Simulation Of Cyclic Degradation Response Of Offshore Wind Power Jacket Foundation

With the development of wind energy gradually moves toward deeper waters where have richer wind energy resources,and the increase in the capacity of per wind turbine(5MW-1OMW),various foundations have begun to be adopted.Due to the extensive use of jacket foundations in the early oil and gas platform development process,this type of foundation has a lot of experience to learn from,and the technology is relatively more mature.At the current stage of wind turbine development in the deep sea,jacket foundations have been more widely used.The difference from the oil and gas platform is that the structure of offshore wind turbine is slender and flexible,and the blades on the top of the tower will bear huge horizontal cyclic loads.The number of cycles can reach a million during its service.At the same time,in extreme conditions such as typhoons,a large overturning moment may be generated at the mudline of the jacket foundation,and the pile group foundation will produce considerable pile-soil weakening,which will affect the normal operation of the wind turbine.In this paper,based on the large-scale commercial finite element software COMSOL Multiphysics 5.5,three-dimensional numerical finite element models including jacket foundation based offshore booster stations,tetrapod piled jacket foundation based offshore wind turbines,and tripod piled jacket foundation based offshore wind turbines have been established.Using the new Q-z model proposed in this paper,the pile-soil nonlinear spring in the API specification,and the cyclic weakening t-z model studied by the group in the early stage,combined with MATLAB for secondary development,this paper studies the force and deformation characteristics of jacket foundations bearing horizontal loads under different environment conditions.The main work and research results are summarized as follows:(1)In sand,according to the stress-strain curve of the soil element in the dynamic triaxial experiment,combined with the concept of the influence area of the pile tip,a new Q-z model that can consider displacement accumulation is constructed.This model is simple and reliable,and the parameters are easy to calibrate.Using this model,for a single pile subjected to axial cyclic load,in the initial loading stage,the soil at the pile tip will produce displacement accumulation.After certain numbers of cycles,the accumulated displacement will no longer increase,and it is considered that the soil reaches the elastic loading and unloading stage.Under the horizontal cyclic load,the jacket foundation based booster statin will produce certain amount of foundation settlement and overall inclination,but no overturning damage will occur.The bearing capacity of the pile foundation should be mainly considered in the design.(2)For the tetrapod piled jacket foundation based offshore wind turbine,the most unfavorable loading direction is to load diagonally;when the pile length is short and the soil is weakened largely,the first nature frequency of the wind turbine under normal loading conditions will drop to the 1P frequency range,affecting the normal operation of the system,causing resonance;The maximum bending moment of the corner pile appears in the range of 5D-6D from the mudline.As the weakening of the soil increases,the maximum value of the pile bending moment will also increase.The increase amplitude is related to the length of the pile.The change of other pile body resistance is similar;under extreme loads,when the pile-soil weakens to a large extent,the rear piles are in danger of pulling out.If the soil is further weakened,the wind turbine may overturn and damage;(3)The offshore wind turbine with tripod piled jacket foundations has relatively little research,the most unfavorable loading direction is that one pile is pulled out and two piles are compressed;considering the cyclic weakening of the pile-soil interface under extreme wind and wave loads,for the pile length of 35m,The tripod piled jacket foundation will overturn and fail when the extreme load cycle reaches 21 times;Before the overturning failure occurs,the natural frequency of the offshore wind turbine in the operation state does not change significantly,and the overall cumulative deformation of the jacket foundation continues to develop with the increase in extreme cycles;With the weakening increase of the pile-soil boundary,the anti-overturning contribution of the t-z spring is significantly reduced,while the Q-z spring's anti-overturning contribution is significantly increased;after the extreme cyclic load is removed,the unloading rebounds of the front pile tip may reverse the resistance of the pile side,and the direction of the bending moment contributed by the t-z spring is the same as the load.