Dynamic submarine cables are used to provide electrical power to floating oil/gas production platforms and to export power from marine renewable energy systems such as floating wind turbines. During installation and life-time operation, the dynamic power cable is subjected to various loads, e.g. axial tension, bending and torsion. A typical three-phase AC cable consist of three helically-interwound power cores with three polymeric fillers to accomplish a circular circumference over which sheaths and tensile armour layers are applied. The present paper focuses on the mechanical behaviour of a dynamic submarine cable subjected to a combined axial tension, radial pressure and bending load. Particular interest is emphasized on the stick-slip behaviour of the helically-interwound power cores. 3D finite element (FE) model has been developed, where the interactions on all layers of the cable are extrinsically taken into account. The stick-slip behaviour of the power cores and the resulting friction stress induced during cable bending can have a large impact on the fatigue life of the dynamic cable. Kinematics of slips between the cable core components resulting from FE simulations will be analyzed and compared to the assumption of fully loxodromic slips typically used in the formulation of analytical model for helically-wound cables. The output of the present analyses will provide a better understanding on the mechanical behaviour of the power cores of dynamic submarine power cables in response to cable bending and provide a basis to verify and develop analytical models that can be used for, e.g. fatigue life assessments.

The bending mechanics of dynamic subsea electrical cables are strongly influenced by the frictional shear interactions that exist between the armour wires and surrounding materials. In some cases the armour wires are clad with bitumen filler material so that the shear mechanics of bitumen are important to the overall bending mechanics of the cable. In this work the shear mechanics of bitumen are studied. Pull-out tests on bitumen-clad armour wires were conducted in-situ on a stub of a dynamic subsea electrical test cable. Test temperatures ranged from 0°C to room temperature and shear velocities ranged from 0.1mm/min to 40mm/min. A thermally activated model of the bitumen shear interaction, incorporating temperature, speed and displacement dependence was proposed and implemented into a finite element code. The testing and modelling was then extended to incorporate the effects of cyclic loading. The bitumen response was highly sensitive to test temperature, applied velocity and cyclic loading with the shear strength varying by approximately two orders of magnitude over the range of conditions studied. The experimental results and model predictions indicate that the shear mechanics of cables containing bitumen clad armour wires differ from the shear mechanics of cables that do not. The bending stiffness displays a velocity and temperature dependence and relaxation of cable stresses is expected during holding events. Because of this, it is recommended that modelling of bitumen clad armour wires be conducted using a suitable bitumen interaction model and not a classic friction model.

No abstract

No abstract