Experimental And Numerical Studies On A Laterally Loaded Monopile Foundation Of Offshore Wind Turbine

With the rapid development of economy in China, the shortage of energy is increasing outstanding. Offshore wind power is a new kind of energy form, brought to the attention of the coastal countries around the world. Developing offshore wind power is an effective way to solve the problem of energy shortage. Monopile with a diameter of 4-8m has been the most popular foundation type for offshore wind turbine due to its rapid construction, economical cost, etc. During lifetime, a monopile foundation of the offshore wind turbine is unavoidable to subject from horizontal loads, originating from wave, wind and currents. In order to ensure the normal operation, the deformation of foundation of the offshore wind turbine is strictly controlled during the period of service life. Thus, in the design of the foundation of an offshore wind turbine, the ultimate bearing capacity is much less critical than controlling the rotational deformation. Traditional design methods for laterally loaded pile are unbefitting to apply to monopile foundation with large diameter. Currently, the character of a monopile foundation under horizontal loads is undefined yet, as well as the design method. Therefore, it is of great practical significance to study the performance of monopile under horizontal loads.Large scale model tests and numerical simulations were conducted to study the pile-soil interaction and deformation characteristics of monopile foundation under horizontal static and cyclic loads, and the corresponding design methods were proposed. The main research works and conclusions are as follows:(1)A series of model tests on the monopile with the prototypical diameter of 5 m and embedded length of 30 m were conducted in saturated silt, based on the physical model test system of MOE Key Laboratory of Soft Soils and Geoenvironmental Engineering of Zhejiang University, including six static loading tests and eight cyclic loading tests. The pile-soil interaction and deformation characteristic of the monopile under horizontal static loading and cyclic loading were studied.(2)Based on the tests results of the static loading tests, the pile-soil interaction and deformation characteristic of a monopile buried in silt under different horizontal static loading were described. It is concluded that the p-y curves of the rigid monopile buried in silt present work hardening type, and the existing expressions to predict the ultimate soil resistance are conservative. During horizontal static loading, the rotational center of the rigid monopile located at the depth of 0.75L-0.85L. Based on the measured soil pressure and pile deformation, an analytical model to compute the soil resistance based on the maximum soil pressure around the pile perimeter for the pile with a circular cross section was proposed. Expressions to calculate coefficient of subgrade reaction associated with the local displacement for silt and sand were established. A new analytical model is developed to estimate the load-displacement curve of the rigid pile based onthe equilibrium of the bending moment about the rotational center.(3)The effects of the cyclic loading level, relative density of the soil and number of the cycles on the pile-soil interaction and accumulated displacement of the monopile were described, based on the test results of the cyclic loading tests. It is found that the rotational center of the rigid monopile under cyclic loading located at the depth of 0.85L. The accumulated displacement was closely related to the cyclic load ratio and seemed to have no obvious relationship with the soil density. The unloading stiffness was related to the relative density of soil and seems independent of the cyclic ratio. Behavior of soil around a rigid monopile under cyclic loading was empirically divided into two stages:the shear stage and the densification stage. Formulations to predict the accumulated deformation and unloading stiffness of the monopile were proposed.(4)To study the effects of the soil parameters and the pile geometry on the lateral bearing capacity of the monopile, a series of three-dimensional numerical model of monopile were constructed, and the soil parameters were obtained by the triaxial tests. The effects of loading height, pile diameter, and embedded length on the location of the rotational center were also described. The determination of the minimum embedded length of the monopile under the ultimate load that the offshore wind turbine may suffer during lifetime was described. It is found that the vertical-tangent criteria and the zero-toe-kick criteria were too conservative to determine embedded length of the monopile. For monopile with diameter of 4-8 m, the embedded length larger than 8D has no significant influence on the development of horizontal deformation.(5) A user subroutine USDFLD was written to incorporate the secant stiffness degradation model of sand for cyclic behavior of the monopile in the finite element program ABAQUS, and cyclic triaxial test results were used to determine the parameters of the secant stiffness degradation model of sand. Then, a series three-dimensional numerical model of monopile was constructed to investigate the effects of the soil parameters and the pile geometry on the cyclic behavior of the monopile. The determination of the minimum embedded length of the monopiH under the maximum cyclic loading that the offshore wind turbine may suffer during lifetime was described. Numerical studies proved that the minimum embedded length determined under static loading may not meet the minimum embedded length of the monopile under cyclic loading, and increasing pile diameter was an effective way to reduce the effect of cyclic loading on the minimum embedded length. Considering the maximum cyclic loading that the offshore wind turbine may suffer during lifetime, the minimum embedded length of monopile under cyclic loading could be taken as 8D.(6) The key problems of the designing of the monopile foundation were discussed, and the designing process and principles were described. An engineering instance was presented to show the designing process. In the designing of monopile foundation, on the premise of meeting deformation at the mud surface, a monopile with small diameter and large embedded length was the first choice, and the diameter of the pile should be firstly determined, the embedded length of the pile was secondly determined through deformation checking calculation.In this research, both experimental and numerical analysis works have been done to study the responses of monopile under horizontal static and cyclic loading. The studies provide a better understanding of the issues and lay a theoretical basis for designing of monopile foundation under horizontal load.