What are the appropriate target safety level and corresponding partial load safety factor for bolted joints on wind turbine tower frame?

Abstract

During preassembly on the quayside, towers are temporarily erected on the tower frame through the supporting structures. The bottom of these structures is bolted on top of the tower frame. The degree of conservatism of applying the generic load safety factor (1.5) for the bolted joints on the tower frame has never been investigated. The objective of this study is to determine the cost-optimal target safety level and calibrate the partial load safety factor for these bolted joints. The target annual reliability index is obtained based upon a simplified cost model. The overturning bending moment is explicitly introduced to the limit state functions through the bending moment coefficient obtained in the wind tunnel tests. The load model uncertainty is quantified by a Bayesian analysis, based upon a full-scale single tower test data. The case study will justify the applicability of the approach and the possibility of decreasing the environmental partial load safety factor for bolted joints. The cost-optimal target safety level and the corresponding load safety factor can significantly help to avoid the unnecessary conservatism in designing the bolted joints on tower frame, which helps the management make decision on economic development strategy of tower frames in the future.