Accelerated reliability testing of articulated cable bend restrictor for offshore wind applications

Abstract

Power cable failures for offshore marine energy applications are a growing concern since experience from offshore wind has shown repeated failures of inter-array and export cables. These failures may be mitigated by dedicated cable protection systems, such as bend restrictors. This paper presents the rationale and the results for accelerated reliability tests of an articulated bend restrictor. The tests are a collaborative effort between the University of Exeter, CPNL Engineering and NSW, supported by the EU MARINET programme. The tests have been carried out at full-scale and exposed the static submarine power cable - bend restrictor specimen to mechanical load regimes exceeding the allowable design loads in order to provoke accelerated wear and component failures. The tested load cases combined cyclic bending motions with oscillating tensile forces. A range of acceleration factors have been applied in respect to the 1:50 years load case, subjecting each of the three restrictor samples to 25,000 bending cycles (50,000 tensile cycles). The static power cable was also loaded beyond its intended use, testing the worst case scenario of repeated dynamic loading, purposely inflicting failure modes for investigation. Throughout the test the static submarine power cable sustained over 77,000 bending cycles. The test demonstrated the integrity of the cable protection system with quantified wear rates obtained through 3D scanning of the individual shells. The static power cable also showed a high reliability level. None of the failure modes, mainly fatigue cracks and fretting, identified by cable dissection would have caused a direct loss of service. The observed failure modes could also be predicted through numerical load analysis, giving confidence in the utilised mechanical modelling and cross-sectional analysis for dynamic applications. Overall this paper shows how dedicated collaborative component testing can make an important contribution to quantify and validate component behaviour in challenging offshore operating environments.