Research On Reliability Of Grouted Connection For Offshore Wind Turbine Pile Foundations

The grouted connection of offshore wind turbines connects the turbine tower to the pile foundation and works as a transition connection in the wind turbine supported structures.Grouted connections are a critical element and weak link of offshore wind turbine supported foundations.The reliability of the grouted connection is directly related to the reliability of the wind turbine.However,up to now,there has been little research on the damage of the grouted connection subjected to ship collision and earthquake motion,and the reliability of the structure.Therefore,studies of the damage mechanism of the structure under ship impact,seismic dynamic and fragility,and reliability analysis of the grouted connection improve the safety and reliability of the wind turbine foundation,and are of great both academic and practical significance.This study is devoted to the investigation of ship collision responses,seismic dynamics,fragility,and structural reliability of the grouted connection of offshore wind turbines.The case study is the grouted connection of a 5 MW monopile offshore wind turbine.The main research contents are as follows:(1)Grouted connections for offshore wind turbine pile foundations at the mean water surface are under a potential threat of ship collision.However,there is little literature on ship collision with the grouted connection of offshore wind turbines at home and abroad.In this study,a solid finite element model of ship collision with offshore wind turbine,which consisted of the pile foundation,the sandwich-type grouted connection,the turbine tower and the striking vessel,was established using the explicit finite element code LS-DYNA.The striking ship model,grouting material model,and grouted connection composite tubular structure subjected to ship impact model were validated by comparing with the published results.Ship collision responses and damages of the grouted connection were studied,taking into account the pile-soil interaction boundary and the non-linear constitutive behavior of the structural materials.It was found that ship collision with the grouted connection can lead to crack and crushing of the grout layer,and subsequently threaten the safety of the wind turbine foundation.The collision damage of the grouted connection was significantly influenced by properties of the ship,initial striking velocity and design of the grouted connection.(2)With regard to the threat on the wind turbine caused by earthquake motions,the reaction forces at the grouted connection subjected to earthquake ground motion and seismic fragility of the grouted connection were studied.A numerical model of a monopile offshore wind turbine was developed using OpenSees.The design ground motions of the severest condition were selected as input earthquake ground motions.The seismic dynamic responses of the wind turbine and the forces at the grouted connection were analyzed.Four potential damage states(DSs)of the grouted connection were defined based on the limit states of the structure.Seismic fragility analysis of the grouted connection was conducted considering both near-and far-field earthquake motion records.Considering the stochastic of wind and waves,a fragility analysis method using the Latin Hypercube Sampling(LHS)method to propagate the uncertainties-of wind and waves was proposed for seismic fragility analysis of the grouted connection.The forces acting at the grouted connection and the probability of exceeding damage states of the structure under different seismic conditions were investigated.It was found that forces acting at the grouted connection subjected to earthquake motions were considerably influenced by the damping of the structure,the earthquake motion direction,and the coupling of wind and waves.The probabilities of the grouted connection exceeded damage states obtained from the fragility analysis considering the uncertainties of wind and waves were lower than the results obtained from the certainty wind and waves(assumed to be the mean values)model of IEC 61400-1 code.(3)For the reliability analysis method of grouted connection,in order to reduce the finite element computational burden,the Support Vector Regressive algorithm was employed to analyse the reliability of the grouted connection in this study.Considering the difficulty in selecting the parameters of the Gauss-vSVR model,the Chaos cloud genetic algorithm(CCGA),which is a modified genetic algorithm using the logistic chaotic map and the cloud drop model,was adopted to select the parameters for the Gauss-vSVR model forming a modified Gauss-vSVR model named CCGA-Gauss-vSVR.A structural reliability analysis method was developed using the CCGA-Gauss-vSVR algorithm to approximate the limit state function and response surface of a structure.The reliability analysis of the grouted layer subjected to local tensile crack and structural fatigue showed that the fatigue reliability of the structure was lower than that of the local tensile crack of the grouted layer.