This paper presents a non-linear spring element in which the extensional and torsional behaviour is coupled. The spring element development relies on a reported local analytical model that predicts the relationship between axial translation and rotation in wire ropes. This model also takes into account the effects of interwire friction by considering two different stiffnesses: no-slip and full-slip. A heave-excited offshore steel cable was analyzed considering four situations: uncoupled and coupled extensionaltorsional behaviour with full-slip or no-slip axial stiffness. The obtained static, dynamic and modal responses were distinct. Furthermore, the implementation of the spring element in a finite element program allows the analysis of composite structures such as mooring lines, as this program is capable of combining different types of elements in the same model.
Steel cables have been used in a great variety of applications in offshore industry due to their high axial strength combined with a low bending stiffness. They are used as structural components of mooring systems in floating production platforms and play an important role in the installation of submarine equipment, flexible pipes and anchor lines. Basically, two types of steel cables are used in offshore applications: spiral strands and wire ropes. It is common knowledge that a tensile load provokes not only extension, but also torque or rotation in spiral strands and wire ropes, due to their helical geometry. This extensional-torsional behaviour is quite important in the structural analysis of steel cables. It is related to some of the most frequent failure mechanisms of these structures such as hockling, birdcaging and kinking. Furthermore, recent investigations (Chaplin et al, 2000) showed that the fatigue life of a six stranded wire rope is significantly affected by the amplitude of torsional twisting and, therefore, is influenced by this combined tension-torsion behaviour.
