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[0001] The present invention relates to the protection of underwater pipes, drill risers, cables or other elongate members.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] In accordance with the present invention, an elongate underwater vortex shedding cladding for an elongate member comprises an outer surface having a plurality of depressions therein.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] In one embodiment, the depressions have a generally circular periphery. In another embodiment, the depressions have an elliptical periphery.
[0010] 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.
[0011] The size of the depressions may vary widely. For example, for circular depressions the diameter may vary from 1 cm to 30 cm and is preferably between 10 cm and 20 cm in diameter. For elliptical depressions, the length of the major axis is preferably between 1 cm and 30 cm with the width of the minor axis being between approximately 0.5 cm and 20 cm. Preferably the length of the major axis is from 10 cm to 20 cm and the length of the minor axis is preferably from 5 cm to 15 cm.
[0012] 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.
[0013] The depressions may be regularly spaced over the cylindrical surface of the cladding or may be randomly spaced. Preferably, the depressions are substantially identical.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] Preferably, the outer surface of the cladding is substantially cylindrical.
[0019] By way of example only, specific embodiments of the present invention will now be described, with reference to the accompanying drawings, in which:
[0020]
[0021]
[0022]
[0023]
[0024]
[0025]
[0026] Referring firstly to
[0027] 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
[0028] 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.
[0029] The outer surface of the assembled cladding comprises a smooth, generally cylindrical surface
[0030] In use, the cladding
[0031] The depressions or recesses
[0032] The embodiment illustrated in
[0033] As for the first embodiment, the outer surface
[0034] The dimensions of the depressions or recesses may vary widely. However, in the illustrated embodiment the length of the major axis is approximately 15 cm and the length of the minor axis is approximately 7.5 cm. 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 1 cm to 30 cm, and more preferably from 10 cm and 20 cm. Preferably, the length of the minor axis is from 0.5 cm to 20 cm, and more preferably from 5 cm to 15 cm.
[0035] The embodiment illustrated in
[0036]
[0037]
[0038] a=40 cms
[0039] b=40 cms
[0040] c=20 cms
[0041] d=25 cms
[0042] r=175 cms
[0043] 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.
[0044] 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.
[0045] As indicated in
[0046] 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
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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%.