Sensitivity analysis of friction and creep deformation effects on preload relaxation in offshore wind turbine bolted connections

A considerable portion of the preload loss in offshore wind turbine foundation to transition piece flanged connections is caused by short-term relaxation of the bolts. This relaxation can be influenced by several factors including the friction coefficient between the contact pairs in the flange bolted connections as well as room temperature creep. The present study focuses on the effect of these two factors on the performance of M72 bolts in offshore wind turbines. A detailed finite element model of a one-bolt-segment of the flanged connection is developed with material properties obtained from the literature. The first analysis examines the response of the bolt to a change in friction coefficient between contact surfaces. In the second analysis, the effect of room temperature creep is investigated over the first 48 hours of operation. Although creep is usually neglected in such low temperatures, for high load levels close to the material's yield stress, significant creep strain rates can occur. Finally, a sensitivity analysis on both friction and creep properties is carried out to investigate the influence of these two factors on the preload relaxation of M72 bolted connections.