Tension-torsion fatigue effects in wire ropes

As oil exploration and production moves to deeper waters, there is increasing reliance on moored floating structures and consequently anchoring Systems are becoming more complex. This complexity inevitably translates also to more difficult Installation procedures. Some components (especially stranded wire ropes), used both as part of permanent mooring Systems and in the raising and lowering of other components, have a tendency to untwist under tensile load. This tendency, if unrestrained can transfer rotation into other parts of the mooring System which are more sensitive to twisting. Dynamic rotations associated with fluctuating tensions are also a potential cause for concern. This article addresses these concerns and in particular describes the results of recent research which has disturbing implications for the reliability of mooring lines where stranded wire rope is allowed to twist without restraint.