Steel wire ropes use individual wires moving relative to each other in a spherical structure. The behaviour of wire ropes exposed to loading, are not as beams or other structural elements. The ropes function is optimal when the individual wires can move in the rope. Several internal and external conditions can prevent the individual wires to move optimal, and change the behaviour of the rope. Based on two double overload failures, the paper describe the condition and behaviour of the failed steel wire ropes mainly as consequences of distorting the behaviour of the individual wires.
Offshore steel wire ropes are made of steel with typically 0.75–0.86% carbon content. The wires are galvanized using zinc. The individual wires are lubricated to fill the spaces between the individual wires. They are helically wound together to form a steel wire rope in specified combinations. If specified by the customer, they add polymer coatings outside the rope. They make end terminations, and the steel wire ropes are tested. The ultimate strength of offshore steel wires is typically about 2000MPa.
We have previously summarized the failures of Norwegian offshore anchoring lines, including the failures of steel wire ropes (Kvitrud, 2014). We have included information about Norwegian steel wire rope failures in lifting and drilling appliances. We are relying heavily on the industry investigative reports. Information about failures in offshore steel wire ropes are in e.g. Ma et al (2013), Leeuwenburgh and Brinkhuis (2014) and Leeuwenburgh (2015).
We describe the circumstances of the two Norwegian double mooring line failures. Then we describe the change of behaviour caused by violation of the individual steel wires possibility to move relatively to each other, as a function of lubrication, wear, corrosion, the distribution of loads over the cross-section of the rope, twist, bending, payout of lines and testing to 100-year load levels. We concentrate on the rope behaviour, and do not discuss the size of the tension. The size of the loads from the waves are currently investigated in the EXWAVE JIP project.