Numerical study of seismic response of the gravity foundation for offshore wind turbines in saturated sandy soils

Offshore wind turbines (OWTs) are one of the approaches that make it easy to use renewable energy sources such as wind to generate energy. In recent years, the use of offshore wind farms has become attractive due to the high quality of offshore wind energy, no need of land extraction, less destructive effects on the environment. Foundation is one of the most important parts of these systems due to the presence of this structure in specific climatic conditions and location, so that include about 34 percent of constructing and execution cost of a wind turbine. These structures foundation is selected based on water depth, distance from the coast and etc. that the gravity based foundation is one of it. This foundation with its high weight in general stability of the structure against slipping and overturning is a good one for these turbines in shallow water and easily transfers the load from the structure to the soil below and around it. Investigation of foundations in marine and seismic zones under earthquake loading is one of the important design criteria. In the marine environment, soils can soften by increasing the pore water pressure under seismic loading. In the worst case, the earthquake causes liquefaction in the soil and leads to a sudden decrease in bearing capacity and lateral stability of the foundation, which can cause settlement or rotation in the foundation and structure. So the occurrence of liquefaction in relatively loose sand caused by rapid earthquake loading should be evaluated. In the present research, a 3D numerical study of gravity based foundation of OWTs with its structure are performed using Opensees software to investigate the behavior of saturated sandy soils at near and far from Foundation under seismic loading. In order to consider the soil saturation conditions, the mentioned software has been used, which has a good ability in simulating the process of changes in excess pore water pressure due to the existence of numerous and suitable soil behavioral models, including PDMY behavioral models and solid-fluid correlated elements. For this purpose, modeling was performed based on laboratory research of Yu et al. (2015) and Validation be done in good agreement and adaption with the laboratory model on soil response in acceleration and ru (ratio of additional pore water pressure Δu to the effective stress σ’), on the response of gravity foundation in settlement and rotation, and horizontal displacement of the structure. Parametric studies are conducted to investigate dimensions and embedded depth of foundation on soil response at near and far from the foundation, foundation and structures performance (settlement and tilt of foundation and structure horizontal displacement). The results show that increasing the foundation dimensions decreases the settlement and tilt of foundation, but the maximum amount of ru in the soil increases and the acceleration does not change. By increasing the embedded depth of foundation, in the maximum value of ru in the center position of the model and near to the foundation is increased. Also caused a decrease in settlement and tilt.