Unplanned OPEX can be in the range of 25-40% of overall maintenance costs for newer wind farms and rising to 40-60% for older assets (typically). However, we often see the forecasts we see for unplanned OPEX assuming a linear slope over time. Analysts are often not taking the correct view of the situation over the long term. Most likely as they don’t have the data they need.
A key driver behind forecasting accurately is understanding the changing failure rates as components get older. See the chart below for example, from a certain turbine technology and siting. For gearboxes, the number of uptower repairs are doubling from year 5 to year 10, then it is flattening out. On the other hand, gearbox and generator exchanges are continually climbing but will flatten out in the last five years of project life. Not all failure instances are variable with time, e.g. blade damage from lightning strike is typically flat over the asset life.
Chart 1 : An example of windfarm failure rates for different components
Pooling GW of turbine data and developing Weibull curves for different failure modes, by technology, component model and siting is critical to accurately forecasting; at P50 values as well as P10/P90. And not all drivetrain failures rise with age. See below the example of Weibull distributions fitted to failure rates experienced by different gearbox components at the same windfarm. The cumulative failure rates are quite different for an early life serial failure mode in a parallel stage (premature) vs. random failures caused by manufacturing defect in a parallel gear set vs. an ageing failure mode in a planetary stage. And this yields a unplanned OPEX cost over time for each case that is markedly different.
Chart 2: Cumulative failure rates for gearbox components
Interested to learn more? We have recently held a webinar, “The Cost of Major Failures to the Wind Industry – What to Expect in 2018 and How to Reduce Them”, where we talk through practical failure issues and associated cost data.
And if you really like the technical details, register for our 2018 EU Technical Symposium or US Wind Turbine Technical Symposium. Events focused for turbine owners, asset/site managers and reliability engineers to get deep into wind farm reliability and maintenance.