GB2551788A

GB2551788A - Bouyancy module and method of manufacture thereof

Info

Publication number: GB2551788A
Application number: GB1611395.3A
Authority: GB
Inventors: Harbison Austin; Lloyd Fox Jonathan
Original assignee: Trelleborg Offshore UK Ltd
Current assignee: CRP Subsea Ltd
Priority date: 2016-06-30
Filing date: 2016-06-30
Publication date: 2018-01-03
Anticipated expiration: 2036-06-30
Also published as: GB2551788B; GB201611395D0

Abstract

A buoyancy element 12 for mounting on an elongate member has a moulded plastics shell 14 containing a core 16 of buoyant material. The shell provides a recess 20 for receiving the elongate member and carries at least one resilient element 36 disposed within the recess and arranged to seat upon the elongate member. The shell has at least one integrally formed first mating feature 40 which mates with a complementary second mating feature of the resilient element to locate the resilient element and to facilitate attachment of the resilient element to the buoyancy element. The first mating element may be male, and it may protrude through the second mating element, where the tip of the male mating element forms the attachment, for example using a fastener. The second mating feature may comprise a counter bore formed in an outer face of the resilient element. Also claimed is a buoyancy module comprising two coupled buoyancy elements, a method of forming the buoyancy element with moulded plastic and a buoyant core material, a buoyancy element with a through going passage for handling purposes and a method of forming such an element, and a buoyancy element with sacrificial upstands to protect the shell.

Description

(54) Title of the Invention: Bouyancy module and method of manufacture thereof Abstract Title: Buoyancy element and method of manufacture (57) A buoyancy element 12 for mounting on an elongate member has a moulded plastics shell 14 containing a core 16 of buoyant material. The shell provides a recess 20 for receiving the elongate member and carries at least one resilient element 36 disposed within the recess and arranged to seat upon the elongate member. The shell has at least one integrally formed first mating feature 40 which mates with a complementary second mating feature of the resilient element to locate the resilient element and to facilitate attachment of the resilient element to the buoyancy element. The first mating element may be male, and it may protrude through the second mating element, where the tip of the male mating element forms the attachment, for example using a fastener. The second mating feature may comprise a counter bore formed in an outer face of the resilient element. Also claimed is a buoyancy module comprising two coupled buoyancy elements, a method of forming the buoyancy element with moulded plastic and a buoyant core material, a buoyancy element with a through going passage for handling purposes and a method of forming such an element, and a buoyancy element with sacrificial upstands to protect the shell.

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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BUOYANCY MODULE AND METHOD OF MANUFACTURE THEREOF

The present invention relates to a buoyancy module for mounting on an elongate underwater member, and to a method of manufacture thereof.

There are many situations in which elongate underwater members, for example pipelines, risers, and umbilicals, need to be provided with buoyancy. It is well known to provide this buoyancy in the form of distributed buoyancy modules, which are large buoyant bodies, often generally cylindrical in shape and formed from two separate buoyancy elements which are coupled to one another around the elongate member by means of straps. The two buoyancy elements together form a passage to receive the elongate member. An example can be found in Figure 1 of international patent application PCT/GB2013/051311, published under no. WO2013/171521 in the name of Trelleborg Offshore UK Ltd. Distributed buoyancy modules of this type are often used to support part of the weight of the risers used in offshore oil and gas extraction, maintaining the riser in a chosen configuration such as the lazy S or steep S configurations, which are known to the skilled person. Risers extend upwards from the seabed to a platform, but there are other situations in which generally horizontally extending elongate members such as jumpers need to be provided with buoyancy.

Buoyancy modules need to be prevented from moving along the length of the elongate member. One known way to achieve this is to secure a clamp to the elongate member and to assemble the buoyancy module around the clamp. Again PCT/GB2013/051311 provides an example. An alternative is to dispense with the clamp and to provide the buoyancy module itself with surfaces which seat upon the elongate member and are urged toward it in the manner of a clamp, so that the engagement of these surfaces with the elongate member secures the buoyancy module against axial movement.

Whatever clamping arrangement is used, it needs to reliably secure the buoyancy in place despite the challenges of the marine environment and over what is often a protracted design lifetime. Problems can be created by dimensional changes. For example the elongate member may suffer thermal expansion and contraction. It may suffer creep over time due to the clamping force. Where the buoyancy module serves as the clamp it too may suffer dimensional changes, including creep. To maintain clamping force despite such factors, dedicated buoyancy clamps can be provided with a compliant element. For example the clamp described in PCT/GB2013/051311 has a number of spring elements through which it seats upon a riser, in use.

It is known to manufacture distributed buoyancy elements by first forming an external hollow shell by rotomoulding (which is also referred to as rotational moulding, and is a process well known to those skilled in the art). The rotomoulded shell is then filled with a suitable low density plastics material, which may be in the form of syntactic foam. This material is well known to those skilled in the art and has a plastics material with an admixture of low density filler typically in the form of microspheres (hollow glass beads). In many cases macrospheres - larger hollow spherical bodies - are also included.

Known buoyancy modules are subject to several problems.

Where the buoyancy module itself seats upon and clamps against the elongate member, it may do so through resilient elements to provide compliance. But in that case attaching the resilient elements to the buoyancy module is challenging. The outer shell of the buoyancy module is typically formed of polyethylene, a material which takes adhesive poorly. One might envisage incorporating some form of anchor into the shell during its rotomoulding, such as a metal insert to which the resilient element can be bolted. But including the anchor in the mould during rotomoulding changes the internal shape of the resultant shell, a bulge being created over the anchor. This bulge is in the path through which the clamping load is transmitted to the buoyancy elements core, and is considered to impair the desired distribution of the load, which may result in failures, especially over a long design lifetime. Another alternative might be to machine the shell after moulding to receive the resilient element, but post-moulding machining operations increase the time and cost involved in manufacture.

A simple and reliable means is needed for attachment of a resilient element to the buoyancy module.

Another group of problems concerns handling of the buoyancy elements prior to their deployment, and protection of the elements from abrasion during handling. If the outer shell of a buoyancy element is excessively abraded during handling, the syntactic core may be exposed, which is not desirable.

Some known distributed buoyancy elements have a pair of open-ended tubes inserted into the shell and running along the axial direction from one end face to the other. A lifting sling is passed down one of these tubes and up the other to enable the element to be lifted, and the element is provided with wooden skids to protect it from abrasion by the supporting surface. Incorporation of the tubes (which are formed separately from the shell of the buoyancy element and inserted in it prior to filling with syntactic foam) adds to the complexity and hence the cost of the manufacturing process. Also the tubes pass through the load path - i.e. the path through which the force of straps or other means used to assemble the buoyancy module is supported by the buoyancy elements. This can impair stress distribution and provide a potential structural weakness. Use of the wooden sleds complicates handling somewhat.

Improved provision for handling the buoyancy elements and for protecting them from abrasion are desirable.

Also desirable is a more straightforward way to provide for engagement of a lifting sling with the buoyancy elements.

The present invention provides buoyancy elements and modules in accordance with the appended claims.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:Figure 1 shows a buoyancy element embodying the present invention;

Figure 2 is a view of the same buoyancy element from above;

Figure 3 is a view of the same buoyancy element from one side;

Figures 4a, 4b and 4c are sections through the buoyancy element in radial planes denoted by lines A, B and C respectively in Figure 2;

Figures 5 and 6 are sections in different radial planes through an assembled buoyancy module comprising a pair of the buoyancy elements depicted in Figures 1 to 4;

Figure 7 is a view of the same assembled buoyancy module from above; and

Figures 8 and 9 are side and top views respectively of a resilient element used in the buoyancy elements.

The term buoyancy element is used herein to refer to the individual buoyant parts which are assembled to one another to form a complete buoyancy module. In the present embodiment a buoyancy module 10 comprises two buoyancy elements 12, although it would be possible to form the buoyancy module from a different number of buoyancy elements, for example three elements or one element.

In the present embodiment the buoyancy elements 12 are moulded items. More specifically they are formed by the type of process described above, in which a hollow plastics shell 14 is formed first by a suitable moulding process, which in the present embodiment is a rotomoulding process. The shell 14 forms the exterior surfaces of the buoyancy element 12. It comprises a shaped wall of small thickness which defines a largely closed internal cavity. This cavity is filled with a mouldable material of low enough density that the element 12 is positively buoyant. In the present embodiment the material in question is syntactic foam, which is poured into the plastics shell 14 as a resin and which sets in situ to form core 16 of the buoyancy element. In this way the shell 14 serves as a mould for the core. The core is structural - it contributes rigidity and the capacity to withstand hydrostatic pressure in use.

The shape of the buoyancy elements 12 may vary from one embodiment to another, but in the present embodiment each is generally semi-cylindrical, providing a generally flat inner face 18 which has a recess 20 extending from a first end 22 to a second end 24. In the assembled buoyancy module 10, the recesses of a pair of buoyancy elements 12 together define a passage for receiving the elongate element. This passage is shaped (which is to say that its diameter varies along its length) to provide:

a first flared portion 26 of frusto-conical shape, diverging toward the first end 22;

a second flared portion 28 of similar shape to the first, diverging toward the second end 24; and a seat portion 30 between the first and second flared portions 26, 28. The clamp seats upon the elongate member through the seat portion 30, which is the smallest diameter region of the passage.

The flared portions 26, 28 enable the buoyancy module 10 to accommodate curvature of the elongate member.

The inner face 18 of the buoyancy element 12 is provided with male and female registration features in the form of pips 32 and recesses 34. In the assembled buoyancy module 10 the pips 32 are received in corresponding recesses 34 to ensure proper alignment of one buoyancy element with the other, as seen in Figure 5. The arrangement of the registration features is such that two identically formed buoyancy elements 12 are able to engage with one another and hence only one shaper of moulding need be produced.

The depth of the pips is such that a separation 35 (refer again to Figure 5 in particular) is maintained between the inner faces 18 of the buoyancy elements 12. This allows for use and removal of a lifting strap. Looking at Figure 4c in particular, it can be seen that open-ended handling passages 38a, 38b are formed in the buoyancy element 12 to receive a lifting strap (which is not shown). These take a different form from the passages described above with reference to the prior art. In those known buoyancy modules, passages used to receive lifting straps extended the full length of the module and were formed by use of tubes inserted into the rotomoulded shell. The applicants have found however that passages of shorter length can be formed more simply, by incorporating them into the shape of the rotomoulding tool used to form the shell 14. That is, the shell 14 is shaped to define the passages, by means of an integral passage wall 39 (see Figure 4c). The handling passages 38a, 38b extend laterally rather than axially in the present embodiment, and so can be of relatively short length. More specifically they extend from the inner face 18 to a shallow cutaway 41 formed in the buoyancy element's outer surface. Even when the buoyancy element 10 is assembled upon the elongate member, lifting straps can be led through the handling passages 38a, 38b, emerging through the separation 35 between the inner faces of the buoyancy elements 12. This facilitates handling of the buoyancy module and/or of the assembly of the buoyancy module and elongate member, especially while both are generally horizontal.

The buoyancy module 10 is able to seat upon an elongate member and to resist movement along it without need of any additional clamp. It does so through resilient elements 36 carried upon the seat portions 30 of buoyancy elements 12. The resilient elements 36 are omitted from Figures 1, 2, 3, and 4 but seen in Figures 6 and 8. They are separately formed from the buoyancy elements 12 and attached to them. To provide for this attachment, the shell 14 of the buoyancy element 12 is provided with integral, moulded first mating features 40. The resilient elements 36 are provided with complementary second mating features 42. Mating of the first and the second mating features 40, 42 serves to locate the resilient element with respect to the buoyancy element 12, and the mating features are also used to secure the resilient elements 36 in place.

More specifically, in the present embodiment the first mating features of the buoyancy element 12 are male, and are received in the second mating features of the resilient elements 36. The first mating features in the illustrated embodiment take the form of male protrusions 40 and project into throughgoing openings 42 formed in the resilient elements 36. The openings 42 take the form of a stepped bore with a narrow lower portion which receives the male mating feature 40 as a close fit, and a wider upper portion 44 (see Figure 8) which provides access to the end of the protrusion 40 to enable it to be secured. By virtue of this formation the protrusion 40 can terminate beneath a seating surface 46 of the resilient element 36, which abuts the elongate member in use. It will be appreciated that the mating of the male protrusions 40 and the openings 42 defines the position of the resilient element 36 upon the buoyancy element 12. The protrusion 40 may be an interference fit in the opening 42, and in the present embodiment this is sufficient to keep the resilient element 36 in position.

In other embodiments an additional component or process is used to form the required attachment and this may take any of several forms. In the present embodiment the exposed tip of the male protrusion 40 is riveted. That is, an enlarged head is formed upon this tip after assembly to engage with a shoulder 48 of the opening 42 (see Figures 8 and 9) and so resist withdrawal of the male protrusion 40 from the opening 42. The riveting process may be carried out by use of ultrasound and/or it may involve application of heat and physical impacts to shape the enlarged head.

In another embodiment the protrusion 40 has a barb which is insertable through the opening 42 but resists withdrawal from it.

In other embodiments the attachment may be formed in different ways. It may be formed by ultrasonic welding or by another plastics welding process. It may be made using a fastener attached to the exposed tip of the male protrusion 40. The fastener may be a threaded fastener such as a nut (to be screwed onto a pre-threaded tip of the male protrusion 40) or a self-tapping nut, to cut its own thread as it is secured onto the said tip. Alternatively the fastener may comprise some form of clip, such as a speed locking clip, which is a device well known to the skilled person having the general form of a washer with internal cut-aways and a slightly domed profile defining a set of fingers which are somewhat inclined with respect to the plane defined by the clip's periphery. The clip can quite easily be slid onto a male member such as the protrusion 40, but attempts to withdraw it in the opposite direction cause the fingers to grip the member and so maintain the clip's position.

Other mechanical fasteners and other forms of attachment may be adopted.

The resilient elements 36 comprise an elastomeric material. Rubber is suitable. They are compressed as the buoyancy module 10 is mounted on an elongate member and act in the manner of springs subsequently, so that if for example creep causes the elongate member's diameter to reduce somewhat over time, the resilient elements 36 expand correspondingly and prevent a catastrophic loss of clamping pressure. In the present embodiment the resilient elements 36 are elongate, square section components but they may take other forms.

The buoyancy elements 12 are secured to one another by use of bands 50, 52, 54a, b, c passed around them and secured under tension (see Figure 7). The buoyancy elements 12 are shaped to provide external semi-annular recesses 57 to receive and locate the bands. Note that in the present embodiment there are three bands 54a, b, c arranged to apply pressure to the region containing the seat portion 30. More specifically, these three bands all lie around the seat portion 30. It is primarily these bands that provide the force needed to clamp the buoyancy element to the elongate member. Use of three bands provides redundancy against the possibility that one or even two of them might fail, and so reduces the risk that the buoyancy module 10 might lose its grip on the elongate member and move in an uncontrolled manner along it. This redundancy may make it possible to specify a more economical form of band.

In this embodiment ends of each are drawn together by an arrangement using a threaded member 60, in a manner which is familiar to the skilled person. This arrangement is accommodated upon a shallow recess 62 formed on the exterior of the buoyancy element 12. Other forms of band may be used, and the buoyancy elements may in other embodiments be attached to one another in different ways, e.g. by means of threaded fasteners.

It was explained above how the buoyancy elements/modules can be handled by means of lifting straps whilst horizontally oriented. It is however desirable to provide for their handling whilst vertically oriented, i.e. while they stand on one end. For this purpose, end face 24a of the buoyancy module 12 is provided with a pair of handling channels 64 whose size, arrangement and separation enables them to receive the forks of a fork lift. The buoyancy elements 12 can thus be easily lifted, transported and released, singly or in groups, using a fork lift.

In order to resist damaging abrasion, e.g. due to contact with the ground during handling, the end face 24a is additionally provided with sacrificial upstands 66. In the illustrated embodiment these take the form of elongate strips, but they may take any of a variety of shapes provided that they stand up from the plane of the end face 24a, ensuring that it is the upstands that contact the ground, provide that it is sufficiently flat, and it is the upstands that are abraded in any rough handling. The upstands are an integral part of the skin 14, formed during its moulding, and thus add nothing to the cost and complexity of manufacture.

The buoyancy module 10 illustrated herein is intended for mounting on a jumper - a flexible or rigid pipe used to connect two subsea structures, which thus typically runs horizontally or on an incline, rather than vertically, but aspects of the present invention are applicable to buoyancy modules of different types and for different purposes.

Claims

1. A buoyancy element for mounting upon an elongate member, the buoyancy element comprising a moulded plastics shell containing a core of buoyant material, the shell being shaped to provide a recess for receiving the elongate member and carrying at least one resilient element disposed within the recess and arranged to seat upon the elongate member in use, wherein the shell comprises at least one integrally formed first mating feature which mates with a complementary second mating feature of the resilient element to locate the resilient element and to facilitate attachment of the resilient element to the buoyancy element.

2. A buoyancy element as claimed in claim 1 in which the first mating element is a male element and the second mating element is a female element.

3. A buoyancy element as claimed in claim 2 in which the first mating element protrudes through the second mating element, which is formed as an opening in the resilient element, and a tip portion of the first mating element is used to attach the resilient element to the buoyancy element.

4. A buoyancy element as claimed in claim 3 in which the tip portion is enlarged to engage with the resilient element and so attach the resilient element to the buoyancy element.

5. A buoyancy module as claimed in claim 3 in which a fastener engages with the tip portion of the first mating element to attach the resilient element to the buoyancy element.

6. A buoyancy element as claimed in claim 5 in which the fastener comprises an internally threaded fastener or a speed locking clip.

7. A buoyancy element as claimed in any of claims 3 to 6 in which the second mating feature comprises a counterbore formed in an outer face of the resilient element and the tip portion of the first mating element lies beneath the said outer face.

8. A buoyancy element as claimed in any preceding claim comprising at least one through-going passage defined by the shell.

9. A buoyancy element as claimed in claim 8 in which the said through-going passage extends along a generally lateral direction, with respect to the elongate member and the in-use orientation of the buoyancy element.

10. A buoyancy element as claimed in any preceding claim having an end face provided with sacrificial upstands for engaging a ground surface when the buoyancy element is stood upon the end face, the upstands being integrally formed with the shell and being able to be sacrificially abraded by contact with the ground surface, protecting the remainder of the shell from abrasion.

11. A buoyancy element as claimed in claim 10 in which the end face further comprises a pair of handling channels for receiving forks of a fork lift.

12. A buoyancy module comprising two or more buoyancy elements as claimed in any of claims 1 to 11 assembled to one another to define together a through-going passage for receiving and embracing the elongate member, and at least one coupling arranged to urge the buoyancy elements together to provide a clamping force upon the elongate member which resists axial movement of the buoyancy module with respect to the elongate member, in use.

13. A method of manufacture of a buoyancy module for mounting upon an elongate member, comprising moulding a plastics shell which is shaped to provide a recess for receiving the elongate member and at least one first mating feature, filling the shell with a buoyant core material, setting of the buoyant core material, manufacture of a resilient element having a second mating feature, mating the first and second mating features to locate the resilient element on the shell, and using the first mating feature to attach the resilient element to the shell.

14. A method as claimed in claim 13 in which the first mating element is a male element and the second mating element is a female element.

15. A method as claimed in claim 14 in which the first mating element protrudes through the second mating element, which is formed as an opening in the resilient element, and a tip portion of the first mating element is used to attach the resilient element to the buoyancy element.

16. A method as claimed in claim 15 further comprising forming an enlarged head on the tip portion after mating of the first and second mating features to attach the resilient element to the buoyancy element.

17. A method as claimed in claim 15 further comprising engaging a fastener with the tip portion of the first mating element to attach the resilient element to the buoyancy element.

18. A method as claimed in claim 17 in which the fastener comprises an internally threaded fastener or a speed locking clip.

19. A buoyancy element as claimed in any of claims 13 to 18 comprising moulding the shell in a shape which defines at least one through-going passage.

20. A method as claimed in claim 19 in which the said through-going passage extends along a generally lateral direction, with respect to the elongate member and the in-use orientation of the buoyancy element.

21. A buoyancy element for mounting upon an elongate member, the buoyancy element comprising a moulded plastics shell containing a core of buoyant material, the shell being shaped to provide a recess for receiving the elongate member and the buoyancy element having at least one throughgoing passage, characterised in that the through-going passage is defined by the shell.

22. A buoyancy element as claimed in claim 21 in which the said through-going passage extends along a generally lateral direction, with respect to the elongate member and the in-use orientation of the buoyancy element.

23. A buoyancy element as claimed in claim 21 or claim 22 in which the through-going passage is arranged to receive a handling strap for handling of the buoyancy element.

24. A method of manufacture of a buoyancy element as claimed in any of claims 21 to 23, comprising moulding the shell with an integral wall defining the through-going passage, and then filling the shell with the core material.

25. A buoyancy element comprising an external shell of plastics material containing a core of structural buoyant material, the buoyancy element having a face which is provided with sacrificial upstands for engaging a ground surface, the upstands being integrally formed with the shell and being able to be sacrificially abraded by contact with the ground surface, protecting the remainder of the shell from abrasion.

26. A buoyancy element as claimed in claim 25 in which the said face further comprises a pair of handling channels for receiving forks of a fork lift.

27. A buoyancy element substantially as herein described with reference to, and as illustrated in, the accompanying drawings.

28. A buoyancy module substantially as herein described with reference to, and as illustrated in, the accompanying drawings.

29. A method of manufacture of a buoyancy element substantially as herein described with reference to, and as illustrated in, the accompanying drawings.

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