A framework for fatigue assessment for tower bolts of floating offshore wind turbine (FOWT) taking into account the impact of preload loss due to ambient temperature creep and plastic deformation is proposed. The ambient temperature creep of high strength steel is modeled in terms of Norton's theory. The residual preload of tower bolt is predicted by a detailed finite element (FE) model of the threaded bolt connection. Subsequently, the residual preload is applied to the FE model of the entire tower segment to achieve the stress range of the bolts. Fatigue life prediction is finally conducted using the Palmgren-Miner's (PM) rule based on S-N curves, with corrections by effects of mean stress and bolt sizes. A fatigue assessment for tower bolts of a 5 MW FOWT is then performed to demonstrate the capability of the proposed framework. Moreover, impact of initial preload, thread friction coefficients and flange gap on preload loss and fatigue life of tower bolts are discussed.
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