energies Article Optimal Maintenance Policy for Offshore Wind Systems Vincent F. Yu 1,2 , Thi Huynh Anh Le 1,3 , Tai-Sheng Su 4 and Shih-Wei Lin 5,6,7, *   Citation: Yu, V.F.; Le, T.H.A.; Su, T.-S.; Lin, S.-W. Optimal Maintenance Policy for Offshore Wind Systems. Energies 2021, 14, 6082. https:// doi.org/10.3390/en14196082 Academic Editor: Eugen Rusu Received: 9 August 2021 Accepted: 14 September 2021 Published: 24 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Industrial Management, National Taiwan University of Science and Technology, Taipei106, Taiwan; vincent@mail.ntust.edu.tw (V.F.Y.); D10901814@mail.ntust.edu.tw(T.H.A.L.) 2 Center for Cyber-Physical System Innovation, National Taiwan University of Science and Technology, Taipei 106, Taiwan 3 Faculty of Project Management, The University of Danang, University of Science and Technology, Danang 550000, Vietnam 4 Department of Industrial Management, National Pingtung University of Science and Technology, Pingtung 912, Taiwan; tyson@mail.npust.edu.tw 5 Department of Information Management, Chang Gung University, Taoyuan 333, Taiwan 6 Department of Industrial Engineering and Management, Ming Chi University of Technology, New Taipei 243, Taiwan 7 Department of Neurology, Linkou Chang Gung Memorial Hospital, Taoyuan 333, Taiwan * Correspondence: swlin@mail.cgu.edu.tw; Tel.: +886-3-211-8800 Abstract: Employing maintenance threshold plays a critical step in determining an optimal main- tenance policy for an offshore wind system to reduce maintenance costs while increasing system reliability. Considering the limited works on this topic, we propose a two-stage procedure to deter- mine the optimal maintenance thresholds for multiple components of an offshore wind power system in order to minimize maintenance costs while achieving the highest possible system reliability. First, using genetic algorithms, a dynamic strategy is developed to determine the maintenance thresholds of individual components where the cost of maintenance and the rate of failure are critical. Then, fuzzy multi-objective programming is applied to find the system’s optimal maintenance threshold considering all components. A variety of factors including weather conditions, system reliability, power generation losses, and electricity market price are carefully considered to enhance the system’s reliability and reduce the costs of maintenance. When maintenance threshold results are compared, component-wise versus system-wise, an average system savings of 1.19% for maintenance cost is obtained while the system reliability is increased by 1.62% on average. Keywords: offshore wind system; fuzzy multi-objective programming; failure rate; maintenance threshold; optimal maintenance policy 1. Introduction Global warming and conventional energy resources have been in the spotlight due to the latter’s scarcity and their negative environmental impact. The situation has led to new energy resources being discovered to improve human life and the natural environment. Renewable energy resources have been promoted for their tremendous potential as a solution for alleviating environmental pollution caused by conventional energy resources. Among the former, wind energy has received seemingly the most interest and investment from many countries. The Global Wind Energy Council (GWEC) reported that annual capacity installation rose from 2015 to 2020 (as shown in Figure 1)[1]. For instance, from the GWEC report in 2020, the installed capacity of wind farms has increased from 435,284 GW in 2015 to more than 650 GW in 2019 and about 744 GW in 2020. Wind farms reached a global installed capacity of 744 GW in 2020, with a net installed capacity of 93 GW added that year [1]. Offshore wind systems are located in harsher conditions, so the costs of installation and maintenance are very high [2,3]. Lin et al. [4] noted that the maintenance of an offshore wind turbine accounts for more than 30% of the total cost of its life cycle. Energies 2021, 14, 6082. https://doi.org/10.3390/en14196082 https://www.mdpi.com/journal/energies