Liquefaction potential analysis is a required task in the foundation design process of offshore wind turbine in Taiwan. The standard penetration test (SPT) is mostly used in the preliminary soil investigation of the pilot offshore wind farm in Chang-Bin, Taiwan. Due to the different experimental conditions and operating conditions, the N value (SPT-N) of SPT varies greatly. This study applies statistical methods in conjunction with the New Japan Road Association simplified-empirical method (NJRA method) to incorporate the uncertainty of SPT-N values into the offshore liquefaction potential assessment to quantify the risk of seabed liquefaction. The study statistics the field experimental geotechnical parameters SPT-N and determines the probability density function of the SPT-N distribution of each layer of soil in the offshore wind farm. In order to quantify the risk of seabed soil liquefaction potential, the Monte Carlo random sampling method is used, and by the NJRA method to carry out the seabed liquefaction potential in Changhua, Taiwan. After comparing, the results obtained by the current SPT-based soil liquefaction potential assessment by deterministic approach are conservative. In the ground investigation of offshore wind farm development, the method proposed in this paper can access the thickness of liquefiable soil layers under any given probability for optimizing offshore wind turbine foundation design.

Introduction

The Chang-Bin offshore wind farm of Taiwan Power Company is located in the offshore area of Changhua. The preliminary geotechnical investigation results show that the seabed of Chang-Bin offshore wind farm in Taiwan is mainly composed of silty sand (SM) and low plasticity clay (CL), it contains a small number of low plasticity silt (ML) and silty poorly graded sand (SP-SM) [CSC (2017), Hai-Shia Offshore Wind Energy Co., LtD. (2017a, 2017b), Chien at al. (2014), TORI (2012), TPC (2012, 2018)].. The soil within 80 meters below the seabed can be roughly divided into three layers. The uppermost layer is mainly loose to medium dense sand (SPT-N<30) and the depth is about 25 meters. The soil conditions of Taiwan’s offshore wind farm are very different from the offshore wind farm in the North Sea [Le at al. (2014)]. For a sandy soil within 20 meters of the surface depth, it is highly likely that soil liquefaction will occur when an earthquake occurs [Seed and Idriss (1971)]. The soil liquefaction potential can assess by deterministic approach in Taiwan, which the SPT-N based soil liquefaction potential semi-empirical analysis method is the common adopted in the engineering design in Taiwan [JRA (1996), CPA (2011)], However, soil liquefaction can be affected by soil layer distribution, soil properties, seismic wave transmission, stress conditions, etc. Therefore, quantifying the risk of soil liquefaction in highly seismic region is essential for the development of offshore wind farms.