GB2581153A

GB2581153A - Thermal insulation of subsea pipelines

Info

Publication number: GB2581153A
Application number: GB1901577.5A
Authority: GB
Inventors: Loentgen Vincent; Lodeho Olivier
Original assignee: Acergy France SAS
Current assignee: Acergy France SAS
Priority date: 2019-02-05
Filing date: 2019-02-05
Publication date: 2020-08-12
Anticipated expiration: 2039-02-05
Also published as: GB2581153B; NO20200132A1; FR3092381A1; GB201901577D0

Abstract

A thermally-insulated subsea pipe, comprising a rigid inner flowline pipe 12; wet insulation 16 disposed around the flowline pipe; spaced-apart bridge structures 22 that extend radially from the flowline pipe toward the wet insulation to hold the wet insulation apart from the flowline pipe, thus defining an annular space 20 between the flowline pipe and the wet insulation; and dry insulation 24 disposed between the bridge structures in the annular space. Aspects include the bridge structures bonded to the wet insulation, for example by welding, and so may be of the same material as the wet insulation or a compatible material that permits welding. The wet insulation may be selected from, or comprise: syntactic foam; polyurethane; polypropylene; or other polymers. The dry insulation is selected from or comprises: a microporous material; an aerogel; Izoflex (RTM); Rockwool (RTM); or a polymer foam.

Description

Intellectual Property Office Application No. GII1901577.5 Rum Date:6 August 2019 The following terms are registered trade marks and should be read as such wherever they occur in this document: lzoflex; Rockwool Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo Thermal insulation of subsea pipelines This invention relates to the thermal insulation of subsea pipelines used in the production of hydrocarbon fluid. A key objective of thermal insulation is to avoid a pipeline becoming clogged or plugged with solids that may appear in the production fluid if its temperature falls too low at a given pressure.

Oil and gas are present in subterranean formations at elevated temperature and pressure, which may be increased by the injection of fluids such as pressurised water.

On 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 across the seabed and eventually flows up a riser to the surface.

Low temperature increases the viscosity of the production fluid and promotes coalescence or precipitation of solid-phase materials from some components present in the production fluid, namely waxes and asphaltenes in crude oil and hydrates in natural gas. Such solid-phase materials tend to deposit and accumulate on the inner wall of the pipeline and may eventually cause plugs, which will interrupt production. Aside from the high cost of lost production, plugs are difficult and expensive to remove and can even sever a pipeline.

During transportation along a pipeline, the temperature and pressure of the production fluid have to be kept high enough to ensure a sufficient flow rate across the seabed and up a riser. In particular, various measures are taken to ensure that the internal temperature of the pipeline remains high, typically above 65°C and in some cases above 200°C, despite thermal exchange with seawater which, for example, is at 4°C below 1000m depth. Plugging becomes a risk if the temperature of the production fluid within the pipeline drops below the wax appearance temperature (VVAT), or below other thresholds at which other solid materials will coalesce from oil or gas.

Designers of subsea pipelines have adopted both active and passive approaches to thermal management, both individually and in combination.

Among active thermal management systems, a trace heating system typically employs resistive electrical wires running along, and in thermal contact with, the outer surface of a steel pipeline. Heat produced by passing an electric current along the wires is conducted through the pipe wall to the production fluid flowing within.

In passive thermal management systems, a pipeline is thermally insulated. One example of a passive thermal management system is a pipe-in-pipe (PiP) assembly comprising a fluid-carrying rigid inner pipe positioned concentrically within a rigid outer pipe. Typically the inner and outer pipes are of steel although one or other of those pipes could instead be a similarly-robust and essentially rigid composite pipe. A composite pipe has a solid, monolithic structure that comprises a polymer resin matrix, for example of polypropylene, reinforced by fibres such as glass fibres or carbon fibres.

A solid polymer liner may be disposed within the reinforced polymer matrix. The matrix may also be surrounded by an outer coating, which may also be of solid polymer.

The inner and outer pipes of a PiP assembly are spaced from each other to define an insulating annulus between them. Typically, thermally-insulating material is disposed in the annulus. It is also possible to draw down a partial vacuum in the annulus to reduce transmission of heat through the annulus.

Thermal insulation material used in subsea pipelines falls into one of two categories, known in the art as wet insulation and dry insulation. Wet insulation can withstand contact with water and also significant mechanical stress. An example of wet insulation is syntactic foam. Conversely, dry insulation should not be in contact with water and cannot bear significant mechanical stress, and so has to be protected both mechanically and chemically. An example of dry insulation is an aerogel.

In terms of thermal insulation, dry insulation is known to be more efficient than wet insulation and is often used where the pipeline comprises a protective external casing, such as the outer pipe of a PiP assembly. For example, WO 2006/133155 describes the use of aerogel insulation in a PiP annulus. In US 7226243, dedicated spacers allow the insertion of a layer of more fragile aerogel insulation. A steel sheet can also protect a microporous insulation layer.

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 by a protective layer for mechanical protection.

The protective layer shields the fragile inner layer from damage during insertion of the insulated inner pipe into the outer pipe of the PiP assembly. Spacers or supports extend between the inner pipe and the protection layer or the outer pipe. In service, the annulus is protected by the outer pipe and so is not subjected to hydrostatic pressure.

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

WO 2008/017147 discloses a thermal insulation system that comprises an inner layer of high-performance thermal insulation that is relatively weak mechanically, covered by an outer layer that has greater mechanical resistance. The inner layer is typically of a microporous insulation material whereas the outer layer is typically of a polymeric foam. However, hydrostatic pressure could deflect the outer layer and thereby crush the inner layer. In this respect, a watertight outer jacket may be provided around the insulation system but such a jacket would not be sufficiently resistant to hydrostatic pressure at great depth.

CN 102705595 also teaches a pipe with a sandwich insulation structure that comprises a first aerogel layer and a second polymeric layer. A similar pipe with a silica aerogel layer and a polymeric layer is used as the inner pipe of a PiP assembly in CN 204328250.

For many years, adopting a PiP assembly has been the default solution in the art when faced with the need for efficient thermal insulation of a pipeline installed at great depth.

However, for some subsea applications, it would be advantageous to adopt a single-wall pipe if a suitably efficient and pressure-resistant thermal insulation system was available. This could avoid the cost, weight and complexity of a PiP assembly in such applications.

Against this background, the invention provides a thermally-insulated subsea pipe, comprising: a rigid inner flowline pipe; wet insulation disposed around the flowline pipe; spaced-apart bridge structures that extend radially from the flowline pipe toward the wet insulation to hold the wet insulation apart from the flowline pipe, thus defining an annular space between the flowline pipe and the wet insulation; and dry insulation disposed between the bridge structures in the annular space. The dry insulation may thereby be sealed in a watertight enclosure within the annular space.

The bridge structures are suitably made of a wet insulation material, or of a material that is chemically compatible with the wet insulation, or indeed of the same material as the wet insulation. The bridge structures are suitably spaced longitudinally along the flowline pipe.

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 1mm.

Advantageously, the wet insulation is bonded to the bridge structures, for example by being fused to the bridge structures. For that purpose, the bridge structures and/or the wet insulation may comprise heatable welding elements.

An anti-corrosion layer may be provided between the dry insulation and the flowline pipe. Such a layer may also promote bonding of the bridge structures to the flowline pipe The wet insulation is suitably capable of defining an outer exposed surface of the pipe.

However, a watertight or protective jacket may be placed around the wet insulation.

A reinforcement layer may support the wet insulation. Such a reinforcement layer suitably extends between the bridge structures and may extend continuously beyond the bridge structures or be interrupted by the bridge structures.

The reinforcement layer may lie radially within the wet insulation, for example between the wet insulation and the bridge structures or within the annular space. Alternatively, the reinforcement layer may be embedded within the wet insulation.

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

The wet insulation may be selected from, or comprise: syntactic foam; polypropylene; polyurethane; or other polymers. Conversely, the dry insulation may be selected from, or comprise: a microporous material; an aerogel; lzoflex (trade mark); rockwool; or a polymer foam.

Advantageously, a single-walled flowline pipe may be capable, in isolation, of bearing 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 suitably extends from the wet insulation to the flowline pipe via the bridge structures.

The dry insulation may be substantially decoupled from that load path.

The inventive concept also embraces a method of making a thermally-insulated subsea pipe. The method comprises: fixing spaced-apart, radially-extending bridge structures to a rigid inner flowline pipe; placing dry insulation around the flowline pipe between the bridge structures; assembling the flowline pipe with wet insulation that surrounds the flowline pipe and that is held apart from the flowline pipe by the bridge structures to define an annular space between the flowline pipe and the wet insulation for encapsulating the dry insulation; and bonding the wet insulation to the bridge structures.

The dry insulation may, conveniently, be placed around the flowline pipe before assembling the flowline pipe with the wet insulation to define the annular space around the dry insulation.

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

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

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 seawater barrier to protect or encapsulate the layer of dry insulation material in a sealed, substantially watertight chamber or enclosure. The outermost layer comprises a layer of wet coating material.

The innermost layer of dry insulation material is regularly spaced between bridges of wet coating material. These bridges provide an external surface for linking the outermost layer to the rigid flowline, for example by fusion or adhesive bonding.

The outer layer of wet coating material achieves a standard thermal insulation performance that adds 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 a standard single pipe with a wet coating, which has a typical U-Value of greater than 2.5 W/m2/K.

Whilst the thermal insulation system of the invention will have some mechanical strength of its own, the flowline does not rely on the presence or contribution of the thermal insulation system to sustain the mechanical loads to which the pipeline will be subjected in service. The thermal insulation system of the invention is therefore to be distinguished from the outer pipe of a PiP assembly, whose presence is necessary to add to the mechanical strength of the inner pipe of the PiP assembly in order that the PiP assembly as a whole will have the requisite mechanical strength.

Embodiments of the invention provide a thermal insulation structure for underwater 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 in a transverse plane.

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

The dry insulation layer is suitably located inside an annulus defined between the outer wall of the pipe and the wet insulation layer. The annulus is defined by the bridging means that maintain the wet insulation layer at a distance from the pipe.

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

The bridging means and/or the wet insulation layer may contain electrical heating cables for fusion bonding of the bridging means with the wet insulation layer.

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

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

A mechanical protection layer or jacket may encapsulate the wet insulation layer.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a schematic side view of a subsea pipe of the invention in longitudinal section, showing a dry insulation layer disposed in an annulus defined between an inner pipe and a wet insulation layer and maintained by longitudinally-spaced bridge structures; Figure 2 corresponds to Figure 1 but shows the wet insulation layer supported by a continuous reinforcement layer; Figure 3 corresponds to Figure 2 but shows the reinforcement layer interrupted longitudinally by the bridge structures that maintain the annulus between the inner pipe and the wet insulation layer; Figure 4 corresponds to Figure 1 but shows reinforcement elements embedded in the wet insulation layer, Figure 5 corresponds to Figure 1 but shows a protective outer layer around the wet insulation layer; Figure 6 corresponds to Figure 1 but shows an anticorrosion and/or bonding layer between the inner pipe and the dry insulation layer; and Figures 7a, 7b and 7c correspond to Figure 1 but show electrical heating elements embedded within and aligned with the bridge structures for fusion bonding or welding of the bridge structures to the wet insulation layer, when the elements are activated as shown in Figure 7c.

The various embodiments of the invention shown in the drawings have several features in common. Like numerals will therefore be used for those like features in the

description that follows.

Figure 1 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 welded end-to-end. A circumferential butt weld 14 shown here between successive pipe joints characterises this method of fabrication. The inner pipe 12 could instead be of a composite material, manufactured in a continuous length between steel end fittings. As is conventional, the inner pipe 12 may also contain a corrosion-resistant liner but such a liner has been omitted from the drawings for simplicity.

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

The wet insulation layer 16 may, for example, be 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.

The wet insulation layer 16 is spaced radially from the outer surface of the inner pipe 12 to define a circumferentially-continuous annulus 20 between them.

The radial spacing between the wet insulation layer 16 and the inner pipe 12 is maintained by a series of bridge structures 22 that are spaced longitudinally along the inner pipe 12 and that act in combination as a load-bearing surface that is resistant to 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 as circumferential rings or hoops.

The bridge structures 22 may also be of a polymer such as polypropylene or polyurethane, In this simplified view, the polymer is solid and homogeneous throughout the cross-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, those polymers are preferably compatible with each other and with any intermediate adhesive or are preferably of the same category as each other, thus both being thermoplastic or thermoset polymers.

In the embodiments illustrated, the wet insulation layer 16 is fixed to the bridge structures 22, for example by being bonded to the bridge structures 22 and/or by being engaged mechanically with the bridge structures 22. Bonding may be effected adhesively, hence by an intermediate layer of adhesive, or by fusion and hence by welding involving melting, intermingling and re-solidification at a mutual interface.

The bridge structures 22 are also fixed to the inner pipe 12, for example by being bonded adhesively or fused to the inner pipe 12. In principle, the bridge structures 22 could instead, or additionally, be fixed mechanically to the inner pipe 12, for example by clamping.

The effect of fixing the wet insulation layer 16 to the bridge structures 22 and fixing the bridge structures 22 to the inner pipe 12 is to transfer all mechanical loads, and not merely radial loads, from the wet insulation layer 16 to the inner pipe 12. Such loads may including axial loads, such as those exerted by a tensioner system or by self-weight, and bending moments such as those experienced during installation.

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

A dry insulation layer 24 is disposed in the annulus 20 under and within the wet insulation layer 16, hence being interposed between the wet insulation layer 16 and the inner pipe 12. The dry insulation layer 24 material may, for example, be a microporous material such as an aerogel or Izoflex (trade mark). Other options for the material of the dry insulation layer 24 include rockwool 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 capability if immersed in water.

The dry insulation layer 24 lies upon, and therefore is in contact with, the inner pipe 12.

Conversely, an optional radial clearance or air gap 26 around the dry insulation layer 24, between the dry insulation layer 24 and the wet insulation layer 16, avoids 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 narrower than 1mm in the radial direction.

More generally, there is no fusion, adhesion or mechanical engagement between the wet insulation layer 16 and the dry insulation layer 24. Thus, beneficially, 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.

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

In the subsea pipe 28 of Figure 2, the wet insulation layer 16 comprises a longitudinally-extending reinforcement layer 30 that may, for example, be metallic or may comprise glass fibres or carbon fibres. In this example, the reinforcement 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 reinforcement layer is interposed between the bridge structures 22 and the remaining thickness of the wet insulation layer 16 In addition to adding mechanical strength to the wet insulation layer 16, the continuous reinforcement layer 30 may help to protect the dry insulation layer 24 from being affected by the manufacturing process that forms the wet insulation layer 16, such as by the effects of pressure and thermal degradation.

Figure 3 shows a subsea pipe 32 in which the reinforcement layer 30 is again offset toward, 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 those 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 fit closely into and extend through the gaps that interrupt the reinforcement layer 30, to join the remaining thickness of the wet insulation layer 16.

It will be apparent from Figure 3 that the longitudinally-interrupted reinforcement layer 30 is disposed in the annulus 20 between the dry insulation layer 24 and the remaining thickness of the wet insulation layer 16. In this example, the reinforcement layer 30 occupies the circumferential space that defines the air gap 26 in other embodiments. However, it would be possible to provide for an air gap between the reinforcement layer 30 and the dry insulation layer 24 in this embodiment too.

Where the reinforcement layer 30 is a tube that is separate from the wet insulation layer 16, the reinforcement layer 30 could be assembled around the inner pipe 12 from part-tubular shell elements such as half-shells.

In the subsea pipe 34 of Figure 4, elongate reinforcement elements 36 such as fibres or wires are embedded in a polymer matrix of the wet insulation layer 16. In this example, the reinforcement elements 36 are arranged in a foraminous layer that is wholly within the radial thickness of the wet insulation layer 16. Thus, the wet insulation layer 16 presents a radially inner surface of substantially solid and homogeneous polymer for direct contact with, and effective bonding to, the bridge structures 22 that extend radially from the inner pipe 12.

Figure 5 shows a subsea pipe 38 that has a longitudinally-and circumferentially-continuous external jacket 40 disposed on the outside of the wet insulation layer 16. The jacket 40 may be metallic or of polymer or composite material. The jacket 40 is substantially impervious 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 tensioners during installation.

Whilst the embodiment shown in Figure 5 is otherwise the same as that shown in Figure 1, the jacket 40 of Figure 5 could be combined with the features of any of the other embodiments shown in the drawings.

Figure 6 shows a subsea pipe 42 that 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. Hence, the anti-corrosion/bonding layer 44 is interposed between the dry insulation layer 24 and the body material of the inner pipe 12. Again, whilst the embodiment shown in Figure 6 is otherwise the same as that shown in Figure 1, the anticorrosion/bonding layer 44 of Figure 6 could be combined with the features of any of the other embodiments shown in the drawings.

Turning finally to Figures 7a to 7c, these drawings show a technique for manufacturing a subsea pipe 46 by fusion-bonding the wet insulation layer 16 to the bridge structures 22 after assembly. First, as shown in Figure 7a, the longitudinally-spaced bridge 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-protruding bridge structures 22. It will be noted that electrical heating elements 48 are embedded into the bridge structures 22.

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 part-tubular pieces around the inner pipe 12, the bridge structures 22 and the dry insulation layer 24. It will be noted that electrical heating elements 48 are also embedded into the wet insulation layer 16 in longitudinal alignment with the bridge structures 22.

Figure 7c then shows the electrical heating elements 48 activated by passing an electric current through them or by inducing an electric current in them. In consequence, resistive heating of the heating elements 48 melts the surrounding thermoplastic polymer of the bridge structures 22 and of 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 electrical heating elements 48 are deactivated by switching off the electric current, the molten interface cools and re-solidifies to weld the wet insulation layer 16 to the bridge structures 22.

Various techniques may be used to activate the electrical heating elements 48, for example by connecting an electric power supply to exposed terminals or by electromagnetic induction using an adjacent coil. Such techniques need not be explained here as they are well understood in the art, in particular for fusion bonding of polymer liner bridges across field joints when fabricating a polymer-lined pipeline.

Whilst Figures 7a to 7c show that both the bridge structures 22 and the wet insulation layer 16 have electrical heating elements 48 embedded into them, it would be possible for only the bridge structures 22 or only the wet insulation layer 16 to have such embedded elements 48. Alternatively, elements 48 may be only partially embedded in or may lie proud of 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.

Again, whilst the embodiment shown in Figures 7a to 7c is otherwise the same as that shown in Figure 1, 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.

Many other variations are possible within the inventive concept. For example, a polymer of the wet insulation layer and/or the bridge structures may be foamed or may contain, embed or encapsulate reinforcing fibres 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 external 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 a radially inner surface and optionally also on a radially outer surface.

The dry insulation layer may be sealed in a watertight enclosure within the annulus to mitigate any migration of seawater through the wet insulation layer due to diffusion. In another option, the dry insulation layer could be injected into the annulus between the inner pipe and the wet insulation after assembling the inner pipe and the wet insulation.

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 protrusions 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, comprising: a rigid inner flowline pipe; wet insulation disposed around the flowline pipe; spaced-apart bridge structures that extend radially from the flowline pipe toward the wet insulation to hold the wet insulation apart from the flowline pipe, thus defining an annular space between the flowline pipe and the wet insulation; and dry insulation disposed between the bridge structures in the annular space.
2. The pipe of Claim 1, wherein the bridge structures are made of a wet insulation material.
3. The pipe of Claim 1 or Claim 2, wherein the bridge structures are made of a material that is chemically compatible with the wet insulation.
4. The pipe of any preceding claim, wherein the bridge structures are made of the same material as the wet insulation.
5. The pipe of any preceding claim, wherein the dry insulation is in contact with the wet insulation.
6. The pipe of Claim 5, wherein the dry insulation fills the annular space substantially completely.
7. The pipe of any of Claims 1 to 4, wherein the wet insulation is spaced from the dry insulation.
8. The pipe of Claim 7, wherein the spacing between the wet insulation and the dry insulation is less than 1mm.
9. The pipe of any preceding claim, wherein the bridge structures are spaced longitudinally along the flowline pipe.
10. The pipe of any preceding claim, wherein the wet insulation is bonded to the bridge structures.
11. The pipe of Claim 10, wherein the wet insulation is fused to the bridge structures.
12. The pipe of Claim 11, wherein the bridge structures and/or the wet insulation comprise heatable welding elements.
13. The pipe of any preceding claim, wherein the wet insulation defines an outer exposed surface of the pipe.
14. The pipe of any of Claims 1 to 12, further comprising a watertight jacket around the wet insulation.
15. The pipe of any preceding claim, further comprising an anti-corrosion and bonding layer between the dry insulation and the flowline pipe.
16. The pipe of any preceding claim, wherein the dry insulation is sealed in a watertight enclosure within the annular space.
17. The pipe of any preceding claim, further comprising a reinforcement layer supporting the wet insulation.
18. The pipe of Claim 17, wherein the reinforcement layer extends between the bridge structures.
19. The pipe of Claim 18, wherein the reinforcement layer extends continuously beyond the bridge structures.
20. The pipe of Claim 18, wherein the reinforcement layer is interrupted by the bridge structures.
21. The pipe of any of Claims 17 to 20, wherein the reinforcement layer lies radially within the wet insulation.
22. The pipe of Claim 21, wherein the reinforcement layer lies between the wet insulation and the bridge structures.
23. The pipe of Claim 21, wherein the reinforcement layer lies within the annular space. 5
24. The pipe of any of Claims 17 to 20, wherein the reinforcement layer is embedded within the wet insulation.
25. The pipe of any of Claims 17 to 23, wherein the dry insulation is in contact with the reinforcement layer.
26. The pipe of any of Claims 17 to 24, wherein the dry insulation is spaced from the reinforcement layer.
27. The pipe of Claim 26, wherein the spacing between the dry insulation and the reinforcement layer is less than 1mm
28. The pipe of any preceding claim, wherein the wet insulation is selected from, or comprises: syntactic foam; polypropylene; polyurethane; or other polymers.
29. The pipe of any preceding claim, wherein the dry insulation is selected from, or comprises: a microporous material; an aerogel; Izoflex (trade mark); rockwool; or a polymer foam.
30. The pipe of any preceding claim, wherein the flowline pipe is capable, in isolation, of bearing substantially all internal pressure, external pressure, axial loads and bending moments to which the pipe will be subjected during installation or use.
31. The pipe of any preceding claim, wherein a load path extends from the wet insulation to the flowline pipe via the bridge structures.
32. The pipe of Claim 31, wherein the dry insulation is substantially decoupled from said load path.
33. A method of making a thermally-insulated subsea pipe, the method comprising: fixing spaced-apart, radially-extending bridge structures to a rigid inner flowline pipe; placing dry insulation around the flowline pipe between the bridge structures; assembling the flowline pipe with wet insulation that surrounds the flowline pipe and that is held apart from the flowline pipe by the bridge structures to define an annular space between the flowline pipe and the wet insulation for encapsulating the dry insulation; and bonding the wet insulation to the bridge structures.
34. The method of Claim 33, comprising placing the dry insulation around the flowline pipe before assembling the flowline pipe with the wet insulation to define the annular space around the dry insulation.
35. The method of Claim 33 or Claim 34, comprising bonding the wet insulation to the bridge structures by welding at a mutual interface.
36. The method of Claim 35, comprising melting material of the wet insulation and the bridge structures by heating one or more heating elements at or adjacent to the mutual interface.