Bitumen Shear Mechanics in a Dynamic Subsea Electrical Cable

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.