Seismic response and spectrum analysis of offshore wind farm sites in Jiangsu Province, China

Abstract

Presently, the offshore wind farms in Jiangsu Province account for 70%–75% of the total installed capacity in China. However, this is a geologically active earthquake area. In this study, we propose a typical site model based on the detailed survey data of an offshore wind farm in Dongtai, Jiangsu Province. Three ground motions (EL Centro, Northridge, and Kobe) were used as model inputs, and the propagation characteristics of an earthquake motion along an elevation were mainly studied. These include acceleration amplification effects, ground motion attenuation effects caused by liquefaction, site filtering effects, and long-period effects of the response spectrum. The results in time domain are as follows: (1) the site shows a nonlinear amplification effect along the elevation under different ground motions, but the amplification laws are different, which may depend on the spectrum components of the input ground motions; (2) along the elevation, the acceleration at the interface of deep clay and surface sand slightly decreased; (3) the ground motion cannot propagate in the time domain when surface soil reached a complete liquefaction state. In the frequency domain: (1) during propagation, the high-frequency components are filtered quickly, and the attenuation of the components in the clay is weaker than that in the sand; (2) the dominant frequency of the input ground motion increases slightly (1∼1.5 Hz) at depths of approximately 40∼30 m, after which the dominant frequency of the ground motion decreases again (0.2∼0.5 Hz); (3) the response spectrum gradually transits from single-peak short-period to double-peak long-period (approximately 3 s). The abovementioned site dynamic analysis can provide the basis for seismic research of offshore engineering sites, such as guiding the seismic design of offshore wind turbine foundations.