Bending-tension fatigue in six-strand wire ropes

This paper discusses a highly damaging yet localised form of rope fatigue termed bending-tension fatigue, which combines low amplitude bending motions (the bending-length) over a sheave with fluctuating tension. A summary of prior work is made, with particular emphasis on the mooring of floating offshore platforms. The review draws together for the first time all the available data for six-strand wire ropes for a range of rope diameters Ø13–Ø127 mm, including previously unpublished data. By normalising the B-T fatigue endurance it is possible to model the B-T fatigue behaviour concisely across the range of rope diameters. In all previous B-T work the bending-length of the rope over the sheave has been assumed to be controlled by the elastic stretch of the rope in response to the fluctuating tensions and it was not seen as practical to model the wrapping motion of the rope on the sheave. This paper describes how a bending-tension fatigue test rig used in the early work has been modified to permit investigation of the effect of active wrapping of the rope on and off the fairlead sheave both in and out of phase with the fluctuating load. Results of testing undertaken to investigate the effect on B-T fatigue of active wrapping–unwrapping such as would occur in service offshore are presented. Finally, the finding that a Ø13 mm rope may be used to model the behaviour of larger six-strand ropes has been used to investigate the service failure of an anchor line on a mobile offshore drilling unit. The loading conditions which the line experienced during the incident were recorded and were available for testing research. The full storm load-time sequence has been run on the test rig to replicate the loads experienced by the rope in service up to failure.