Risk-based derivation of target reliability levels for life extension of wind turbine structural components

The main wind turbine design standard IEC61400-1 ed. 4 includes an annual target reliability index for structural components of 3.3. Presently, no standards specify specific reliability requirements for existing wind turbines, to be used in relation to verification of structural integrity for life extension or continued operation. For existing structures in general, both economic and sustainability considerations support differentiation in reliability targets, as it is generally more expensive and requires more resources to improve the reliability. ISO2394 "General Principles on Reliability for Structures" includes tables with differentiated reliability targets depending on the consequences of failure and costs of improving reliability, which are derived using risk-based economic optimization. However, the assumptions behind these tables do not match the specific problem of life extension of wind turbines. In this paper, the risk-based approach is applied to derive specific target reliability levels for life extension of wind turbines, and a target annual reliability index around 3.1 is proposed.