Thermal insulation of underwater pipelines

Abstract

A thermally insulated subsea pipe (10) includes a rigid inner flow pipe (12) and wet insulation (16) disposed around the flow pipe (12). Spaced-apart bridge structures (22) extend radially from the flow pipe (12) toward the wet insulation (16) to keep the wet insulation (16) away from the flow pipe (12). Dry insulation (24) is disposed between the bridge structures (22) in the resulting annular space between the flow pipe (12) and the wet insulation (16). The bridge structures (22) are bonded to the wet insulation (16), for example by welding, and thus may be made of the same material as the wet insulation (16) or a compatible material that allows welding. Figure for abstract: Fig.1

Description

Title of the invention: Thermal insulation of subsea pipelines

[0001] The present invention relates to the thermal insulation of subsea pipelines used in the production of hydrocarbon fluid. A key objective of thermal insulation is to prevent a pipeline from becoming clogged or obstructed by solids that could appear in the production fluid if its temperature decreases too much at a given pressure.

[0002] Oil and gas are present in subterranean formations at elevated temperature and pressure, which can be increased by the injection of fluids such as pressurized water. In the production of oil or gas from subsea fields, the hot production fluid emerges from a subsea wellhead and enters a subsea pipeline in a multiphase state. The production fluid then flows in the pipeline through the seabed and eventually rises to the surface through a riser.

[0003] Low temperature increases the viscosity of the production fluid and promotes the coalescence or precipitation of solid phase materials from certain components present in the production fluid, namely waxes and asphaltenes in crude oil and hydrates in natural gas. These solid phase materials tend to deposit and accumulate on the inner wall of the pipeline and may eventually form plugs, which has the effect of interrupting production. In addition to the high cost associated with lost production, plugs are difficult and expensive to remove and may even damage the pipeline.

[0004] During subsequent transport along a pipeline, the temperature and pressure of the production fluid must be maintained at a sufficiently high level to ensure sufficient flow over the seabed and in the riser. In particular, various measures are taken to ensure that the internal temperature of the pipeline remains high, generally above 65°C and in some cases above 200°C, despite heat exchange with seawater which, for example, is at 4°C at a depth greater than 1000 m. Plugging becomes a risk if the temperature of the production fluid inside the pipeline falls below the wax onset temperature (WAT), or below other thresholds at which other solid materials will fuse with the oil or gas.

[0005] Subsea pipeline designers have adopted active and passive approaches to thermal management, both individually and in combination.

[0006] As active thermal management systems, a trace heating system typically uses resistive electrical wires along and in thermal contact with the outer surface of a steel pipeline. The heat produced in passing an electric current along the wires is conducted through the pipe wall to the production fluid flowing through it.

[0007] In passive thermal management systems, a pipeline is thermally insulated. An example of a passive thermal management system is a multi-walled pipeline (PWP) assembly comprising a rigid inner pipe carrying fluids positioned concentrically within a rigid outer pipe. Typically, the inner and outer pipes are made of steel, although either pipe may instead be a composite pipe of equal strength and substantially rigidity. A composite pipe has a solid monolithic structure that comprises a matrix of polymer resin, for example polypropylene, reinforced with fibers such as glass fibers or carbon fibers. A solid polymer liner may be disposed within the reinforced polymer matrix. The matrix may also be surrounded by an outer covering, which may also be made of a solid polymer.

[0008] The inner and outer pipes of a PiP are spaced apart to define an insulating annular space between them. Typically, a thermally insulating material is disposed in the annular space. It is also possible to create a partial vacuum in the annular space to reduce heat transmission through the annular space.

[0009] The thermal insulation material used in subsea pipelines falls into one of two categories known in the prior art as wet insulation and dry insulation. Wet insulation can withstand contact with water and also significant mechanical stresses. An example of wet insulation is syntactic foam. Conversely, dry insulation must not be in contact with water and cannot withstand significant mechanical stresses, and therefore must be protected both mechanically and chemically. An example of dry insulation is an aerogel.

[0010] In terms of thermal insulation, dry insulation is known to be more effective than wet insulation and is often used when the pipeline includes a protective outer casing, such as the outer pipe of a PiP assembly. For example, WO 2006 / 133155 describes the use of aerogel-based insulation in a PiP annular space. In US 7226243, dedicated spacers allow the insertion of a more fragile aerogel-based insulation layer. A steel sheet can also protect a microporous insulation layer.

[0011] Document US 2009 / 301596 describes a thermal insulation system for a PiP assembly. An inner layer of thermal insulation material, such as an aerogel, is placed around an inner pipe of the assembly and is then covered with a protective layer for mechanical protection. The protective layer protects the fragile inner layer from damage when inserting the insulated inner pipe into the outer pipe of the PiP assembly. Spacers or supports extend between the inner pipe and the protective layer or outer pipe. In service, the annular space is protected by the outer pipe and is therefore not subject to hydrostatic pressure.

[0012] Multi-layer thermal insulation of pipelines combining chemically compatible materials is known in the state of the art. For example, in document WO 2010 / 009559, a first thermal insulation layer comprises polypropylene and a second thermal insulation layer comprises polybutylene.

[0013] WO 2008 / 017147 describes a thermal insulation system that comprises an inner layer of high-performance thermal insulation that is relatively mechanically weak, covered by an outer layer that has greater mechanical strength. The inner layer is typically made of a microporous insulating material while the outer layer is typically made of polymer foam. However, hydrostatic pressure could deflect the outer layer and thus crush the inner layer. In this regard, a waterproof outer jacket may be provided around the insulation system but such a jacket would not be sufficiently resistant to hydrostatic pressure at great depth.

[0014] Document CN 102705595 also teaches a pipe having a sandwich insulation structure that includes a first aerogel layer and a second polymer layer. A similar pipe with a silica aerogel layer and a polymer layer is used as an inner pipe of a PiP assembly in document CN 204328250.

[0015] US 4921018 and US 5722462 describe a thermally insulated pipe surrounded by an outer casing which is spaced from the inner pipe by partitions. Similarly, WO 99 / 05447 describes an insulated pipe in which an inner pipe is surrounded by a syntactic foam which is enclosed by an outer pipe.

[0016] WO 99 / 05447 and US 2003 / 127148 describe insulated pipes in which an inner pipe is insulated with a syntactic foam or a syntactic element. The insulating material is then surrounded by an outer casing. CN 105299334 describes an insulated pipe in which the inner pipe is insulated with a polyurethane layer.

[0017] For many years, the adoption of a PiP assembly has been the default state-of-the-art solution to the need for effective thermal insulation of a pipeline installed at great depth. However, for certain subsea applications, it would be advantageous to adopt a single-wall pipe if a thermal insulation system suitably efficient and pressure-resistant was available. This could avoid the cost, weight and complexity of a PiP assembly in such applications.

[0018] In this context, the invention provides a thermally insulated subsea pipe, comprising: a rigid internal flow pipe; wet insulation disposed around the flow pipe; spaced-apart deck structures extending radially from the flow pipe toward the wet insulation to hold the wet insulation away from the flow pipe, thereby defining an annular space between the flow pipe and the wet insulation; and dry insulation disposed between the deck structures in the annular space. The dry insulation may thus be sealed in a watertight enclosure within the annular space.

[0019] The bridge structures are suitably made of a wet insulation material, or a material chemically compatible with the wet insulation, or the same material as the wet insulation. The bridge structures are suitably spaced longitudinally along the drainpipe.

[0020] The dry insulation may be in contact with the wet insulation. For example, the dry insulation may fill the annular space substantially completely. Alternatively, the wet insulation may be spaced from the dry insulation, for example with a spacing between the wet insulation and the dry insulation of less than 1 mm.

[0021] Advantageously, the wet insulation is bonded to the bridge structures, for example by being fused to the bridge structures. For this purpose, the bridge structures and / or the wet insulation may comprise heated welding elements.

[0022] An anti-corrosion layer may be provided between the dry insulation and the drain pipe. Such a layer may also promote the bonding of bridge structures to the drain pipe.

[0023] The wet insulation may suitably define an exposed exterior surface of the pipe. However, a waterproof or protective sheath may be placed around the wet insulation.

[0024] A reinforcing layer may support the wet insulation. Such a reinforcing layer suitably extends between the deck structures and may extend continuously beyond the deck structures or be interrupted by the deck structures.

[0025] The reinforcing layer may be located radially inside the wet insulation, for example between the wet insulation and the bridge structures or inside the annular space. Alternatively, the reinforcing layer may be embedded in the wet insulation.

[0026] The dry insulation may be in contact with the reinforcing layer or spaced from the reinforcing layer, for example with a spacing between the dry insulation and the reinforcing layer of less than 1 mm.

[0027] The wet insulation may be selected from, or include: syntactic foam; polypropylene; polyurethane; or other polymers. Conversely, the dry insulation may be selected from, or include: microporous material; aerogel; Izoflex (registered trademark); rock wool; or polymer foam.

[0028] Advantageously, a single-walled drainpipe may be capable, in isolation, of withstanding substantially all of the internal pressure, external pressure, axial loads, and bending moments to which the pipe will be subjected during installation or use. A load path conveniently extends from the wet insulation to the drainpipe via the bridge structures. The dry insulation may be substantially decoupled from this load path.

[0029] The inventive concept also encompasses a method of manufacturing a thermally insulated subsea pipe. The method comprises: attaching spaced apart, radially extending deck structures to a rigid internal flow pipe; placing dry insulation around the flow pipe between the structures; joining the flow pipe with wet insulation that surrounds the flow pipe and is held away from the flow pipe by the deck structures to define an annular space between the flow pipe and the wet insulation for encapsulating the dry insulation; and bonding the wet insulation to the deck structures.

[0030] The dry insulation may conveniently be placed around the drain pipe prior to assembly of the drain pipe with the wet insulation to define the annular space around the dry insulation.

[0031] The wet insulation may be bonded to the structures by welding at a mutual interface. For example, the material of the wet insulation and the bridge structures may be melted by heating at least one heating element positioned at or adjacent to the mutual interface.

[0032] In summary, a subsea pipeline of the invention comprises a rigid flowline for transporting fluids such as hydrocarbon production fluids. The flowline is capable of withstanding the mechanical loads to which the pipeline will be subjected during its installation and use, namely internal and external pressure, axial loads and bending moments. The invention is characterized by an external thermal insulation system or layer that surrounds the flowline.

[0033] The thermal insulation system of the invention comprises an innermost layer of dry insulation material that achieves high thermal insulation performance or low thermal conductivity. The thermal insulation layer further comprises an outermost layer that provides lower thermal insulation performance but acts as a barrier to seawater to protect or encapsulate the layer of dry insulation material in a sealed, substantially airtight chamber or enclosure. The outermost layer comprises a layer of wet facing material.

[0034] The innermost layer of dry insulation material is regularly spaced between the bridges of wet liner material. These bridges provide an outer surface for connecting the outermost layer to the rigid flowline, for example by fusion or bonding.

[0035] The outer layer of wet lining material achieves a standard thermal insulation performance which is in addition to the thermal insulation performance achieved by the inner layer of dry insulation material. This synergistic aggregation allows the invention to achieve a U-value of between 1 W / m2 / K and 2.5 W / m2 / K. Such a U-value is not achievable by means of a single standard pipe with a wet lining, which has a typical U-value greater than 2.5 W / m2 / K.

[0036] While the thermal insulation system of the invention will have its own mechanical strength, the flowline is not dependent on the presence or contribution of the thermal insulation system to support the mechanical loads to which the pipeline will be subjected during its service. The thermal insulation system of the invention must therefore be distinguished from the outer pipe of a PiP assembly, the presence of which is necessary to increase the mechanical strength of the inner pipe of the PiP assembly so that the PiP assembly as a whole has the required mechanical strength.

[0037] Embodiments of the invention provide a thermal insulation structure for a subsea pipe, comprising: at least one inner dry insulation layer around the pipe; at least one wet insulation layer above the dry insulation layer; and bridging means extending between the pipe and the wet insulation layer. The bridging means may be located in a transverse plane.

[0038] An anti-corrosion and / or bonding layer may be present between the pipe wall and the inner layer.

[0039] The dry insulation layer is suitably located within an annular space defined between the outer wall of the pipe and the wet insulation layer. The annular space is defined by the bridging means which maintain the wet insulation layer away from the pipe.

[0040] The bridging means may be bonded to the wet insulation layer by fusion or chemical bonding. Preferably, the bridging means are made of the same material as the wet insulation layer.

[0041] The bridging means and / or the wet insulation layer may contain electric heating cables for fusion bonding the bridging means with the wet insulation layer.

[0042] The dry insulation layer is suitably a microporous material such as an aerogel or a silica-based microporous material.

[0043] The wet insulation layer is suitably made of a polymer such as polypropylene or polyurethane and may comprise at least one polymer layer. The wet insulation layer may also be made of a syntactic foam comprising beads encapsulated in a foam or polymer matrix.

[0044] A mechanical protective layer or envelope may encapsulate the wet insulation layer.

[0045] To facilitate understanding of the invention, reference will now be made, by way of example, to the attached drawings, in which:

[0046] [Fig.l] [Fig.l] is a schematic side view of a subsea pipe of the invention in longitudinal section, showing a dry insulation layer disposed in an annular space defined between an inner pipe and a wet insulation layer and held by longitudinally spaced deck structures;

[0047] [Fig.2] [Fig.2] corresponds to [Fig.l] but shows the wet insulation layer supported by a continuous reinforcing layer;

[0048] [Fig.3] [Fig.3] corresponds to [Fig.2] but shows the reinforcement layer interrupted longitudinally by the bridge structures which maintain the annular space between the inner pipe and the wet insulation layer;

[0049] [Fig.4] [Fig.4] corresponds to [Fig.l] but shows reinforcing elements embedded in the wet insulation layer;

[0050] [Fig.5] [Fig.5] corresponds to [Fig.l] but shows an outer protective layer around the wet insulation layer;

[0051] [Fig.6] [Fig.6] corresponds to [Fig.l] but shows an anti-corrosion and / or bonding layer between the inner pipe and the dry insulation layer; and

[0052] [Fig.7] Figures 7a, 7b and 7c correspond to [Fig.l] but show electrical heating elements integrated within and aligned with the bridge structures for fusion bonding or welding the bridge structures to the wet insulation layer, when the elements are activated as shown in Figure 7c.

[0053] The various embodiments of the invention shown in the drawings have several features in common. Identical numbers will therefore be used for these same features in the following description.

[0054] [Fig.l] shows a thermally insulated subsea pipe 10 of the invention in longitudinal section. The pipe 10 shown here comprises a single-walled inner pipe 12 of steel, which is typically fabricated from a series of pipe joints butt welded. A circumferential butt weld 14 shown here between successive pipe joints characterizes this manufacturing method. The inner pipe 12 could instead be made of a composite material, fabricated in a continuous length between steel end fittings. As is conventional, the inner pipe 12 may also contain a corrosion-resistant coating but such a coating has been omitted from the drawings for simplicity.

[0055] A wet insulation layer 16 surrounds the inner pipe 12 in concentric relationship about a common central longitudinal axis 18. As will be understood by those skilled in the art, "wet" does not require that the wet insulation layer 16 be immersed or otherwise exposed to water. In this context, "wet" simply requires that the wet insulation layer 16 be suitable for immersion in water or exposure to water without experiencing a significant reduction in its thermal insulation capability.

[0056] The wet insulation layer 16 may, for example, be made of a polymer such as polypropylene or polyurethane. In this simplified view, the polymer of the wet insulation layer 16 is solid and homogeneous throughout the thickness of the wet insulation layer 16.

[0057] The wet insulation layer 16 is radially spaced from the outer surface of the inner pipe 12 to define a circumferentially continuous annular space 20 therebetween.

[0058] The radial spacing between the wet insulation layer 16 and the inner pipe 12 is maintained by means of a series of bridge structures 22 which are spaced longitudinally along the inner pipe 12 and which act in combination as a bearing surface resisting radially inward loads. The bridge structures 22 extend radially between the radially outer surface of the inner pipe 12 and the radially inner surface of the wet insulation layer 16. In this example, the bridge structures 22 extend continuously around the inner pipe 12 in the form of circumferential rings or circles.

[0059] The bridge structures 22 may also be made of a polymer such as polypropylene or polyurethane. In this simplified view, the polymer is solid and homogeneous over the entire section of each bridge structure 22. Preferably, a polymer that constitutes the bridge structures 22 is the same as a polymer that constitutes the wet insulation layer 16. At least, these polymers are preferably compatible with each other and with any intermediate adhesive or are preferably of the same category with respect to each other, thus both being thermoplastic or thermosetting polymers.

[0060] In the illustrated embodiments, the wet insulation layer 16 is attached to the bridge structures 22, for example by being bonded to the bridge structures 22 and / or by being mechanically engaged with the bridge structures 22. The bonding can be carried out adhesively, i.e. by means of an intermediate layer of adhesive, or by fusion and i.e. by welding involving fusion, intermingling and re-solidification at a mutual interface.

[0061] The bridge structures 22 are also attached to the inner pipe 12, for example by being glued to the inner pipe 12 or melted thereto. In principle, the bridge structures 22 could instead, or in addition, be mechanically attached to the inner pipe 12, for example by clamping.

[0062] The attachment of the wet insulation layer 16 to the deck structures 22 and the attachment of the deck structures 22 to the inner pipe 12 have the effect of transferring all mechanical loads, and not just radial loads, from the wet insulation layer 16 to the inner pipe 12. Such loads may include axial loads, such as those exerted by a tensioning system or by self-weight, and bending moments such as those experienced during installation.

[0063] In the illustrated embodiments, the bridge structures 22 taper in a radial direction, for example in a radially outward direction as illustrated. This reduction in their thickness minimizes heat conduction through the bridge structures 22 between the inner pipe 12 and the wet insulation layer 16.

[0064] A dry insulation layer 24 is disposed in the annular space 20 under and inside the wet insulation layer 16, thus interposed between the wet insulation layer 16 and the inner pipe 12. The material of the dry insulation layer 24 may for example be a microporous material such as an aerogel or Izoflex (registered trademark). Other options for the material of the dry insulation layer 24 include rock wool or polyurethane foam. In this context, "dry" implies to the skilled reader that the dry insulation layer 24 would lose a substantial amount of its thermal insulation capacity if immersed in water.

[0065] The dry insulation layer 24 rests on, and is therefore in contact with, the inner pipe 12. Conversely, a possible radial clearance or air gap 26 around the dry insulation layer 24, between the dry insulation layer 24 and the wet insulation layer 16, prevents any contact between the dry insulation layer 24 and the wet insulation layer 16. Thus, the bridge structures 22 extend radially to a greater extent than the thickness of the dry insulation layer 24. Advantageously, the air gap 26 prevents direct conduction of heat between the inner pipe 12 and the wet insulation layer 16 through the dry insulation layer 24. The air gap 26 is typically less than 1 mm in the radial direction.

[0066] More generally, there is no fusion, adhesion or mechanical engagement between the wet insulation layer 16 and the dry insulation layer 24. Thus, Advantageously, there is no significant transfer of mechanical loads from the wet insulation layer 16 to the dry insulation layer 24. Instead, the dry insulation layer 24 is substantially decoupled from a load path that extends from the wet insulation layer 16 to the inner pipe 12 via the bridge structures 22.

[0067] Figures 2, 3 and 4 show various arrangements for reinforcing the wet insulation layer 16 to increase its mechanical strength. For example, the reinforcement may improve the resistance of the wet insulation layer 16 to radially inward buckling or collapse under hydrostatic pressure and other mechanical loads, such as those exerted by a tensioning system or by self-weight. The reinforcement may also improve the stiffness or flexural rigidity of the wet insulation layer 16 where it spans the longitudinal space between successive bridge structures 22.

[0068] In the subsea pipe 28 of [Fig. 2], the wet insulation layer 16 includes a longitudinally extending reinforcing layer 30 which may, for example, be metallic or may include glass fibers or carbon fibers. In this example, the reinforcing layer 30 is continuous both longitudinally and circumferentially and is offset toward, or disposed on, the radially inner side of the wet insulation layer 16. Thus, the reinforcing layer 30 is interposed between the deck structures 22 and the remaining thickness of the wet insulation layer 16.

[0069] In addition to adding mechanical strength to the wet insulation layer 16, the continuous reinforcement layer 30 may help protect the dry insulation layer 24 from the effects of the manufacturing process that forms the wet insulation layer 16, such as the effects of pressure and thermal degradation.

[0070] [Fig. 3] shows a submarine pipe 32 in which the reinforcing layer 30 is again offset to, or disposed on, the radially inner side of the wet insulation layer 16. In this example, the reinforcement layer 30 is discontinuous, being interrupted by longitudinally spaced gaps aligned with the bridge structures 22, but is otherwise continuous between these gaps. Thus, the reinforcement layer 30 is no longer interposed between the bridge structures 22 and the remaining thickness of the wet insulation layer 16; instead, the bridge structures 22 closely fit into and extend through the gaps that interrupt the reinforcement layer 30, to join the remaining thickness of the wet insulation layer 16.

[0071] It will be apparent from [Fig. 3] that the longitudinally interrupted reinforcing layer 30 is disposed in the annular space 20 between the dry insulation layer 24 and the remaining thickness of the wet insulation layer 16. In this example, the reinforcing layer 30 occupies the circumferential space which defines the air gap 26 in other embodiments. of embodiment. However, it would also be possible to provide an air gap between the reinforcing layer 30 and the dry insulation layer 24 in this embodiment.

[0072] Where the reinforcing layer 30 is a tube which is separate from the wet insulation layer 16, the reinforcing layer 30 could be assembled around the inner pipe 12 from partially tubular shell members such as half-shells.

[0073] In the subsea pipe 34 of [Fig. 4], elongated reinforcing elements 36 such as fibers or yarns are embedded in a polymer matrix of the wet insulation layer 16. In this example, the reinforcing elements 36 are disposed in a perforated layer that is located entirely within the radial thickness of the wet insulation layer 16. Thus, the wet insulation layer 16 has a radially inner surface of substantially solid and homogeneous polymer for direct contact and effective bonding with the deck structures 22 that extend radially from the inner pipe 12.

[0074] [Fig. 5] shows a subsea pipe 38 that has a longitudinally and circumferentially continuous outer jacket 40 disposed outside the wet insulation layer 16. The jacket 40 may be metallic or made of a polymeric or composite material. The jacket 40 is substantially impermeable to water to resist diffusion or migration of water into and through the wet insulation layer 16 with prolonged immersion of the pipe in deep water. The jacket 40 also provides mechanical protection, for example against abrasion or gouging of the wet insulation layer 16 due to engagement of the pipe 38 with turnbuckles during its installation.

[0075] While the embodiment shown in [Fig.5] is otherwise the same as that shown in [Fig.l], the jacket 40 of [Fig.5] could be combined with the features of any of the other embodiments shown in the drawings.

[0076] [Fig. 6] shows a subsea pipe 42 which has a continuous anti-corrosion and / or bonding layer 44, for example of fusion bonded epoxy, applied to the exterior of the inner pipe 12. Accordingly, the anti-corrosion / bonding layer 44 is interposed between the dry insulation layer 24 and the body material of the inner pipe 12. Again, while the embodiment shown in [Fig. 6] is otherwise the same as that shown in [Fig. 1], the anti-corrosion / bonding layer 44 of [Fig. 6] could be combined with the features of any of the other embodiments shown in the drawings.

[0077] Turning finally to Figures 7a-7c, these drawings show a technique for manufacturing a subsea pipe 46 by fusion bonding the wet insulation layer 16 to the deck structures 22 after assembly. First, as shown in Figure 7a, the longitudinally spaced deck structures 22 are bonded to the inner pipe 12. Then, a longitudinally discontinuous but circumferentially continuous layer 24 of dry insulation material is placed around the inner pipe 12 between the radially projecting bridge structures 22. It will be noted that electric heating elements 48 are embedded in the bridge structures 22.

[0078] Next, as shown in Figure 7b, the inner pipe 12 carrying the bridge structures 22 and the dry insulation layer 24 is inserted into a continuous tube of the wet insulation material that forms the wet insulation layer 16. Alternatively, but less preferably, the tubular wet insulation layer 16 could, in principle, be assembled from partially tubular pieces around the inner pipe 12, the bridge structures 22 and the dry insulation layer 24. It will be noted that the electric heating elements 48 are also embedded in the wet insulation layer 16 in longitudinal alignment with the bridge structures 22.

[0079] Figure 7c then shows the electric heating elements 48 activated by passing an electric current through them or by inducing an electric current in them. Accordingly, the resistive heating of the heating elements 48 melts the surrounding thermoplastic polymer of the bridge structures 22 and the wet insulation layer 16. The molten polymer of the bridge structures 22 and the wet insulation layer 16 intermingles at their mutual interface. When the electric heating elements 48 are deactivated by turning off the electric current, the molten interface cools and solidifies to weld the wet insulation layer 16 to the bridge structures 22.

[0080] Various techniques may be used to activate the electrical heating elements 48, for example, by connecting a power supply to the exposed terminals or by electromagnetic induction using an adjacent coil. These techniques need not be explained here as they are well understood in the state of the art, particularly for fusion bonding polymer coating bridges across field joints during the manufacture of a polymer coated pipeline.

[0081] While Figures 7a-7c show that both the bridge structures 22 and the wet insulation layer 16 have integrated electric heating elements 48, it would be possible for only the bridge structures 22 or only the wet insulation layer 16 to have such integrated elements 48. Alternatively, the elements 48 may be only partially embedded in or may lie proximate to the bridge structures 22 and / or the wet insulation layer 16. Thus, the elements 48 may be positioned at the interface between the bridge structures 22 and the wet insulation layer 16.

[0082] Again, while the embodiment shown in Figures 7a to 7c is otherwise the same as that shown in [Fig.l], the heating elements 48 of Figures 7a to 7c could be combined with the features of any of the other embodiments shown in the drawings.

[0083] Many variations are possible in the concept of the invention. For example, a polymer of the wet insulation layer and / or the bridge structures may be foamed or may contain, incorporate or encapsulate reinforcing fibers and / or thermally insulating particles, spheres or beads. If not solid and homogeneous throughout their thickness, the wet insulation layer and / or the bridge structures may nevertheless comprise an outer layer of solid and homogeneous polymer. For example, the wet insulation layer may comprise at least one layer of solid and homogeneous polymer on at least one radially inner surface and optionally also on a radially outer surface.

[0084] The dry insulation layer may be sealed in a watertight enclosure in the annular space to mitigate any migration of seawater through the wet insulation layer due to diffusion. Alternatively, the dry insulation layer could be injected into the annular space between the inner pipe and the wet insulation after joining the inner pipe and the wet insulation.

[0085] The bridge structures could take other forms. For example, the bridge structures need not extend continuously around the inner pipe or extend circumferentially around the inner pipe. The bridge structures could instead be interrupted or defined by isolated projections or pillars that are spaced circumferentially and / or longitudinally around and along the inner pipe. In other arrangements, the bridge structures could instead be ridges, fins, or flanges that extend longitudinally or helically along the inner pipe.

Claims

Claims

1. A thermally insulated subsea pipe (10) comprising: - a rigid inner flow pipe (12); - wet insulation (16) disposed around the flow pipe (12); - spaced apart deck structures (22) which are longitudinally spaced along the flow pipe (12) and which extend radially from the flow pipe (12) towards the wet insulation (16) in order to keep the wet insulation (16) outside the flow pipe (12), thereby defining an annular space between the flow pipe (12) and the wet insulation (16); - the wet insulation (16) being bonded to the structures (22); - the deck structures are made of the same material as the wet insulation; and - dry insulation (24) is disposed between the deck structures (22) in the annular space.

2. A pipe according to any preceding claim, wherein the dry insulation (24) is in contact with the wet insulation (16).

3. A pipe according to claim 2, wherein the dry insulation (24) fills the annular space substantially completely.

4. A pipe according to claim 1, wherein the wet insulation (16) is spaced from the dry insulation (24).

5. A pipe according to claim 4, wherein the spacing between the wet insulation (16) and the dry insulation (24) is less than 1 mm.

6. A pipe according to any preceding claim, wherein the wet insulation (16) is fused to the bridge structures (22).

7. A pipe according to claim 6, wherein the bridge structures (22) and / or the wet insulation (16) comprise heatable welding elements.

8. A pipe according to any preceding claim, wherein the wet insulation (16) defines an exposed outer surface of the pipe.

9. A pipe according to any one of claims 1 to 7, further comprising a watertight jacket (40) around the wet insulation (16).

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23. A pipe according to any preceding claim, further comprising an anti-corrosion and bonding layer (44) located between the dry insulation (24) and the flow pipe (12). A pipe according to any preceding claim, wherein the dry insulation (24) is sealed in a watertight enclosure within the annular space. A pipe according to any preceding claim, further comprising a reinforcing layer (30) supporting the wet insulation (16). A pipe according to claim 12, wherein the reinforcing layer (30) extends between the bridge structures (22). A pipe according to claim 13, wherein the reinforcing layer (30) is interrupted by the bridge structures (22). A pipe according to any one of claims 12 to 14, wherein the reinforcing layer (30) is radially inside the wet insulation (16). A pipe according to claim 15, wherein the reinforcing layer (30) is located within the annular space. A pipe according to any one of claims 12 to 14, wherein the reinforcing layer (30) is embedded in the wet insulation (16). A pipe according to any one of claims 12 to 16, wherein the dry insulation (24) is in contact with the reinforcing layer (30). A pipe according to any one of claims 12 to 17, wherein the dry insulation (24) is spaced from the reinforcing layer (30). A pipe according to claim 19, wherein the spacing between the dry insulation (24) and the reinforcing layer (30) is less than 1 mm. A pipe according to any preceding claim, wherein the wet insulation (16) is selected from or comprises: syntactic foam; polypropylene; polyurethane; or other polymers. A pipe according to any preceding claim, wherein the dry insulation (24) is selected from or comprises: a microporous material; an aerogel; Izoflex (registered trademark); rock wool; or a polymer foam. A pipe according to any preceding claim, wherein the flow pipe (12) is capable, in isolation, of withstanding substantially all of the internal pressures, external pressures, axial loads and bending moments to which the pipe (12) will be subjected during its installation or use.

24. A pipe according to any preceding claim, wherein a load path extends from the wet insulation (16) to the flow pipe (12) via the bridge structures (22).

25. The pipe of claim 24, wherein the dry insulation (24) is substantially decoupled from said load path.

26. A method of manufacturing a thermally insulated subsea pipe (10), the method comprising: - attaching spaced and radially extending deck structures (22) to a rigid inner flow pipe (12) such that the deck structures (22) are longitudinally spaced along the flow pipe (12); - placing dry insulation (24) around the flow pipe (12) between the deck structures (22); - assembling the drain pipe (12) with wet insulation (16) surrounding the drain pipe (12) and held away from the drain pipe (12) by the deck structures (22) to define an annular space between the drain pipe (22) and the wet insulation (16) to encapsulate the dry insulation (24), the wet insulation (16) being made of the same material as the deck structures (22); and - bonding the wet insulation (16) to the deck structures (22).

27. A method according to claim 26, comprising placing the dry insulation (24) around the drain pipe (12) before gathering the drain pipe (12) with the wet insulation (16) to define the annular space around the dry insulation (24).

28. A method according to claim 26 or claim 27, comprising bonding the wet insulation (16) to the bridge structures (22) by welding at a mutual interface.

29. A method according to claim 28, comprising fusing the material of the wet insulation (16) and the bridge structures (22) by heating at least one heating element (48) at or adjacent to the mutual interface.

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