FR 3092381 A1 20200807 FR 2001037 A 20200203 Description Title of Invention: Thermal insulation of subsea pipelines [0001] This invention relates to the thermal insulation of subsea pipelines used in the production of hydrocarbon fluid. A key objective e of thermal insulation is to avoid a pipeline becoming clogged or plugged with solids that may appear in the production fluid if its temperature fans too low at a given pressure. [0002] Oil and gas are present in subten-anean 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 Ilows in the pipeline across the seabed and eventually llows up a riser to the surface. [0003] Low temperature increases the viscosity of the production fluid and promotes coa- lescence 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 plues, which will interrupt production. Aside from the high cost of lost production, plues are difficult and expensive to remove and can even sever a pipeline. [0004] Durine transportation along a pipeline, the temperature and pressure of the production fluid have to be kept high enough to ensure a sulTicient flow rate across the seabed and up a riser. In particular, various measures are taken to ensure that the internai 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 (WAT), or below other thresholds at which other solid materials will coalesce from oil or gas. [0005] Designers of subsea pipelines have adopted both active and passive approaches to thermal management, both individually and in combination. [0006] Among active thermal management systems, a trace hcating system typically employs resistive clectrical wircs running along, and in thermal contact with, the outcr surface of a steel pipeline. Hcat produccd by passing an clectric current along the wires is conducted through the pipe wall to the production fluid tlowing within. [0007] In passive thermal management systems, a pipeline is thcrmally insulated. One example of a passive thermal management system is a pipe-in-pipe (Pi P) assembly comprising a fluid-carrying rigid inner pipe positioncd concentrically within a rigid outcr pipe. Typically the inner and outcr pipes arc of steel although one or other of 2 thosc pipes couic! instcad be a similarly-robust and essentially rigid composite pipe. A composite pipe has a solid, monolithic structure that comprises a polymer rcsin matrix, for example of polypropylene, reinforced by fibres such as glass fibres or carbon fibres. A solid polymer liner may be disposcd within the reinforced polymer matrix. The matrix may aise) bc surrounded by an outer coating, which may also lie of solid polymer. [0008] The inner and outer pipes of a PiP asscmbly arc spaccd from cadi othcr to &fine an insulating annulus between thcm. Typically, thermally-insulating material is disposcd in the annulus. it k also possible to draw down a partial vacuum in the annulus to reduce transmission of hcat through the annulus. [0009] Thermal insulation material uscd in subsca pipelines falls into one of two categories, known in the art as wct insulation and dry insulation. Wet insulation can withstand contact with watcr and also significant mechanical stress. An example of wct insulation k syntactic foam. Conversely, dry insulation should not be in contact with water and cannot bcar significant mechanical stress, and so has to bc protected both mcchanically and chcmically. An example of dry insulation is an aerogel. [0010] -In terris of thermal insulation, dry insulation is known to lie more efficient than wct insulation and k often uscd where the pipeline comprises a protective external casing, such as the outer pipe of a PiP assembly. For example, WO 2006/133155 &scribes the use of acrogel insulation in a PiP annulus. In US 7226243, dedicated spaccrs allow the insertion of a layer of more fragile aerogel insulation. A steel sheet can also protect a microporous insulation layer. [0011] 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. [0012] 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. [0013] 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 3 the inncr layer. In this respect, a watertight enter jackct may bc provided around the insulation system but such a jackct would not be sufficicntly rcsistant to hydrostatic pressure at grcat dcpth. [0014] CN 102705595 also teaches a pipe with a sandwich insulation structure that comprises a first acrogel layer and a second polymcric layer. A similar pipe with a sil ica acrogel layer and a polymeric layer is uscd as the inncr pipe of a PiP asscmbly in CN 204328250. [0015] US 4921018 and US 5722462 &scribe a thermally insulatcd pipe surroundcd by an outcr casing that is spaced apart from the inncr pipe by partitions. Similarly, WO 99/05447 &scribes an insulatcd pipe in which an inncr pipe is surrounded by a syntactic foam which is encloscd by an outcr pipe. [0016] WO 99/05447 and US 2003/127148 &scribe insulatcd pipes in which an inncr pipe is insulatcd by a syntactic foarn or a syntactic ciment. The insulation material is thcn surroundcd by an outcr casing. CN 105299334 &scribes an insulatcd pipe in which the inncr pipe is insulatcd by a polyurethanc layer. [0017] For tnany ycars, adopting a PiP asscmbly has been the default solution in the art when faccd with the nced for efficient thermal insulation of a pipeline installed at grcat dcpth. However, for sonie subsca 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, wcight and complexity of a PiP asscmbly in such applications. [0018] Against this background, the invention provides a thermally-insulated subsea pipe, comprising: a rigid inner tlowline pipe; wet insulation disposed around the flowline pipe; spaced-apart bridge structures that extend radially from the tlowline pipe toward the wet insulation to hold the wet insulation apart from the tlowline pipe, thus defining an annular space between the tlowline 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 annulai- space. [0019] The bridge structures are suitably made of a wet insulation material, or of a material that is chernically 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. [0020] The dry insulation may be in contact with the wet insulation. For example, the dry in- sulation may fill the annular space substantially completely. Altematively, 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 lmm. [0021] 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. [0022] An anti-corrosion layer may be provided between the dry insulation and the flowlinc pipe. Such a layer may also promote bonding of the bridge structures to the flowlinc pipe. [0023] The wet insulation is suitably capable of dcfining an outer exposed surface of the pipe. However, a watcrtight or protective jackct may be placed around the wet insulation. [0024] A reinforcement layer may support the wet insulation. Such a reinforcement layer suitably extcnds between the bridge structures and may extend continuously bcyond the bridge structures or bc interrupted by the bridge structures. [0025] 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 bc embedded within the wet insulation. [0026] The dry insulation may be in contact with the reinforcement layer or spaccd from the reinforcement layer, for example with a spacing between the dry insulation and the reinforcement layer of less than lmm. [0027] The wet insulation may be selected from, or comprise: syntactic foam; polypropylene; polyurethane; or othcr polymcrs. Converscly, the dry insulation may bc selected from, or comprise: a microporous matcrial; an acrogel; Izoflex (trade mark); rockwool; or a polymer foam. [0028] Advantageously, a single-walled flowline pipe may be capable, in isolation, of bearing substantially ail of the internai pressure, extemal 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. [0029] 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 tlowline pipe; placing dry insulation around the tlowline pipe between the bridge structures: assembling the tlowline pipe with wet insulation that surrounds the tlowline pipe and that is held apart from the tlowline pipe by the bridge structures to define an annular space between the tlowline pipe and the wet insulation for encapsulating the dry insulation; and bonding the wet insulation to the bridge structures. [0030] The dry insulation may, conveniently, be placed around the flowline pipe before as- sembling the tlowline pipe with the wet insulation to define the annular space around the dry insulation. [0031] 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 hcating elements positioncd at or adjacent to the tnutual interface. [0032] -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 subjectcd during installation and in use, namely, internai and external pressure, axial loads and bending moments. The invention is characterised 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 matcrial 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 scaled, substantially watcrtight chamber or enclosure. The outermost layer comprises a layer of wet coating matcrial. [0034] The innermost layer of dry insulation material is regularly spaccd between bridges of wet coating matcrial. Thcse bridges providc an external surface for linking the outermost layer to the rigid flowlinc, for example by fusion or adhcsive bonding. [0035] The outcr layer of wet coating matcrial achieves a standard thermal insulation per- formance 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. [0036] 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. [0037] 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. [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 inside an annulus defined between the 6 outer wall of the pipe and the wet insulation layer. The annulus is defined by the bridging mcans that maintain the wet insulation layer at a distance from the pipe. [0040] The bridging mcans may be bonded to the wet insulation layer by fusion or chcmical bonding. Preferably the bridging mcans arc of the same matcrial as the wet insulation layer. [0041] The bridging mcans and/or the wet insulation layer may contain clectrical heating cables for fusion bonding of the bridging tneans with the wet insulation layer. [0042] The dry insulation layer is suitably a microporous matcrial such as an acrogel or a silica-based microporous matcrial. [0043] The wet insulation layer is suitably of a polymer such as polypropylene or polyurcthane and may comprise at least one polymeric layer. The wet insulation layer may also be of a syntactic foam comprising bcads encapsulatcd within a polymeric foam or matrix. [0044] A mcchanical protection layer or jackct may encapsulate the wet insulation layer. [0045] -In ordcr that the invention may be more rcadily understood, reference will now be made, by way of example, to the accompanying drawings in which: [0046] [fig.1] Figure 1 is a schematic side vicw of a subsca pipe of the invention in lon- gitudinal section, showing a dry insulation layer disposcd in an annulus defined between an inner pipe and a wet insulation layer and rnaintained by longitudinallyspaced bridge structures; [0047] [fig.2] Figure 2 corresponds to Figure 1 but shows the wet insulation layer supported by a continuous reinforcement layer; [0048] [fig.3] Figure 3 corresponds to Figure 2 but shows the reinforcement layer in- terrupted longitudinally by the bridge structures that maintain the annulus between the inner pipe and the wet insulation layer; [0049] [fig.4] Figure 4 corresponds to Figure 1 but shows reinforcement elements embedded in the wet insulation layer; [0050] [fig.5] Figure 5 corresponds to Figure 1 but shows a protective outer layer around the wet insulation layer; [0051] [fig.6] Figure 6 corresponds to Figure 1 but shows an anticorrosion and/or bonding layer between the inner pipe and the dry insulation layer; and [0052] [fig.7] Figures 7a, 7b and 7c con-espond 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. [0053] The various embodiments of the invention shown in the drawings have several features in cormnon Lice numerals will therefore be used for those like features in the description that follows. 7 [0054] Figure 1 shows a thermally-insulated subsea pipe 10 of the invention in longitudinal section. The pipe 10 shown herc 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 hem between successive pipe joints charactcriscs this method of fabrication. The inner pipe 12 could instead be of a composite matcrial, manufacturcd 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 bas becn omittcd from the drawings for simplicity. [0055] 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, `wee does not requirc the wct insulation layer 16 to be immersed in or otherwise exposed to water. Tn this context, simply requircs the wet insulation layer 16 to be suitable for immersion in or cxposure to watcr without expericncing a significant reduction of its thermal insulation capability. [0056] The wct insulation layer 16 may, for example, be of a polymer such as polypropylene or polyurethane. In this simplified view, the polymer of the wct insulation layer 16 is solid and homogcncous throughout the thickness of the wct insulation layer 16. [0057] The wct insulation layer 16 is spaced radially from the outcr surface of the inner pipe 12 to define a circumferentially-continuous annulus 20 between thern. [0058] 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 radiallyouter 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 limer pipe 12 as circumferential rings or hoops. [0059] The bridge structures 22 may also be of a polymer such as polypropylene or polyurethane, In this simplified view, the polymer is solicl 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. [0060] 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. 8 [0061] The bridge structures 22 arc also fixcd to the inncr pipe 12, for example by being bonded adhesively or fuscd to the inner pipe 12. In principlc, the bridge structures 22 could instcad, or additionally, be fixcd mechanically to the inncr pipe 12, for example by clamping. [0062] The effcct of fixing the wct insulation layer 16 to the bridge structures 22 and fixing the bridge structures 22 to the inncr pipe 12 is to transfer ail mcchanical loads, and not merely radial loads, from the wct insulation layer 16 to the inncr pipe 12. Such loads may including axial loads, such as thosc exerted by a tensioner system or by selfwcight, and bending moments such as thosc experienced during installation. [0063] In the embodiments illustratcd, the bridge structures 22 taper in a radial direction, for example in a radially-outward direction as shown. This rcduction in thcir thickncss minimises conduction of hcat through the bridge structures 22 between the inncr pipe 12 and the wct insulation layer 16. [0064] A dry insulation layer 24 is disposcd in the annulus 20 undcr and within the wct in- sulation layer 16, hence bcing intcrposed between thc wet insulation layer 16 and the inncr pipe 12. The dry insulation layer 24 matcrial may, for example, be a microporous material such as an aerogel or Izoflex (trade mark). Other options for the matcrial of the dry insulation layer 24 includc rockwool or polyurethane foam. In this context, 'dry' impl ics to the skillcd rcader that the dry insulation layer 24 would losc a substantial amount of its thermal insulation capability if immersed in watcr. [0065] The dry insulation layer 24 lies upon, and therefore is in contact with, the limer 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 ah- gap 26 prevents direct conduction of heat between the inner pipe 12 and the wet insulation layer 16 tlu-ough the dry insulation layer 24. The air gap 26 is typically narrower than 1mm 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, 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. [0067] 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 9 system or by self-weight. Reinforcement may also improvc the bending rigidity or stiffncss of the wet insulation layer 16 whcre it spans the longitudinal gap between successive bridge structures 22. [0068] -In the subsca pipe 28 of Figure 2, the wet insulation layer 16 comprises a longi- tudinally-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 continuons 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 30 is interposed between the bridge structures 22 and the remaining thickness of the wet insulation layer 16. [0069] -In addition to adding mcchanical strength to the wet insulation layer 16, the continuons reinforcement layer 30 may help to protcct 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. [0070] Figure 3 shows a subsca pipe 32 in which the reinforcement layer 30 is again offset toward, or disposed on, the radially-inner sicle of the wet insulation layer 16. In this example, the reinforcement layer 30 is discontinuous, being interruptcd by longitudinally-spaced gaps aligncd with the bridge structures 22, but is otherwise continuons between thosc gaps. Thus, the reinforcement layer 30 is no longer intcrposed 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. [0071] It will be apparent from Figure 3 that the longitudinally-inten-upted 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 rein-forcement layer 30 and the dry insulation layer 24 in this embodiment too. [0072] 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. [0073] 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. [0074] Figure 5 shows a subsca pipe 38 that has a longitudinally- and circumferentially- continuons external jacket 40 disposcd on the outside of the wet insulation layer 16. The jacket 40 may be [limaille or of polymer or composite material. The jacket 40 is substantially impervious to water to rcsist 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 providcs mcchanical protection, for example against abrasion or gouging of the wet insulation layer 16 duc to engagement of the pipe 38 with tensioners during installation. [0075] Whilst the embodiment shown in Figure 5 is otherwise the sanie as that shown in Figure 1, the jacket 40 of Figure 5 could he combincd with the features of any of the othcr embodirnents shown in the drawings. [0076] Figure 6 shows a subsca pipe 42 that has a continuons anti-corrosion and/or bonding layer 44, for example of fusion-bondcd epoxy, applied to the exterior of the inner pipe 12. Hence, the anti-corrosion/bonding layer 44 is interposcd between the dry insulation layer 24 and the body material of the inner pipe 12. Again, whilst the embodi ment shown in Figure 6 is otherwisc 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. [0077] Turning finally to Figures 7a to 7c, thcse drawings show a technique for manu- facturing a subsea pipe 46 by fusion-bonding the wet insulation layer 16 to the bridge structures 22 atter 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. [0078] Next, as shown in Figure 7b, the inner pipe 12 canying the bridge structures 22 and the dry insulation layer 24 is inserted into a continuous tube of the wet insulation material that tbrms the wet insulation layer 16. Altematively, 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. [0079] 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 11 molten polyrner of the bridge structures 22 and the wet insulation layer 16 intermingles at thcir mutual interface. When the electrical hcating elements 48 arc dcactivated by switching off the clectric current, the molten interface cools and re-sol idifies to weld the wct insulation layer 16 to the bridge structures 22. [0080] Various techniques may he used to activatc the electrical hcating elements 48, for example by connecting an clectric power supply to exposed terminais or by clectromagnetic induction using an adjacent cou. Such techniques need not be explained hem as thcy arc well understood in the art, in particular for fusion bonding of polymer liner bridges across field joints when fabricating a polymer-lined pipeline. [0081] Whilst Figures 7a to 7e show that both the bridge structures 22 and the wet insulation layer 16 have electrical heating elements 48 cmbedded into thon 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 maylic proud of the bridge structures 22 and/or the wet insulation layer 16. Thus, the elements 48 may be positioned at the interface betwecn the bridge structures 22 and the wet insulation layer 16. [0082] Again, whilst the embodiment shown in Figures 7a to 7e is otherwise the same as that shown in Figure 1, the hcating elements 48 of Figures 7a to 7e could he combined with the featurcs of any of the other embodimcnts shown in the drawings. [0083] Many other variations arc 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 tlu-oughout 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. [0084] 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 atter assembling the limer 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 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, REVENDICATIONS1. A thermally-insulatcd subsca pipe (10), comprising: - a rigid inner flowlinc pipe (12); - wct insulation (16) disposcd around the flowlinc pipe (12); - spaccd-apart bridge structures (22) that arc spaccd longitudinally along the flowlinc pipe (12) and that extend radially from the flowline pipe (12) toward the wct insulation (16) to bold the wct insulation (16) apart from thc flowlinc pipe (12), thus dcfining an annular space betwecn the flowlinc pipe (12) and thc wct insulation (16); - wherein the wet insulation (16) is bonded to the bridge structures (22); - the bridge structures (22) are made of the same material as the wet insulation (16); and - dry insulation (24) is disposed between the bridge structures (22) in the annulai- space. 2. The pipe of Cairn 1, wherein the bridge structures (22) are made of a wet insulation material. 3. The pipe of Cairn 1 or Claim 2, wherein the bridge structures (22) are made of a material that is chemically compatible with the wet insulation (16). 4. The pipe of any preceding daim, wherein the dry insulation (24) is in contact with the wet insulation (16). 5. The pipe of Cairn 4, wherein the dry insulation (24) fills the annular space substantially completely. 6. The pipe of any of Claims 1 to 3, wherein the wet insulation (16) is spaced from the dry insulation (24). 7. The pipe of Cairn 6, wherein the spacing between the wet insulation (16) and the dry insulation (24) is less than lmm. 8. The pipe of any preceding daim, wherein the wet insulation (16) is fused to the bridge structures (22). 9. The pipe of Claim 8, wherein the bridge structures (22) and/or the wet insulation (16) comprise heatable weldine elements. 10. The pipe of any preceding daim, wherein the wet insulation (16) defines an outer exposed surface of the pipe. 11. The pipe of any of Claims 1 to 9, further comprising a watertight jacket (40) around the wet insulation (16). 12. The pipe of any preceding daim, further comprising an anti-corrosion 13 and bonding layer (44) between the dry insulation (24) and the flowlinc pipe (12). 13. The pipe of any preceding claim, wherein the dry insulation (24) is scaled in a watertight enclosure within the annular space. 14. The pipe of any preceding claim, furthcr comprising a reinforcement layer (30) supporting the wet insulation (16). 15. The pipe of Claim 14, wherein the reinforcement layer (30) extends between the bridge structures (22). 16. The pipe of Claim 15, wherein the reinforcement layer (30) extends continuously beyond the bridge structures (22). 17. The pipe of Claim 15, wherein the reinforcement layer (30) is in- tcrrupted by the bridge structures (22). 18. The pipe of any of Claims 14 to 17, wherein the reinforcement layer (30) lies radially within the wet insulation (16). 19. The pipe of Claim 18, wherein the reinforcement layer (30) lies between the wet insulation (16) and the bridge structures (22). 20. The pipe of Claim 18, wherein the reinforcement layer (30) lies within the annular space. 21. The pipe of any of Claims 14 to 17, whercin the reinforcement layer (30) is cmbedded within the wet insulation (16). 22. The pipe of any of Claims 14 to 20, wherein the dry insulation (24) is in contact with the reinforcement layer (30). 23. The pipe of any of Claims 14 to 21, wherein the dry insulation (24) is spaced from the reinforcement layer (30). 24. The pipe of Claim 23, wherein the spacing between the dry insulation (24) and the reinforcement layer (30) is less than Imm. 25. The pipe of any preceding daim, wherein the wet insulation (16) is selected from, or comprises: syntactic foam; polypropylene; polyurethane; or other polymers. 26. The pipe of any preceding daim, wherein the dry insulation (24) is selected from, or comprises: a microporous material; an aerogel; Izotlex (trade mark); rockwool; or a polymer foam. 27. The pipe of any preceding daim, wherein the tlowline pipe (12) is capable, in isolation, of bearing substantially all internai pressure, extemal pressure, axial loads and bending moments to which the pipe (12) will be subjected during installation or use. 28. The pipe of any preceding daim, wherein a load path extends from the wet insulation (16) to the flowline pipe (12) via the bridge structures 14 (22). 29. The pipe of Claim 28, whcrein the dry insulation (24) is substantially decoupled from said load path. 30. A method of making a thermally-insulated subsca pipe (10), the ulethod comprising: - fixing spaccd-apart, radially-extending bridge structures (22) to a rigid inncr flowlinc pipe (12) such that the bridge structures (22) arc spaccd longitudinally along the flowlinc pipe (12); - placing dry insulation (24) around the flowlinc pipe (12) betwoen the bridge structures (22); - assembling the flowlinc pipe (12) with wet insulation (16) that surrounds the flowlinc pipe (12) and that is held apart from the flowlinc pipe (12) by the bridge structures (22) to daine an annular space between the flowlinc pipe (12) and the wet insulation (16) for encapsulating the dry insulation (24), wherein die wet insulation (16) is made of die same matcrial as the bridge structures (22); and - bonding the wet insulation (16) to the bridge structures (22). 31. The method of Claim 30, comprising placing the dry insulation (24) around the flowlinc pipe (12) beforc assembling the flowlinc pipe (12) with the wet insulation (16) to define the annular space around the dry insulation (24). 32. The method of Claim 30 or Claim 31, comprising bonding the wet in- sulation (16) to the bridge structures (22) by welding at a mutilai interface. 33. The method of Claim 32, comprising melting material of the wet in- sulation (16) and the bridge structures (22) by heating one or more heating elements (48) at or adjacent to the mutual interface.