Description

Method for manufacture of a pipe-in-pipe pipeline

Technical field

[0001] This invention relates to a method of joining lengths of pipes to form a pipeline. In particular the invention relates to the use of a mechanical connector to join the outer pipe lengths of a pipe-in-pipe system for underwater pipe laying.

Background art

[0002] Pipe-in-pipe assemblies are commonly used for the transport of fluids that require temperature control, in particular in offshore operations in the oil and gas industry. The inner pipe provides the facility to transport the fluid which can occur at high levels of temperature and pressure. The primary functions of the outer pipe are to resist the external pressure forces due to the underwater installation of the pipe-in-pipe arrangement, and also to contribute to the structural capacity of the pipe-in-pipe assembly to resist the effects of various forms of mechanical loading that occur during installation and operation of the pipeline. The annulus formed between the inner and outer pipes may be empty or contain an insulating material. The annulus provides thermal insulation that can be designed for any particular pipeline to provide the temperature control required by the fluid being transported in the inner pipe.

[0003] The inner pipe is usually formed into a continuous pipeline by welding lengths of pipe together. The outer pipe is also usually formed by welding lengths of pipes together to form a continuous pipeline.

[0004] Pipelines designed for installation and operation at very deep water locations can be installed by connecting new sections of pipe-in-pipe on the vessel that is used to lay the pipe, and suspending the pipe, at a low angle to the vertical, from the vessel. As each new section of pipe is added the manufactured pipe-in-pipe is lowered and eventually, as the vessel is moved forward, or backward, the pipeline is laid on the seabed and forms a continuous pipeline between the seabed and the installation vessel. This is commonly known as the J-lay method for pipeline

installation. A major cost of manufacturing and installing this type of pipe- in-pipe system from an offshore installation vessel is the cost of carrying out the welding and weld inspections for the inner and outer pipes. The welding and weld inspections of both the inner and outer pipelines are time consuming and as such increases the time for which the vessel is offshore. This is especially relevant when producing long pipelines, such as what is required when laying pipelines in deep underwater environments.

[0005] Therefore it is an object of the invention to provide a method that reduces the time required on a vessel to form a pipe-in-pipe assembly. In particular the invention provides a mechanical mechanism for joining the ends of the outer pipes together of a pipe-in-pipe assembly.

Disclosure of the invention

[0006] Accordingly a first aspect of the invention comprises a method for joining lengths of pipes to form a pipe-in-pipe assembly, wherein the assembly comprises outer pipe sections and inner pipe sections located within the outer pipe, the method comprising: joining a first inner pipe section to a second inner pipe section; abutting the end of a first outer pipe section to the end of second outer pipe section to form a joint; providing a collar around the joint; and contracting the collar around the joint such that the inner surface of the collar engages the outer surface of adjacent outer pipes to resist separation of the ends of the adjacent pipes.

[0007] Preferably the thickened end portion is a flange extending radially from the end of the outer pipe. More preferably the flange is tapered.

[0008] In one embodiment the inner surface of the collar and the outer surface of the thickened end section of the outer pipe have interengaging teeth.

[0009] Contracting the collar may comprise subjecting the collar to cold forging.

[0010] Placing the collar around the joint may comprise slipping the collar over one end of an outer pipe.

[0011] In one embodiment the collar is a segmented collar. Placing the collar around the joint comprises connecting the collar segments together to form a collar around the joint of the outer pipe.

[0012] Preferably the thickened end portions are tapered and the inner surface of the collar comprises a complementary tapered groove.

[0013] Contracting the collar can also comprise tightening a connecting mechanism between the collar segments. The connecting mechanism may comprise a nut and bolt or other connecting mechanisms.

[0014] Preferably the method further comprises inserting a sealing ring between the first and second outer pipes. The sealing ring may be inserted into a corresponding groove on the face of the abutting pipe ends.

[0015] Preferably joining the first and second inner pipes comprises welding the ends of the inner pipes together.

[0016] The method may further comprise joining a pipe connector comprising the thickened portion to the end of the outer pipes before joining adjacent outer pipes together. Joining the pipe connectors may comprise welding the connector to the end of the outer pipes.

[0017] A second aspect of the invention comprises a connector system for joining adjacent pipe sections to form a pipe in pipe assembly comprising: pipe connectors for connection to the end of an outer pipe and including abutment ends that can be brought together to form a joint; and a collar positionable around the joint, such that when the collar is placed around adjacent pipe connectors and contracted, the collar can engage the outer surfaces of the pipe connectors and resist separation of the abutment ends..

[0018] Preferably the thickened end sections are flanges extending radially from the end of the pipe.

[0019] In one embodiment the collar and the flanges have interengaging teeth. In another embodiment the collar is a segmented collar formed from at least two segments joinable together to form the collar.

[0020] Preferably the thickened sections are tapered and the collar comprises a complementary tapered groove in its inner surface.

[0021] Further aspects of the invention will be apparent from the detailed description below.

Brief description of the drawings

[0022] Figures 1 shows a cut away view of a first embodiment of the invention;

Figure 2 shows reverse view of Figure 1 ;

Figure 3 shows a cut away view of a second embodiment of the invention prior to joining of the outer pipes; and

Figure 4 show a cut away view of a second embodiment of the invention after joining of the outer pipes.

Mode(s) for carrying out the invention

[0023] A pipe-in-pipe assembly comprises an outer pipeline, comprising a series of outer pipe sections 1 , 2 joined together, and an inner pipeline, comprising a series of inner pipe sections 3, 4 joined together and located within the outer pipeline. The inner pipeline may lay concentric with the outer pipeline, however other arrangements of inner pipelines can also be used. The sections of outer pipe 1 , 2 may have a pipe connector portion 5 at the ends of the pipe and adjacent sections of outer pipe are joined together by a collar 6 that is attached to adjacent pipe connectors 5.

[0024] The pipe connector 5 comprises a short pipe length having a thickened section at one end. The thickened section of the pipe is sufficiently thicker than the remaining length of the pipeline and protrudes from the outer surface of the pipe. As shown in the figures the thickened section of the pipe connector 5 may be a flange 7 extending radially from one end of the main body of the pipe. The pipe connectors are relatively short compared to the full length of a section of outer pipe. These connectors 5 may be welded to the outer pipe. The welding and inspection of these welds 8 can be carried out onshore, this reduces the time required to form the pipeline once on the vessel. The pipe connector can also be formed integral to the outer pipe. The flanges at the end of the outer pipe section being formed by heating and compressing the outer pipe lengths and then machining to shape. In this situation no welding is required to form the outer pipe sections.

[0025] To form a pipe-in-pipe assembly on the vessel initially an inner pipe section is joined to an adjacent inner pipe section. These pipes are typically joined by welding. A section of outer pipe is joined to a further section of outer pipe. Adjacent outer pipes are joined together via the connecting collar.

[0026] With reference to figures 1 and 2 the collar can be formed from a number of segments 9, 10. These segments are placed around the abutting flanges 7 of adjacent outer pipe sections 1 , 2 to form a collar. Initially the diameter of the collar formed is greater than the diameter of the flanges. Once the collar is in positioned surrounding the flanges the diameter of the collar is decreased so that the inner surface of the collar engages with the outer surfaces of the flanges.

[0027] The means for causing the connecting collar 6 to contract radially can be a nut 12 and bolt 11 assembly inserted through the lugs 13 of adjacent collar segments that when tightened cause the collar segments 9, 10 to move together. However other means can be used to join the collar segments together. For example some of the segments of the collar can be connected by hinged attachments. The segments can then be pulled together using an arrangement of bolts and nuts or hydraulic tensioners.

[0028] The collar 6 and the flanges 7 may connect via a tapered fit. The inner surface of the collar may have a tapered groove, while the flanges are shaped so that when the faces of adjacent flanges contact one another the two flanges 7 form a tapered shaped complementary to the shape of the groove in the inner surface of the collar. Once the collar has contracted the outer surface of the flanges will engage with the inner surface of the collar, with the collaring pulling the ends of the pipe together so that adjacent outer pipe sections are connected together without the need for welding.

[0029] In one form of a connecting collar 6 as shown in figures 1 and 2, the lugs 13 and tightening bolts 11 and the flanges 7 at the pipe ends are designed such that the connecting collar will have a strength at least equivalent to the basic pipe and can withstand the effects of the applied pressure and other forces with any yielding of the material.

[0030] The collar, lugs and flanges are dimensioned such that the effects of any cyclic loading will not induce stress levels that could cause the inception or growth of fatigue cracks. The assessment of the fatigue limit state of the completed joint includes all possible effects such as crevice corrosion.

[0031] Sealing the connection to prevent ingress of seawater into the annulus between the inner 3, 4 and outer 1 , 2 pipes can be effected by machining one or more grooves 14 into the faces of the flanges 7 of the pipes. Suitable flexible or soft metal sealing rings 15 are fitted in the grooves and compressed when the mechanical jointing is completed. The size of the grooves and the rings are designed to provide sealing appropriate to the pressure applied by the seawater.

[0032] Testing of the efficacy of the seals can be carried out by accessing the space between the rings and applying a suitable level of pressure. Any leakage of the seals would be detected by a reduction in the applied pressure. An advantage of the mechanical method of joining the pipe with a connecting collar as shown in figures 1 and 2 is that if it is found that the seal has not been adequately formed, it enables the connector to be undone, the seals remade and then the connector to be re-applied to the pipe.

[0033] Alternatively, the seal could be effected by pumping a liquid elastomeric material into the space between the seal rings. The elastomeric material would harden and ensure an adequate permanent sealing capability.

[0034] Figures 3 and 4 show a second embodiment of the mechanical joining method for the outer pipe lengths of a pipe-in-pipe assembly. The flanges 7 of the adjacent outer pipe section 1 , 2 are connected by a connecting collar 6 which is non-segmented formed from a continuous material. Teeth 18 and grooves are machined into the inner surface of the collar 6. Complementary teeth 17 and grooves are machined into the outer surface of the flanges 7. The teeth 18 on the inner surface of the collar can mesh with the spaces between the teeth 17 on the outer surface of the flanges 7 to connect the adjacent outer pipe sections 1 , 2.

[0035] During the joining process the collar 6 is placed over the adjacent flanges 7 of the outer pipe sections 1 , 2 and the outer face of the collar is subjected to radial forces of such a magnitude as to cause the material to exceed its yield stress and for the collar to reduce in diameter. This process is well-known in mechanical engineering and is called 'cold forging'. Figure 3 shows the connection prior to cold forging and figure 4

shows the completed connection. Once cold forging is completed the two pipes are permanently joined. One method that can be used to connect the pipes is prior to slipping the collar on the pipe the collar is heated causing it to expand. The collar is then positioned over the flanges and as the collar cools, it contracts and engages with the outer surface of the flanges.

[0036] The collar can be slipped over the end of one outer pipe before the second outer pipe is brought towards the first outer pipe length, and then the collar member positioned over the abutting flanges of the two outer pipes. Alternatively the two outer pipes can be brought together and the collar member slipped onto the pipeline from one end and positioned over the abutting flanges.

[0037] The teeth and spaces of the flanges and collar are designed such that the meshing and reduction in the diameter of the collar causes the pipe to be pulled together. The geometry of the interlocking teeth and the thickened section of the ends of the two pipes are designed such that the joint has the strength at least equivalent of the basic pipe and can withstand the effects of the applied pressure and other forces without any yielding of the material.

[0038] The teeth are also dimensioned such that the effects of any cyclic loading will not induce stress levels that could cause the inception or growth of fatigue cracks. The assessment of the fatigue limit state of the completed joint includes all possible effects such as crevice corrosion.

[0039] Sealing of the faces between the ends of the pipes can occur by one or more elastomeric sealing rings, or soft iron sealing rings, and corresponding grooves in each pipe, as a described above. The efficacy of the seal can be tested by applying pressure to the space between the sealing rings. Following completion of a successful leak test the space can be filled with a suitable elastomeric material.

[0040] One aspect of the J-lay method is that the suspended weight of the pipeline needs to be supported, which for very deepwater locations can mean that a very great force has to be carried by the vessel. Typically a method for supporting the pipeline comprises the use of a collar,

commonly called a hang-off collar, installed into or on the outer pipe at intervals along its length.

[0041] An advantage of the present invention is that the connecting collar being used to join the sections of outer pipe can also act as a hang-off collar so that the connecting collar can be used to support the weight of the suspended section of the pipe when being installed using the J-lay method.

[0042] The outer pipe in a pipe-in-pipe pipeline installed in very deep-water locations has to also be designed to resist the effect of the seawater pressure and avoid the possibility of external pressure collapse and propagation buckling. In the event of accidental loading or damage to the outer pipe it is necessary to incorporate a buckle arrestor in the outer pipe to limit the extent of damage or collapse of the pipe. A buckle arrestor is usually a short section of pipe with a wall thickness greater than the outer pipe.

[0043] An advantage of the present invention is that that the connecting collar used to mechanically join the lengths of the outer pipe can also act as a buckle arrestor in the event of accidental loading or damage to the outer pipe.

[0044] The second embodiment using the connecting collar formed from the continuous band of material as well as allowing rapid joining of the outer pipe the mechanical connector is particularly suitable for also acting as a hang-off collar and as a buckle arrestor.

[0045] The cost of special installation vessels, such as used for J-lay installation of pipe-in-pipe pipelines in deep-water locations, is largely governed by the time taken to perform the connection of the inner and outer pipes. By carrying out these connection operations simultaneously the time to complete the installation of the pipeline would be much reduced compared to the time if the were carried out sequentially, for example with the weld and inspection of the inner pipe requiring to be completed before the commencement of the connection for the outer pipe.

[0046] Therefore it is preferred that the installation procedures on the installation vessel should be arranged such that the connection by welding of the pipe

lengths forming the inner pipeline should be carried out simultaneously with the connection by mechanical means of the pipe lengths forming the outer pipeline.