Centrifuge And Numerical Studies On Ocean Wave-monopile-seabed Interactions

The development of offshore wind energy in deeper waters has gained increasing momentum in recent years for the sake of high yields and low environmental impact.Monopiles with larger diameter are required to withstand the extremely large lateral and rotational loads acting on the large turbines.On the one hand,horizontal load and overturning moment are exerted on pile by ocean wave,current,etc.On the other hand,the seabed around the foundation might liquefy caused by twave loads.these two effects combine together,which makes the prediction of capacity of superstructure more complex.In addition,the increase of pile diameter(more than 5 m)causes different drainage condition compared with small diameter piles,which alters the pile-soil interaction.Centrifugal model test and finite element numerical simulation were carried out in this paper to study the lateral weakening/liquefaction characteristics of pile and the horizontal bearing characteristics of pile foundation under extreme wave loads:(1)Centrifuge model tests have been carried out to study the lateral pile-soil interaction in dry and saturated sand.The influence of pile embedment depth,delative density of sand,loading history and partial drainedge effect has been studied.The cumulative characteristics of the pore pressure around the pile and the horizontal loading characteristics of the pile under different loading rates were studied.A parameter η,representing the excess pore pressure stiffness,was used to evaluate the excess pore pressure response at pile front,and a linear relationship between η and vD/cv in the semi-logarithmic coordinates can be established.(2)A numerical model for pile-soil interaction was established based on effective stress based coupling analysis program DBLEAVES.The parameters for the cyclic mobility constitutive model were obtained through comparing with the triaxial results.The method to exert wave loads on irregular grid was developed,and some validation were made,which shows the well performance of the numerical model.(3)A parametric study primarily focusing on the effects of loading rates were conducted.The initial stiffness of the p-y curve was found to increase with the loading rate whilst the bearing capacity shows the inverse,and the mechanism behind this phenomenon is examined in detail.Then an explicit model was developed to evaluate the development of excess pore pressure in the pile front upon lateral loading,and an upper boundary of normalized loading rate was identified to distinguish fully and partially drained conditions.(4)Numerical simulation was conducted to study the effect of pile-soil interaction on liquefaction of seabed around pile.Different constitutive model were adopted and the advantage of cyclic mobility model proposed by Zhang et al.(2007)was demonstrated.The excess pore pressure for seabed without pile and seabed around a single pile were compared.It has been found that seabed around the pile liquefies faster than that without the pile.(5)A centrifuge wave tank was developed based on ZJU400 geotechnical centrifuge.Preliminary tests were carried out to eliminate reflection of wave due to the length of wave tank.Centrifuge tests on wave-induced liquefaction of seabed have been carried out and the liquefaction of seabed was witnessed.