Analysis of corrugated radial water barriers in submarine dynamic cables for FOWF towards design optimisation using FEA.

In the current green transition, shifting to renewable and sustainable energy generation sources is vital and achieving carbon neutrality by 2050 requires an unprecedented rate of transformation. Wind power represents one of the cleanest and yet least exploited resources we have and further potential could be harnessed by moving to open seas. To comply with the increasing water depths, floating offshore wind farms are necessary and more advanced submarine dynamic cables must be engineered for a more efficient power transmission. On this regard, this study aims at identifying the technical requirements of dynamic cables and their actual loading conditions in harsh marine working environments. The most critical component in dynamic cables is the corrugated radial water barrier which must ensure water-tightness despite the extensive plastic deformation due to the cold work. In particular, the focus of this thesis is studying the mechanical reliability of this component during operations (i.e. cyclic loading due to sea waves). Hence, we explored the corrugation process and the in-service fatigue loading. First, we examined the problem from a theoretical perspective: a review of hardening relations and most common fatigue models, including stress, strain and energy based formulations, is provided. Then, a FE axisymmetric model of the corrugation process is introduced and the resulting corrugated geometry is compared with the production. The fatigue analysis of the obtained geometry is then performed using different models, namely the Octahedral invariant, Fatemi-Socie, Kanbil-Brown-Miller and Liu. The objective is to identify a better model for fatigue life estimation by comparing the models´ predictions with experimental data. Finally, a search grid optimisation has been performed with the aim of maximising the component lifetime for different geometries, still taking into account the intrinsic constraints of the corrugation process. The results show that the corrugation model provides good prediction of the corrugated geometry corresponding to the production results. The main geometrical and process parameters affecting the obtained corrugation are the targeted corrugation pitch and depth together with the feeding speed ratio and indenting tool radius. Among the considered fatigue models, Kanbil-Brown-Miller provides the best life estimates. Lastly, the optimisation grid search has proved that changes in the corrugation depth are more significant than any other design variable. In general, larger pitches, larger depths and smaller speed ratios provide longer fatigue lives. Larger indenting tools, instead, are preferred for more sinusoidal corrugation profiles.