Fretting wear-fatigue in submarine power cable conductors for floating offshore wind turbines

This thesis presents finite element modelling and experimental testing for fretting wear and fatigue in copper conductors for submarine power cables for floating offshore wind turbines. A finite element methodology for global and local analyses of fretting wear-fatigue between copper conductors in multi-strand cables for floating offshore wind turbine is presented. This involves the development of a systematic model that bridges the gap between offshore submarine cables and a representative fretting wear laboratory test setup. Local finite element, frictional contact models are developed for characterisation of local wire-to-wire fretting damage. These two-dimensional and three-dimensional representative models, based on configurations commonly employed in laboratory-scale fretting tests, inherit boundary conditions derived from the global dynamic model, allowing for micro-scale analysis of global dynamic effects on local fretting contact. A beamelement, dynamic global finite element model, which simulates the scenario of a floating wind turbine connected to a dynamic cable, using Flexcom Wind, captures the representative design loading conditions. A key component in the global-local framework is a simplified SPC finite element model for quantifying the fraction of load carried by the SPCs. Local inter-wire slip and contact traction condition are identified via a seven-wire frictional contact sub-model which includes the helical geometry. A critical-plane, multiaxial fretting-fatigue life methodology has been implemented to assess and quantify fretting-fatigue life. Two- and three-dimensional fretting wear models are developed within the UMESHMOTION adaptive mesh subroutine in Abaqus and validated against piezoelectric fretting wear tests on crossed cylinder arrangements. The latter tests were also used to quantify coefficient of friction and wear coefficient evolution under representative loading conditions for C101 copper. The critical-plane methodology is combined with the two- and three- dimensional wear models for life prediction of SPC copper contacts under IEC design load conditions. The effects of lay angle, contact size, wire diameter, friction and slip are investigated. The findings reveal a general trend where increases in lay angle, contact size, wire diameter, and friction result in reductions in fretting fatigue life. A key contribution of this work is the establishment of fretting wear-fatigue life prediction, drawing connections between localised modelling achieved by matching contact pressure and global modelling. The work also provides insight into the relative impact of contact size on relevant transverse- and longitudinal-type contacts for SPCs. In summary, this work employs local computational modelling to replicate crossed-cylinder laboratory fretting test arrangements, utilising results from the global assessment as boundary conditions to predict potential fretting wear-fatigue damage in SPCs. A design study was undertaken to evaluate lazy-wave and catenary configurations in both normal sea state and extreme sea state conditions, in compliance with the IEC standard. The analysis revealed that the lazy-wave configuration offers a more efficient design due to its ability to significantly reduce tension and bending moments, as evidenced by simulation results. Following this framework, the key functional relationship between FOWTs and dynamic SPCs was established, applying to various laboratory test configurations, including cylinder on flat, cylinder on cylinder, and crossed cylinder arrangements. These tests provided a computationally efficient approach to assessing fretting variables under representative load conditions and predicting fretting wear-fatigue life. The results of fretting wear tests on C101 pure copper, were integrated with fatigue data from previously published research. Notably, this data addresses a gap in the existing literature, as fretting wear data (including coefficient of friction, wear coefficient data) for C101 pure copper was not previously available. This newly acquired information was then incorporated into the local representative models. It is shown that the floating wind operational conditions and wire designs (wire diameter and crossing angle) have a substantial impact on the predicted fretting wear-fatigue life. The results indicate the important role played by fretting wear in SPC failure and the significance of this phenomenon in the overall service life of these SPCs.

Publisher

University of Galway

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Attribution-NonCommercial-NoDerivatives 4.0 International

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University of Galway Theses (PhD Theses)