Abstract
Under certain harsh environmental conditions, jacket structures supporting offshore wind turbines might be exposed to plunging breaking waves, causing slamming forces that affect the structural integrity and fatigue life. The slamming forces should thus be properly considered during the design, but a suitable force model specifically for jacket structures is currently in absence. In this study, a five-parameter force model is developed for estimating global slamming forces due to plunging breaking waves on jacket structures, based on statistical analyses of experimental data from the WaveSlam project. The force model is developed by considering a total of 176 individual breaking waves, under six wave conditions. For each individual breaking wave, the time history of the slamming force is calculated based on hammer test data in addition to wave test data, and the wave parameters are acquired from a wave elevation measurement. The acquired time histories and wave parameters are then used to determine the parameters involved in the force model, including two exponential parameters (i.e. α1 and α2) and three dimensionless coefficients for the expressions of wave-dependent parameters (i.e. duration coefficient ζ1, rising time coefficient ζ2, and peak force coefficient ζ3). It is found that α1, α2, ζ1 and ζ2 are approximately constant, and ζ3 follows a lognormal distribution. The quantile that determines ζ3 should be carefully selected so as to provide a conservative prediction. A quantile of 95% is suggested in this paper, and it is found to be conservative based on the verification of the developed force model. Therefore, for a given sea state, this force model can give a deterministic and conservative prediction of the slamming force time history, regardless of the randomness of slamming forces. Challenges for the application of the force model are also addressed.
Highlights
The development of offshore wind energy is mainly in shallow or intermediate water, where bottom-fixed substructures are mainly used
A force model was developed for estimating global slamming forces due to plunging breaking waves on jacket structures, based on statistical analyses of experimental data from the WaveSlam project
For each individual breaking wave, the time history of the resulting slamming force was reconstructed based on hammer test data and wave test data, and the wave parameters were acquired from a wave elevation measurement
Summary
The development of offshore wind energy is mainly in shallow or intermediate water, where bottom-fixed substructures (e.g. monopiles and jacket structures) are mainly used. The von Karman method and Wagner method are usually used to determine the slamming coefficient of the slamming force for cylindrical structures, but the time history of the slamming force is important when considering the effect of the slamming force. Six different wave conditions from the WaveSlam project are further analyzed to propose a more accurate global slamming force model for jacket structures for offshore wind applications. Based on the experimental data, global slamming forces acting on a jacket structure model are inversely calculated by using the method developed by Tu et al [13]. Verification and application of this developed force model are addressed
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