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A Methodology for Estimating the Position of the Engineering Bedrock for Offshore Wind Farm Seismic Demand in Taiwan

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A Methodology for Estimating the Position of the Engineering Bedrock for Offshore Wind Farm Seismic Demand in Taiwan energies Article A Methodology for Estimating the Position of the Engineering Bedrock for Offshore Wind Farm Seismic Demand in Taiwan Yu-Shu Kuo 1,*, Tzu-Ling Weng 1, Hui-Ting Hsu 1, Hsing-Wei Chang 2, Yun-Chen Lin 1, Shang-Chun Chang 3, Ya-Jhu Chuang 1, Yu-Hsiu Tseng 4 and Yih-Ting Wong 1 1 Department of Hydraulic and Ocean Engineering, National Cheng Kung University, Tainan 701, Taiwan; [email protected] (T.-L.W.); [email protected] (H.-T.H.); [email protected] (Y.-C.L.); [email protected] (Y.-J.C.); [email protected] (Y.-T.W.) 2 Taiwan Semiconductor Manufacturing Co., Ltd., Hsinchu 300, Taiwan; [email protected] 3 CECI Engineering Consultants, Inc., Taipei 114, Taiwan; [email protected] 4 Cheng Da Environment and Energy Ltd., Taipei 104, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-6-2757575 (ext. 63271) Abstract: Taiwan lies in the circum-Pacific earthquake zone. The seabed soil of offshore wind farms in Taiwan is mainly composed of loose silty sand and soft, low-plasticity clay. The seismic demand for offshore wind turbines has been given by the local code. Ground-motion analysis is required to consider the site effects of the soil liquefaction potential evaluation and the foundation design of offshore wind turbines. However, the depth of the engineering bedrock for ground motion analysis Citation: Kuo, Y.-S.; Weng, T.-L.; is not presented in the local code. In this study, we develop a three-dimensional ground model Hsu, H.-T.; Chang, H.-W.; Lin, Y.-C.; of an offshore wind farm in the Changhua area, through use of collected in situ borehole and PS Chang, S.-C.; Chuang, Y.-J.; Tseng, (P wave (compression) and S (shear) wave velocities) logging test data. The engineering bedrock is Y.-H.; Wong, Y.-T. A Methodology for the sediment at the depth where the average shear wave velocity of soil within 30 m, Vsd30, is larger Estimating the Position of the than 360 m/s. In this ground model, the shear wave velocity of each type of soil is quantified using Engineering Bedrock for Offshore the seismic empirical formulation developed in this study. The results indicate that the engineering Wind Farm Seismic Demand in bedrock lies at least 49.5–83 m beneath the seabed at the Changhua offshore wind farm. Based on Taiwan. Energies 2021, 14, 2474. these findings, it is recommended that drilling more than 100 m below the seabed be done to obtain https://doi.org/10.3390/ en14092474 shear wave velocity data for a ground response analysis of the seismic force assessment of offshore wind farm foundation designs. Academic Editors: Jesús Manuel Riquelme-Santos and Keywords: ground model; offshore wind farm; seismic demand Adrian Ilinca Received: 20 February 2021 Accepted: 20 April 2021 1. Introduction Published: 26 April 2021 Taiwan lies in the circum-Pacific earthquake zone. Offshore wind farms in the western sea area are affected by earthquakes and active faults. In order to ensure the stability of Publisher’s Note: MDPI stays neutral the offshore wind turbine foundations, site effects and soil liquefaction must be taken into with regard to jurisdictional claims in consideration in its design. published maps and institutional affil- The seismic demand for offshore wind turbines is given in the local code for offshore iations. wind farm seismic demand (CNS 15176–1) by the Bureau of Standards, Metrology, and Inspection in the Ministry of Economic Affairs [1]. AppendixA of CNS 15176–1 states that, when evaluating the offshore wind farm seismic force and the potential for soil liquefaction, a site-specific seismic hazard analysis is required. Copyright: © 2021 by the authors. The duration of the seismic acceleration applied to the offshore wind turbine foun- Licensee MDPI, Basel, Switzerland. dation can be determined by calculating the amplified response of the seismic wave This article is an open access article transmitted from the engineering bedrock to the seabed surface. AppendixA of CNS distributed under the terms and 15176–1 [1] suggests that the soil beneath the seabed can be treated as engineering bedrock conditions of the Creative Commons for ground response analysis when the shear wave velocity value (Vsd30) of the 30 m soil Attribution (CC BY) license (https:// profile reaches 360 m/s. However, the CNS standard does not present a recommended creativecommons.org/licenses/by/ 4.0/). depth of engineering bedrock for offshore wind farms in Taiwan. Energies 2021, 14, 2474. https://doi.org/10.3390/en14092474 https://www.mdpi.com/journal/energies Energies 2021, 14, 2474 2 of 17 To perform a seismic force analysis for the offshore wind farm before a detailed foundation design is done, we need to determine the depth of the engineering bedrock, according to limited soil borehole data. In the early development stage of offshore wind farms in Taiwan, the standard penetration test (SPT test) is often used for site investigation. Ohta & Goto (1978) [2], Seed and Idriss (1981) [3], Lee (1992) [4], Dikmen (2009) [5], the Construction and Planning Agency (2011) [6], Silvia et al. (2015) [7], and others have provided recommendations for estimating the shear wave velocity of soil. Table1 indicates the recommended soil conditions proposed by various scholars for onshore soil data, which are not the same as the range of SPT-N values available for offshore constructions. To design onshore buildings, considering the soil characteristics of Taiwan, the Con- struction and Planning Agency (2011) [6] recommends calculating the shear wave velocity of soil using Equations (1) and (2): Cohesive soil: 0.36 120qu ; Ni < 2 Vsi = 1/3 , (1) 100Ni ; 2 ≤ Ni ≤ 25 Cohesionless soil: 1/3 Vsi = 80Ni ; 1 ≤ Ni ≤ 50, (2) where Ni is the N-value of the ith soil layer obtained by the standard penetration test 2 (SPT) and qu is the unconfined compression strength (kg/cm ). The empirical formula of the Construction and Planning Agency (2011) [6] applies to the calculation of shear wave velocity for cohesionless soil with N-value less than 50 and for cohesive soil with N-value less than 25. According to the empirical formula in Table1, the shear wave velocity of offshore wind farm #29 in the Changhua area varies with depth, as shown in Figure1. A comparison is provided for the distribution trend of shear wave velocity with depth, calculated by the empirical formula with the experimental data of a resonant column test and the measured values of PS logging. At a depth of 5.5 m, the results obtained from the empirical formula of Dickmen (2009) [5] were close to that of the resonant column test. Meanwhile, at a depth of 9 m, the shear wave velocity calculated using the formulation suggested by Silvia et al. (2015) [7] was similar to that of the PS logging test results. At a depth of 18–40 m, the Construction and Planning Agency (2011) [6] and Lee (1992) [4] predicted the shear wave velocity as the measured values of PS logging. Seed and Idriss (1981) [3] and Dickenson (1994) [8] proposed empirical formulae specific to sand. While the range of the SPT-N value of Ohta and Goto (1978) [2] met the engineering requirements, their shear wave velocity estimation was more conservative. Table 1. Empirical formulae for wave velocity and SPT-N values proposed in previous research [2–8]. Vs (m/s) Range of Area Researcher(s) Sand Clay Silt SPT-N Japan Ohta and Goto (1978) 85.35 N0.348 0 < N < 50 USA Seed and Idriss (1981) 61.4 N0.5 – – 0 < N < 50 Taiwan Lee (1990) 57 N0.49 114 N0.31 105.64 N0.32 0 < N < 50 USA Dickenson (1994) 88.4 (N + 1)0.3 – – 5 < N <90 Turkey Dikmen (2009) 73 N0.33 44 N0.48 60 N0.36 0 < N < 50 Construction Taiwan and Planning Agency 80 N1/3 100 N1/3 – 0 < N < 50 (2011) Italy Silvia et al. (2015) 149.3 N0.192 110.5 N0.252 – 0 < N < 60 Considering the difference between the application scope of the soil conditions and the analysis results proposed by various scholars to use the SPT-N value to estimate the shear wave velocity, this research compares the measured values of PS logging in an offshore wind farm in the Changhua area with the results of resonant column testing. A shear wave velocity prediction method for the soil of the offshore wind farm in Taiwan Energies 2021, 14, x FOR PEER REVIEW 3 of 19 Table 1. Empirical formulae for wave velocity and SPT-N values proposed in previous research. [2–8] Vs (m/s) Range of Area Researcher(s) Sand Clay Silt SPT-N Japan Ohta and Goto (1978) 85.35 N0.348 0 < N < 50 USA Seed and Idriss (1981) 61.4 N0.5 – – 0 < N < 50 Energies 2021, 14, 2474 Taiwan Lee (1990) 57 N0.49 114 N0.31 105.64 N0.32 0 < N <3 50 of 17 USA Dickenson (1994) 88.4 (N + 1)0.3 – – 5 < N <90 Turkey Dikmen (2009) 73 N0.33 44 N0.48 60 N0.36 0 < N < 50 is proposed. TheConstruction depth of the engineering bedrock is determined using predicted shear waveTaiwan velocities. and Planning By collecting Agency the existing80 N borehole1/3 data100 N of1/3the offshore– Changhua0 < N < wind 50 farm, we established(2011) a three–dimensional ground model for the depth of the engineering bedrockItaly that canSilvia be et used al. (2015) to analyze ground149.3 N0.192 motion 110.5 during N0.252 an earthquake.– 0 < N < 60 Figure 1.
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