Modelling of a Dynamic Power Cable

Abstract

This Master's thesis explores the modelling of dynamic power cables (DPCs) used in floating offshore wind turbines (FOWTs), focusing on how lower- and higher-order models capture the cables' dynamic behaviour.

The study is motivated by the increasing use of floating offshore wind systems in deeper waters, where dynamic cables are required to transmit the power. To manage tension and reduce fatigue failure, a lazy wave configuration (LWC) is commonly employed as configuration for a dynamic power cable.

Two modelling techniques are compared: a custom-developed linear finite element model based on the Modal Superposition Method (MSM) and a more computationally intensive, non-linear model using OrcaFlex. The MSM approach is based on the linearisation of the system’s dynamic behaviour using mode shapes, which significantly reduces computational cost compared to high-fidelity methods like OrcaFlex. In fact, the MSM model runs over twelve times faster than OrcaFlex, making it particularly suitable for early-stage analysis. Results from both models are analysed in the time domain, with focus on displacements, bending moments and axial tension to identify fatigue-prone areas.

The findings show that the lower-order MSM model, while limited by linear assumptions, accurately captures displacement in tension-dominated regions and effectively identifies fatigue-prone locations. Compared to the OrcaFlex model, MSM computes conservative fatigue life estimates, as demonstrated through simulations across multiple surge spectra, imposed at the top-end. In contrast, the higher-order OrcaFlex model, which accounts for geometric non-linearities, offers more accurate predictions under larger wave loads and is better suited for detailed fatigue analysis of selected sea states.