WO2003098093A1

WO2003098093A1 - Thermally insulated, rigid pipe-in-pipe systems

Info

Publication number: WO2003098093A1
Application number: PCT/GB2003/001580
Authority: WO
Inventors: Michael Antoine Bell; Sylvain Denniel
Original assignee: Coflexip SA; Technip France SAS; Coflexip Stena Offshore Ltd; Technip Offshore UK Ltd
Current assignee: Technip Energies France SAS; Technip UK Ltd
Priority date: 2002-05-17
Filing date: 2003-04-10
Publication date: 2003-11-27
Anticipated expiration: 2004-11-17
Also published as: GB2388641A; GB2388641B; GB0211299D0; AU2003224273A8; AU2003224273A1

Abstract

A pipe-in-pipe assembly according to the invention comprises a rigid inner pipe (10); a rigid outer pipe (12); an annulus defined between the inner and outer pipes; and at least one thermal insulation layer located in said annulus, wherein, said at least one thermal insulation air comprises a plurality of elongate bars (14, 16) wound along the length of said inner pipe, said bars including a first plurality of bars (14) formed from a first material having a first lambda value and a first compressive strength and at least one additional bar (16) formed from a second material having a second lambda value greater than said first lambda value and having a second compressive strength greater than said first compressive strength. Also disclosed is a method of forming a rigid, thermally insulated pipe-in-pipe assembly.

Description

THERMALLY INSULATED, RIGID PIPE-IN-PIPE SYSTEMS

The present invention relates to pipe-in-pipe systems of the type comprising a rigid inner pipe surrounded by a rigid outer pipe and having thermally insulating material provided in the annulus between the inner and outer pipes. The invention relates especially to such systems and associated methods particularly for subsea pipelines laid from a pipelaying vessel by the reel pipelay method.

Pipe-in-pipe systems are double walled pipe structures typically used in subsea production lines (particularly for the transport of hydrocarbon products) that require the high level of thermal insulation which can be provided by filling the annulus of the pipe-in-pipe structure with thermally insulating material. Furthermore, the interior of the annulus of the pipe-in-pipe structure is dry, so that the thermal insulation material is protected from water. Also, the interior of the annulus may be maintained at atmospheric pressure, so that the insulating material may not be subject to the hydrostatic pressure that is supported by the external pipe ("carrier pipe"), or to the internal pressure of the fluid in the internal pipe ("flowline") . For these reasons, the rigid pipe-in- pipe structure provides an arrangement that allows the use of a wide variety of thermal insulation materials with good insulation properties (low lambda) .

It is necessary to insulate subsea production lines because of the tendency of heavy oils and the like to solidify while being transported between the subsea production well and the surface, as a result of heat losses from the submerged pipeline. Insulation is also necessary to avoid the formation of hydrates that can occur when there is a cooling of certain types of crude oil; e.g. during interruptions to production.

Hitherto, two types of thermal insulation means have been used in pipe-in-pipe systems. US-A-3547161 discloses an arrangement whereby the annulus is completely filled with injected insulation material. Another method employs half shells of insulating material that are assembled longitudinally on the flowline, with spacers between longitudinally adjacent pairs of half shells. Conventionally, two main techniques are used to lay subsea pipelines from pipelaying vessels: the J-lay (or S-lay) method and the reeling method. In the J- lay (or S-lay) method, the pipeline is assembled on the vessel by welding together lengths of pipe before laying them in the sea. This technique is slow and expensive, requiring a crew of welders on board the vessel. The reeling technique comprises assembling the pipeline on shore and spooling the pipeline onto a reel which is subsequently transported to the production site where it is unreeled along the required pipeline path. This allows the pipeline to be laid much more quickly, so that the vessel is not required for as long a period as in the case of J-lay/S-lay .

In the reeling technique, the rigid pipeline is subjected to plastic deformation while being spooled onto the reel, and the plastic deformation is removed by straightening mechanisms while the pipeline is being unreeled. During spooling of a pipe-in-pipe assembly, tension is applied to the pipeline to conform the carrier pipe to the reel. The carrier pipe is bent through direct contact with the reel drum, or with previously spooled layers of the pipeline.

The bending forces are transferred from the carrier pipe to the flowline by spacers or annular walls disposed along the length of the pipeline in the annulus. When unreeling, the pipeline is straightened by the carrier pipe being passed through straightening means, such as opposed rollers or roller track assemblies (as are well known in the art) . The straightening forces are applied to the exterior of the carrier pipe, and are only transferred to the flowline at the locations of the spacers or annular walls inside the annulus. Accordingly, the carrier pipe can be fully straightened but it is not possible to fully straighten the flowline inside the carrier pipe. The flowline therefore retains some residual curvature after the straightening process, depending on the pitch of the spacers in the annulus. The resulting relative displacement of the flowline inside the carrier pipe results in local reductions in the annulus gap between the two pipes, leading to potential compression of the insulating material.

If reeling techniques are applied to pipe-in-pipe structures of the type with injected thermal insulation, continuous longitudinal lengths of the insulation material have to be separated by annular walls, and the pitch of the walls must be sufficiently small (the walls must be sufficiently closely spaced) to effectively transmit the bending forces from the carrier to the flowline and so avoid local reductions in the annulus gap between the two pipes.

If reeling techniques are applied to pipe-in-pipe structures of the type with "half shell" thermal insulation, local compression of the insulating material leads to a reduction in insulating properties and so to a global reduction in the thermal properties of the pipeline. In order to counter such losses, it is necessary to add more spacers (reducing the spacer pitch to reduce the compression) or to modify the pipeline design so as to increase the amount of insulating material (by increasing the internal diameter of the carrier and/or reducing the external diameter of the flowline) , but such solutions create many additional problems. Adding spacers reduces the total amount of insulating material, and changing the pipe diameters is usually unacceptable because it leads to excessive costs and/or reduces the flow capacity of the pipeline.

The problem of providing adequate thermal insulation becomes more severe for long lengths of pipeline laid in very deep water. The use of high performance insulating materials (with low lambda values) is the best solution, but such materials tend to have low density and hence low mechanical strength. Furthermore, the installation of spacers or other accessories (such as waterstops or bulkheads) is expensive and time-consuming, offsetting the savings obtained by use of the reeling technique.

The present invention seeks to provide improved thermally insulated rigid pipe-in-pipe systems that are suitable for spooling onto a reel and for subsequent straightening in reel-type pipelaying operations . In accordance with a first aspect of the invention, there is provided a rigid, thermally insulated pipe- in-pipe assembly comprising: a rigid inner pipe; a rigid outer pipe; an annulus defined between the inner and outer pipes ; and at least one thermal insulation layer located in said annulus; wherein: said at least one thermal insulation layer comprises a plurality of elongate bars wound along the length of said inner pipe, said bars including a first plurality of bars formed from a first material having a first lambda value and a first compressive strength and at least one additional bar formed from a second material having a second lambda value greater than said first lambda value and having a second compressive strength greater than said first compressive strength.

A rigid, thermally insulated pipe-in-pipe assembly in accordance with the first aspect of the invention may be laid using the reel pipelaying method by spooling the pipe-in-pipe assembly onto a reel by plastically bending the pipe-in-pipe assembly around the reel, and subsequently unreeling the pipe-in- pipe assembly from the reel along a pipeline path while straightening the plastically bent pipe-in- pipe assembly.

In accordance with a second aspect of the invention there is provided a method of forming a rigid, thermally insulated pipe-in-pipe assembly comprising: providing a length of rigid inner pipe; applying at least one thermal insulation layer to the external surface of said inner pipe by winding a plurality of elongate bars along the length of said inner pipe, said bars including a first plurality of bars formed from a first material having a first lambda value and a first compressive strength and at least one additional bar formed from a second material having a second lambda value greater than said first lambda value and having a second compressive strength greater than said first compressive strength; and inserting said inner pipe with said at least one thermal insulation layer into a rigid outer pipe.

Other aspects and preferred features of the invention are defined in the appended claims.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a partially cut away perspective view of a first embodiment of a pipe-in-pipe assembly in accordance with the present invention;

Fig. 2 is a partial cross sectional view of a pipe- in-pipe assembly illustrating further preferred features of embodiments of the invention;

Fig. 3 is a partial cross sectional view of a pipe- in-pipe assembly illustrating a further embodiment of the invention; Figs. 4a - 4e are schematic illustrations of alternative cross sections of insulating strip materials employed in the present invention; and

Fig. 5 is a partially cut away perspective view of a further embodiment of a pipe-in-pipe assembly in accordance with the present invention.

Referring now to the drawings, Fig. 1 shows a thermally insulated pipe-in-pipe structure comprising a rigid inner pipe or flowline 10, a rigid outer pipe or carrier 12, and thermally insulating material disposed in the annulus between the flowline 10 and carrier 12. The thermally insulating material comprises a plurality of bars or strips of a first material 14 and at least one bar or strip of a second material 16.

The bars 14 and 16 are wound upon the flowline so as to extend along its longitudinal axis in a serpentine manner. The bars 14, 16 may be wound helically upon the flowline. However, the apparatus required for continuous helical winding is relatively complex and expensive. It is preferred that, as illustrated, the bars 14, 16 are wound in a so-called "S/Z" or "reversing lay" ("pseudo- helical") configuration (as described, for example, in GB-A-2219063, in relation to forming flexible, multi -conductor lines) , in which the direction of winding is reversed periodically. This technique does not require the flowline 10 to be rotated continuously relative to the bars 14, 16, simplifying the required apparatus. As used herein, the term "helical winding" means either genuine, continuous helical winding or pseudo-helical winding such as S/Z winding. In Fig. 1, for the sake of clarity, the direction of winding is shown as being reversed about every 90 degrees. In other embodiments, the direction of winding is reversed after approximately a complete turn of the bars 14, 16 about the flowline 10.

The machine used for winding the insulated bars 14 , 16 in continuous helical winding or pseudo-helical winding can be any one of a variety of many different types. It may be a classical helical lay or spiralling machine using a rotating device sharing the reels that supply the insulated bars to be wound. It can also be a winding machine where the bars are provided to a rotating twist head through a plurality of rotating plates which hold the insulated bars around the flowline and in parallel relationship. The plates are co-ordinated in rotation and in certain cases are able to advance simultaneously, the bars being assembled by predetermined length. In this type of machine, the insulated bars could be assembled step by step (by a predetermined length) as the process of winding is going on, contrary to the classical helical lay machine where the insulated bars are stored on reels shared on a rotating device (i.e. a continuous process) . In the case of S/Z winding of the bars, the winding machine can use a plurality of spaced rotating plates co-ordinated in rotation but with fixed reels for supplying the insulated bars. This kind of winding machine also avoids the need for a rotating device sharing the supply reels of the classical spiralling machine.

The first material 14 comprises a material having relatively high thermal insulation properties (low lambda) and, correspondingly, relatively low mechanical (compressive) strength. The second material 16 comprises a material having relatively lower thermal insulation properties (higher lambda) and, correspondingly, relatively higher mechanical (compressive) strength. The first material 14 covers the majority of the surface area of the flowline 10, providing good thermal insulation. The second material covers the remainder of the surface area of the flowline 10, and serves to transmit bending forces applied to the carrier 12 to the flowline 10 during spooling and subsequent straightening of the pipe assembly. The second material 16 is sufficiently strong to resist being crushed during bending of the pipe assembly, and protects the first material against excessive compressive stresses. The second material should occupy as little as possible of the annulus volume, so as to maximise the thermal insulation properties, whilst providing the required mechanical support between the flowline 10 and carrier 12. In certain embodiments, the second material may constitute 20% or more of the volume of the insulating layer. The bars of the second material 16 thus act as helical spacers, avoiding the need for conventional annular spacers. The number and arrangement of the helical spacers 16 is selected so as to limit any deformation (crushing) of the first insulating material 14 during bending and/or straightening to an acceptable value. The spacers 16 may have a radial depth substantially equal to that of the first material 14 or, as shown in Fig. 2, the radial depth h2 of the second bars 16 may be greater than that, hi, of the first bars 14, in order to better protect the first bars 14 from crushing during bending/straightening.

Depending upon the winding pattern, the winding pitch, and the degree of compression of the first material 14 that is acceptable, there should be at least one bar of the second material 16. For example, where a particular pipeline design requires the presence of a spacer every two meters along the length of the flowline 10, a single high strength bar 16 could be used with a winding pitch of two meters, or two bars 16 could be used with a winding pitch of four meters. Where more than one bar of the second material 16 is employed, these will generally be spaced equidistantly around the circumference of the flowline 10, with a plurality of bars of the first material 14 located therebetween. In the illustrated embodiments, three bars of the second material 16 are used. During bending/straightening of the pipe assembly, the thermally insulating, high strength bars 16 serve to maintain the flowline 10 in a concentric position in the carrier pipe 12, limiting any displacement of the flowline, and limiting any heat loss through the spacers 16 themselves.

The invention has a number of other advantages related to the manufacturing of the pipeline. The insulating materials may be applied in a substantially continuous process by means of a winding machine, without the need of a step-by-step process to apply discrete spacers and/or insulating elements as in previous methods. As shown in Figs. 2 and 5, the method also makes it easy to incorporate heating cables, or other secondary hoses for active heating, conduits or electrical or optical cables or the like 18 in the insulating layers (e.g. disposed in recesses 20 formed in the bars, preferably in the inner faces of the bars, and most preferably in the bars of the second material 14) .

The pipe-in-pipe structure of the invention may be formed by winding the materials 14, 16 onto the exterior surface of a length of the inner pipe 10 and the inner pipe 10 with the materials 14, 16 applied thereto may then be inserted into the outer pipe 12. A plurality of inner pipe sections may be assembled together prior to applying the materials 14, 16, and a plurality of outer pipe sections may be assembled together prior to inserting the insulated inner pipe therein.

The cross sectional size and shape of the first and second bars 14 and 16 may be substantially identical, although it may be desirable for the second bars 16 to have a greater radial depth than the first bars 14, as discussed above. As shown in Fig. 4, the radially opposed faces of the bars 14, 16 may be curved to match the curvatures of the flowline 10 and carrier pipe 12, and their lateral faces may have complementary curvatures and/or overlapping features to reduce convection along the radial gaps between adjacent bars.

As shown in Fig. 3, multiple (generally two or three) layers of insulating materials 14 and 16 may be applied. The high strength bars 16 of the various layers should cross one another in order to transmit bending forces through the crossing points from the outer layer to the flowline 10. For this purpose, the bars of adjacent layers may be wound out of phase and/or with differing pitches and/or in opposite directions.

For most reel pipelaying applications, the second material 16 will generally have to be capable of supporting compressive forces up to about 300 kN. The second material 16 might suitably have a shore hardness (D) greater than 50 and a lambda value less than 0.3 W/Km. For example, the second material 16 might suitably comprise a polyamide reinforced by glass fibre.

The first material suitably has a lambda value less than 0.1 W/Km and may comprise, for example, polyurethane (PU) foam, rubber foam, PVC foam, aerogel or syntactic foam.

The bars 14, 16 are preferably wound with a relatively long pitch (i.e. with a winding angle less than about 35 degrees) .

Improvements and modifications may be incorporated without departing from the scope of the invention as defined in the claims appended hereto.

Claims

1. A rigid, thermally insulated pipe-in-pipe assembly comprising: a rigid inner pipe; a rigid outer pipe; an annulus defined between the inner and outer pipes; and at least one thermal insulation layer located in said annulus; wherein: said at least one thermal insulation layer comprises a plurality of elongate bars wound along the length of said inner pipe, said bars including a first plurality of bars formed from a first material having a first lambda value and a first compressive strength and at least one additional bar formed from a second material having a second lambda value greater than said first lambda value and having a second compressive strength greater than said first compressive strength.

2. A rigid, thermally insulated pipe-in-pipe assembly as claimed in claim 1, wherein said bars are helically wound along said inner pipe.

3. A rigid, thermally insulated pipe-in-pipe assembly as claimed in claim 1 or claim 2, wherein said bars are wound in an S/Z configuration.

4. A rigid, thermally insulated pipe-in-pipe assembly as claimed in any preceding claim, including a second plurality of bars of said second material .

5. A rigid, thermally insulated pipe-in-pipe assembly as claimed in any preceding claim, including a plurality of thermal insulation layers, each comprising a plurality of said elongate bars wound along the length of said inner pipe.

6. A rigid, thermally insulated pipe-in-pipe assembly as claimed in any preceding claim, wherein said at least one bar of said second material has a radial depth greater than said bars of said first material .

7. A rigid, thermally insulated pipe-in-pipe assembly as claimed in any preceding claim, wherein radially opposed faces of the bars are curved to match the curvatures of the inner and outer pipes.

8. A rigid, thermally insulated pipe-in-pipe assembly as claimed in any preceding claim, wherein lateral faces of the bars have complementary, mutually interlocking configurations.

9. A rigid, thermally insulated pipe-in-pipe assembly as claimed in any preceding claim, including at least one cable, hose or conduit incorporated in said at least one thermal insulation layer.

10. A rigid, thermally insulated pipe-in-pipe assembly as claimed in any preceding claim, wherein said second material has a shore hardness (D) greater than 50.

11. A rigid, thermally insulated pipe-in-pipe assembly as claimed in any preceding claim, wherein said first material has a lambda value less than 0.1 W/Km.

12. A rigid, thermally insulated pipe-in-pipe assembly as claimed in any preceding claim, wherein said second material has a lambda value less than 0.3 W/Km.

13. A rigid, thermally insulated pipe-in-pipe assembly as claimed in any preceding claim, wherein said second material costitutes at least 20% of the volume of the at least one thermal insulation layer.

14. A method of forming a rigid, thermally insulated pipe-in-pipe assembly comprising: providing a length of rigid inner pipe; applying at least one thermal insulation layer to the external surface of said inner pipe by winding a plurality of elongate bars along the length of said inner pipe, said bars including a first plurality of bars formed from a first material having a first lambda value and a first compressive strength and at least one additional bar formed from a second material having a second lambda value greater than said first lambda value and having a second compressive strength greater than said first compressive strength; and inserting said inner pipe with said at least one thermal insulation layer into a rigid outer pipe.

15. A method as claimed in claim 14, further comprising: assembling a plurality of inner pipe sections to provide said length of rigid inner pipe prior to applying said at least one thermal insulation layer to the external surface thereof; and assembling a plurality of outer pipe sections to provide a length of said rigid outer pipe before inserting said inner pipe with said at least one thermal insulation layer therein.