Site liquefaction characteristics of offshore wind farms under earthquakes in the South Yellow Sea, China

Abstract

Jiangsu offshore belongs to the South Yellow Sea, which is the most concentrated area of offshore wind farms in China. At present, it accounts for about 70 ∼ 75% of the total installed capacity of China. The overburden layer thickness in this area is quite large; soils along the pile depth (generally 40 m ∼ 60 m) are mainly silty sand, silt, and silty clay. Especially, there are 20 m thick liquefiable silty sand layer below the mud surface. The South Yellow Sea is also the earthquake activity area in China. Therefore, this paper researched the seismic liquefaction characteristics of offshore wind farms in this area. Firstly, the drilling borehole data of 50 offshore wind turbines in Dongtai, South Yellow Sea were analyzed, and a representative formation model was established. Secondly, the soil dynamic parameters for site dynamic analysis were calibrated based on dynamic triaxial test and resonance column test. Then, the site liquefaction was researched, the El Centro wave was put into the model, and the peak ground accelerations (PGAs) near the ground surface were adjusted to 0.05 g, 0.1 g, 0.2 g, and 0.4g, respectively. The characteristics of excess pore pressure ratio, total settlement, and layered settlement under earthquakes were analyzed. It was found that the strata in this area consisted of liquefiable soils. When input PGA = 0.05 g, the excess pore pressure ratio of each layer is less than 1.0, and the total settlement is about 1 cm; when PGAs = 0.1 g and 0.2 g, only the surface layer (within 12 m) is completely liquefied, and the total settlement is 10 cm and 17 cm, respectively; when PGA = 0.4 g, the 20 m strata below the mud surface are completely liquefied and the total settlement is about 30 cm. Under different earthquakes, the settlement of surface soil (12 m) accounts for the largest proportion (more than 95% when PGAs = 0.1 g and 0.2 g). Therefore, the site liquefaction characteristics under earthquakes should be considered for pile foundation design of offshore wind turbines.