Subsea Riser Systems

Abstract

A pliant link to mitigate fatigue-inducing motion of a subsea catenary riser has an articulated spine having a longitudinal series of interconnected rigid segments. The spine can be coupled to upper and lower sections of the riser to transmit loads along the riser through the link on a load path that extends through the segments. The link also has a pliant pipe terminating in end fittings that can be joined, respectively, to the upper and lower sections of the riser for fluid communication along the riser through the link.

Claims

1. A pliant link for a subsea riser, the link comprising: an articulated spine having a longitudinal series of interconnected rigid segments that is capable of being coupled to upper and lower sections of the riser to transmit loads along the riser through the link on a load path that extends through the segments; and a pliant pipe terminating in end fittings that are capable of being joined, respectively, to the upper and lower sections of the riser for fluid communication along the riser through the link.

2. The link of claim 1, wherein the pliant pipe is substantially decoupled from the load path.

3. The link of claim 1, wherein the segments surround the pliant pipe.

4. The link of claim 1, wherein the end fittings are engaged rigidly with respective ends of the spine, such that the load path extends at least partially through the end fittings.

5. The link of claim 4, wherein interface formations of the end fittings are adapted for mechanical and fluid coupling with the upper and lower sections of the riser.

6. The link of claim 5, wherein the interface formations comprise bevelled ends.

7. The link of claim 4, wherein anchor formations of the end fittings engage within sleeves at the ends of the spine.

8. The link of claim 1, wherein the spine is arranged to transmit compressive, tensile and torsional loads along the riser through the link.

9. The link of claim 1, wherein segments of the spine are connected to adjoining segments of the spine for relative pivotal movement between those segments.

10. The link of claim 1, wherein each segment of the spine comprises first and second parts that are connected to each other for relative pivotal movement.

11. The link of claim 10, wherein the first part of an intermediate segment is pivotably coupled to the second part of an adjoining segment and the second part of the intermediate segment is pivotably coupled to the first part of another adjoining segment.

12. The link of claim 11, wherein the first part is a ring and the second part is a pull head, the pull head comprising a body and first projections extending longitudinally from the body to support the ring for coupling the ring to the pull head of an adjoining segment.

13. The link of claim 12, wherein the pull head further comprises second projections extending from the body in a longitudinal direction opposed to the first projections for coupling the pull head to the ring of an adjoining segment.

14. The link of claim 13, wherein the second projections are offset angularly from the first projections about a longitudinal axis extending through the body.

15. The link of claim 9, wherein relative pivotal movement between adjoining segments is possible about two mutually orthogonal axes.

16. The link of claim 9, wherein relative pivotal movement between adjoining segments is limited to prevent the spine bending beyond a minimum bend radius of the pliant pipe.

17. The link of claim 1, wherein the spine is substantially fixed in length.

18. The link of claim 1, wherein the pliant pipe is no longer than the spine.

19. The link of claim 18, wherein the end fittings extend beyond the length of the spine.

20. The link of claim 1, wherein the pliant pipe is selected from: unbonded flexible pipe; bonded flexible pipe; polymer tubing; composite pipe; and coiled tubing.

21. The link of claim 1, wherein the pliant pipe is free for movement relative to the segments of the spine.

22. The link of claim 1, wherein the segments of the spine are structurally distinct from each other.

23. A subsea riser comprising the link of claim 1 situated between upper and lower sections of the riser.

24. The riser of claim 23, wherein the upper and lower sections are of rigid pipe.

25. The riser of claim 23, wherein the link is situated between the surface and the seabed.

26. The riser of claim 25, wherein the link is situated at a sag bend of the riser.

27. The riser of claim 23, wherein the pliant pipe has lower bending stiffness than the upper and lower sections of the riser.

28. A subsea installation comprising the riser of claim 23.

29. The installation of claim 28, wherein the riser hangs as a catenary from a buoyant support at an upper end of the riser.

30. The installation of claim 29, wherein the support floats at the surface.

31. A method of installing a subsea riser, the method comprising incorporating an articulated pliant link in-line while manufacturing the riser from a succession of pipe joints on an installation vessel offshore, which link comprises an articulated spine having a longitudinal series of interconnected rigid segments that surround a pliant pipe terminating in end fittings, wherein incorporating the link comprises joining the end fittings to respective pipe joints of the riser, the link thereby transmitting mechanical loads between, and effecting fluid communication between, the pipe joints to which the link is joined.

32. The method of claim 31, comprising manufacturing and partially overboarding a lower section of the riser; joining the link to a trailing end of the lower section; joining the link to a leading end of an upper section of the riser; and manufacturing and overboarding the remainder of the riser, including the link and the upper section.

33. The method of claim 31, performed in an S-lay or J-lay operation.

34. The method of claim 31, comprising bending the link around a stinger supported by the installation vessel.

35. A method of installing a subsea riser, the method comprising: incorporating an articulated in-line pliant link between pipeline sections, which link comprises an articulated spine having a longitudinal series of interconnected rigid segments that surround a pliant pipe terminating in end fittings, wherein incorporating the link comprises joining the end fittings to respective pipeline sections, the link thereby transmitting mechanical loads between, and effecting fluid communication between, the pipeline sections to which the link is joined; and spooling the riser including the link onto a reel, while bending the link around the reel.

36. The method of claim 35, comprising: manufacturing onshore a first pipeline section of the riser; spooling the first pipeline section onto a reel; manufacturing onshore a second pipeline section of the riser; joining the link to a trailing end of the first pipeline section and a leading end of the second pipeline section; and spooling the remainder of the riser comprising the link and the second pipeline section onto the reel.

37. The method of claim 35, further comprising installing the riser by a reel-lay method performed on an installation vessel offshore, in which the link is unspooled from the reel with the pipeline sections.

38. A method of installing a subsea riser, the method comprising: providing an articulated in-line pliant link that comprises an articulated spine having a longitudinal series of interconnected rigid segments that surround a pliant pipe terminating in end fittings; joining one of the end fittings to at least one pipeline section on an installation vessel offshore; and installing the riser by a reel-lay method in which at least one pipeline section joined to the link is unspooled from a reel.

39. The method of claim 38, wherein the link is joined to a spooled pipeline section onshore and is joined to at least one other pipeline section on the installation vessel offshore to transmit mechanical loads, and to effect fluid communication, between those pipeline sections.

40. The method of claim 38, wherein the link is joined to two pipeline sections on the installation vessel offshore to transmit mechanical loads, and to effect fluid communication, between those pipeline sections.

41. The method of claim 38, comprising: manufacturing onshore a first pipeline section of the riser; spooling the first pipeline section onto a reel; manufacturing onshore a second pipeline section of the riser; and spooling the second pipeline section onto a reel.

42. The method of claim 41, comprising spooling the first and second pipeline sections onto the same reel.

43. The method of claim 41, comprising: joining an end of the link to a leading end of the second pipeline section, before, after or while partially installing the first pipeline section; joining another end of the link to a trailing end of the first pipeline section; and installing the remainder of the riser comprising the link and the second pipeline section.

Description

[0069] 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:

[0070] FIG. 1 is a perspective view of an articulated link in accordance with the invention;

[0071] FIG. 2 is a perspective view of an outer articulated spine of the link shown in FIG. 1;

[0072] FIG. 3 is a perspective view of a pliant inner conduit of the link shown in FIG. 1, that fits within the spine shown in FIG. 2;

[0073] FIG. 4 is a perspective view of a segment of the spine shown in FIG. 2;

[0074] FIG. 5 is a side view of three interconnected segments shown in FIG. 4, deflected angularly relative to each other to illustrate how the spine shown in FIG. 2 can bend along the length;

[0075] FIGS. 6a to 6d are a sequence of schematic side views of an articulated link being incorporated into a catenary riser during onshore fabrication and spooling onto a reel of a reel-lay vessel in preparation for installing the riser;

[0076] FIGS. 7a to 7d are a sequence of schematic side views of an articulated link being incorporated into a catenary riser offshore aboard a reel-lay vessel, shown here in the process of installing the riser;

[0077] FIGS. 8a to 8d are a sequence of schematic side views of an articulated link being incorporated into a catenary riser offshore aboard an S-lay vessel, shown here in the process of fabricating and installing the riser;

[0078] FIG. 9 is a schematic side view that compares the catenary shape of a conventional SCR with the catenary shape of an equivalent riser incorporating the articulated link of the invention;

[0079] FIG. 10 is an enlarged schematic side view corresponding to Detail X of FIG. 9; and

[0080] FIG. 11 is a graph comparing levels of von Mises stress for conventional risers against a riser that incorporates an articulated link in accordance with the invention.

[0081] Referring firstly to FIGS. 1 to 3 of the drawings, a pliant articulated link 10 of the invention shown in FIG. 1 comprises an articulated spine 12 shown in FIG. 2 and an elongate pliant inner conduit 14 shown in FIG. 3. FIG. 1 shows a human figure beside the articulated link 10 as a non-limiting indication of its typical scale.

[0082] As will be explained below with reference to later drawings, the link 10 is designed to be incorporated into a catenary riser between, and in series with, upper and lower pipe sections of the riser. Thus, the invention can adapt a traditional SCR by the addition of the articulated link 10 at the most efficient and beneficial location along the length of the SCR, to create a gimbal joint riser or GJR. This minimises compression loads and stress spikes experienced in the riser at and around the touch-down point and improves the fatigue endurance of the riser for its service life.

[0083] The spine 12 comprises a longitudinal series of interconnected rigid vertebrae, joints or segments 16. The series of segments 16 comprises multiple body segments 16B in a row between two end segments 16E. As will be explained below with reference to FIGS. 4 and 5, the segments 16 are attached to neighbouring segment(s) 16 in the series in a manner that allows limited relative pivotal movement between neighbouring segments 16 in any direction.

[0084] The successive segments 16 of the spine 12 together form an interrupted elongate tube that surrounds and defines a central lumen. The inner conduit 14 is received telescopically and concentrically within the lumen of the spine 12 such that the conduit 14 is surrounded and protected by the segments 16, much as human vertebrae surround and protect the spinal cord.

[0085] The inner conduit 14 comprises a length of pliant pipe 18 that extends between tubular steel end fittings 20 at opposed ends of the pliant pipe 18.

[0086] The pliant pipe 18 may take various forms depending upon the application, examples being: unbonded flexible pipe; bonded flexible pipe; polymer tubing; thermo-composite pipe (TCP) or other composite pipe; and coiled tubing.

[0087] Each end fitting 20 of the pliant pipe 18 comprises an enlarged inner anchor portion 22 and a narrower outer end portion 24. When the link 10 is assembled as shown in FIG. 1, the inner anchor portions 22 are engaged within respective end segments 16 of the spine 12 whereas the outer end portions 24 protrude axially from the respective end segments 16.

[0088] By virtue of their engagement with the end segments 16, the inner anchor portions 22 of the end fittings 20 couple the pliant pipe 18 mechanically to the spine 12. Specifically, each end segment 16 of the spine 12 comprises a tubular sleeve 26 that surrounds and engages a respective inner anchor portion 22. This engagement allows axial forces, such as compressive shock loads or tension due to the weight of the riser, to be transmitted along the spine 12 rather than through the pliant pipe 18 of the inner conduit 14.

[0089] Conversely, the outer end portions 24 of the end fittings 20 protrude axially from the sleeves 26 of the end segments 16 of the spine 12. The outer end portions 24 are adapted to be joined in fluid-tight manner to respective lengths of rigid pipe of a catenary riser into which the link 10 is to be incorporated. For example, where the link 10 is to be incorporated into the riser by welding, tubular outer interface formations of the end portions 24 may have bevelled outer ends.

[0090] The end segments 16 of the spine 12 may also be adapted to interface with equipment handling devices onshore or offshore for safe lifting, orientation and insertion into a riser.

[0091] In this example, the spine 12 resists relative axial movement between neighbouring segments 16. Thus, longitudinal or axial extension or contraction of the link 10 is not possible, except to the minimal extent that may be permitted by an accumulation of tolerances between the interconnecting parts. In any event, it is preferred that any minor axial extension or contraction of the spine 12 is not such as to transfer substantial additional axial loads to the pliant pipe 18. Thus, whilst the pliant pipe 18 could be under slight axial tension or compression when assembled into the articulated link 10, the spine 12 should carry substantially all of the transient axial loads to which the articulated link 10 will be subjected in use.

[0092] The gimbal joint elements or segments 16 in the spine 12 of the link 10 of the invention are responsible for articulating with minimum stiffness and for supporting the tension of the catenary. The internal flexible element or pliant conduit 14 conducts fluid and provides pressure containment while also conferring lower localised bending stiffness on the system. In particular, the pliant conduit 14 has lower bending stiffness than the adjoining steel flowline of the riser.

[0093] Turning next to FIG. 4 of the drawings, this shows one of the body segments 16B of the spine 12 in detail. The body segment 16B comprises a pull head 28 and an inner ring 30. The inner ring 30 is pivotably supported by the pull head 28. Each end segment 16E of the spine 12 also supports an inner ring 30 that is pivotably attached to the adjoining body segment 16B.

[0094] The inner ring 30 is centred on a central longitudinal axis 32 that extends through the body segment 16B. The inner ring 30 is penetrated by four radially-extending holes 34 that are equi-angularly spaced in two pairs around the inner ring 30 at 90-degree intervals.

[0095] When it is in a central, neutral position relative to the pull head 28, the inner ring 30 lies in a plane that is orthogonal to the central longitudinal axis 32 as shown. This neutral position of the inner ring 30 corresponds to the spine 12 being straight.

[0096] The pull head 28 comprises an annular body 36 that is of substantially circular cross-section, also centred on the central longitudinal axis 32. The pull head 28 further comprises two fork formations, each comprising a pair of smoothly-radiused, diametrically-opposed projections 38 that are integral with the body 36. The fork formations extend in mutually-opposed axial directions from respective ends of the body 36.

[0097] The fork formations are offset angularly from each other by a quarter turn about the central longitudinal axis 32. In other words, one pair of projections 38 at one end of the body 36 is turned through 90 about the central longitudinal axis 32 with respect to the other pair of projections 38 at the other end of the body 36.

[0098] The projections 38 of each pair are penetrated by respective holes 40 that are in mutual alignment. The holes 40 of a first pair are aligned along a first pivot axis 42. The holes 40 of a second pair are aligned along a second pivot axis 44 that is orthogonal to, and spaced longitudinally from, the first pivot axis 42.

[0099] The inner ring 30 is pivotably connected to the pull head 28 by diametrically-opposed pivot pins 46 that are received by aligned holes 34, 40 in the inner ring 30 and in the first pair of projections 38 of the pull head 28. One pivot pin 46 is shown here in one of the first pair of projections 38; the pivot pin 46 in the opposite aligned holes 34, 40 has been omitted.

[0100] This leaves two other holes 34 in the inner ring 30 that are similarly connected to the pull head 28 of a neighbouring body segment 16B via the holes 40 of its second pair of projections 38. Again, one pivot pin 46 is shown here in one of the second pair of projections 38; the pivot pin 46 in the opposite hole 40 has been omitted.

[0101] Thus, with reference now also to FIG. 5, when the spine 12 comprising a longitudinal series of such body segments 16B has been assembled, the pull head 28 is pivotably connected by pivot pins 46 to the inner ring 30 of the preceding body segment 16B of the series. Similarly, the inner ring 30 is pivotably connected by pivot pins 46 to the pull head 28 of the next body segment 16B of the series. Thus, each pull head 28 is connected to two inner rings 30 and each inner ring 30 is connected to two pull heads 28.

[0102] Two-axis angular deflection of segments 16 relative to their neighbours allows the spine 12, and hence the articulated link 10 also comprising the pliant inner conduit 14, to bend away from a straight longitudinal axis 32 along a curved path in any direction. In this respect, each projection 38 occupies substantially less than 90 of arc in a plane containing the respective pivot axis 42, 44. This ensures clearance for relative angular movement between the segments 16 as the spine 12 bends.

[0103] The extent to which the spine 12 can bend is limited because parts of neighbouring segments 16 will eventually abut and bear against each other as shown in FIG. 5. This prevents further angular deflection of the spine 12. In this way, the spine 12 protects the pliant pipe 18 of the inner conduit 14 by ensuring that the MBR of the pliant pipe 18 cannot be exceeded. In this example, the minimum angle between successive segments 16 is 167.08, corresponding to an angular deflection from one segment 16 to the next of 12.92.

[0104] Turning next to FIGS. 6a to 6d, these schematic and much-simplified drawings show a reel-lay vessel 48 floating on the surface 50 of the sea when visiting a coastal spoolbase 52. The vessel 48 is shown here spooling a rigid pipeline onto a reel 54, subsequently to carry the pipeline offshore for laying as a catenary riser.

[0105] Other reel-lay equipment of the vessel 48 such as a pipe straightener and a lay ramp has been omitted from these drawings for clarity. A vertical-axis reel, also known as a carousel, could be used instead of the horizontal-axis reel 54 shown here.

[0106] The spoolbase 52 comprises a pipeline factory 56 at which pipe joints are assembled end-to-end into pipe stalks and the pipe stalks are assembled end-to-end into pipeline sections. The pipeline sections may be several hundred metres long.

[0107] FIG. 6a shows a first pipeline section 58 emerging from the pipeline factory 56 and being spooled onto the reel 54 as the reel 54 turns.

[0108] FIG. 6b shows the reel 54 now stationary as only a trailing end portion of the first pipeline section 58 remains to be spooled onto the reel 54. A leading end portion of a second pipeline section 60 is shown ready to be pulled from the pipeline factory 56. A link 10 in accordance with the invention is shown interposed in a gap between the pipeline sections 58, 60.

[0109] FIG. 6c shows the link 10 now joined to the trailing end of the first pipeline section 58 and to a leading end of the second pipeline section 60. Typically, joining will be effected by forming circumferential butt welds between the pipeline ends and bevelled interface formations of the end portions 24 of the link 10.

[0110] FIG. 6d shows the reel 54 now turning again as the link 10 has been pulled onto the reel 54 between the pipeline sections 58, 60 that it joins. The ability of the spine 12 to flex along its length enables the link 10 to be spooled onto the reel 54, while the spine 12 protects the pliant pipe 18 of the inner conduit 14 against damage or exceeding its MBR during spooling.

[0111] When spooling of the second pipeline section 60 has been completed, the vessel 48 can depart for an installation site at which the riser will be laid using well-known reel-lay techniques. During installation, the second section 60 will be unspooled and overboarded first, as a lower section of the riser to lie on the seabed. The second section 60 is followed by the link 10 and finally by the first section 58. The first section 58 will then serve as an upper section of the riser extending from the link 10 to a supporting floating surface vessel or installation, such as an FPSO or a platform.

[0112] As will be explained later with reference to FIGS. 9 and 10 of the drawings, the link 10 is suspended above the seabed as part of the free span of the riser that extends between the seabed and the surface. This decouples motion of the supporting FPSO or platform from the touch-down point where the riser meets the seabed. Thus, the link 10 is at a level above the seabed and hence at a level above the touch-down point.

[0113] Preferably, the link 10 is at the sag bend of the riser, which is typically a few tens of metres above the seabed.

[0114] FIGS. 7a to 7d show that the link 10 need not be joined to both of the pipeline sections 58, 60 at a coastal location such as a spoolbase. Instead, the link 10 could be joined to one or both of the pipeline sections 58, 60 aboard a reel-lay vessel 48 offshore. The link 10 may previously have been joined to one of the pipeline sections 58, 60 or to neither of the pipeline sections 58, 60.

[0115] Like numerals are used for like features in FIGS. 7a to 7d, which now shows basic reel-lay equipment on the vessel 48. That equipment comprises: a pipe straightener 62 for straightening the pipeline sections 58, 60 as they are unspooled from the reel 54; a lay ramp 64 whose inclination matches that of a launch axis for the pipeline sections 58, 60; and a tensioning system 66 for applying hold-back tension to the length of the pipeline sections 58, 60 suspended from the vessel 48. The tensioning system 66 may comprise a combination of clamps and/or tensioners acting on the pipeline sections 58, 60, as will be well understood by those skilled in the art.

[0116] FIG. 7a shows a lower pipeline section 60 of the riser being unspooled, straightened and overboarded first after passing along the lay ramp 64 and through the tensioning system 66.

[0117] FIG. 7b shows the lower pipeline section 60 of the riser now fully unspooled and almost entirely overboarded. An upper end of the lower pipeline section 60 is supported by the tensioning system 66. The upper pipeline section 58 of the riser is also shown being unspooled from the reel 54 and fed into the pipe straightener 62. Optionally, however, the upper pipeline section 58 could be unspooled from a different reel.

[0118] In FIG. 7c, the link 10 has been lifted onto the lay ramp 64 and joined to the facing ends of the upper and lower pipeline sections 58, 60. The techniques used to lift and join the link 10 are routinely used for inserting in-line accessories into a reel-laid pipeline. Being well understood by those skilled in the art, those techniques need no elaboration here.

[0119] FIG. 7d shows the lower pipeline section 60 of the riser and the articulated link 10 now launched fully beneath the surface 50 of the sea as unspooling and straightening of the upper pipeline section 58 continues. The tensioning system 66 has released the pipeline sections 58, 60 to allow the link 10 to pass along the launch axis.

[0120] When the riser has been installed fully, a major portion of the lower pipeline section 60 will lie on the seabed. A minor portion of the lower pipeline section 60 adjoining the link 10 will be suspended above the seabed where it extends beyond the touch-down point. The upper pipeline section 58 will extend through the water column from the link 10 to a supporting floating surface vessel or installation, such as an FPSO or a platform.

[0121] Moving on now to FIGS. 8a to 8d, these drawings show how the link 10 may be incorporated into a pipe string 68 as the pipe string 68 is being fabricated aboard a pipelaying vessel 70 offshore. In this example, the pipelaying vessel 70 is configured for S-lay operations although it will be clear to those skilled in the art that the same principle can also be applied to J-lay operations.

[0122] Again, the pipelaying vessel 70 shown schematically in these drawings is not to scale and is much-simplified. In particular, the pipelaying vessel 70 has only basic equipment on its working deck, comprising a welding station 72 and a tensioning system 74 downstream of the welding station 72. Downstream of the tensioning system 74, the pipeline 68 is launched over a stinger 76, curving through an overbend, to hang in the water beneath the stinger 76 as a catenary.

[0123] In practice, there will be a succession of welding stations 72, testing stations and coating stations on the working deck upstream of the tensioning system 74. The tensioning system 74 will typically comprise multiple tensioners.

[0124] At the welding station 72, standard-length pipe joints 78 as shown in FIG. 8a are welded to an upper end of an already-fabricated pipe string 68, to which the tensioning system 74 applies hold-back tension that supports the weight of the suspended catenary.

[0125] FIG. 8b shows a link 10 being inserted into the pipe string 68 in place of a pipe joint 78. Conveniently, the link 10 may be of a similar length to, or at least no longer than, a pipe joint 78 or a multiple thereof so as not to disrupt the stepwise fabrication and processing steps that are performed aboard the vessel 70. Again, the techniques used to join the link 10 to the pipe string 68 are routinely used for inserting in-line accessories such as in-line tees (ILTs) into a pipeline during S-lay or J-lay operations. Being well understood by those skilled in the art, those techniques need no elaboration here.

[0126] FIG. 8c now shows the link 10 having been welded at both ends to pipe joints 78 of the pipe string 68 and advancing past the welding station 72 toward the tensioning system 74. Another pipe joint 78 is shown being readied for welding to the upper end of the growing pipe string 68 that now incorporates the link 10.

[0127] As further pipe joints 78 are welded to the upper end of the pipe string 68, the link 10 advances through the tensioner system 74 and over the stinger 76 to be launched beneath the surface 50 of the sea as shown in FIG. 8d. Advantageously, the elongate, narrow dimensions of the link 10 facilitate its passage through the tensioning system 74 and along the stinger 76. Its flexibility also allows the link 10 to follow the overbend curvature as the pipe string 68 advances along and beyond the stinger 76, while protecting the pliant conduit 14 within the articulated spine 12.

[0128] The pipe string 68 comprises a portion downstream of the link 10 and a portion upstream of the link 10, having regard to the launch direction. The downstream portion of the pipe string 68 constitutes the lower section of the riser, a major portion of which will lie on the seabed when the riser has been installed fully. Again, a minor portion of the lower section adjoining the link 10 will be suspended above the seabed where it extends beyond the touch-down point. The upstream portion of the pipe string 68 constitutes the upper section of the riser, which again extends through the water column from the link 10 to a supporting surface vessel or other floating installation, such as an FPSO or a platform.

[0129] As noted above, FIGS. 9 and 10 show the link 10 in a static configuration, suspended above the seabed as part of the free span of a rigid steel catenary riser 80 that hangs from a supporting FPSO 82 at the surface 50 and extends from there to the seabed 84. The link 10 is shown here at the sag bend of the riser 80, a few tens of metres above the seabed 84, and hence at a level above the touch-down point 86 where the riser 80 meets the seabed 84.

[0130] By way of comparison, FIGS. 9 and 10 also show the curvature of a corresponding conventional SCR 88 in dashed lines. It will be apparent that the weight of the link 10 changes the shape of the riser 80 in the sag bend region, making the sag bend curvature more acute than in a conventional SCR 86. Like a known weight-distributed riser, the weight of the link 10 contributes significantly to the dynamic behaviour of the riser 80. However, the articulation of the spine 12 accommodates the concentrated weight of the link 10 to avoid over-stressing the riser 80 around the mass concentration point.

[0131] Turning finally to FIG. 11, the graph shown here plots von Mises stress against the arc length of a riser for various types of equivalent risers in a simulated dynamic marine environment. A limiting von Mises stress level of approximately 360000 kPa is shown as a horizontal line.

[0132] It will be apparent that in the case of a conventional SCR, the limiting stress is exceeded at an arc length of between 2700 and 2900 metres, which corresponds to the sag bend region approaching the touch-down point in this example.

[0133] Before the present invention, the standard approach when designing a riser system for use in this dynamic environment would be to opt for a more complex system than a conventional SCR, such as a steel lazy-wave riser (SLWR), to keep the von Mises stress below the limiting level as shown. However, such a solution is significantly more expensive than a conventional SCR.

[0134] The invention, modelled here as a gimbal joint riser or GJR, keeps the von Mises stress well below the limiting level, to a similar or better extent than the SLWR, yet at very much lower cost than an SLWR.