On the Design of Mooring Lines for Deep Water Applications

As ocean engineers begin to extend their analytical and design capabilities from the shallow regions of the oceans to the deep regions it becomes necessary to examine the established computational algorithms and determine to what extent they can be applied to problems of current interest. In this spirit, this paper generalized the catenary equations for inextensible mooring lines, by examining them in a dimensionless form, which allows the generation of certain design curves applicable to a broad class of naval off-shore operations. As an example, candidate mooring lines for an offshore platform are considered in water depths ranging from 1000 to 4000 ft with the maximum tension prescribed at 5 percent excursion from station. The candidate mooring lines consist of single component (all wire rope, all chain) as well as multiple component (wire rope/chain) combinations. For this example it was observed that for wire rope/chain mooring lines a large restoring force could be obtained and that the magnitude of the restoring force was dependent upon the depth of water and the amount of wire rope outboard of the offshore platform. Further it was observed that certain wire rope/chain combinations offer some desirable characteristics as the water depth increases.