includes an insulation hanger support 71. The insulation hanger support suspends the insulation to prevent the weight of the insulation from damaging the insulation on the vertical sections of the pipeline system. 
     The insulation hanger support 71 is shown in detail in FIG. 16. It comprises a metal ring 72 attached, such as by welding, to the interior of the outer jacket. A steel band 73 is fitted over the GRE covering 74 over the insulation and is locked in place by GRE rings 75 above and below the steel band 73. A second annular ring 76 is attached to the outside of the steel band and has holes drilled through it to permit bolts 77 to extend through the second ring. The bolt holes, which may be eight to ten or more in number as may be appropriate, are symmetrically spaced around the second annular ring. An annular support channel 78 is mounted on top of the first annular ring 72 and the bolts 78 are attached to and extend upwardly from the inside of the support channel 78. The number and size of the bolts corresponds with the number of size of holes through the second annular ring 76. Each of the bolts 78 has a spring 79 over it to support the weight of the insulation. Preferably the bolt includes a nut 80 threaded onto the end of the bolt to prevent the assembly from possibly coming apart. This method of spring mounting permits some relative axial movement between the first annular support ring 72 and the insulation. 
     The other construction details of the corner module are the same as for the straight and pressure vessel modules, with one exception. Obviously, the design of the insulation in the corner module will be different. For example, it is preferable that the bent portions of the outer jackets be provided in two parts, cut longitudinally, so that the inner pipe may be insulated and the outer jacket corner welded around it during construction of the corner modules. However, the exact design of the corner insulation is not considered part of to the invention and, since it is within the abilities of those of ordinary skill in the art, will not be described in detail. 
     When the pipeline system is installed, it will preferably include a sliding support 81 under the horizontal portion of the corner module. The sliding support is shown schematically in FIG. 14. It could be constructed in several ways. For example the corner module could be placed over a number of pilings driven into the sea bed. Alternatively, a concrete pier or foundation could be poured and covered with a steel beam or cap on which the bottom of the corner module would rest. The sliding support will permit some axial movement of the end of the pipeline system. This movement should be minimal because the frame is buried in the sea bed. 
     As an alternative to the corner module shown in FIG. 14, a corner module could be constructed utilizing expansion joints on each of the pipelines in the system. In this event, the radii of the ends in the pipelines could be significantly reduced and a pressure vessel would be required surrounding the expansion joints in a manner similar to that described above. Howver, this would add additional cost to the system. The biggest benefit of the alternative corner module design is that it is not necessary to use large radii bends in the pipelines. 
     A riser module is shown in FIG. 15. The riser module is designed to be connected to the vertical end of the corner module. Since the riser module is substantially vertical, each of the pipelines includes at least one insulation hanger support 71 to prevent insulation damage due to its own weight. Each of the pipelines in the system has a main anchor support 70 at the upper end of the module. Each pipeline also includes an expansion joint 30 below the main anchor 70 on the riser module and above the main anchor 70 on the vertical section of the corner module. The expansion joint 30 must be capable of handling the contraction between those main anchor supports. The other construction details of the riser module are similar to the details of the pressure vessel module that was discussed above. 
     The riser module also includes a pressure vessel 9 surrounding the expansion joints. A number of crossmembers 27 support two main guide supports 28 for each pipeline, one on each side of each expansion joint. As was described with regard to the pressure vessel modules, the insulation under the sliding main guide supports 28 is preferably a higher density polyurethane foam that is better able to resist bending moments adjacent the ends of the expansion joints. 
     A section of a second embodiment of the invention is illustrated in FIG. 17, 18 and 19. In the second embodiment, the frame is comprised of a plurality of elongate members 85 interconnected by a plurality of braces 86. In FIG. 17, there are four members 85 configured into a rectangular truss and the pipeline system includes two pipelines. However, this embodiment is not limited to rectangular trusses or two pipelines. Although only a section of the second embodiment of the invention is shown, it is apparent that the features and principles discussed above with regard to the preferred embodiment are equally applicable to the second embodiment. For example, the second embodiment of the invention would also preferably be provided in modular sections, i.e. straight, pressure vessel, corner and inner modules, for assembly on-site and the details of construction of the modules will be apparent from the foregoing description. 
     The elongate members 85 are preferably attached to the pressure vessel 9, such as welding, so that the pressure vessel is an integral part of the frame. The outer jacket 17 of each pipeline is also attached to the pressure vessel 9. Inside the pressure vessel, a plurality of lateral crossmembers 27 are attached to the inside of the pressure vessel and support the primary intermediate pipe anchors 29 of the pipelines. The pressure vessel 9 also includes a pressure relief valve 31 and a water tight hatch 10 to make the interior of the pressure vessel accessible to divers or submersibles. Each pipeline has a pair of expansion joints 30 located inside the pressure vessel, with one expansion joint on each side of the primary intermediate pipe anchor. Main guide supports 28 support the inner pipe 25 and insulation and protect the ends of the expansion joints 30 from bowing and bending loads. About midway between successive pressure vessels, secondary intermediate pipe anchors 29 attach the inner pipe 25 to the truss structure. The construction of these intermediate anchors can be that shown in FIGS. 17, or 6 and 7 or they could be constructed as shown in FIGS. 10 and 11. If the latter construction is used, some type of saddle would be needed that would attach to the outer jacket 17 of the pipeline to the lateral crossmember 27. 
     In a third embodiment, the invention comprises an inner pipe constructed out of Invar steel. Since the coefficient of thermal expansion of Invar is so low, an Invar inner pipe can be fixed at regular intervals without danger of yielding the material when it is cooled rom ambient to cryogenic temperatures. Expansion joints are therefore not needed in an Invar pipeline system. The pipeline system in the third embodiment would have the design characteristics of the straight module shown in FIG. 5. The system would thus consist of a plurality of similar straight modules connected end to end. The preferred length between secondary intermediate anchors for an Invar system is 36.0 m. (118 ft.). The corner and riser modules for an Invar system would be like those described for the first embodiment above, but without expansion joints. A disadvantage of an Invar system is that since design codes for Invar pipelines do not exist at this time, construction of such a system would have to await code approval. 
     It is believed that for all embodiments of the invention, the soil around the pipeline may freeze over a long period of time as a result of heat transfer from the soil. Since such freezing could cause frost heaving and thus damage the pipeline system, heat tracing should be considered if the design warrants it. However, such heat tracing is not considered a part of this invention. 
     Specific embodiments of the invention have been described above. Naturally, certain modifications of the above specified embodiments may be suggested to those skilled in the art and it is intended that this application cover all such modifications that fall within the scope of the attached claims.