Availability Evaluation And Improvements Of An Offshore Wind Farm Considering Accessibility Problems

China is rich of offshore wind resources,with 7500 GW capacity nearshore waiting to be explored,which is three times of onshore wind resources.Besides,closer geographical distribution of offshore wind farms and load centers represents a shorter transmission distance and the advantage of satisfying the local consumers' requirements.Offshore wind exploration is now one of the important contents of energy strategy and marine strategy in China.There is a bright future for offshore wind power in China.As the investment of an offshore wind project is very expensive,with huge expenses of constructions as well as operations and maintenance,the capability and possibility of offshore wind turbines operating reliably have been a key index for the feasibility and economical efficiency of an offshore wind project.How to evaluate the reliability of an offshore wind farm and how to operate and maintain the offshore wind turbines properly are now two critical problems waiting to be solved urgently.Comparing with onshore wind farms,there is a distinctive feature of offshore wind farms which is the low accessibility.Poor accessibility introduces a lot of difficulties in availability evaluations.Firstly,poor accessibility prolongs the downtime of wind turbines and makes the downtime of a failure an uncertain parameter.The downtime will be influenced by many uncertain factors.Secondly,the power produced by wind turbines is determined my wind speed and the possibility of wind turbines staying normal.As the accessibility of an offshore wind farm is determined by the wind speed,there is a nonlinear correlation between the power losses and the wind speed,failures,and maintenance strategies.Besides,the maintenance of offshore wind turbines may be postponed unexpectedly by the poor accessibility.Find out the influences of the limited accessibility and the availability and capability of power produced by the wind turbines,and then make a more efficient maintenance strategy is the main problem that the wind farm operators concern.This thesis takes the theories and methods of evaluating and improving the availability of offshore wind farms as research objectives,mainly focus on the time availability and energy availability evaluations of an offshore wind farm and then the improvement strategies.This thesis carries out the research from the point view of data analysis,modeling constructions,calculation researches and method suggestions.The main contributions are as follows:(1)Wind speed and accessibility data analysis based on a real offshore wind farmThe main factors that determined the accessibility of an offshore wind farm are analyzed and categorized.Based on several years operational experience of Shanghai Donghai Offshore wind farm,the characteristics of offshore wind resources and accessibilities are calculated and analyzed,and then the influences of the limited accessibility to the availability and maintenance strategies of the offshore wind farm summarized qualitatively.(2)Modeling of time availability evaluation of offshore wind turbines considering accessibilitiesThis paper presents a Markov-Chain-Based availability assessing model of offshore WT considering accessibility problems.Based on procedure analysis of corrective maintenance offshore,the contribution of poor accessibility to availability is summarized into two independent aspects: stochastic offshore weather and inadequate maintenance resources.A three-state model of offshore WT is established.A Poisson process-based algorithm is presented to calculate a transition rate of the three-state model.A three-integer field representing a wind farm with NWT WTs is defined for the Markov chain.Mean availability of WT is obtained by solving a Markov transition matrix.Results of the case study show that the proposed model provides an effective approach for the availability assessment of WT in a multi-turbine wind farm influenced by stochastic offshore weather and different logistic policies.(3)Energy availability evaluations of an offshore wind farm based on corrective maintenanceAs the produced energy is the most concerns of the wind farm operators,and corrective maintenance is an indispensable and time-consuming way to maintain the wind turbines,this thesis presents an energy availability evaluation method for the offshore wind farms based on corrective maintenance.Firstly,this paper points out that the energy production of an offshore wind farm that wind farm operators,wind turbine manufacturers,and power networks really concerns is not an accurate value but a general range.A method of evaluating the confidence intervals of energy availability is proposed here.Then,Monte-Carlo simulation is introduced to deal with the stochastic failures of offshore wind turbines and the procedure of corrective maintenance.The results show that the stochastic failure causes a great change of energy availability of the offshore wind farm,and taking the advantage of accessibility to keep the failure happens in low wind speed and high accessible period is an effective way to improve the energy availability.(4)Joint optimization of spare part inventory and wind turbine maintenance strategiesCondition maintenance is an effective way to take advantage of the deterioration information of wind turbines to perform the maintenance for a certain purpose.But for the wind industry,the immaturity of the condition monitoring system makes it still require a lot of state examinations on site by technicians.Considering for these realities,this thesis develops a joint optimization of condition maintenance strategy and spare part inventory.Firstly,the time-delay concept is introduced to optimize the schedule of state examinations of offshore wind turbines.And then a Markov Chain of the offshore wind farm considering corrective replacement,preventive replacement and spare part inventory is presented.The schedules of state examinations and the spare part inventory are optimized by the nesting optimization.Comparing with traditional spare part management coordinating with the corrective maintenance,the case study shows that the proposed method will increase the power generation profits and cut down the spare part inventory.