Study On Lateral Loading Behaviors Of Jacket Foundation For Offshore Wind Turbines

The development and utilization of offshore wind power is an important choice of energy strategy in our country. With the increase of water depth in wind farm and wind turbine capacity, jacket foundation is more widely applied. The lateral bearing capacity and cyclic cumulative deformation of foundation for offshore wind turbines are the key design issues for foundation under the lateral cyclic loads of wind, wave, current, etc. The pile-soil interaction is the core of analyzing and solving the above issues. Therefore, the paper aims to investigate the lateral load behavior and the pile-soil interaction of jacket foundation for offshore wind turbines by carrying out centrifuge model tests, field tests, and numerical analysis.In offshore regions of China, double-layered seabed areas that consist of an upper soft clay layer and a lower sand layer are widespread. By means of both centrifuge model tests and field tests of double-layered soil ground, this paper examines the effectiveness of centrifuge model tests for this typical soil ground and presents a study of both laterally monotonic and cyclic pile-soil interactions in soft clay, and a set of hyperbolic tangent p-y curves for large-diameter piles in soft clay are proposed. Finally, a cyclic p-y curve analysis approach related to the number of cycles and the cyclic soil reaction ratios is proposed for large-diameter piles subjected to laterally cyclic loads.The failure of a jacket foundation is mainly caused by the lateral loads of wind, wave, current, earthquake, etc. By carrying out centrifugal model tests on a jacket foundation loaded along the border and the diagonal respectively in saturated sand soil and double-layered soil, it obtained the distribution laws of internal forces, the laws of pile-soil interaction and deformation characteristics for piles. The test results show that the piles of a jacket foundation will be pulled out more easily under diagonal loading and the lateral deflection of each pile is basically same. Under the same lateral load, the up-lifted displacement of the back-row pile of the jacket foundation in double-layered soil is larger than that in sand, and the shear force and bending moment ratios of the front-row pile to the back-row pile of the jacket foundation in double-layered soil are obviously smaller than these in sand for the diagonal loading. As for a5.8-diameter spaced jacket foundation, because of the pile group effect and the up-lifting force which reduces the effective stress in soil around the pile, the reaction of shallow layer soil around the up-lifted pile is60%of that around the downward-pushed pile under border loading, which is only40%of that under diagonal loading. As for the soft clay layer, the reaction of shallow layer soil around the up-lifted pile is75-85%of that around the downward-pushed pile under both diagonal loading and border loading. Therefore, it suggested that the p-y curves for the pile in soft clay are not affected by the axial force, and only affected by pile group effect.A one-way and laterally cyclic loading centrifuge model test on a tetrapod jacket foundation subjected to loads along the diagonal direction which is the most unfavourable direction was carried out in saturated sand. The test shows the displacement of the jacket foundation is accumulated under cyclic loads. After the occurrence of the upward displacement, the back-row pile is difficult to get back to the initial position, which eventually leads to the tilt of the jacket foundation. The shallow soil around pile degrades seriously, and the degradation degree of the soil around the front-row pile is severer than that of the soil around the back-row pile. Moreover, the distribution law of internal forces is changed by cyclic loads on the jacket foundation.Numerical analysis is one of the effective means to investigate the issues in geotechnical engineering. The paper analyzes the laterally monotonic and cyclic load behaviors of jacket foundation in saturated sand by SACS software, of which results were compared to the results of the centrifuge model tests. The analysis results show that the results computed by the hyperbolic p-y curves considering the pile group effect and axial force effect are in good agreement with the centrifuge model test results, the bearing capacities computed by API approach are larger than the test values, and the bearing capacities computed by the hyperbolic p-y curves without considering the pile group effect and axial force effect are smaller than the test values. As for the jacket foundation in saturated snad under cyclic loads, the lateral deflections computed by API approach are smaller than the actual values, which is not safe in engineering application. Finally, Monotonic and cyclic analysis approaches are proposed for jacket foundation under lateral loads Based on the previous research results, the proposed p-y curve model and analysis approaches were adopted to analyze the lateral behavior of jacket foundation for offshore wind turbines on the Guishan Offshore Wind Farm in Guangdong Province of China.