Session: 02-12-02 Structural Analysis and Optimization 2

Paper Number: 79467

79467 - Crack Propagation in Dynamic Power Cables 

The offshore energy sector is installing floating offshore wind turbines and farms with tidal and wave energy devices. A challenge for these installations is the cable that hangs from the installations’ base to the seabed or a hub. This arrangement exposes the cable to the dynamics of the marine environment it is installed in, creating concern for failure of these dynamic cables. 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 tree defects is studied by considereing 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.

An example of 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. The COMSOL software is used to simulate how the electric field strength near the tip of the water tree increases as the length of the water tree increases (growing from the outside of the insulation material towards the conductor). A water tree growth model based on cyclic Maxwell stresses is employed to estimate the the time it takes for the water tree to grow until a short circuit of the cable occurs. To investigate if mechanical stresses of the dynamic cable shorten the cable’s life further, numerical simulations in the DNV software SESAM and the finite element software ABAQUS are carried out to simulate and analyse the cable’s motions and stress responses for several sea states and ocean current loading conditions. 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. Finally, a simplified superposition model is proposed which considers the combined effects of Maxwell and mechanical stresses. It is used to discuss under which conditions, from a water tree perspective, the influence of mechanical stresses on the cable’s operation life are essential to include.

Presenting Author: Zhiyuan Li Chalmers University of Technology

Authors:

Zhiyuan Li Chalmers University of Technology
Jonas W. Ringsberg Chalmers University of Technology
Erland Johnson RISE Research Institutes of Sweden
Yuriy Serdyuk Chalmers University of Technology