@misc{Schnackenberg_2021,
howpublished={Presentation},
  type={Conference presentation},
 abstract = {Main bearings of offshore wind turbines (WTG) are exposed to high loads over a long service life. They are initially lubricated and are continuously relubricated via an automatic system. A seal is designed to prevent the ingress of moisture. In practice, a visual inspection to check the condition of the lubricant and the seal is carried out by a service technician during regular maintenance. However, they are often unable to make a clear assessment of the grease condition. The ingress of moisture due to a damaged seal requires subsequent examination in the laboratory. A Grease Check System (GCS) developed by the Schaeffler Group (SG) solves this problem by checking the lubricant for moisture via an optical sensor, temperature, and aging. This is a shift from visual lubricant assessment to condition-based maintenance. 

To estimate the expected economic gain, an offshore wind farm with all WTGs main bearings and seals is analyzed over a lifetime of 25 years. Main bearing damage is modeled as a dependent condition. Only when the seal is in state {\textquotedbl}defective{\textquotedbl}, main bearing life degradation progresses. The subsequent analysis of the scenarios is based on time series. By simulating seal damage and main bearing failures along weather and electricity price, the costs are determined. Monte Carlo simulations were performed to infer probability distributions. The basic scenarios reflect the classical approach in the field. The parameters of the failure probability distributions of the main bearing and seal are estimated. To take uncertainties into account, they are varied in the base scenarios. In the GCS scenarios, each WTG of the offshore reference farms is equipped with the system. We assume that a defective seal is detected two weeks after moisture penetration in the lubricant with a detection probability of 85{\%}. A team of technicians then repairs the seal damage. The simulation of the total costs includes the costs for components (seal and main bearing), technicians, daily charter rate of a CTV and a jack-up vessel. The investments into GCS are included as well as the production losses due to turbine downtime.

Main bearing damage induced by defective seals is significantly reduced in all scenarios, but they are not completely avoided. Both result from the fact that seal damage is most likely to be detected within two weeks and replacement is initiated quickly. In the basic scenarios, damage is detected during the next scheduled maintenance and the seal is replaced much later. Consequently, over the total lifetime of plants equipped with GCS, the seals are changed more frequently. Production losses are lower using GCS. The total cost distributions are particularly striking. They consist of several individual bell curves due to the number of main bearing damages which have a great influence on the total costs, showing up to 80{\%} costs saving using GCS. The statistical evaluations of the simulation results lead to the conclusion that the use of Grease Check System is economical. The most relevant cost drivers are main bearing damages, which at the same time are leading to high production losses. These damages are significantly reduced.},
 author = {Schnackenberg, Marco and Kaczenski, Jonas and Meyer, Tobias and Mangold, Andreas},
 date = {2021-05-25},
 year = {2021},
 title = {Impact of dependent failures on wind turbine reliability},
 address = {online},
 series = {Wind Energy Science Conference},
 institutions = {{European Academy of Wind Energy}}
  note = {Slides available at: \url{http://www.tobi-meyer.de/Schnackenberg_2021.pdf}},
}