Abstract
Reassessment of the fatigue life for wind turbine structural components is typically performed using deterministic methods with the same partial safety factors as used for the original design. However, in relation to life extension, the conditions are generally different from the assumptions used for calibration of partial safety factors; and using a deterministic assessment method with these partial safety factors might not lead to optimal decisions. In this paper, the deterministic assessment method is compared to probabilistic and risk-based approaches, and the economic feasibility is assessed for a case wind farm. Using the models also used for calibration of partial safety factors in IEC61400-1 ed. 4, it is found that the probabilistic assessment generally leads to longer additional fatigue life than the deterministic assessment method. The longer duration of the extended life can make life extension feasible in more situations. The risk-based model is applied to include the risk of failure directly in the economic feasibility assessment and it is found that the reliability can be much lower than the target for new turbines, without compromising the economic feasibility.
Highlights
Wind turbine towers are typically designed for a fatigue life of 20–25 years
For cases where the design assumptions differ from the assumptions originally used for calibration of partial safety factors, there could be variation at 25 years of lifetime; and the models could coincide at another fatigue life
This paper investigates how probabilistic and risk-based methods can be applied for life extension assessment; and examines how the use of these alternatives to deterministic assessment can enhance the economic feasibility of life extension by allowing for longer life extension periods
Summary
Wind turbine towers are typically designed for a fatigue life of 20–25 years. This section briefly outlines the background on deterministic, probabilistic, and riskinformed assessment. 4 [6], and the riskinformed assessment model was first presented in [36] for the derivation of a target reliability index for life extension. Fatigue assessment is based on SN curves, where the relation between the number of cycles to failure N under constant amplitude loading with stress range ∆σ is given as: N = K ∆σ−m (1). Miner’s rule for linear damage accumulation is applied. The fatigue damage D resulting from variable loading from k stress ranges ∆σi , i = 1 : k, with each ni stress cycles is given by: k
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