Research On Degradation Of Lateral Bearing Capacity For Offshore Wind Turbine Monopile Under Cyclic Loading

Because of its huge potential, that the offshore wind power has been the core of the future national energy strategy. An offshore wind turbine monopile will subject to long-term lateral cyclic loading such as wind, wave, flow loads and other horizontal loads, the cyclic loading will lead to degradation of horizontal bearing capacity and the increases of cumulative deformation. How to analyze the foundation stability under Iong-term low-frequency cyclic loading properly has become a huge challenge. Therefore, it is important to research the deterioration of bearing capacity for a wind turbine monopile under long-term cyclic loading. Focusing on the questions, tests on a model pile embedded in silt were carried out. This paper studied the degradation of bearing capacity and deformation law for a monopile based on p-y curve method. The main research achievements are listed as follows:(1)Tests on a model pile embedded in rapidly deposited of saturated Yellow River delta silt were carried out under lateral loadings, and the lateral displacement and moment of the pile shaft are studied, the p-y curves at different depths were obtained on the basis of relationships among the moment of the pile shaft. The proposed p-y curve were compared to the existing curves and evaluated with the experimental date, the ultimate lateral soil resistance is larger than API method in shallow soil layer but between Matlock method and O’Neill method in deep soil. Two different pile installation methods were investigate to quantify the effects on the p-y curves. Results indicate that the driven pile p-y curves have relatively smaller initial stiffness but larger ultimate soil-pile interaction; it is revealed that the driven pile have significant influence on shallow soil ultimate lateral resistance.(2) The characteristics of monopile deformation with different amplitude and frequency under horizontal cyclic loads were researched based on the results of model tests in soil box. The development of pile displacement rapid in first few cycles and it becomes stabilized gradually in the late. The big amplitude, the more difficult to achieve stable. The low frequency, the big displacement of pile head.(3)Tests results show that the soil resistance per unit length (p) of a pile is almost linearly decreasing logarithmically with number of cycles. The relationship between the degradation index and the cumulative strain were depicts with a curve. Only when cumulative plastic strain was greater than 0.02, the soil resistance will decrease. The modulus of subgrade reaction will remained stable when the amplitude was small, but with the decrease of frequency, the modulus of subgrade reaction will become smaller.(4) Both the modulus of subgrade reaction and ultimate soil resistance degradation under cyclic loading. The impact on the ultimate degradation of soil resistance of cyclic loading is not great, although the soil stiffness degradation happened around the pile but on a wider range the soils still able to provide adequate soil resistance. However, the degradation of soil stiffness around pile is more great, which a greater soil displacement just produce smaller soil resistance. This phenomenon need pay more attention.(5) Based on soil resistance degradation model and research of other scholars, a degradation model based on accumulated plastic strain is adopted to model die cyclic degradation of the soil stiffness and undrained soil strength. With this model, we can got the cyclic p-y curve. The availability of degradation soil stiffness model was also proved by comparing with laboratory model test results. Practical guidance for design a monopile foundation in the Yellow River Delta offshore wind turbine can be proposed.