Factors Influencing The Endurance Of Steel Wire Ropes For Mooring Offshore Structures

Abstract

The service factors influencing the endurance of steel wire rope moorings for offshore structures are discussed. A tensiontension fatigue SoN curve for six strand ropes is presented. consideration of cumulative damage resulting from a typical TBP spectrum shows tensile fatigue as a non-critical upper bound on endurance. However, combined bending-tension fatigue at fairlead arrangements is considered to represent the most severe fatigue loading. This mechanism is discussed in relation to existing knowledge of wire rope fatigue behaviour which provides an indication of the influence of the principal variables. The effect of the seawater environment is also reviewed and it is concluded that adequate protection can be provided by combinations of galvanizing, blocking compounds and impressed current systems, while jacketing may also provide long term protection. Caution is expressed regarding the use of bright wires and the consequences of zinc dissolution resulting from extended exposure. The problems of inspection and NOT are also discussed and recommendations are made as to how this may be effected for the critical fairlead region with the mooring line in place. Recommendations are also made as to what further work is necessary to give operators and certifying authorities greater confidence in the endurance of wire ropes for permanent moorings.