Bolted ring flanges in offshore-wind support structures - in-situ validation of load-transfer behaviour

Offshore wind turbines are subjected to cyclic loads from and waves, ultimately resulting in a fatigue-driven design. These result shell forces above the bolted ring flange into loading of bolt tension forces. To model transfer bolt, Load Transfer Functions (LTF) used, which tri-linear function according Schmidt/Neuper is most known approximation but more contemporary approaches exist. As LTF has significant impact on estimated fatigue lifetime it relevant investigate accuracy currently used LTF. This paper uses in-situ monitoring data three offshore at Nobelwind windfarm, each equipped with an instrumented monopile transition piece connection strain-gauges said flange. Long-term derive empirical load coefficients (LTC), i.e. derivatives In parallel finite element (FE model) was developed. The found measurements were compared values obtained FE model. It concluded that observed LTC farm favourable expected Schmidt/Neuper. matched closely, especially when MP-flange inclination included. inclusion had positive effect final LTF, confirmed by measurements.