Assessment and monitoring of reliability and robustness of offshore wind energy converters

The present paper provides an overview of existing approaches for design and assessment of offshore structures and on this basis proposes a framework for the risk based assessment and monitoring of the performance of offshore wind energy converters in operation. In recognition of the fact that there are presently no established best practices for the assessment and monitoring of offshore wind energy converters the first part of the paper provides an overview of best practices from related engineering fields specifically addressing typically applied procedures for the design of offshore wind energy converters, general codes and guidelines for the assessment of offshore structures and finally some requirements to the inservice monitoring of offshore wind energy converters in operation as prescribed from the side of a certification society. Based on the reviewed best practices a risk based assessment and monitoring framework for offshore wind energy converters is proposed. In terms of structural analysis the proposed framework builds upon an overall dynamic analysis model of the wind energy converter which is typically established already in the design phase for wind energy converters in the megawatt class. The structural reliability is analysed utilizing a stochastic finite-element representation of the structure in conjunction with modern response surface technologies. The structural system characteristics, the systems reliability and the risks are assessed through Bayesian probabilistic network representations of the scenarioswhich through the loading and deterioration processes may lead to structural damages, overall structural collapse and loss of operation and thereby result in economical losses. The risk assessment framework facilitates that the robustness of the wind energy converter system as such can be quantified. This in turn facilitates that efficient monitoring schemes can be identified which in combination with optimized thresholds for structural response characteristics can be utilized to reduce in operation risks. The framework is illustrated through an example considering a prototype of the Multibrid 5MW offshore wind energy converter. The numerical results from the example indicate that the structure has a high level of structural reliability and that the overall risks are low.