Crack propagation in dynamic power cables

Abstract

Renewable energy has been the focus of recent years to decrease carbon emissions and global warming. Among them, wave energy has a high potential of being one of the primary sources of sustainable energy. Wave energy is harnessed using wave energy converters (WECs) which have multiple possible configurations. All these technologies need to transport the energy produced to the distribution centres on land. This step is carried out by dynamic subsea cables. Their service life is crucial for the profit of these solutions. The presence of water molecules in the insulating material causes a crack filled with water called a water tree, which grows with time and decreases the service life of these cables. This thesis studies the possible parameters that influence the water tree propagation: cyclic mechanical loading and Maxwell stresses. The working environment for a dynamic cable connected to a WEC causes motions in the cable that subjects it to mechanical loads. These cable motions are studied by simulations of various sea states for the Waves4Power WEC WaveEL 3.0 installed in Runde, Norway, using the SIMA software. The results from this global model simulations are post-processed in MATLAB, and the fatigue life of the cable is determined over its length to determine the fatigue-critical locations. A local FE model of the cable is created in the ABAQUS software to simulate and analyse a segment of the cable. The stress responses in the conductor’s insulation material are analysed in MATLAB using an in-house fatigue crack propagation code based on linear elastic fracture mechanics (LEFM). The results show only short propagation of the initial crack introduced to the insulation material for the simulated sea states and cable configuration. Hence, it was concluded the motion-induced stresses in the current case study have negligible influence on the cable’s service life. The flow of current causes an electric field in the cable’s conductors which give rise to cyclic variations of Maxwell stresses. A model was developed in the COMSOL Multiphysics software to simulate this electric field. The model allows studying the electric field variation due to the presence of a water tree. The cyclic variation of the simulated Maxwell stress was used in a water tree growth model to determine its limitation in service life due to water tree growth using an in-house MATLAB code. The results show that water tree growth due to the cyclic variation of Maxwell stresses has a more considerable impact on the cable’s service life than motions-induced stresses.