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 a first aspect of the present invention an underwater cladding for an elongate member comprises a substantially cylindrical outer surface having a plurality of grooves therein and extending therealong.

It has been found that the grooves cause disruption of flow around the cladding in use, reducing vortex induced vibration. In certain circumstances the grooves are also found to beneficially reduce drag forces on the cladding and hence on the elongate member. These effects associated with the presence of the grooves have been found to occur in certain circumstances in both steady and fluctuating flow.

In a preferred embodiment, the grooves extend substantially parallel to the longitudinal axis of the cladding. Other arrangements of the grooves are however possible, within the scope of the invention. In one such embodiment the grooves are helical.

The grooves may be mutually parallel. They may be equally mutually spaced, but an unequal or even random groove spacing is also within the scope of the present invention.

Preferably, the grooves are arranged around the whole periphery or circumference of the cladding. Circumferential coverage of the grooves ensures suppression of vortex induced vibrations arising from omnidirectional flows.

The groove depth is to be chosen with reference to the diameter of the cladding but preferably lies between lcm and 30cm.

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 grooves 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 grooves 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 grooves may be moulded into the outer surface of the cladding. Alternatively, the cladding may be moulded without the grooves, which may be cut, machined or otherwise formed into the outer surface subsequently. The cladding may be formed as thermal insulation.

While the first aspect of the present invention relates to cladding for an underwater member, the advantage of vortex induced oscillation reduction can also be achieved where no such cladding is provided.

In accordance with a second aspect of the present invention there is an elongate underwater member comprising a substantially cylindrical outer surface having a plurality of grooves therein and extending therealong.

Here again the grooves serve to disrupt flow around the outer surface and to reduce vortex induced vibration, and in some cases drag.

The elongate underwater member may for example be a pipe, riser, drill riser, cable or drill riser buoyancy.

As for the first aspect of the invention, the grooves may extend substantially parallel to the longitudinal axis of the cladding or may for example be helical.

The grooves may be mutually parallel. They may be equally mutually spaced, but an unequal or random groove spacing is also within the scope of the second aspect of the invention.

Preferably, the grooves are arranged around the whole periphery or circumference of the cladding.

The groove depth is to be chosen with reference to the diameter of the elongate member but preferably lies between lcm and 30cm.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Fig. 1 is a perspective view of a first underwater cladding embodying the present invention; Fig. 2 is a similar perspective view of a further underwater cladding embodying the present invention; Figs. 3 (a)- (d) are fragmentary views in cross section of four different claddings embodying the present invention; Fig. 4 is a perspective view of still a further underwater cladding embodying the present invention and being constructed in sections; Fig. 5 is a side view of yet a further underwater cladding embodying the present invention and again being constructed in sections; and Fig. 6 is a cross section along arrows X-X in Fig. 5.

Referring firstly to Fig. 1, a cladding or sheath 10 for an underwater elongate member such as a pipe, drill riser, cable etc (not itself illustrated) can be seen to have a cylindrical outer surface 12 into which are formed several axially extending grooves 14. In the illustrated embodiment the grooves are equally spaced about the circumference of the cladding 10, although in other embodiments the circumferential spacing of the grooves may be unequal or even random.

An alternative form for the grooves is illustrated in Fig. 2 where the cladding 10'again has a cylindrical outer surface 12'but in this embodiment a plurality of grooves 14'extend helically. Thus the grooves extend both along the length of the cladding and around its circumference.

The profile of the grooves can be chosen to provide appropriate hydrodynamic characteristics. Figs. 3 (a)- (d) show four different embodiments of the grooves according to the invention. Here the cladding's outer surface is seen at 12"and the grooves 14 (a-d) are respectively part-circular (14a), oblong (14b), tapered flat- bottomed (14c) and"V"shaped (14d).

The examples 10,10'of cladding shown in Figs. 1 and 2 are formed as tubes, each having an internal diameter chosen to receive the elongate underwater member (pipe, drill riser, cable etc) to which they are to be applied. Clearly with such embodiments the elongate underwater member must be inserted axially into the cladding which can be problematic.

It is known instead to construct cladding from a set of part-circular interlocking sections which can be assembled and secured around the elongate underwater member to form a generally tubular cladding. Our own published application GB-A-2335248 discloses such a construction in detail and Fig. 4 of the present application illustrates how cladding according to the present invention can be constructed in a similar manner.

The cladding 20 in question forms a tubular, flexible, impervious, polyurethane casing comprising a plurality of identical, preformed, releasably engaged semi-tubular sections 22 which are arranged with respect to one another to provide a cylindrical passage 24 whose diameter is chosen to receive the relevant elongate underwater member.

The required length of cladding is assembled by arranging by arranging the appropriate number of sections 22 along the length of the underwater member. As illustrated schematically in Fig. 4, opposed semi-tubular sections 22 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 opposed end (as shown in GB-A-2335248), whereby longitudinally adjacent sections are secured to one another by fitting a reduced diameter end spigot of one section into a complementarily-shaped enlarged inner diameter end socket portion of the adjacent section.

The assembled cladding forms a substantially cylindrical outer surface 24 but in common with previously described embodiments there are several grooves 26 formed in the outer surface and extending therealong. In the illustrated example these grooves are straight and axial but other groove formations (eg helical) may again be used.

In use, the cladding 20 is assembled on a pipe, drill riser, cable or other elongate underwater 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 grooves 26 may be moulded into the outer surface of the preformed semi- tubular sections 22. Alternatively, the preformed sections may be moulded with a smooth exterior surface (i. e. without the grooves) and the grooves may be cut or machined into the outer surface subsequently, either before or after the preformed sections 22 are assembled on the pipe, drill riser, cable or other elongate member.

The embodiment illustrated in Figs. 5 and 6 is somewhat similar to that of the Fig. 4 embodiment. However, instead of being formed in two semi-tubular sections which are subsequently clamped together, cladding 30 here comprises a plurality of preformed, flexible, impervious, polyurethane tubular cladding sections 32, each of which is provided with a single longitudinally-extending slit 34 which passes along the length of the cladding section, parallel to its longitudinal axis. In order to fit the cladding section onto an underwater pipe, cable or other elongate member, the section is opened at the slit 34 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. 4 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. 4, 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 in previously described embodiments substantially cylindrical outer surface 36 of the cladding has formed in it a set of grooves 3 8, these being straight and axial in the illustrated example.

In each of the embodiments, when the cladding is in position underwater, the provision of the grooves 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 to enable the cladding to serve as a buoyancy module for a drill riser. The use of syntactic foam offsets much of the riser weight and the provision of grooves in the outer surface of the cladding in accordance with the present invention reduces or eliminates vortex induced vibrations on the riser.

The material from which the cladding is made may incorporate an anti-fouling agent which retards the build-up of material in the grooves 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 conventional of 1-5%, more preferably 2-3%.

While the above described embodiments of the present invention all concern ducting of one form or another for receiving an elongate member-pipe, drill riser, cable etc. -it is to be understood that the grooves of the present invention may, in situations where such an underwater elongate member is used without such cladding, be formed in the outer surface of the elongate member itself. The appearance of such a member may for example be generally in accordance with Fig. 1 or Fig. 2.