Crack propagation in dynamic power cables
Typ
Examensarbete för masterexamen
Program
Naval architecture and ocean engineering (MPNAV), MSc
Publicerad
2021
Författare
Harikumar, Jayakrishnan
Roux, Rémi
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
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.
Beskrivning
Ämne/nyckelord
Fatigue life , Fracture mechanics , Maxwell stresses , Subsea dynamic cable , Water tree , Wave Energy