Mechanical design of dynamic cables for floating wind – Technology Qualification combining testing and cross section modelling

Currently, only a limited number of dynamic cables, with voltage ratings above 66 kV, are in operation. No dynamic cables with the necessary power for export, i.e., with voltage ratings above 220 kV, have yet been deployed.

Statistics from the offshore wind installations shows that approximately 80% of insurance claimed cost is attributed to power cable failures. Observations suggest that a significant number of these incidents are due to unanticipated dynamic motions of the cables. Reported experience from decades of operation of similar structures, as un-bonded flexible risers, indicate the threats and failure mechanisms are complex and that an inadequate qualification may cause a significant operational consequence.

Dynamic cable systems are one of the most novel technology elements in the development of floating offshore substation [1]. In the development of dynamic cable systems for floating offshore wind farms, it is crucial to focus on potential mechanical failure mechanisms caused by dynamic loadings and responses. Today, the design specifications for dynamic cables typically may provide reference to a diverse selection of industry codes, in the form of standards, recommended practises and guidelines.

Where different industry codes are intended as basis for different design aspects. Where codes may originate from different industries, as onshore power production, to subsea oil and gas, the terminology and approach may also differ. In the absence of uniformed set of codes, a general methodology for technology qualification can be used as process to ensure and document the performance and reliability of a dynamic cable design.

A general technology qualification process will assess the technology and threats and develop a set of activities required to prove the reliability of the design. The selected qualification approach, i.e. set of activities, will depend on what needs to be proven. A qualification approach by pure product testing is called test based, while a qualification approach utilizing testing and numerical methods is called analysis based. A test based qualification approach is considered more likely relevant to document reliability against a limited number of deterministic load conditions, while an analysis based qualification approach is likely more relevant to document reliability against a larger number of stochastic load conditions.

Developing cables for dynamic applications requires assessing and adequately documenting threats related to dynamic behaviour. Industry experience shows that the metallic layers are most likely the limiting elements for the integrity of a typical dynamic cable design. An analysis based qualification approach is outlined, by applying it to document reliability for cable elements threaten by the mechanical response due the dynamic loading. The advantages with using an analysis based qualification approach are discussed by the means of the response in the conductor.