WO2002018709A1

WO2002018709A1 - Protection of underwater elongate members

Info

Publication number: WO2002018709A1
Application number: PCT/GB2001/003792
Authority: WO
Inventors: Robert Gibson
Original assignee: Crp Group Limited
Priority date: 2000-08-30
Filing date: 2001-08-23
Publication date: 2002-03-07
Also published as: GB2385648A; AU2001282324A1; GB0304462D0; US20040013473A1; GB2385648B

Abstract

An underwater cladding (10) for an elongate member such as a pipe, has a substantially cylindrical outer surface with a plurality of depressions (18) therein. The provision of depressions in an otherwise cylindrical outer surface of a cladding interrupts or reduces vortex induced vibrations and the absence of projections facilitates use with conventional pipe-laying equipment.

Description

DESCRIPTION

PROTECTION OF UNDERWATER ELONGATE MEMBERS

The present invention relates to the protection of underwater pipes, drill

risers, cables or other elongate members.

When water flows past an underwater pipe, drill riser, cable or elongate

member of circular cross section, vortices may be shed alternately from each side.

The effect of these vortices is to induce fluctuating, across-flow forces on the

structure. If the natural frequency of the structure is close to the shedding frequency

of the vortex the member can be caused to vibrate with a large oscillation amplitude.

Such oscillations not only cause the pipe, drill riser, cable or member to bend

more than is desirable, but can also induce unwanted forces on a connector (either

underwater or above water) to which the pipe, drill riser, cable or the like is secured.

In extreme cases, the coupling between the pipe, drill riser, cable or the like and the

connector is damaged.

Also, if there are intermediate connections or joints (e.g. welds), then similar

problems can arise. One solution to the above problem is found in our co-pending

patent application published as GB-A-2335248. The arrangement disclosed therein

works extremely well but in view of the fact that the cladding disclosed therein

comprises a series of helical strakes, problems can arise when a clad pipe, drill riser,

cable or other elongate member is fed through conventional pipe-laying apparatus or

a vessel moonpool. Whilst it is possible to overcome such problems, there is a desire

to avoid such problems altogether. In accordance with the present invention, an elongate underwater cladding for

an elongate member comprises an outer surface having a plurality of depressions

therein.

It has been found that the formation of depressions in an otherwise cylindrical

outer surface of a cladding interrupts or reduces vortex induced vibrations and in

view of the fact that there are no strakes or other projections extending outwardly

from the otherwise cylindrical outer surface, the aforementioned problems which can

be encountered with pipe-laying apparatus are avoided.

The presence of the depressions has also been found to reduce the drag of the

elongate member in certain circumstances in both a steady and a fluctuating current.

Preferably, the depressions are arranged around the whole periphery or

circumference of the cladding. Circumferential coverage of the depressions ensures

suppression of vortex induced vibrations arising from omnidirectional flows.

In one embodiment, the depressions have a generally circular periphery. In

another embodiment, the depressions have an elliptical periphery.

In some embodiments, the depressed or recessed surfaces of the depressions

can be smooth and it is convenient if each of the depressions is smoothly concave

or dished when viewed in cross-section. However, in other, currently more preferred

embodiments the sides of the depressions are angular as this can help to "trip" the

vortices, moving from regular to irregular shedding.

The size of the depressions may vary widely. For example, for circular

depressions the diameter may vary from 1cm to 30cm and is preferably between 10cm and 20cm in diameter. For elliptical depressions, the length of the major axis

is preferably between 1cm and 30cm with the width of the minor axis being between

approximately 0.5cm and 20cm. Preferably the length of the major axis is from 10cm

to 20cm and the length of the minor axis is preferably from 5cm to 15cm.

Preferably, for cladding of circular cross-section, the largest dimension of a

depression (e.g. the diameter of a circular depression or the length of the major axis

of an elliptical depression) is from 5% to 50% (and more preferably between 10%

and 30%) of the external diameter of the cladding.

The depressions may be regularly spaced over the cylindrical surface of the

cladding or may be randomly spaced. Preferably, the depressions are substantially

identical.

If desired, the cladding may comprise positively buoyant material. Syntactic

foam (either with or without the inclusion of macrospheres) would be particularly

useful in this regard. The use of positively buoyant material is particularly useful in

the context of drill risers. Thus, the cladding of the present invention can comprise

a buoyancy module for a drill riser. Such a buoyancy module would offset much of

the weight of the drill riser whilst additionally providing protection against vortex

induced vibrations.

The cladding may comprise preformed sections which are subsequently

assembled on, and secured to, the elongate member to be protected. For example, the

preformed sections may comprise semi-tubular sections. Alternatively, the

preformed sections may comprise tubular sections. If tubular sections are used, the sections are preferably split along their length (e.g. a longitudinal or helical split) to

allow the sections to be located at any point along the length of the elongate member

to be protected.

In either case, the depressions can be moulded into the outer surface of the

preformed sections. Alternatively, the preformed sections may be moulded with a

smooth outer surface and the depressions may be cut or machined or otherwise

formed into the outer surface subsequently, either before or after the preformed

sections are assembled on the elongate member.

In another embodiment, the cladding may be moulded directly onto the outer

surface of the elongate member to be protected. The depressions or recesses may be

moulded into the outer surface of the cladding as it is moulded. Alternatively, the

cladding may be moulded without the depressions, which may be cut, machined or

otherwise formed into the outer surface subsequently.

Preferably, the outer surface of the cladding is substantially cylindrical.

By way of example only, specific embodiments of the present invention will

now be described, with reference to the accompanying drawings, in which:-

Fig. 1 is a perspective view of a first embodiment of cladding in

accordance with the present invention;

Fig. 2 is a side view of a second embodiment of pipe cladding in

accordance with the present invention;

Fig. 3 is a cross section of the pipe cladding of Fig. 2, looking in the

direction of arrows X-X in Fig. 2; Fig. 4 is a side view of a third embodiment of cladding in accordance

with the present invention;

Figs 5a to 5d show examples of four possible orientations of the

recesses along the cladding length; and

Fig 6 illustrates diagrammatically a typical cross-section of a cladding

moulding show the shapes and relative dispositions of depressions.

Referring firstly to Fig. 1, a protective ducting 10 for a pipe (not illustrated)

comprises a tubular flexible, impervious, polyurethane casing comprising a plurality

of identical, preformed, releasably engaged semi-tubular sections 12 which are

arranged with respect to one another to provide a cylindrical passage 14 therethrough

which is dimensioned and shaped to receive the pipe. The internal diameter of the

protective ducting or cladding varies with the diameter of the pipe to be clad. Thus,

in the embodiment illustrated in Fig. 1 the internal diameter is approximately 0.75

metres.

The required length of cladding is assembled by arranging the appropriate

number of preformed sections along the length of a pipe. As illustrated schematically

in Fig. 1, opposed semi-tubular sections of the cladding are held together by means

of metal bands B passing around the outer surface of the assembled cladding.

It should also be noted that, in use, diametrically opposed sections of the cladding may be "staggered" by approximately half the length of the section to ensure

that the vertical joints between two longitudinal adjacent sections are not aligned with

the vertical joints between diametrically opposed longitudinally adjacent sections. Moreover, each cladding section may be provided with a reduced-diameter spigot

portion at one end and an enlarged diameter socket portion at the opposite end (as

shown in GB-A-2335248), whereby longitudinally adjacent sections are secured to

one another by fitting a reduced diameter end spigot portion of one section into a

complementarily-shaped enlarged inner diameter end socket portion of the adjacent

section.

The outer surface of the assembled cladding comprises a smooth, generally

cylindrical surface 16. However, it will be observed that the cladding sections are

provided with a plurality of identical evenly-spaced depressions or recesses 18 in the

outer surface. In the embodiment illustrated in Fig 1 , the periphery of the depressions

is circular and the depressions are smoothly concave or dished. In the embodiment

illustrated, the diameter of the depressions is approximately 15cm and the maximum

depth of the depressions is approximately 3 cm. However, the diameter and depth of

the depressions may vary widely. Preferably the diameter of the circular depressions

is from 1cm to 30cm and more preferably from 10cm to 20cm, but it is not intended

that the invention is limited in any way to these preferred ranges.

In use, the cladding 10 is assembled on a pipe, drill riser, cable or other

elongate member and the clad elongate member is then positioned underwater. The

fact that the outer surface of the cladding is devoid of projections greatly facilitates

the laying of the clad elongate member by conventional pipe laying equipment which

removes the requirement for modification of the pipe laying equipment and improves

the reliability of the laying operation. The depressions or recesses 18 may be moulded into the outer surface of the

preformed semi-tubular sections 12. Alternatively, the preformed sections may be

moulded with a smooth exterior surface (i.e. without the depressions or recesses 18)

and the depressions or recesses may be cut or machined into the outer surface

subsequently, either before or after the preformed sections 12 are assembled on the

pipe, drill riser, cable or other elongate member.

The embodiment illustrated in Figs. 2 and 3 is very similar to that of the first

embodiment. However, it will be observed that instead of being formed in two semi-

tubular sections which are subsequently clamped together, the cladding 10' comprises a plurality of preformed, flexible, impervious, polyurethane tubular cladding sections

20, each of which is provided with a single longitudinally-extending slit 22 which

passes along the length of the cladding section, parallel to its longitudinal axis. In

order to fit the cladding section onto a pipe, cable or other elongate member, the

cladding section is opened at the slit 22 and manipulated onto the elongate member

to be protected. The cladding member may then be held in the shut position by the

use of metal bands (not illustrated) identical to those used in the embodiment of Fig.

1 or by any other suitable fixing means, such as two semi-circular half shells, coupled

together at their ends by fastenings, such as bolts. As for the embodiment of Fig. 1,

opposite ends of the cladding section may be provided with a spigot portion and a

complementarily-shaped socket portion respectively to assist in the connecting of

longitudinally adjacent sections.

As for the first embodiment, the outer surface 24 of the cladding section is generally smooth and cylindrical but is provided with a plurality of identical, evenly-

spaced elliptical depressions or recesses 26. In the embodiment illustrated, the major

axis of each elliptical depression is aligned parallel with the longitudinal axis of the

cladding section, although this need not be the case.

The dimensions of the depressions or recesses may vary widely. However,

in the illustrated embodiment the length of the major axis is approximately 15cm and

the length of the minor axis is approximately 7.5cm. The depth of the depressions

can also vary widely but in the embodiment illustrated it is approximately 3 cm.

Preferably, the length of the major axis is from 1cm to 30cm, and more preferably

from 10cm and 20cm. Preferably, the length of the minor axis is from 0.5cm to

20cm, and more preferably from 5cm to 15cm.

The embodiment illustrated in Fig. 4 comprises a cladding section 28 which

is moulded directly onto the outer surface of a pipe, drill riser, cable or other elongate

member, rather than being pre-formed and subsequently fitted as in the first and

second embodiments. As for the first two embodiments, the embodiment of Fig. 4

comprises a generally cylindrical smooth outer surface 30 but in contrast to the first

two embodiments, comprises a plurality of randomly-spaced depressions or recesses

32, formed in the cylindrical outer surface. The dimensions of the circular recesses

32 correspond to those for the first embodiment. The depressions or recesses 32 may

be moulded into the outer surface as the cladding section is moulded. Alternatively,

the cladding may be moulded with a smooth cylindrical exterior surface (i.e. without

the depressions or recesses 32) and the depressions or recesses may be cut or machined into the outer surface subsequently.

Fig 5a - 5d shows examples of possible orientations of the depressions along

the length of the cladding, Fig 5a shows the depressions in regular lines and columns,

Fig 5b shows lines and columns which are offset, Fig 5 c shows the depressions

arranged in "waves" which are in phase in the various rows; and Fig 5d shows the

depressions arranged in "waves" which are out of phase in the various rows.

Fig 6 is a partial section of a typical moulding showing depressions which

have sides which are substantially at right angles with flat bases of the depressions.

Purely by way of example only, the dimensions a, b, c, d and r can be as follows in

such a moulding:

a = 40 cms

b = 40 cms

c = 20 cms

d = 25 cms

r = 175cms

In each of the embodiments, when the cladding is in position underwater, the

provision of the depressions or recesses in the generally cylindrical outer surface of

the cladding interrupts or reduces vortex induced vibration. Moreover, in each case,

because the outer surface of the cladding is devoid of projections, the clad pipe, or

other elongate member, can be laid underwater using conventional laying

mechanisms which do not require modification.

In the above embodiments, the material from which the cladding is made need not be polyurethane but could, in fact, be any material which is sufficiently flexible

and impervious for the intended use. For example, in the above embodiments it

would be possible to make the cladding from a syntactic foam, e.g. a mixture of glass

microspheres and a thermoset resin matrix (with or without the inclusion of larger

macrospheres). A cladding in accordance with the present invention which is made

from syntactic foam would have increased buoyancy which can be desirable in some

circumstances. Indeed, the use of such a cladding is particularly suitable as a

buoyancy module for a drill riser. The use of syntactic foam offsets much of the riser

weight and the provision of depressions in the outer surface of the cladding in

accordance with the present invention reduces or eliminates vortex induced vibrations

on the riser.

As indicated in Fig. 7, one embodiment of drill riser buoyancy module in

accordance with the present invention comprises a plurality of identical, preformed,

releasably engaged semi-tubular sections 12' having an internal profile which, when

the sections are arranged with respect to one another, form an aperture for receipt of

the drill riser, auxiliary lines, riser clamps and the like. The sections 12' are moulded

from syntactic foam made from a mixture of glass microspheres and a thermoset resin

matrix (either with or without the inclusion of macrospheres). The required length

of buoyancy modules is assembled by arranging the preformed sections 12' along the

length of the drill riser. As indicated in Fig. 7, opposed semi-tubular sections of the

cladding are held together by means of metal bands B' passing around the outer

surface of the assembled cladding. Alternatively, the buoyancy module halves may be bolted together.

The outer surface of the assembled buoyancy module is cylindrical, but as for

the previous embodiments the cylindrical surface is provided with depressions or

recesses 18'. The number, size, shape and pattern of the depressions or recesses 18'

can, for example, be as for the previous embodiments, but are not restricted to those

details. As for the previous embodiments, the depressions or recesses 18' can be

formed integrally with the moulded preformed sections 12' or can be formed

subsequently, either before or after assembly on the drill riser.

The invention is not restricted to the details of the foregoing embodiments.

For example, the depressions as shown in each of the four embodiments, could be

used in each of the other embodiments. Moreover, the number, shape, dimensions

and pattern of the depressions or recesses can vary widely from those illustrated.

Preferably, for cladding of circular cross-section, the largest dimension of a

depression (e.g. the diameter of a circular depression or the length of the major axis of an elliptical depression) is from 5% to 50% (and more preferably between 10%

and 30%) of the external diameter of the cladding.

The periphery of the depressions or recesses could be polygonal or could be

any other shape which interrupts or reduces vortex induced vibrations. Moreover,

the depressions or recesses need not be smooth or dished and, for example, may

comprise side walls extending generally perpendicularly or at same angle or angles

to the cylindrical surface of the cladding and a flat or curved (e.g. part-cylindrical,

concentric with the cylindrical surface of the cladding) base wall. Furthermore, the material from which the cladding is made may incorporate

an anti-fouling agent which retards the build-up of material in the depressions which

might otherwise impair their effectiveness at reducing vortex induced vibrations. An

example of a suitable anti-fouling agent is tributyl tin (TBT) which is typically added

to the material used to manufacture the cladding in a concentration of 1 - 5%, more

preferably 2 - 3%.

Claims

1. An elongate underwater cladding (10) for an elongate member, comprising

an outer surface having a plurality of depressions (18) therein.

2. An underwater cladding as claimed in claim 1, wherein the depressions

(18) are arranged around the whole periphery or circumference of the cladding (10).

3. An underwater cladding as claimed in claim 1 or claim 2, comprising a

plurality of depressions (18) having a generally circular periphery .

4. An underwater cladding as claimed in claim 3, wherein the diameter of the

circular depressions is from 1cm to 30cm.

5. An underwater cladding as claimed in claim 4, wherein the diameter of the

circular depressions is from 10cm to 20cm.

6. An underwater cladding as claimed in any of the preceding claims,

comprising a plurality of depressions having an elliptical periphery.

7. An underwater cladding as claimed in claim 6, wherein the length of the

major axis of the elliptical depressions is from 1cm to 30cm.

8. An underwater cladding as claimed in claim 7, wherein the length of the

major axis of the elliptical depressions is from 10cm to 20cm.

9. An underwater cladding as claimed in any of claims 6 to 8, wherein the

length of the minor axis of the elliptical depressions is from 0.5cm to 20cm.

10. An underwater cladding as claimed in claim 9, wherein the length of the

minor axis of the elliptical depressions is from 5cm to 15cm.

11. An underwater cladding as claimed in any of the preceding claims, comprising an external circular cross-section and wherein the largest diameter of a

depression is from 5% to 50% of the external diameter of the cladding.

12. An underwater cladding as claimed in claim 11, wherein the largest

diameter of a depression is from 10% to 30% of the external diameter of the cladding.

13. An underwater cladding as claimed in any of the preceding claims,

comprising a plurality of depressions having a smooth depressed or recessed surface.

14. An underwater cladding as claimed in claim 13, comprising a plurality

of depressions which are smoothly concave or dished in cross-section.

15. An underwater cladding as claimed in claim 13, comprising a plurality

of depressions having a planar base wall.

16. An underwater cladding as claimed in claim 15, wherein the depressions

further comprise a side wall inclined to the planar base wall and extending to the

periphery of the depression.

17. An underwater cladding as claimed in claim 16, wherein the side wall

extends perpendicularly to the planar base wall.

18. An underwater cladding as claimed in any of the preceding claims,

wherein the depressions are substantially identical.

19. An underwater cladding as claimed in any of the preceding claims,

wherein the depressions are randomly spaced over the surface of the cladding.

20. An underwater cladding as claimed in any of claims 1 to 18, wherein the

depressions are regularly spaced over the surface of the cladding.

21. An underwater cladding as claimed in claim 20, wherein the depressions are arranged in a plurality of rows and/or columns.

22. An underwater cladding as claimed in claim 21, wherein the depressions

of the row or column are offset from those in an adjacent row or column.

23. An underwater cladding as claimed in claim 20, wherein the depressions

are arranged in a plurality of waves.

24. An underwater cladding as claimed in claim 23, wherein adjacent waves

of depressions are in phase with one another.

25. An underwater cladding as claimed in claim 23, wherein adjacent waves

of depressions are out of phase with one another.

26. An underwater cladding as claimed in any of the preceding claims,

comprising positively buoyant material.

27. An underwater cladding as claimed in claim 26, wherein the buoyant

material comprises syntactic foam.

28. An underwater cladding as claimed in claim 26 or claim 27, wherein the

cladding comprises solely syntactic foam.

29. An underwater cladding as claimed in claim 26 or claim 27, further

comprising macrospheres.

30. An underwater cladding as claimed in any of the preceding claims,

comprising a plurality of preformed sections assembled on, and secured to, the

elongate member to be protected.

31. An underwater cladding as claimed in claim 30, wherein the preformed

sections comprise semi-tubular sections.

32. An underwater cladding as claimed in claim 30, wherein the preformed

sections comprise tubular sections.

33. An underwater cladding as claimed in claim 32, wherein the tubular

sections are split along their length.

34. An underwater cladding as claimed in claim 33, wherein the split is

longitudinal.

35. An underwater cladding as claimed in claim 33, wherein the split is helical.

36. An underwater cladding as claimed in any of claims 1 to 29, wherein the

cladding is moulded directly onto the outer surface of the elongate member to be

protected.

37. An underwater cladding as claimed in any of the preceding claims,

wherein the depressions are moulded into the cladding.

38. An underwater cladding as claimed in any of claims 1 to 36, wherein the

depressions are cut, machined or otherwise formed into the outer surface of the

cladding after moulding the cladding.

39. An underwater cladding as claimed in any of the preceding claims,

wherein the cladding is profiled to receive objects other than the elongate member being protected.

40. An underwater cladding as claimed in any of the preceding claims,

wherein the outer surface is substantially cylindrical.