On risk-based operation and maintenance of offshore wind turbine components

Abstract

Operation and maintenance are significant contributors to the cost of energy for offshore wind turbines. Optimal planning could rationally be based on Bayesian pre-posterior decision theory, and all costs through the lifetime of the structures should be included. This paper contains a study of a generic case where the costs are evaluated for a single wind turbine with a single component. Costs due to inspections, repairs, and lost production are included in the model. The costs are compared for two distinct maintenance strategies, namely with and without inclusion of periodic imperfect inspections. Finally the influence of different important parameters, e.g. failure rate, reliability of inspections, inspection interval, and decision rule for repairs, is evaluated.

Introduction

The costs of operation and maintenance (O&M) of offshore wind turbines are significant contributors to the cost of energy—up to 30% of the cost of energy. Optimal planning of O&M should include the use of inspections and monitoring results to make decisions that minimize the expected total costs through the lifetime of the structures. For offshore wind turbines it is especially important because of the dependence on weather windows for inspections and repairs to be possible. The aim of this work is to demonstrate the effect of the use of condition-based maintenance compared to the use of corrective maintenance for a generic case. For simplicity, only a single wind turbine with a single component is included in model, but the relative influence of different parameters are considered to be generic.

In general maintenance activities can be divided into corrective and preventive maintenance. Corrective maintenance is performed if a component has failed, and preventive maintenance is performed to avoid failure. Preventive maintenance can be divided into scheduled and condition-based maintenance. Scheduled maintenance is performed on scheduled times, and could e.g. be lubrication, tightening bolts, and changing filters [1]. Condition-based maintenance is performed on the basis of the actual health of the component, and thus it requires a condition-monitoring system with online monitoring and/or inspections, see e.g. [2], [3]. For offshore wind turbines service visits are performed on a scheduled basis, where scheduled maintenance are performed, and at the same time inspections can be performed at a relatively low additional cost.

The use of corrective maintenance is the most simple strategy, but it has several flaws. The failure of one minor component can cause escalated damage to a major component, which gives large repair/replacement costs. Further failures will often happen during a period with large wind loads, and the site will be unaccessible during that period, which will cause lost production, see e.g. [4]. Thus the costs for corrective maintenance are associated with much larger uncertainty than preventive maintenance [2].