Assessment of allowable sea states for offshore wind turbine blade installation using time-domain numerical models and considering weather forecast uncertainty

Abstract

Installation of offshore wind turbines, particularly blades, is challenging and should be executed within the allowable limit of sea states to ensure the safety and the efficiency of installation. During the execution phase where weather forecast will be used in the decision-making process, uncertainty in weather forecast is an important issue required to be dealt with. The purpose of this paper is to assess allowable sea states for offshore blade installation, with emphasis on both considering weather forecast uncertainty and using time-domain numerical models for installation response analysis. The general procedure is presented, which includes generation of the response-based alpha-factor αR (that is a reduction indicator reflecting weather forecast uncertainty) in the planning phase based on time-domain modelling and analysis of blade installation, and assessment of corresponding allowable sea states in the execution phase. Single blade installation by a semi-submersible crane vessel at the North Sea center is studied in this paper, and two typical limiting response parameters, namely the blade root radial motion and velocity, are considered. The αR factors correspond to these two limiting parameters are first generated respectively, based on the forecast uncertainty quantification of sea states, the quantitative assessment of system dynamic responses and the estimation of their characteristic values through probabilistic analysis. The characteristic values of the responses that correspond to a certain exceedance probability (10−2 or 10−4) could be generated with and without the consideration of the weather forecast uncertainty. Finally, the allowable sea states with and without the weather forecast uncertainty are obtained when the characteristic value reaches the allowable response limit. These could be further used to identify overall workable weather windows for the offshore blade installation and support operation decision-making. Results indicate that there is a significant difference between the allowable sea states with and without considering weather forecast uncertainty. As the forecast lead time increases, the allowable sea states gradually decrease. Hence, it is necessary to apply αR to involve and quantify the effect of weather forecast uncertainties on operations. Moreover, since time-domain simulations are used for dynamic response analysis of the installation system, the effect of statistical uncertainty related to the use of a limited number of simulations to derive the characteristic response values is also investigated and is found small.