Subsea pipe-in-pipe structures

Abstract

A pipe-in-pipe assembly with thermally-insulating spacers positioned in an annulus to act radially between inner and outer pipes is disclosed. The spacers have at least one circumferentially-extending array of circumferentially-spaced ribs that define longitudinally-extending passageways in gaps between neighbouring ribs of the array. Cables including heating elements extend longitudinally along, the annulus outside the inner pipe. The cables extend longitudinally along the passageways. At least one insulation layer disposed radially outboard of the cables has insulating elements disposed in the gaps between the ribs and/or an insulating layer extending around the inner pipe, positioned radially outboard of the ribs and bridging the gaps. Bands encircle and retain components of the insulation layer. Insulation may also be disposed on the inner pipe between first and second arrays of ribs, those arrays being spaced longitudinally from each other.

Claims

1. A pipe-in-pipe assembly comprising: inner and outer pipes in spaced concentric relation to define a thermally-isolating annulus between them, the inner pipe being of steel; a plurality of cables extending longitudinally along the annulus outside the inner pipe; and thermally-insulating spacers positioned in the annulus to act radially between the pipes; wherein: the spacers comprise first and second circumferentially-extending arrays of circumferentially-spaced ribs that define longitudinally-extending passageways in gaps between neighbouring ribs of the arrays, those arrays being spaced longitudinally from each other; the cables extend longitudinally along the passageways; at least one cable is a heating element positioned for direct contact with the inner pipe in a passageway; the radial thickness of the ribs is greater than the thickness of any of the cables; and at least one insulation layer is disposed radially outboard of the cables.

2. The assembly of claim 1, wherein the insulating layer comprises insulating elements disposed in the gaps between the ribs.

3. The assembly of claim 1, wherein the insulating layer comprises at least one layer of insulation extending around the inner pipe, positioned radially outboard of the ribs and bridging the gaps.

4. The assembly of claim 3, further comprising at least one band or sleeve encircling the or each layer of insulation and retaining the or each layer of insulation on the ribs.

5. The assembly of claim 1 and comprising at least one layer of insulation extending around the inner pipe, positioned longitudinally between the arrays and radially outboard of the cables.

6. The assembly of claim 5, further comprising at least one band or sleeve encircling the or each layer of insulation between the arrays to clamp the or each layer of insulation and the cables against the inner pipe.

7. The assembly of claim 1, wherein the cables are free for longitudinal or angular movement with respect to the pipe in the gaps between the ribs.

8. The assembly of claim 1, wherein the ribs of each array are joined by a circumferentially-extending band or sleeve.

9. The assembly of claim 8, wherein the band or sleeve joining the ribs is flexible.

10. The assembly of claim 8, wherein the band or sleeve joining the ribs is substantially rigid.

11. The assembly of claim 8, wherein the band or sleeve joining the ribs is separate from the ribs.

12. The assembly of claim 1, wherein the cables extend substantially parallel to each other.

13. The assembly of claim 1, wherein the cables are substantially angularly offset from a central longitudinal axis of the inner pipe.

14. The assembly of claim 13, wherein the passageways defined between the ribs are substantially angularly offset from the central longitudinal axis of the inner pipe.

15. A method of assembling a pipe-in-pipe structure, the method comprising: positioning first and second circumferentially-extending arrays of thermally-insulating spacers at longitudinally spaced locations around a steel inner pipe, the arrays comprising circumferentially-spaced ribs defining longitudinally-extending passageways in gaps between neighbouring ribs of the arrays; positioning a plurality of cables on the outside of the inner pipe such that the cables extend longitudinally along the passageways, wherein the radial thickness of the ribs is greater than the thickness of any of the cables and wherein at least one cable is a heating element positioned for direct contact with the inner pipe in a passageway; and positioning at least one insulation layer radially outboard of the cables.

16. The method of claim 15, wherein the arrays of spacers are positioned before positioning the plurality of cables.

17. The method of claim 15, comprising positioning insulating elements in the gaps between the ribs.

18. The method of claim 15, comprising positioning the arrays of spacers around the inner pipe with the spaced ribs joined by a transversely-extending band or a sleeve.

Description

(1) Reference has already been made to FIG. 1 of the appended drawings, which is a cut-away perspective view of a length of electrically trace-heated PiP pipeline known in the prior art. In order that the invention may be more readily understood, reference will now be made, by way of example, to the remaining drawings in which:

(2) FIG. 2 is a schematic end view of an array of spacer ribs in accordance with the invention, to which a tape is attached;

(3) FIG. 3 is a schematic perspective view of the array shown in FIG. 2, but inverted;

(4) FIGS. 4 to 6 are schematic end views of segmented spacer rings incorporating spacer ribs in accordance with the invention;

(5) FIG. 7 is a perspective view of a segment of a spacer ring of the invention;

(6) FIGS. 8 to 13 are schematic sectional views of the array of spacer ribs shown in FIGS. 2 and 3, in situ within a PiP assembly, showing different arrangements of the array and of insulation within the assembly; and

(7) FIGS. 14 and 15 are schematic views of a PiP assembly of the invention in longitudinal section, with insulation partially cut away.

(8) Where appropriate, like numerals are used for like parts in FIGS. 2 to 15. With the exception of FIG. 7, these drawings are schematic, simplified views that show only a few heating elements 10, widely spaced for clarity. In practice, there will be more heating elements 10, relatively closely spaced, as FIGS. 1 and 6 will make clear.

(9) FIGS. 2 and 3 show a spacer array 24 before installation into a PiP assembly, whereupon the spacer array 24 is wrapped around the inner pipe 12 of the assembly.

(10) The spacer array 24 comprises parallel elongate ribs 26 that are spaced apart from neighbouring ribs 26 to define gaps 28 between them. Each rib 26 has opposed faces 30, 32 and is moulded of a thermally- and electrically-insulating plastics material such as polyurethane or polyamide. The faces 30, 32 of the ribs 26 lie in parallel planes, each plane being shared by corresponding faces 30, 32 of the ribs 26. When the spacer array 24 is wrapped around the inner pipe 12 as part of the PiP assembly, the opposed faces 30, 32 of the ribs 26 bear against the outside of the inner pipe 12 and face toward the inside of the outer pipe 18.

(11) The ribs 26 are spaced along a flexible band or tape 34 that is attached to the faces 30, 32 on one side of the ribs 26. The tape 34 extends transversely, preferably orthogonally, with respect to the alignment of the ribs 26. Thus, when the spacer array 24 is wrapped around the inner pipe 12 with the tape 34 on a plane orthogonal to the central longitudinal axis of the inner pipe 12, the ribs 26 are aligned in parallel with respect to that axis. If the ribs 26 are instead to be angled with respect to the central longitudinal axis of the inner pipe 12 to suit a helical arrangement of heating elements 10, the ribs 26 may instead lie in diagonal relation to the tape 34 as shown in FIG. 15.

(12) The tape 34 may be integral with the ribs 26 or may be attached to the ribs 26 by bonding with adhesives or by welding or fusing. Fastenings 36 at the ends of the tape 34 may be engaged to each other when the tape 34 encircles the inner pipe 12 of a PiP assembly, for example by adhesion of overlapping ends.

(13) It will be noted that the spacer array 24 is shown inverted in FIG. 3 with the tape 34 on top of the ribs 26 rather than underneath the ribs 26 as shown in FIG. 2. In this respect, the tape 34 may be wrapped around the inner pipe 12 of a PIP assembly either on the radially outer side of the ribs 26 or on the radially inner side of the ribs 26. The lengths of the tape 34 between the ribs 26 may be varied to suit. As will be explained later, the former possibility is shown in FIGS. 8 to 10 whereas the latter possibility is shown in FIGS. 11 to 13.

(14) FIGS. 4 to 7 show the ribs 26 supported by pre-shaped curved webs 38 of self-supporting rigidity to form semi-circular segments 40. The segments 40, which are preferably identical, can be assembled as shown in FIGS. 4 to 6 to form a complete circular ring. Again, the ribs 26 may be integral with the webs 38, for example as a one-piece plastics moulding, or may be attached to the webs 38, which may for example comprise curved strips of metal. Whilst essentially rigid, the webs 38 suitably have elastic resilience so as to bow slightly outwardly during assembly to hold the segments 40 on the inner pipe 12.

(15) FIG. 4 shows the webs 38 disposed on the radially inner side of the ribs 26. FIGS. 5 and 7 show the webs 38 disposed on the radially outer side of the ribs 26. FIG. 6 shows the webs at a radially intermediate position between the inner and outer faces 30, 32 of the ribs 26.

(16) FIG. 7 shows a semi-circular segment 42 of a spacer array 24, intended to be assembled with a similar segment 42 around the inner pipe 12 to form a full-circumference spacer array 24. It will be apparent here that an inner side of the segment 42 has a circumferential array of channels 44 extending longitudinally through the segment 42 between neighbouring ribs 26. In this example, the channels 44 are oriented to lie parallel to the central longitudinal axis of the inner pipe 12 when the spacer array 24 is assembled around the inner pipe 12. Again, however, the channels 44 could instead be angled to suit a helical arrangement of heating elements 10 if desired.

(17) In this example, each channel 44 has an arched cross-section. The channels 44 are shaped and dimensioned to accommodate typical electric heating elements 10 used in electrically trace-heated PiP systems, preferably without the heating elements 10 touching the sides of the channels 44. The channels 44 may also accommodate some insulation although this is not essential.

(18) Turning now to FIGS. 8 to 13, these show spacer arrays 24 as shown in FIGS. 2 and 3 in situ within a PiP assembly 46. FIGS. 8 to 10 show the tape 34 wrapped around the inner pipe 12 of the PiP assembly on the radially outer side of the ribs 26. This has the advantage that the tape 34 helps to retain the heating elements 10 and any insulating elements in the gaps 28 between the ribs 26. Conversely, FIGS. 11 to 13 show the tape 34 wrapped around the inner pipe 12 of the PiP assembly on the radially inner side of the ribs 26. This has the advantage that the heating elements 10 may more easily be laid in the gaps 28 between the ribs 26 after the spacer array 24 has been wrapped around the inner pipe 12.

(19) FIGS. 8 to 13 have features in common, notably that the electric heating elements 10 are disposed equi-angularly around the inner pipe 12 of the PiP assembly 46. Thus, the heating elements 10 lie within the annulus 16 defined between the inner pipe 12 and the outer pipe 18 of the PiP assembly 46. Air may be evacuated from the annulus 16. Other cables such as fibre-optic data cables may be positioned beside and between the heating elements 10 but have been omitted for clarity.

(20) Each heating element 10 lies within a respective longitudinal passageway defined by a gap 28 between neighbouring ribs 26 of the array. The thickness of the heating elements 10 is less than the radial thickness of the ribs 26, so that the ribs 26 protect the heating elements 10 from crushing and pinching if the annulus 16 narrows as the PiP assembly 46 bends during installation.

(21) FIGS. 8 to 10 show the heating elements 10 in direct contact with the inner pipe 12. Conversely, FIGS. 11 to 13 show the heating elements 10 separated from the inner pipe 12 by the tape 34, which should therefore be of a heat-resistant material. The tape 34 is thin enough to conduct heat effectively from the heating elements 10 to the inner pipe 12. In any event, the tape 34 is narrow enough that it only separates the heating elements 10 from the inner pipe 12 over an insignificantly short longitudinal distance. Thus, the presence of the tape 34 does not materially affect the efficient transfer of heat from the heating elements 10 to the inner pipe 12.

(22) In each of FIGS. 8 to 13, the annulus 16 also contains insulation that overlays the heating elements 10 at least and that may also overlay the ribs 26. Specifically, in FIG. 8 and its counterpart FIG. 11, an insulating element 48 lies in each of the gaps 28 between neighbouring ribs 26. Each insulating element 48 lies on the radially outer side of the heating element 10 that is also housed in that gap 28, and substantially fills the full circumferential width of the gap 28. On their radially inner sides, the insulating elements 48 have concave curvature in cross section to conform with the convex shape of the underlying heating elements 10.

(23) In FIG. 8, the insulating elements 48 are held in the gaps 28 by the surrounding tape 34. In FIG. 11, the insulating elements 48 are held in the gaps 28 by an additional tape 50 that wraps around the radially outer side of the ribs 26 and spans the gaps 28 between the ribs 26, covering the insulating elements 48 and the heating elements 10 that lie in the gaps 28.

(24) In FIG. 9 and its counterpart FIG. 12, insulating elements 48 are omitted from the gaps 28. Instead, insulation is provided by a wrap or layer of insulation 52 such as an insulating blanket. The layer of insulation 52 extends circumferentially around the radially outer side of the ribs 26 and spans the gaps 28 between the ribs 26, covering and retaining the heating elements 10 that lie in the gaps 28. The layer of insulation 52 may be retained by encircling tapes or bands 54 as shown in FIGS. 14 and 15. There may be more then one layer of insulation.

(25) FIG. 10 and its counterpart FIG. 13 combine the insulating systems of FIGS. 8, 9, 11 and 12. Thus, insulating elements 48 are present in the gaps 28 and additional insulation is provided by a circumferentially-extending layer of insulation 52 around the radially outer side of the ribs 26. The layer of insulation 52 spans the gaps 28 between the ribs 26 to cover and retain the insulating elements 48 and the heating elements 10 that lie in the gaps 28. Again, the layer of insulation 52 may be retained by encircling tapes or bands 54 as shown in FIGS. 14 and 15.

(26) Once assembled in this way, the cylindrical assembly of the inner pipe 12 and the surrounding spacer array 24, heating elements 10, insulating elements 48 and/or layers of insulation 52 may be inserted telescopically into an outer pipe 18. To enable sliding movement, a small clearance, preferably of less than 10 mm, is left between the cylindrical assembly and the inside of the outer pipe 18. A low-friction sliding material such as nylon or PTFE may be wrapped around the cylindrical assembly or applied to its radially outer extremities, notably the radially outer faces of the ribs 26.

(27) Turning finally to FIGS. 14 and 15, these show PiP assemblies of the invention in longitudinal section, with insulation partially cut away for clarity. They differ in that the PiP assembly 56 of FIG. 14 shows the heating elements 10 extending substantially parallel to each other and to a central longitudinal axis 58 of the PiP assembly whereas the PiP assembly 60 of FIG. 15 shows the heating elements 10 angularly offset at an angle of substantially greater than 10 relative to the central longitudinal axis 58. Specifically, FIG. 15 shows a helical arrangement in which parallel heating elements 10 twist around the inner pipe 12 as they extend along its length. Preferably, the ribs 26 lie at the same angle with respect to the central longitudinal axis 58 as shown in FIG. 15. The resulting diagonal relationship between the ribs 26 and the tape 34 of the spacer arrays 24 is evident in this view.

(28) The arrangements shown in FIGS. 14 and 15 are akin to those shown in FIG. 10, with tape 34 on the radially outer side of the ribs 26, insulating elements 48 in the gaps 28 between the ribs 26 and a further layer of insulation 52 wrapped around the ribs 26, the heating elements 10 and the insulating elements 48. In both cases, the further layer of insulation 52 is shown retained by an encircling tape or band 54, which is one of several such tapes or bands 54 that will be spaced along the length of the assembly 56, 60.

(29) FIGS. 14 and 15 show that there may be more than one circumferentially-extending spacer array 24, each comprising rings of circumferentially-spaced ribs 26 encircling the inner pipe 12. Those spacer arrays 24 are shown here spaced longitudinally or axially along the inner pipe 12, with an exemplary distance between them of more than 1.50 m.

(30) The heating elements 10 may be exposed in the longitudinal gap between the spacer arrays 24 but they are preferably insulated by a further annular layer of insulation 62 that encircles the inner pipe 12 to surround the heating elements 10 in that gap. Again, this layer of insulation 62 is retained by an encircling tape or band 64. Advantageously, tightening the encircling tape or band 64 conforms the layer or insulation 62 to the heating elements 10 and clamps the heating elements 10 against the inner pipe 12. This ensures good thermal contact between the heating elements 10 and the inner pipe 12.

(31) An advantage of the helical arrangement of the heating elements 10 in FIG. 15 is that whilst the heating elements 10 are clamped to the inner pipe 12 between the spacer arrays 24, the heating elements 10 are still able to slide relative to the inner pipe 12 elsewhere, for example between the ribs 26 of the spacer arrays 24. At those locations, axial elongation or contraction of the heating elements 10 can be accommodated simply by allowing the angle of the heating elements 10 to change slightly relative to the central longitudinal axis 58.

(32) It will be apparent that by virtue of the longitudinal passageways defined by the gaps 28 between the ribs 26, the PiP assembly of the invention allows heating elements 10 in the annulus 16 to extend continuously along a pipeline through the spacer arrays 24.

(33) In particular, there is no need for additional electrical connections or to interrupt thermal management. The use of insulating elements 48 or layers of insulation 52, 62 between or around the spacer arrays 24 or ribs 26 also enables effective insulation to extend continuously along the pipeline across the spacer arrays 24 and all around the inner pipe 12.

(34) In addition to the variations described above, other variations are possible within the inventive concept. For example, webs, tapes or strips of fabrics may be provided between ribs 26 or between insulating elements 48. Also, as an example of a method of installation, a tubular heat-shrinkable sleeve or web could be slid either with ribs 26 or over pre-installed ribs 26 around the pre-installed heating elements 10, whereupon the web can be heated to shrink it around the ribs 26 and the heating elements 10. Insulation may be disposed radially inside and/or radially outside the web.

(35) Whilst the currently preferred embodiment corresponds to FIGS. 9 and 15 of the drawings, any combination of the design of the ribs, the support band technology and containment band can be considered.