Crack Propagation in Dynamic Power Cables

Abstract

Abstract One failure mechanism of subsea cables is the degradation of the insulation layers by so-called “water treeing” induced by strong electric fields in defects in the material. Usually, the growth of water trees is studied by considering the cyclic variation of Maxwell stresses at the tip of the defect. However, there is a concern for dynamic power cables (i.e., cables with low bending stiffness) what effects the additional contribution from cyclic mechanical stresses will have on the cable’s structural and electrical integrity and operational life. A dynamic cable installed on a wave energy converter is studied. A vented water tree defect is (assumed to be) initiated in the XLPE insulation material of one of three conductors of a dynamic power cable. COMSOL Multiphysics software is used to simulate how the electric field strength near the tip of the water tree channel increases as the length of the water tree increases. A water tree growth model based on cyclic Maxwell stresses is employed to estimate the time it takes for the water tree to grow until a short circuit of the cable occurs. Numerical simulations in the hydrodynamic software SIMA and the finite element software ABAQUS are carried out for several sea states and ocean current loading conditions to investigate if mechanical stresses due to motions of the dynamic cable shorten the cable’s lifetime. The results from these simulations are used in a fracture mechanics-based model to see how fast a defect in the insulation material grows only due to mechanical stresses. It is found that under the investigated load conditions, the mechanical stresses in the insulation layers are not large enough to contribute to crack growth. It implies that despite the cable motions and the cyclic mechanical stresses in the cable components, the water tree failure of this dynamic cable is mainly associated with the electric field and the resultant cyclic Maxwell stress.