Assessing loading regimes and failure modes of marine power cables in marine energy applications

Abstract

Highly reliable marine power cables are imperative for the cost-effective operation of marine energy conversion systems. Cable manufacturers and installers have considerable experience with marine power cables when deployed to operate under static or dynamic load conditions, but highly dynamical power cables for marine renewable energy converters have large uncertainties. The mechanical loadings of a power cable attached to a floating marine energy converter will be considerably different to the present applications like remotely operated vehicles (ROVs) or oil and gas umbilicals. The floating structure responds to the wave action and transfers this dynamic motion to the attached power cable. Moreover the frequency of response will be at the wave period (linear case) leading to considerable cyclic effects. At present the loading regime in such applications is not well understood, due to a lack of field experience. The paper describes the parameters and results of a dynamic computational model that investigates the umbilical load conditions for a generic wave energy converter. Two geometric configurations of a double armoured power cable are considered, a catenary and a Lazy Wave shape. The model is set up using the dynamic analysis package OrcaFlex and uses top-end motions measured in 1:20 wave tank tests. While the simple catenary shape experiences high tensional forces at the attachment point and considerable compression, the maximum tensional forces can be significantly reduced and compression is avoided with a Lazy Wave shape. For this configuration the highest tension occurs near the attachment point and at the transition points of the buoyancy section. For the modelled conditions, the power cable accumulated a significant number of tension and bending load cycles, indicating that power cables in floating marine energy applications will operate in a high cycle regime (in the order of 10^6 cycles) likely to accumulate several million load cycles during a single year of operation.