The author has fairly recently reported details of theoretical predictions for the axial fatigue life of large-diameter (e.g. 127 mm O.D.) spiral strands based on first principles. The predictions have been verified against a very extensive set of test data for spiral strands with diameters ranging from 25 to 164 mm. The paper presents newly developed S-N curves which, unlike the previously available design S-N curves, take variations in the lay angles of large-diameter (i.e., realistic) spiral strands into account, and also cater for the effects of end terminations. The proposed S-N curves are based on extensive theoretical parametric studies using the newly proposed model. In addition, the question of size effects as regards the axial fatigue performance of spiral strands will be addressed in some detail. Finally, the proposed S-N curves are compared with those recommended by others.
The safety of the many deep-water platform concepts is, among other considerations, strongly dependent on the reliability of the anchoring systems which should have a high level of integrity and whose costs of installation and replacement are very high. Steel cables are used for mooring certain types of offshore platforms such as semi-submersibles, guyed towers, etc., and are proposed as elements of mooring systems of wave energy devices. The loading spectrum on the individual mooring lines is obviously very complex: It depends mainly upon the type of structure, its location and type of mooring system adopted. The most significant aspects of the service conditions are long lives (in excess of perhaps 20 years) and the random nature of imposed loading. Permanent immersion of most of the cable in seawater is another important consideration. Cable design and manufacture are often considered to be an art rather than a science. The limits of validity of present design and calculating routines, largely based on commercial experience, are far from clear and it is an area where the rule of thumb reigns supreme.