Research On The Pile-soil Interaction Of Offshore Wind Farm Foundation Under Lateral Load

The energy crisis and air pollution caused by traditional energy resources have led to an urgent need of clean and renewable resources.Under this situation,offshore wind farms have become an important source of clean energy.It also has the strategic significance in the energy filed of China as well as other countries.In offshore wind farms,the most popular supporting structures are steel pipe piles.However,there are many problems on the design method of these piles when appealing to the traditional mechanic theory of laterally loaded piles.The traditional model of pile-soil interaction cannot completely reflect the real situation.Therefore,it has important theoretical significance and practical value to propose a more reasonable pile-soil interaction model.This topic,which comes from National Key Technology R&D program(No.2017YFC0703408)”,the Chinese National Natural Science Foundation(No.51678145),is focusing on the development of pile-soil interaction model of piles with large diameters under lateral load.An energy-based method in layered elastic soil is used to solve pile response under a lateral load and/or an overturning moment,it provides a more efficient approach to calculate lateral deflection of steel pipe pile in offshore wind farm projects.The theoretical derivations were verified by in-situ pile tests and numerical studies(FLAC3D),then the calculation methods of soil reaction around piles under small-strain condition were given,the main conclusions are as follows:(1)A pile-soil interaction model of large-diameter flexible steel pipe pile under lateral load was improved.On the original theoretical framework,the method determining the elastic modulus of soil was extended;both vertical and horizontal loads were considered by improving the hypothesis function of soil displacement;the influence of soil in pipe pile was also considered.Some cases of flexible pile were selected.The results obtained from this paper were compared with that from measured data and FLAC3D;The results show that they are all in good agreement;the influence of vertical load on the pile response can be ignored under smallstrain condition.(2)A pile-soil interaction model of large diameter rigid steel pipe pile under lateral load was established.For rigid pile and semi-rigid pile,a Timoshenko beam element was used;the three-dimensional displacement assumption of soil domain was introduced,and the displacement along the depth direction of soil layer was considered.By improving the hypothesis function of soil displacement,a variational method was proposed,which considers both vertical and horizontal loads.To verify the calculation results,typical cases of caisson foundation were selected,and the results of variational method were compared with the measured results and FLAC3 D simulation results.The three-dimensional displacements distribution of the soil around the caisson were compared with that from numerical simulation to prove the correctness of the method.By comparing the results of rigid beam,Euler Bernoulli beam and Timoshenko beam,the model is suitable for Timoshenko beam with slenderness ratio greater than 1.7.(3)The soil resistance around the large-diameter slender steel pipe pile under lateral loads were studied.The influence of different soil parameters and load position on the pile response were systematically studied;The functions of horizontal soil resistance and additional moment around the pile are given;The influence of various soil resistances on the overall lateral bearing characteristics of piles is studied.results show that the horizontal soil resistance at the pile side is the dominant factor in the total soil resistance,the influence of the shear action between each soil layers on the total horizontal soil resistance is about 33%;The additional moment have little effect on the lateral capacity of piles.(4)The soil resistance around the large diameter rigid steel pipe pile under lateral loads and its influencing factors are studied.The influence of parameters and horizontal loading position on the lateral response of short piles is analyzed;The functions of horizontal soil resistance,additional moment at pile side,horizontal resistance at pile end and resistance moment of rigid pile are given,and the influence of the position of Fa on the overall lateral bearing capacity of piles is systematically studied;the results show that the lateral soil resistance is the dominant factor,the influence of the shear action between each soil layers on the total horizontal soil resistance is about 20%.(5)The modified expression of the initial stiffness of soil resistance around the pile was given to modify the multi-spring mechanical model of the pile and soil.The initial stiffness in p-y curve was modified.Finally,the results were verified by several cases of typical flexible pile and rigid pile;The modified expression of the initial stiffness can also be used to calculate the natural frequency of the wind turbines considering pile-soil interaction,and the results were verified by some real projects.