Title: Stabilizing lines or cables

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Abstract: A line having a central load-supporting member is provided with a stabilizing system to counteract instabilities caused by currents in a fluid surrounding the line. The stabilizing system comprises a single start helical formation projecting radially from the central member. BACKGROUND OF THE INVENTION This invention relates to lines or cables which, in use, are subjected to the forces of a fluid current. A particularly important example of lines is a deep-sea mooring line as used, for example, to retain a buoy at a predetermined depth and location in the ocean. Such lines are subject to a current-induced vibration (often referred to as "strumming" which reduces the lifetime of the line and which can cause other problems in various line applications (e.g., interference with the effective operation of a hydrophone array secured to the line). Although various arrangements have been proposed for reducing strumming, each of these has suffered from one or more of various drawbacks. Thus, heretofore it has been proposed to provide a thin rigid plate projecting radially from the outer surface of the line and pointing in the downstream direction of the current. Such an arrangement, however, would greatly increase the difficulty in paying-out or taking-up line over conventional drums and sheaves and would result in the generation of substantial forces of drag and/or lift where the plate was not parallel to the current (owing to the inevitable twist in a line of substantial length, variations of current direction with depth, etc.). In another proposal, the rigid plate is replaced by extended (i.e., of length several times the diameter of the cable) tufts of yarn secured to a braided outer wrapping of the line. Again there are lift and drag problems when the plane of the yarn is not parallel to the current at any depth. Additionally, deterioration of the yarn results from agitation produced by vortices created as the fluid flows past the line. In another arrangement a wing-like enclosure surrounds the line. The great expense of this arrangement, coupled with lift generation and obvious handling problems, has limited its usefulness. More obscure proposals have included the suggestion of a plurality of fins or wires, of very small diameter relative to that of the line, which extend either longitudinally or as a multiple-start helix around the line. While this proposal avoided the problems discussed above, it was a failure in not reducing the strumming to an acceptable level. In view of the foregoing, it is a principal object of the present invention to provide a system for stabilizing a line or cable which experiences current forces, the system being effective to substantially reduce strumming, while not materially increasing drag or lift forces on the line. Further objects include the provision of such a system which permits the line to be handled by conventional drums and sheaves, which has a long operating lifetime, which achieves the principal objects independent of current direction relative to the line, and which is inexpensive. SUMMARY OF THE INVENTION A line constructed in accordance with the present invention comprises a load-supporting member and a stabilizing system which comprises a single start helical formation disposed around the load-supporting member. Preferably, that stabilizing formation projects from the central member outer surface by a distance equal to at least about (10/.pi.)% of the circumferential length of the central member; the central member has a circular cross section and the repeat length of the helical formation is of the order of a plurality of diameters of the central member; the helical formation is provided either by a flexible elongated member wound around the central member or by a formation integral with an outer sheath of the central member; and the helical formation has either a polygonal or circular cross-sectional shape. Other objects, features, and advantages of the invention will appear from the following description of particular preferred embodiments illustrated in the accompanying drawings.


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