WO2002040875A2

WO2002040875A2 - Protection of underwater elongate members

Info

Publication number: WO2002040875A2
Application number: PCT/GB2001/005024
Authority: WO
Inventors: Robert Gibson
Original assignee: Crp Group Limited
Priority date: 2000-11-15
Filing date: 2001-11-15
Publication date: 2002-05-23
Also published as: WO2002040875A3; GB0308660D0; GB2386662B; GB2386662A; AU2002223806A1; GB0027858D0

Abstract

An underwater cladding for an elongate member such as a pipe (10) has outwardly projecting flexible members (16, 24, 28) which are deformed by a current flow to guide water around the elongate member. The elongate members may be in the form of strands or fingers (16), narrow flaps (24) or longitudinally extending flaps (28).

Description

DESCRIPTION

PROTECTION OF UNDERWATER ELONGATE MEMBERS

The present invention relates to the protection of underwater pipes,

cables or other elongate members.

When water flows past an underwater pipe, cable or other elongate

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

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

structure. If the natural f equency 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, 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, cable or the like is secured. In extreme

cases, the coupling between the pipe, cable or the like and the connector is damaged.

One technique for reducing the vortex induced vibrations involves

cladding the pipe, cable or other elongate member with an elongate fairing which is

hydrodynamically shaped to guide a current of water around the pipe without causing

vortices to be shed, thereby eliminating or greatly reducing the occurrence of vortex

induced vibrations. However, cladding of this type is unidirectional, i.e. only suitable

for producing the desired flow for a current flowing in one direction past the pipe,

cable or other elongate member. Omnidirectional variations of the cladding are

known, in which the fairing is rotatably mounted and rotates under the influence of

the water current to adopt the position in which the smoothest flow occurs. However, in an underwater environment such pivotally mounted fairings tend to seize up in one

position, often increasing the level of vortex-induced vibrations and substantially

increasing the drag of the system.

It is an object of the present invention to provide a cladding for an

underwater pipe, cable or other elongate member which eliminates or reduces vortex

induced vibrations without the problems associated with the prior art.

In accordance with the present invention, an underwater cladding for

an elongate member comprises a plurality of outwardly projecting flexible members

which are deformable by a current flow to guide water around the elongate member.

Preferably, a plurality of the flexible members overlap when deformed

by the current flow.

Preferably, the cladding comprises a generally tubular casing for

enclosing the elongate member and the flexible members project from the casing.

The casing may comprise a flexible material, e.g. polyurethane. In one embodiment

the casing comprises a generally planar mat which is wrapped around an elongate

member to be clad and has means for securing the mat in position around the

elongate member, e.g. one or more circumferential securing bands.

The flexible members may be moulded onto the casing. The flexible

members may also be neutrally buoyant and may comprise polyester or

polypropylene.

The length of the flexible members in their projecting direction is

preferably at least 0.5 times the diameter of the elongate member to be clad and is preferably at least 150mm.

Preferably the flexible members are elongate.

In one embodiment the flexible members comprise strands or fingers.

The strands or fingers preferably extend generally radially with respect to the

elongate member on which the cladding is mounted.

The strands or fingers are preferably circular in cross-section, e.g.

having a diameter from 3mm to 5mm. They may be arranged in rows which are

preferably parallel to one another. The rows may be parallel to the longitudinal axis

of the elongate member.

In another embodiment, the flexible members comprise flaps. The

thickness of the flaps is preferably from 1.0 to 1.5mm.

The flaps may be elongate in their proj ecting direction. Preferably the

flaps are arranged side-by-side. They may be arranged in a plurality of rows which

are preferably parallel to each other and parallel to the longitudinal axis of the

cladding and/or the elongate member. The width of the flaps is preferably

approximately from 5mm to 15mm, preferably approximately 10mm wide.

In another embodiment the flaps are elongate along the length of the

cladding. They may extend along substantially the whole length of a cladding

section.

The present invention also includes an elongate member, e.g. a pipe,

clad with an underwater cladding in accordance with the present invention

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 pipe cladding in

accordance with the present invention, shown in a relaxed condition;

Fig. 2 is a cross-sectional end elevation of the cladding of Fig. 1

mounted on a pipe, when placed in a flow of water;

Fig. 3 is a perspective view of a second embodiment of cladding in

accordance with the present invention; and

Fig. 4 is a perspective view of a third embodiment of pipe cladding

in accordance with the present invention.

Referring firstly to Figs. 1 and 2, a conventional tubular pipe 10 of

circular cross section is enclosed in a protective elongate cladding 12. The cladding

12 comprises a flexible rectangular polyurethane mat 14 which is wrapped around the

pipe 10 to form a tubular cladding and is held in position by means of conventional

circumferentially-extending metal securing bands (not illustrated).

A plurality of flexible strands or fingers 16 extend radially from the

cylindrical outer surface of the deformed mat 14 and in a relaxed state, as illustrated

in Fig. 1, the strands or fingers 16 project substantially radially with respect to the

longitudinal axis of the cladding. The strands 16 are flexible in that they can be

easily deformed by the flow of water to lie against the tubular outer surface of the

cladding 12. The dimensions of the strands or fingers 16 depend to a large extent on

the diameter of the pipe to be protected. The length of the strands or fingers 16 in the projecting, radial direction should preferably be at least 0.5 times the diameter of the

pipe to be protected and preferably at least 150mm in length. In the embodiment

illustrated, for a pipe of 325mm diameter, the strands are approximately 250mm in

length and are circular in cross-section with a diameter from 3 to 5mm.

The strands are arranged in a series of parallel rows 18 which extend

along the outer tubular surface of the rolled mat 14 parallel to the longitudinal axis

of the tubular cladding and of the pipe 10. Adjacent rows 18 are spaced apart by a

distance from about 10 to 15mm and adjacent strands or fingers 16 in each row are

separated by approximately the same distance. The strands or fingers are neutrally

buoyant so that when they are not deformed by a current flow they project generally

radially outwardly. They may conveniently be formed from polypropylene but may

be formed from other materials, for example, polyester. The strands or fingers 16

may be moulded onto the mat 14 which forms the cladding or may be secured by

other means, for example by stitching or glueing the strands in place.

When the cladding of Fig. 1 is secured in position on a pipe 10 to be

protected, in the absence of a flow of water past the pipe, the strands or fingers 16

adopt the relaxed position shown in Fig. 1, in which they extend generally radially

with respect to the longitudinal direction of the pipe.

However, when a current of water (indicated by arrow C in Fig. 2)

flows past the pipe, the strands or fingers 16 are deformed by the flow of water such

that they overlap one another, and, as seen in Fig. 2, form a profile which is akin to

that of a fairing and which guides the water around the pipe smoothly, with the absence of, or with significantly reduced, vortices. The occurrence of vortex-induced

vibrations is thus greatly reduced or even eliminated.

As will be apparent, the cladding is omnidirectional since the

flexibility of the strands or fingers 16 allows them to form a profile having a rounded

head H upon which the flow of water is incident and a streamlined tail T around

which the water flows smoothly.

A variation of the embodiment of Fig. 1 is shown in Fig. 2, in which

the strands or fingers 16 of the first embodiment are replaced with a plurality of

elongate flaps 24. The flaps are formed from the same material as the strands 16 of

Fig. 1 (and thus are neutrally buoyant) and are of the same length in the projecting,

radial direction. However, the flaps are approximately from 5mm to 15mm

(preferably approximately 10mm) wide and from 1.0 to 1.5mm thick. As for the first

embodiment, the flaps 24 are arranged side-by-side (or edge to edge) in rows 26

which are spaced from one another by from 10 to 15mm and which extend parallel

to the longitudinal direction of the cladding and the pipe 10. However, adjacent flaps

in each row are separated from one another by approximately 5mm.

The second embodiment operates in an identical manner to the first

embodiment and, as illustrated in Fig. 3, the flaps 24 overlap one another to assume

a streamlined shape in the presence of a flow of water. However, the provision of

flaps 24 rather than strands or fingers 16 provides a much greater surface area and

significantly improves the streamline flow of water past the clad pipe 10.

A third embodiment is illustrated in Fig. 4 which is a variation of the embodiment of Fig. 3. In the embodiment of Fig. 4, instead of having a plurality of

narrow flaps arranged in a row, a plurality of elongate rectangular flaps 28 is

provided. The flaps 28 preferably (but not necessarily) extend along the whole length

of the mat 14 to which they are secured and are secured along one long edge which

extends parallel to the longitudinal direction of the cladding and the pipe 10. The

radial length of the flaps is the same as that for the first and second embodiments.

Moreover, adjacent flaps 28 are spaced from one another by a distance which is the

same as the distance between adjacent rows of strands or fingers in Fig. 1. The

material and the thickness of the flaps 28 are the same as that of the narrow flaps 24

of Fig. 3 and the flaps 28 are neutrally buoyant.

In the relaxed state, when no current of water is flowing, the flaps

extend generally radially. As for the first two embodiments, when placed in a flow

of water the flaps 28 overlap one another and assume a streamline shape as illustrated

in Fig. 4. The use of much longer flaps 28 further increases the surface area of the

profiled cladding and thereby improves the streamlined shape of the cladding,

resulting in a reduction in the occurrence of vortices.

Claims

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

plurality of outwardly projecting flexible members (16, 24, 28) which are deformable

by a current flow to guide water around the elongate member.

2. An underwater cladding as claimed in claim 1, wherein a plurality of the

flexible members (16, 24, 28) overlap when deformed by the current flow.

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

generally tubular casing (14) for enclosing the elongate member (10), the flexible

members (16, 24, 28) projecting from the casing.

4. An underwater cladding as claimed in claim 3, wherein the casing (14)

comprises a flexible material.

5. An underwater cladding as claimed in claim 4, wherein the casing (14)

comprises polyurethane..

6. An underwater cladding as claimed in claim 4 or claim 5, wherein the

casing (14) comprises a generally planar mat which is wrapped around an elongate

member (10) to be clad and means for securing the mat in position around the

elongate member (10).

7. An underwater cladding as claimed in claim 6, wherein the means for

securing the mat in position comprises one or more circumferential securing bands.

8. An underwater cladding as claimed in any of claims 3 to 7, wherein the

flexible members (16, 24, 28) are moulded onto the casing.

9. An underwater cladding as claimed in any of the preceding claims, wherein the projecting flexible members (16, 24, 28) are neutrally buoyant.

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

wherein the projecting flexible members (16, 24, 28) comprise polyester or

polypropylene.

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

wherein the length of the flexible members in their projecting direction is at least 0.5

times the diameter of the elongate member to be clad.

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

wherein the length of the flexible members in their projecting direction is at least

150mm.

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

wherein the flexible members (16, 24, 28) are elongate.

14. An underwater cladding as claimed in claim 13, wherein the elongate

flexible members comprise strands or fingers (16).

15. An underwater cladding as claimed in claim 14, wherein, in the

undeformed state, the strands or fingers (16) extend generally radially with respect

to the elongate member on which the cladding is mounted.

16. An underwater cladding as claimed in claim 14 or claim 15, wherein the

strands or fingers (16) are substantially circular in cross-section.

17. An underwater cladding as claimed in claim 16, wherein the diameter of

the strands or fingers (16) is from 3mm to 5mm.

18. An underwater cladding as claimed in any of claims 14 to 17, wherein the strands or fingers (16) are arranged in a plurality of rows (18).

19. An underwater cladding as claimed in claim 18, wherein the rows (18)

are substantially parallel to each other.

20. An underwater cladding as claimed in claim 19, wherein the rows (18)

are substantially parallel to the longitudinal axis of the elongate member (10).

21. An underwater cladding as claimed in any of claims 1 to 13, wherein the

flexible members comprise flaps (24, 28).

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

the flaps is from 1.0 to 1.5mm.

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

flaps (24) are elongate in their projecting direction.

24. An underwater cladding as claimed in claim 23, comprising a plurality

of flaps (24) arranged side-by-side.

25. An underwater cladding as claimed in claim 24, wherein the flaps (24)

are arranged in a plurality of rows (26).

26. An underwater cladding as claimed in claim 25, wherein the rows (26)

are substantially parallel to each other.

27. An underwater cladding as claimed in claim 26, wherein the rows (26)

28. An underwater cladding as claimed in any of claims 21 to 27, wherein the

width of the flaps (24) is from 5mm to 15mm.

29. An underwater cladding as claimed in claim 28, wherein the width of the flaps is approximately 1 Omm.

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

flaps (28) are elongate along the length of the cladding.

31. An underwater cladding as claimed in claim 30, wherein the flaps (28)

extend along substantially the whole length of a section of cladding.

32. An elongate member clad with an underwater cladding as claimed in any

of the preceding claims.

33. A pipe clad with an underwater cladding as claimed in any of the preceding claims.