Marine engineering equipment is usually located far away from the coastline. The complicated and varied marine environment not only increases the failure rate of components, but also brings great inconvenience to maintenance work. The construction, operation and maintenance of offshore wind farms are affected by the weather accessibility due to the special weather environment [
1]. Different weather and sea conditions are important factors restricting maintenance activities. In general, offshore operations are difficult, expensive and time-consuming. Favorable operation conditions are the basis to ensure the accessibility of offshore engineering site and offshore operation. Professional marine transportation resources and unpredictable weather lead to poor accessibility of offshore wind farms, and operations can only be carried out in appropriate weather, resulting in large shutdown loss and high maintenance cost. Compared with onshore wind power, operation and maintenance cost of offshore wind farm is much higher, which can account for about 30% of the average energy cost [
2]. Meanwhile, the current maintenance models are idealized or simple. In general, equipment status after maintenance is somewhere between perfect maintenance and minimum maintenance, that’s to say, wind turbine status after preventative maintenance will be better than that before maintenance, but it can’t return to the initial intact status [
3,
4]. In addition, once the maintenance engineering team arrives at the offshore wind farm, attention should also be paid to multiple wind turbine systems contained in the wind farm, and each offshore wind system also contains multiple equipment under simultaneous operation. Economic relevance differences between system and system, equipment and equipment should be comprehensively taken into consideration so that maintenance activities between systems and equipment in the wind farm can coordinate and cooperate with each other, so as to reduce outage loss and reduce fixed maintenance cost of the offshore wind farm [
5].
In view of the complexity of the maintenance model for offshore wind farm, existing offshore wind farm maintenance methods, which can consider weather conditions, maintenance uncertainty and economic relevance of the maintenance, have achieved few researches progresses. Carlos et al. [
6] optimized the maintenance cost for onshore wind farms based on the stochastic model. Laura and Vincente [
7] analyzed the lifecycle cost for offshore wind farms, and Carroll [
8] provided failure rate, repair time and unscheduled O&M cost analysis of OWTs. Lu et al. [
9] proposed a rolling horizon approach for OWTs, the maintenance planning can be updated to take short-term information into account, which could be changed with time, however, maintenance uncertainty is not considered in this study. Ding and Ting [
10] compared imperfect maintenance and perfect maintenance based on replacement due to failure in preventive maintenance of wind turbines, and the study showed that imperfect maintenance method not only fitted engineering practice but also could save maintenance cost when compared with perfect maintenance mode based on replacement. Research by Huang et al. [
11] showed that meaningful wave height is the main factor limiting landing system work and maintenance personnel landing, which determines the validity window of marine maintenance. Tian [
12] developed a Condition based maintenance (CBM) strategy for WTs to optimize maintenance cost, and Lu et al. [
13] optimized OWTs by the CBM strategy, the results showed that the CBM strategy is more effective than the time-based maintenance policies. Wu et al. [
14] established a weather-based Markov maintenance model of an offshore wind system, estimated accessibility of offshore wind power maintenance and considered correlation between equipment maintenance, but the study object was not expanded to offshore wind farm, and maintenance imperfection in actual engineering was neglected. Hagen [
15] proposed a multivariate Markov chain model for state estimation of sea conditions based on observation time sequences, weather conditions were expressed by wave height, wind velocity and wind direction, and maintenance strategies of the offshore wind farm were optimized. This model considered the influence of weather accessibility on the maintenance work, but neglected economic relevance between systems and equipment, and consequently, overall maintenance cost of the wind farm was not obviously reduced. Zhu et al. [
16] proposed a failure model that included multiple failure modes with different failure consequences, logistic delays and weather conditions were also obstacles for performing maintenance activities, however, the research scheme had not been extended to offshore wind farms.
In this paper, the availability, uncertainty, economic relevance and outage loss of maintenance in practical offshore engineering are comprehensively considered, and the optimal maintenance scheme is formulated for the purpose of maximizing the power production of offshore wind farm.
Section 2 gives concrete handling methods of imperfect equipment maintenance; In
Section 3, considering the influence of weather environment accessibility on offshore wind turbine maintenance, based on the parameter estimation theory of L-moment, the expected waiting window time is calculated by the range of significant wave height in maintenance condition; In
Section 4, a comprehensive maintenance model of offshore wind farm based on rolling horizon approach is established. The maintenance improvement factor, weather waiting time, downtime and other variables are introduced into the mathematical model. Taking optimal economical efficiency as the objective function, this model considers economic relevance between equipment and equipment, system and system during the maintenance process, which then improves accuracy of the decision-making model of total expected offshore wind farm maintenance cost; taking Dafengtian wind farm of Goldwind Technology Co., Ltd. as an example,
Section 5 verifies the proposed maintenance optimization model, and results show that considering weather accessibility not only accords with actual offshore wind farm maintenance work, but moreover, compared with independent repair of wind turbines or equipment, the opportunistic grouping maintenance scheme can effectively reduce the cost, the reasonability and applicability of this model are verified.