Study On The Deformation Of Mono-pile For Offshore Wind Turbines With Liquefied Reduction On P-y Curves

In recent years, offshore wind power farm has been rapid developed in many countries withrich offshore wind resources. Offshore wind turbine foundation account for a large proportion ofthe cost and take the main engineering risks, especially mono-pile foundation. Multiple forms offoundations are applied in existing wind farms both at domestic and abroad, but according toincomplete statistics, the mono pile foundation make up the majority. There is still less study onthe deformation and stability of the pile foundation of offshore wind turbines. Offshore windturbine brought a complex load condition in three directions; and the six degrees of freedom onthe pile head node are applied load; but the design methods such asp-y curve method are used in aplane; therefore more study of feasibly engineering three-dimensional design method is necessary.The wave induced pore pressure calculation method gradually developed; but the influence ofwave induced pore pressure on the pile foundation in complex stress conditions remains to bestudied. Problems above are studied by mathematical modeling and numerical analysis in thispaper; the main contents and conclusions are as follows：1) For the calculation of inclined load condition brought by wind turbine, a mathematicalprogramming model with strong adaptability was established according to the p-y curve method,which recommended by the American Petroleum Institute (API) and China Port pile foundationdesign specification. Under the assumption of the nonlinear soil spring, integral formulas of p-ycurves are built in the generalized energy equation to represent the soil resistance acting work.Modeling with the principle of minimum potential energy and the foundation beam deformationcompatibility conditions, the original calculus problem in FEM method can be turned into anoptimization (programming) problem and solved by MATLAB optimization toolbox. Consideringthe p-delta effect induced by secondary bending moment, this model provides a deformationanalysis method for the pile of bridge, offshore platforms and offshore wind turbines. This paperalso discussed some means to calculate the deformation, including simplex method, BFGSquasi-Newton method, DFP quasi-Newton method and the steepest descent method.2) This paper tries to extend the traditional p-y curve method in to3dimensions by considering ‘soil resistance direction (angle, implied)’ and developing a compound bendingfoundation beam model. Every node in the proposed mathematical programming model containsfive degrees of freedom (DOF)(x, y, and Z absolute displacement, corner around the x-axis andcorner around the y-axis); compare with the6DOF beam unit in conventional finite element, theproposed model ignored torsion Mz. This model calculates the value and direction of the soilresistance acting on the unit. Analysis showed that only when the concentrated force ratio of x,y-direction in proportion to ratio of bending moment at the pile head, the soil resistance can beapproximately regarded as in one plane. The actual offshore wind turbines load condition is verycomplex, and often the six loads at pile head are out of proportion in means that soil resistance isnot in a plane. The traditional design of pile foundation are completed in the plane may causesome errors. Therefore the p-y curve method considering the direction of soil resistance and thecompound bending of the pile is very necessary.3) Pile foundations with liquefiable soil in seismic conditions and the ultimate seaconditions are analyzed to study the effects of the development of excess pore water pressure. Thecalculations are based on the cu discount method (Liu L1995). Results show that in the extremesea condition, excess pore pressure induced soil resistance reduction, resulting in deformationincreases, is not significant; On the contrary, in the seismic conditions and extreme sea conditionsthe load increase (Inertia force and wind-force) is a major impact factor. In seismic condition, thedevelopment of excess pore water pressure is obvious. Without considering the seismic inertiaforce，the pile deformation calculated by cu method nearly doubled. Consideration of soilresistance reeducation should be given in seismic condition. Similar parameters of the sands areused in the calculation for cooperation, but there are still many assumptions in the model toprovide a reference for the pile foundation design.