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Steady state motion analysis of an offshore wind turbine transition piece during installation based on outcrossing of the motion limit state

Abstract:

Installation of OffshoreWind Turbine structural components need to be executed in sea states for which their dynamic responses are expected to remain within a safe domain or perform a limited number of outcrossings from the safe boundary beyond which the responses may lead to unsafe working conditions, large impact loads or even structural failure. A critical installation activity limiting the installation of a Transition Piece TP is often the motion monitoring phase of the mating points until its landing on the foundation. The operational limit is normally given by the horizontal displacement and the safe domain could conveniently be defined by a circle of radius r in the horizontal plane. This paper presents an existing general accurate method and its solution to estimate the outcrossing rate of dynamic responses for a circular safe boundary in short crested seas which is applicable for the motion monitoring phase of offshore wind turbine components prior to mating. The required input is calculated from spectral analysis in the frequency domain and the solution is derived for Gaussian processes. It is found that both 1st and 2nd order responses have to be included and that the Gaussian assumption for the slow drift motions is not valid so that its real PDF is required. Also wave spreading has large influence in the outcrossing rate and should realistically be applied. The suggested approach is in agreement with real offshore practice, and is efficient when compared with time domain simulations. Then, the outcrossing rate method could help on Marine Operations decision making during critical installation activities.