Hardware and system for supporting subsea flexible lines such as cables, umbilicals and risers

Abstract

A jacket assembly 100 is provided for attachment to a subsea flexible line for facilitating passive control of movement of the flexible line. The jacket assembly comprises a plurality of jackets (FIG

Description

Technical field The disclosure relates to subsea flexible lines, including cables, umbilicals and risers, used to provide connectivity to or from a floating structure that moves responsive to sea behaviour. Background Off-shore installations may comprise a floating structure that is attached to the seabed using a mooring system that allows vertical and lateral movement of the floating structure responsive to sea behaviour. In addition to a mooring system, off-shore installations may also require connectivity either between the off-shore installation and the mainland or between a plurality of off-shore installations. Such connectivity may be required to transport, for example, electrical power, communications, hydraulics and / or fluid materials such as oil or gas. One or more flexible lines may be used to facilitate such connectivity. In this disclosure, the term flexible line is used to refer to a flexible line that provide connectivity such as a cable, an umbilical and a riser, whereas the term mooring system is used to describe the apparatus by which movement of a floating off-shore installation is limited. In some arrangements, a flexible line may run from one floating structure to another. In other arrangements, a flexible line may run from one floating structure to the sea bed. In either case, the flexible line needs to accommodate movement of the floating structure within the scope permitted by the mooring system. Where an average vertical distance between the floating structure and the seabed is of the order of 50 to 100 metres, the degree of movement of the floating structure allowed within the restrictions of the mooring system may be proportionally large relative to the average distance between the floating structure and the seabed. Accordingly, it may be that flexible line assemblies need to accommodate a variation in straight line distance between the two ends of the flexible line that equates, in use, to perhaps an increase of 20 %, 30 %, 40 % or maybe 50 % of the average water depth. In one known arrangement, shown in Figure 1 (which employs a mooring system that is not shown in the Figure), a flexible line 10 is allowed to hang freely between a first end that is fixed to the floating structure 20 and a second end that is anchored to the seabed using a seabed anchor 30. This may be known as a Free Hanging Catenary arrangement. One disadvantage of this arrangement is that movement of the floating structure 20 is accommodated by a greater or lesser extent of flexible line 10 making contact with the seabed. This risks damage to the flexible line 10. It is desirable to provide a flexible line assembly that facilitates potential movement of one or both ends of the flexible line 10 whilst avoiding scenarios whereby the flexible line 10 may be at risk of dragging along the seabed and / or protruding above the waterline, both of which are likely to cause damage to the flexible line 10 and reduce its useable life. In order to provide a flexible line system that enables sufficient degree of flexibility whilst at the same time avoiding scenarios whereby the flexible line 10 is at risk of dragging along the seabed and / or protruding above the waterline, it is known to provide buoyancy devices 40 distributed along a length of the flexible line such that the flexible line 10 takes an indirect route that can become more or less direct dependent upon sea conditions. In this way, the flexible line 10 acquires a spring-like quality, such that the flexible line 10 has a degree of resilient flexibility to accommodate movement of one or both ends of the flexible line 10. One known approach may be called a Lazy Wave, as shown in Figure 2 (again, a mooring system is employed but not shown in the Figure). The flexible line 10 extends between a floating structure 20 and a seabed anchor 30, whereby it is anchored horizontally relative to the seabed. A plurality of buoyancy devices 40 is distributed along a length of the flexible line in region A, each buoyancy device 40 being fastened to the flexible line 10 in a manner that prevents movement relative to the flexible line 10. The buoyancy devices 40 are distributed so as to facilitate what is known in the art as a hog bend in the flexible line 10, wherein the hog bend has a large radius that exceeds a minimum flexible line radius that the flexible line is designed to withstand. Then, in region B, no buoyancy devices are provided, such that the flexible line 10 droops under its own weight in region B, so as to form what is known in the art as a sag bend. In this way, with the hog bend and the sag bend, the flexible line 10 takes on a doublecurved form that facilitates stretching in order to accommodate movement of the floating structure 20, both vertically and horizontally, towards or away from the seabed anchor 30. The reason for requiring a plurality of buoyancy devices 40 in region A (rather than only a single buoyancy device) is that it enables the radius of curvature of the flexible line 10 to be relatively large, which reduces the risk of the flexible line 10 bending to a radius smaller than that which it is designed to accommodate. Distribution of the plurality of buoyancy devices 40 within the region A may be non-uniform. In particular, the distribution of buoyancy devices at the top of the curved region A may be closer than at the edges of the curved region A. This contributes to the provision of a gentle curvature, which again contributes to reduction in the risk of the flexible line 10 bending to a radius smaller than that which it is designed to accommodate. Use of a plurality of buoyancy devices 40 is costly, in terms of hardware cost, manufacturing cost, installation cost and maintenance cost. Another known approach may be called a Steep Wave, as shown in Figure 3 (again, a mooring system is employed but not shown in the Figure). This is similar to the Lazy Wave shown in Figure 2, except that end of the flexible line 10 anchored to the seabed is anchored in a vertical orientation rather than in a horizontal orientation. A further known approach may be called the Lazy-S, which is shown in Figure 4 (again, a mooring system is employed but not shown in the Figure). This is similar to the Lazy Wave shown in Figure 2, except that instead of a plurality of buoyancy devices 40 distributed along the flexible line and fastened thereto, there is a single buoyancy device 50 that is connected to the seabed using a buoyancy restraint 60. The flexible line 10 is laid over the buoyancy device 50 without being attached to it. The buoyancy device 50 has a relatively large diameter, sufficiently large to prevent curvature of the flexible line 10 beyond that which it is designed to accommodate. This arrangement avoids the need for a plurality of buoyancy devices 40. However, the size of the buoyancy device 50 (particularly so as to prevent curvature of the flexible line 10 beyond that which it is designed to accommodate) means that the single buoyancy device 50 is costly, in terms of hardware cost, manufacturing cost, installation cost and maintenance cost. Furthermore, given the need for tethering of the buoyancy device 50 to the seabed requires substantial seabed preparation such as anchor piles or gravity bases, which are also costly. A further known approach may be called the Steep-S, which is shown in Figure 5 (again, a mooring system is employed but not shown in the Figure). This is similar to the steep wave shown in Figure 2, except that instead of a plurality of buoyancy devices 40 distributed along the flexible line 10 and fastened thereto, there is a single buoyancy device 50 that is connected to the seabed using a buoyance device in the same manner as for the Lazy-S of Figure 4. The flexible line 10 is laid over the buoyancy device 50. The buoyancy device 50 has a relatively large diameter, sufficiently large to prevent curvature of the flexible line 10 beyond that which it is designed to accommodate. A still further known approach is disclosed in US 4,906,137. This approach may be known as a Pliant Wave and is shown in Figure 6 (again, a mooring system is employed but not shown in the Figure). Commonly owned patent publication GB2582601 discloses the use of a series of buoyancy jackets along a length of cable, as shown in Figure 7 (alongside an equivalent Lazy Wave arrangement per Figure 2). Each buoyancy jacket comprises a first bend protection element at a first end of the buoyancy jacket and a second bend protection element at a second end of the buoyancy jacket that is opposite the first end. The first and second bend protection elements are configured to distribute flexible line curvature through the bend protection elements whilst facilitating multidirectional freedom to accommodate movement of the subsea flexible line. All of these prior art arrangements have in common a desire to facilitate a broad range of movement of the floating structure 20 whilst minimising risk of damage to the flexible line 10. The approach to avoiding flexible line damage includes: preventing the flexible line 10 from making contact with the seabed (other than at its anchor point); preventing the flexible line 10 from protruding above the waterline; and preventing the flexible line 10 from encountering situations where the flexible line 10 will bend to a radius that is smaller than that which it is designed to accommodate. Summary of the disclosure Against this background, there is provided a jacket for attachment to a subsea flexible line for facilitating passive control of movement of the flexible line in accordance with claim 1. Accordingly, the jacket assembly is providable in a modular form. This means that the jacket assembly can be configured differently for different circumstances. More than two options may be available fluid displacing elements, having more than two fluid displacement values. In this way, the first and subset of fluid displacing elements may be selected from a larger menu of options and appropriately chosen to suit the circumstances of the installation. Furthermore, the jacket assembly can be formed at any appropriate length by adding further components. By contrast with buoyancy arrangements with non-modular forms, each component is smaller and more easily manufacturable. Furthermore, controllability of manufacture of the components is more straightforward and consistency is higher. The components of the jacket assembly that are configured to surround the subsea flexible line may be provided as split components. This means that the jacket assembly may be assembled around the flexible line rather than needing to be inserted from one end. In a second aspect of the disclosure there is provided a jacket assembly kit in accordance with claim 13. In this way, the kit may be configured differently for different applications. In a third aspect of the disclosure there is provided a method of facilitating passive control of movement of a subsea flexible line in accordance with claim 18. Brief description of the drawings Specific embodiments of the invention, in the context of various prior art arrangements, will now be described with reference to the following drawings in which: Figure 1 shows a prior art flexible line arrangement between the seabed and a floating structure, the arrangement referred to as a Free Hanging Catenary; Figure 2 shows a prior art flexible line arrangement between the seabed and a floating structure, the arrangement referred to as a Lazy Wave; Figure 3 shows a prior art flexible line arrangement between the seabed and a floating structure, the arrangement referred to as a Steep Wave; Figure 4 shows a prior art flexible line arrangement between the seabed and a floating structure, the arrangement referred to as a Lazy-S; Figure 5 shows a prior art flexible line arrangement between the seabed and a floating structure, the arrangement referred to as a Steep-S; Figure 6 shows a prior art flexible line arrangement between the seabed and a floating structure, the arrangement referred to as a Pliant Wave; Figure 7 shows a prior art subsea flexible line arrangement comprising three buoyancy jackets distributed at intervals along the length of the flexible line and, shown for comparison, is an equivalent Lazy Wave arrangement (similar to that of Figure 2) that requires 10 buoyancy devices to provide for an equivalent freedom of movement; Figure 8 shows a subsea flexible line assembly in accordance with the present disclosure, the subsea flexible line assembly comprising three arc-forming jacket assemblies; Figure 9 shows the subsea flexible line assembly of Figure 8 wherein the floating structure is at a near extent within its mooring system; Figure 10 shows the subsea flexible line assembly of Figure 8 wherein the floating structure is at a far extent within its mooring system; Figure 11 shows the subsea flexible line assembly of Figure 8 wherein buoyancy has been selected too be too large such that there is a risk that the flexible line may protrude above the surface of the water; Figure 12 shows the subsea flexible line assembly of Figure 11 and having the same buoyancy profile as that of Figure 11 but wherein each of the three arc-forming jacket assemblies is tethered to the seabed; Figure 13 shows an arrangement in accordance with the disclosure wherein the flexible line extends between a pair of floating structures; Figure 14 shows a highly schematic three dimensional representation of an arc-forming jacket assembly in accordance with the disclosure; Figure 15 shows a central section of the arc-forming jacket assembly of Figure 14; Figure 16 shows an exploded view of the components of a central jacket of the arc-forming jacket assembly plus a stiffener on either side thereof; Figure 17 shows a two-dimensional schematic representation of the arc-forming jacket assembly of Figure 14; Figure 18 shows a two-dimensional cross sectional view of the arc-forming jacket assembly of Figure 14. Specific description An arc-forming jacket assembly 100 in accordance with the present disclosure is shown in a highly schematic form in Figure 14. The jacket assembly 100 is configured to fasten around a flexible line 10. The arc-forming jacket assembly 100 comprises a plurality of fluid displacing elements 110 that are configured to extend radially and axially around the flexible line 10. The arcforming jacket assembly 100 further comprises a plurality of stiffener elements 120 configured to extend radially and axially around the flexible line. In the arrangement of Figure 14, the fluid displacing elements 110 and the stiffener elements 120 are arranged such that they alternate along the flexible line. Figure 15 shows the central five fluid displacing elements 110 of the arc-forming jacket assembly 100 of Figure 14 together with four stiffener elements 120. Referring to Figure 16, the arc-forming jacket assembly 100 further comprises a plurality of spools 130 configured to extend radially and axially around the flexible line and radially within each fluid displacing element 110. Each of the spools 130, the fluid displacing elements 110 and the stiffener elements 120 is provided in a first half and a second half, each of which extends 180 ° around the flexible line 10 such that the first half and the second half combined enclose 360 ° of the flexible line 10. Specifically, the fluid displacing elements 110 each comprise a first fluid displacing element half 110a and a second fluid displacing element half 110b. Similarly, each stiffener element 120 comprises a first stiffener half 120a and a second stiffener half 120b. Also similarly, each spool 130 comprises a first spool half 130a and a second spool half 130b. In this way, each of these components is configured to be assembled around a flexible line, whereas if each of these components were provided as a single part, installation of the components on the flexible line would require them to be slid along from one end of the flexible location to their intended positions. It may be that only one of the spools 130 is fastened to the flexible line. Specifically, it may be that only a centre-most spool 130 in the arc-forming jacket assembly 100 is fastened to the flexible line. Moving axially away from the spool that is fastened to the flexible line, the components may be connected axially to the adjacent component rather than being radially fastened to the flexible line. This allows for differential expansion and contraction of the arc-forming jacket assembly 100 relative to the flexible line 10 since the flexible line 10 is able to expand or contract whilst facilitating axial movement of all components either side of the one spool 130 that is fastened to the flexible line. Each fluid displacing element 110 comprises a first edge 111, a second edge 112, and a circumferential interior surface between the first edge 111 and the second edge 112. In one embodiment, the distance between the first edge 111 and the second edge 112 may be 500 mm. Each fluid displacing element 110 may comprise an exterior circumferential recess 114 configured to accommodate a strap (not illustrated) to retain the two halves of the fluid displacing element 110 together once assembled around the flexible line. Similarly, each stiffener 120 may comprise an exterior circumferential recess 124 configured to accommodate a strap (again not illustrated) to retain the two halves of the stiffener 120 together once assembled around the flexible line. Each spool 130 may have an exterior profile which is configured to cooperate with an interior profile of its fluid displacing element 110. Thus, as shown in Figure 16, an exterior circumferential surface 135 of the spool 130 may comprise a radial recess 136 and an interior circumferential surface 115 of the fluid displacing element 110 may comprise a radial protrusion 116. In this way, with the fluid displacing element 110 in situ around the spool 130 the protrusion 116 sits in the recess 136 and axial movement of the fluid displacing element 110 relative to the spool 130 is thereby prevented. Each spool 130 may comprise a first flange 138 at a first end of the spool 130 and a second flange 139 at a second end of the spool. The flanges 138, 139 may each comprise apertures through which fixing elements (such as bolts) may be inserted for fastening the spool 130 to the adjacent stiffener 120. A first annular recess 119 may be provided in the first edge 111 of the fluid displacing element 110 to accommodate the first flange 138 of the spool 130 and a second annular recess may be provided in the second edge 112 of the fluid displacing element 110 to accommodate the second flange 139 of the spool 130. These may provide restriction on axial movement of the fluid displacing element 110 relative to the spool 130. Each stiffener 120 may comprise an appropriate shore hardness to facilitate a maximum degree of bending of the stiffener 120. In this way, a limited degree of bending of the flexible line within the stiffener 120 may be facilitated. As shown in Figures 14, 17 and 18, the fluid displacing elements 110 in the arc-forming jacket assembly 100 are not all the same as each other. In particular, the fluid displacing elements 110 located in a central region of the arc-forming jacket assembly 100 have a greater fluid displacement effect than those located away from the central region of the arcforming jacket assembly 100. In the illustrated embodiment, the fluid displacing elements 110 have a consistent width in the axial direction and variable radii. In alternative arrangements which are not illustrated, the fluid displacing elements may have a consistent radius and a variable width. In a further alternative, the fluid displacing elements may have variable radius and variable width. In the illustrated embodiments, the fluid displacing elements 110 located in a central region of the arc-forming jacket assembly 100 have a greater fluid displacement effect in that they provide a greater buoyancy than the fluid displacing elements 110 located away from the central region of the arc-forming jacket assembly 100. In this way, the greatest buoyancy effect is achieved in the middle of the arc-forming jacket assembly 100 but more modest buoyancy is provided either side of the centre of the arcforming jacket assembly 100. Thus, there is a gradual reduction in buoyancy rather than an immediate step change in buoyancy. It is this which facilitates the arc-forming property of the arc-forming jacket assembly. One consequence of this arrangement is that it means that it avoids a need for bend protection elements on either side of a single fluid displacing element 100. Instead, the bend protection functionality is effectively provided by the series of reducing buoyancy value fluid displacing elements when moving away from the central most fluid displacing element(s) 100. Although the illustrated embodiments all show the arc-forming jacket assembly 100 performing a buoyancy function, it is equally possible to deploy the arc-forming jacket assembly 100 to perform a ballast function. In such a scenario, each fluid displacing element 110 comprises ballast. Those fluid displacing elements 110 located in a central region of the arc-forming jacket assembly 100 have a greater fluid displacement effect in that they provide a greater ballast than the fluid displacing elements 110 located away from the central region of the arc-forming jacket assembly 100. Thus, arc-forming functionality is effectively provided by the series of reducing ballast value fluid displacing elements when moving away from the central most fluid displacing element(s) 100. Figure 8 shows a subsea flexible line assembly having three hog bends, each of which is provided by an arc-forming jacket assembly 100 of the kind illustrated in Figure 14 and wherein each fluid displacing element 110 comprises a buoyancy element 110. The subsea flexible line assembly of Figure 8 also comprises three sag bends, alternating with the hog bends. In the Figure 8 arrangement, each sag bend is facilitated simply by the weight of the flexible line between the hog bends. However, in an alternative arrangement, one or more of the sag bends may be provided by an arc-forming jacket assembly 100 in accordance with the present disclosure, wherein each fluid displacing element 110 comprises a ballast device. Figure 9 shows the same subsea flexible line assembly of Figure 8 wherein the floating structure is at a near excursion limit within its mooring system. Tethering (not shown) of the subsea flexible line will prevent the floating structure 20 from moving closer to the seabed anchor 30 than shown in the near excursion limit scenario of Figure 9. Similarly, Figure 10 shows the same subsea flexible line assembly of Figure 8 wherein the floating structure is at a far excursion limit within its mooring system. Again, tethering (not shown) of the subsea flexible line will prevent the floating structure 20 from moving further away from the seabed anchor 30 than shown in the far excursion limit scenario of Figure 10. In both scenarios (far excursion limit and near excursion limit) as well as in selected scenarios in between those limits, the highest point of a hog bend in the equivalent Lazy Wave arrangement (see Figure 2) would be closer to the surface of the sea than the highest point of any of the three hog bends of the subsea flexible line assembly in accordance with the present disclosure (Figure 10). Also, in the near extent scenario (Figure 9) as well as in selected scenarios in between those limits (including that shown in Figure 8), the lowest point of the sag bend of the equivalent Lazy Wave would be closer to the seabed than any of the three hog bends of the subsea flexible line assembly in accordance with the present disclosure (Figure 10). Further, in the near extent scenario (Figure 9), the three hog bends of the subsea flexible line assembly in accordance with the present disclosure are further from the seabed then the hog bend of the equivalent Lazy Wave. Accordingly, the same extent of lateral and vertical freedom of movement between the two ends of the subsea flexible line assembly is provided in the arrangement of the present disclosure as compared with the Lazy Wave arrangement, but the risk of the flexible line touching the seabed or protruding from the surface of the water in extreme conditions is reduced. It is known that marine life builds up on the flexible line 10 with time, which will act against the buoyancy of buoyancy-providing fluid displacing elements 110. Thus a distance, d, between the top of the highest hog bend and the surface of the sea will increase with time as the marine life builds up and the flexible line 10 drops. Eventually, the lowest position of the lowest sag bend will be too close to the seabed. In order to mitigate this to some extent, one option is provide more buoyancy than would ordinarily be used and to tether each hog bend to the seabed so as to prevent the hog bends from travelling any closer to the surface of the sea than would be safe. Figure 11 shows a hypothetical arrangement whereby additional buoyancy is provided meaning that the highest sag bend moves too close to the surface of the sea, and would therefore not be appropriate for deployment. Figure 12 shows the same degree of buoyancy but with each hog bend tethered to the seabed, thus preventing the hog bends from moving closer to the surface of the sea than would be appropriate. This means that initial build up of marine life will not cause downward movement of the flexible line but will instead simply reduce tension on the tether lines. This will extend the usable life of the arrangement. Figure 13 shows an arrangement in accordance with the disclosure wherein the flexible line 10 extends between two floating structures 20 and does not extend to the seabed. In an alternative arrangement to those illustrated, the disclosure also includes an arcforming jacket assembly without stiffeners located between the fluid displacing elements. In such arrangements, over-bending of the flexible line is prevented by the fluid displacing elements being sufficiently close together that when the flexible line bends between adjacent fluid displacing elements, a radially outer part of the first edge 111 of one fluid displacing element comes into contact with a radially outer part of the second edge 112 of 5 the adjacent fluid displacing element at an angle sufficiently acute that a radius of curvature of the flexible line between those fluid displacing elements is appropriately limited. In some embodiments, the stiffness of each stiffening element 120 in the arc-forming jacket assembly 100 may be the same as the stiffness of each of the stiffening elements 120 in 10 the arc-forming assembly. In other embodiments, different stiffnesses of stiffening element 120 may be selected in order to accommodate a changing bend angle so as to facilitate a gentle arc shape of the arc-forming jacket assembly.

Claims

1. A jacket assembly for attachment to a subsea flexible line for facilitating passive control of movement of the flexible line, the jacket assembly comprising a plurality of jackets, each jacket comprising:a fluid displacing element extending longitudinally between a first edge and a second edge and having a circumferential interior surface between the first edge and the second edge, the circumferential interior surface configured to bound a first inner cylindrical volume;wherein the plurality of fluid displacing elements comprises a first subset of fluid displacing elements and a second subset of fluid displacing elements, and wherein the first subset of the plurality of fluid displacing elements has a first fluid displacement value a second subset of the plurality of fluid displacing elements has a second fluid displacement value different from the first fluid displacement value.

2. The jacket assembly of claim 1 wherein the fluid displacing element comprises a first element and a second element, wherein the first element extends around a first radial part of the first inner cylindrical volume and the second element extends around a second radial part of the first inner cylindrical volume, such that the first element and the second element together bound the first inner cylindrical volume.

3. The jacket assembly of claim 1 or claim 2 wherein one or more of the plurality of jackets further comprises a spool for attachment to a subsea flexible line, the spool configured to bound a second inner cylindrical volume for surrounding an exterior circumferential surface of the flexible line.

4. The jacket assembly of claim 3 wherein the spool comprises an outer profile that cooperates with the first inner cylindrical volume of at least one of the fluid displacing elements.

5. The jacket assembly of claim 3 or claim 4 wherein the spool comprises a first radial spool part that extends around a first radial part of the second inner cylindrical volume and a second radial spool part that extends around a second radial part of the second inner cylindrical volume, such that the first radial spool part and the second radial spool part together bound the second inner cylindrical volume.

6. The jacket assembly of claim 3, claim 4 or claim 5 wherein the spool comprises a first flange at a first axial end and a second flange at a second axial end.

7. The jacket assembly of claim 3, claim 4 or claim 5 wherein the spool is one of a plurality of spools.

8. The jacket assembly of any preceding claim further comprising a stiffener element located between a pair of adjacent jackets of the plurality of jackets.

9. The jacket assembly of claim 8 wherein the stiffener is one of a plurality of stiffeners, wherein the jacket assembly comprises alternating jackets and stiffeners in an axial direction of the jacket assembly.

10. The jacket assembly of any preceding claim wherein the first fluid displacement value is higher than the second fluid displacement value and the second subset of fluid displacing elements is distributed either side of the first subset of fluid displacing elements.

11. The jacket assembly of any preceding claim wherein the plurality of fluid displacing elements further comprises a third subset of fluid displacing elements each having a third fluid displacement value, different from both the first fluid displacement value and the second fluid displacement value.

12. The jacket of claim 11 wherein the third fluid displacement value is lower than the second fluid displacement value, and wherein the third subset of fluid displacing elements is located axially further from the first subset than the second subset.

13. A jacket assembly kit comprising:a plurality of fluid displacing elements, each extending longitudinally between a first edge and a second edge and having a circumferential interior surface between the first edge and the second edge, the circumferential interior surface configured to bound a first inner cylindrical volume;wherein the plurality of fluid displacing elements comprises a first subset of fluid displacing elements and a second subset of fluid displacing elements, and wherein the first subset of the plurality of fluid displacing elements has a first fluid displacement value asecond subset of the plurality of fluid displacing elements has a second fluid displacement value different from the first fluid displacement value.

14. The jacket assembly kit of claim 13 further comprising a third subset of fluid displacing elements each having a third fluid displacement value, different from both the first fluid displacement value and the second fluid displacement value15. The jacket assembly kit of claim 13 or claim 14 further comprising a plurality of spools each configured to bound a second inner cylindrical volume for surrounding an exterior circumferential surface of a flexible line.

16. The jacket assembly kit of any of claims 13 to 15 wherein each spool comprises an outer profile that cooperates with the first inner cylindrical volume of at least one of the fluid displacing elements.

17. The jacket assembly kit of claim 15 or claim 16 further comprising a plurality of stiffener elements configured for axial connection to one or more of the spools.

18. A method of facilitating passive control of movement of a subsea flexible line that extends between a first end and a second end, the method comprising:fastening a first spool to a flexible line;fastening a first fluid displacing element around the spool;fastening a first stiffener element to a first axial end of the first spool and fastening a second stiffener element to a second axial end of the spool;fastening a second spool to the first stiffener at an opposite end of the first stiffener from the first spool;fastening a second fluid displacing element around the second spool;fastening a third spool to the second stiffener at an opposite end of the second stiffener from the first spool; andfastening a third fluid displacing element around the third spool.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS17CLAIMS:

1. A jacket assembly for attachment to a subsea flexible line for facilitating passive control of movement of the flexible line, the jacket assembly comprising a plurality of5 jackets, each jacket comprising:a fluid displacing buoyancy element extending longitudinally between a first edge and a second edge and having a circumferential interior surface between the first edge and the second edge, the circumferential interior surface configured to bound a first inner cylindrical volume;10 wherein the plurality of fluid displacing buoyancy elements comprises a first subsetof fluid displacing buoyancy elements and a second subset of fluid displacing buoyancy elements, and wherein the first subset of the plurality of fluid displacing buoyancy elements has a first buoyancy value a second subset of the plurality of fluid displacing buoyancy elements has a second buoyancy value different from the first buoyancy value.LO 15CM2. The jacket assembly of claim 1 wherein the fluid displacing buoyancy element 'l“ comprises a first element and a second element, wherein the first element extends around a first radial part of the first inner cylindrical volume and the second element extends CO around a second radial part of the first inner cylindrical volume, such that the first element 20 and the second element together bound the first inner cylindrical volume.

3. The jacket assembly of claim 1 or claim 2 wherein one or more of the plurality of jackets further comprises a spool for attachment to a subsea flexible line, the spool configured to bound a second inner cylindrical volume for surrounding an exterior25 circumferential surface of the flexible line.

4. The jacket assembly of claim 3 wherein the spool comprises an outer profile that cooperates with the first inner cylindrical volume of at least one of the fluid displacing buoyancy elements.

305. The jacket assembly of claim 3 or claim 4 wherein the spool comprises a first radial spool part that extends around a first radial part of the second inner cylindrical volume and a second radial spool part that extends around a second radial part of the second inner cylindrical volume, such that the first radial spool part and the second radial spool part35 together bound the second inner cylindrical volume.08 01 256. The jacket assembly of claim 3, claim 4 or claim 5 wherein the spool comprises a first flange at a first axial end and a second flange at a second axial end.

7. The jacket assembly of claim 3, claim 4 or claim 5 wherein the spool is one of a plurality of spools.

8. The jacket assembly of any preceding claim further comprising a stiffener element located between a pair of adjacent jackets of the plurality of jackets.

9. The jacket assembly of claim 8 wherein the stiffener is one of a plurality of stiffeners, wherein the jacket assembly comprises alternating jackets and stiffeners in an axial direction of the jacket assembly.

10. The jacket assembly of any preceding claim wherein the first fluid displacement buoyancy value is higher than the second fluid displacement buoyancy value and the second subset of fluid displacing buoyancy elements is distributed either side of the first subset of fluid displacing buoyancy elements.

11. The jacket assembly of any preceding claim wherein the plurality of fluid displacing buoyancy elements further comprises a third subset of fluid displacing buoyancy elements each having a third buoyancy value, different from both the first buoyancy value and the second buoyancy value.

12. The jacket of claim 11 wherein the third buoyancy value is lower than the second buoyancy value, and wherein the third subset offluid displacing buoyancy elements is located axially further from the first subset than the second subset.

13. A jacket assembly kit configured to produce a jacket assembly in accordance with any preceding claim comprising:a plurality of fluid displacing elements for use with the jacket assembly of any preceding claim, each fluid displacing element extending longitudinally between a first edge and a second edge and having a circumferential interior surface between the first edge and the second edge, the circumferential interior surface configured to bound a first inner cylindrical volume;08 01 25wherein the plurality of fluid displacing buoyancy elements comprises a first subset of fluid displacing buoyancy elements and a second subset of fluid displacing buoyancy elements, and wherein the first subset of the plurality of fluid displacing buoyancy elements has a first buoyancy value a second subset of the plurality of fluid displacing buoyancy elements has a second buoyancy value different from the first buoyancy value.

14. The jacket assembly kit of claim 13 further comprising a third subset of fluid displacing buoyancy elements each having a third buoyancy value, different from both the first buoyancy value and the second buoyancy value15. The jacket assembly kit of claim 13 or claim 14 further comprising a plurality of spools each configured to bound a second inner cylindrical volume for surrounding an exterior circumferential surface of a flexible line.

16. The jacket assembly kit of any of claims 13 to 15 wherein each spool comprises an outer profile that cooperates with the first inner cylindrical volume of at least one of the fluid displacing buoyancy elements.

17. The jacket assembly kit of claim 15 or claim 16 further comprising a plurality of stiffener elements configured for axial connection to one or more of the spools.

18. A method of facilitating passive control of movement of a subsea flexible line that extends between a first end and a second end, the method comprising:fastening a first spool to a flexible line;fastening a first fluid displacing buoyancy element around the spool, wherein the first fluid displacing buoyancy element has a first buoyancy value;fastening a first stiffener element to a first axial end of the first spool and fastening a second stiffener element to a second axial end of the spool;fastening a second spool to the first stiffener at an opposite end of the first stiffener from the first spool;fastening a second fluid displacing buoyancy element around the second spool, wherein the second fluid displacing buoyancy element has a second buoyancy value;fastening a third spool to the second stiffener at an opposite end of the second stiffener from the first spool; andfastening a third fluid displacing buoyancy element around the third spool, wherein the third fluid displacing buoyancy element has a third buoyancy value;wherein the second buoyancy value is greater than the first buoyancy value andgreater than the third buoyancy value.

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