Numerical Modelling On Tetrapod Piled Jacket Foundations For Offshore Wind Turbines Subject To Lateral Monotonic And Cyclic Loads

Offshore wind energy is acknowledged as a sustainable resource to address global energy shortages and reduce carbon dioxide emissions.In an offshore wind farm,the design and installation of the foundations for offshore wind turbines(OWTs)are particularly critical,because the cost of foundation alone accounts for 15%to 40%of the total construction cost.As the water depth of many wind farms exceeds 30 m and the capacity of per turbine has improved to 5 MW or greater nowadays,tetrapod piled jacket(TPJ)foundations,consisting of four corner piles interconnected with slender steel bracings to form a lattice tower,have attracted increasing interest.This foundation type has shown good potential due to the generally high stiffness,and the low construction cost owing to the easy assembling of structural elements before floated for installation.The design approaches for TPJ foundations are borrowed from the prevailing concepts in offshore oil and gas platforms,whereas,unlike the latter,OWTs are primarily subjected to lateral load induced by wind,waves,ice,etc,over its lifetime with millions of loading cycles.Therefore,it is necessary to study the loading behaviors of TPJ foundations under lateral loads and establish the monotonic loading and cyclic loading analysis model.In this paper,a three-dimensional finite element model corresponding to an OWT prototype in Guishan Wind farm is first established using ABAQUS.The reliability of the numerical model is verified by centrifuge tests carried out previously.After that,to improve the modeling efficiency,a Python script for TPJ foundations is written and the systematically parametric analysis is performed both in satruarted sand and in soft clay.The main research work and conclusions are summarized as follows:(1)In saturated soft clay,the rotation center of the TPJ foundations under lateral monotonic loads gradually approaches the downwind piles that are under compression,leading to more upward displacements of the upwind piles than the downward displacements of the downwind piles;The downwind piles bear slightly more lateral loads than the upwind piles;the ultimate bearing capacity of TPJ foundations decreases when the lateral loading direction shifts from along the orthogonal to along the diagonal line of the jacket in the horizontal plane,and the latter is thus defined to be the most unfavorable loading direction of TPJ foundations.In particular,when laterally loaded along the diagonal direction,three piles of the foundation will be subjected to tension.The distribution of pm along soil depth is not uniform and depends largely on the pile spacing and pile deflection.Existing studies on the quantification of pm considering these factors are very limited and do not specifically aim for TPJ foundations for offshore wind turbines.Based on these models and the parametric study results,an improved approach for the quantification of pm at different depths and pile deflections is then proposed,with the pile spacing being taken as the key parameter.(2)In saturated sand,it is needed to pay much attention on the design of upwind piles for TPJ foundations under lateral monotonic load.The unfavorable loading direction in sand is along the diagonal direction.In the saturated soft clay,the axial force of corner piles has little effect on p-y curves,whereas the downforce of the downwind piles(upwind piles)would increase(weaken)the soil effective stress around corner piles in sand.Thus,the soil resistace on dwonwind piles is always larger than that on upwind piles.The bearing capacity of TPJ foundation would increase with pile length and pile diameter.(3)In saturated sand,a stiffness attenuation model embedde into ABAQUS is used to achieve the soil stiffness weakening under lateral cyclic loads.The analysis results show that the cyclic cumulative deformation of TPJ foundations has a linear relationship with the logarithm of loading cycles.Unlike the design method suggested by API code,the cyclic p-y curves is related to the number of cycles.The depth of maximum bending moment of corner piles will move downwards with increasing cycles.The smaller the length of the corner pile,the larger the vertical cumulative displacement.