Consideration of ice drift in determining the contribution of iceinduced vibrations to structural fatigue

Ice-induced vibrations have to be considered in the design of vertically sided offshore structures which may encounter drifting sea or lake ice during their lifetime. One particular aspect is the contribution of ice-induced vibrations to the fatigue of such structures. Estimation of the duration of events is often difficult, due to limited available data on ice drift, leading to conservative assumptions.In this paper, the approach followed for assessing the fatigue resulting from frequency lock-in vibrations in the design stage of a recent offshore wind project is presented. The project concerned offshore wind turbines with jacket support structures consisting partly of vertical structural members.The severity of ice-induced vibrations for the structures is first assessed using a simulation model. Following this, ice drift is included in the assessment to obtain an estimate of the number of cycles of frequency lock-in over the lifetime of the structure. Results show that sitespecific combinations of ice floe size and driving forces significantly influence the expected number of cycles of frequency lock-in. It is concluded that for this project limited conditions exist in which sustained vibrations can develop and that the contribution of frequency lock-in to structural fatigue is therefore limited as well