"During the lifetime of a wind farm the costs of O&M have to be considered continuously. During the planning phase of a wind farm an estimate of the expected O&M cost over the life time has to be made to support the financial decision making, and furthermore quite often an initial O&M strategy has to be set up. To support this process ECN has developed the O&M Tool, which is commonly used by the wind industry. With this computer program developed in MS-Excel it is possible to calculate the average downtime and the average costs for O&M over the life time of the wind farm. During the operational phase of a wind farm it is important (1) to monitor the actual O&M effort and (2) to control and to optimise future O&M costs. To be able to control and subsequently to optimise the future O&M costs of these wind farms, it is necessary to accurately estimate the O&M costs for the next coming period of e.g. 1, 2 or 5 years, taking into account the operational experiences available at that moment. Several reasons are present for making accurate cost estimates of O&M of (offshore) wind farms. Examples are: ­ to make reservations for future O&M costs (this is especially important for the party who is responsible for the financial management of the maintenance); ­ operating experiences may give indications that changing the O&M strategy will be profitable, and then the costs need to be determined accurately in order to compare the adjusted strategy with the original one; ­ before the expiration of the warrantee period, a wind farm owner needs to decide how to continue with servicing the wind turbines (new contract with turbine supplier or to take over the total responsibility) after the warranty period; ­ if a wind farm is going to be sold to another investor, the new owner wants to have detailed information on what O&M costs he can expect in the future. To support the process of monitoring, control, and optimisation ECN is developing the O&M Cost Estimator (OMCE). To handle both aspects, processing of operational data and prediction of future O&M costs two major parts can be distinguished: 1. OMCE Building Blocks for processing of operational data, where each building block covers a specific data set. Currently BB’s are being developed for the following data sets: • Operation and Maintenance; • Logistics; • Loads and Lifetime; • Health Monitoring. The main objective of these building blocks is to process all available data in such a way that useful information is obtained, which can be used on the one hand as input for O&M modelling and on the other hand to monitor certain aspects of the wind farm. 2. OMCE-Calculator for the assessment of the expected O&M effort and associated costs for the coming period, where amongst others all relevant information provided by the OMCE Building Blocks is taken into account. The Building Blocks ‘Operation & Maintenance and ‘Logistics’ have the main goal of characterisation and providing general insight in the corrective maintenance effort that can be expected for the coming years. With respect to corrective maintenance important aspects are the failure frequencies of the wind turbine main systems, components, and failure modes. Furthermore, other parameters that are needed to describe the corrective maintenance effort are for instance the length of repair missions, delivery times of spare parts and mobilisation times of equipment. Within the OMCE project procedures and software have been developed that can be used by operators to analyse their data sets and generate input data needed for O&M modelling but only if the data are collected in a structured way, f.i. in accordance with the Event List specifications described in the current report. For estimating the expected future condition based maintenance work load the Building Blocks ‘Loads & Lifetime’ and ‘Health Monitoring’ have been developed. The main goal of these Building Blocks is to obtain insight in the condition or, even better, remaining lifetime of the main wind turbine systems or components. Within the OMCE project it was concluded that it was not possible to develop a software tool for analysing the data sets that apply to ‘Loads & Lifetime’ and ‘Health Monitoring’. These data are often obtained from measurement systems from third parties and require experts to draw meaningful conclusions. The results need to be interpreted carefully and combined for instance with inspection results. Such procedures appeared to be too complicated to incorporate in software that can be used straightforwardly by wind farm operators. For the further development of the BB ‘Loads & Lifetime’ the Flight Leader concept for Wind Farm Load Counting is being developed by ECN and for the BB ‘Health Monitoring’ the description is limited to the lessons learned, some general procedures, and references to further reading. In contrary to the ECN O&M Tool, the OMCE-Calculator is meant to be used during the operational phase of a wind farm, to estimate the required O&M effort for the coming period of 1 to 5 years, taking into account the operational experiences of the wind farm acquired during the operation of the wind farm so far. This implies that for the OMCE model it is not sufficient to determine long term yearly average numbers, but that another approach has to be followed, viz. simulation in the time domain, taking into account the random behaviour of the weather conditions and the random occurrence of failures. This approach allows that all kind of optimisation studies can be carried out, f.i. for contracting of ships or for setting up a warehouse for spares. It may be clear that such a tool with these features is not of interest for operators only, but also for other stakeholders (owners of wind farms, wind turbine manufacturers, etc.). The OMCE is described intensely in the current report with the aim to provide the reader not only information concerning the features of the tools, but especially to provide background information on the models applied and also to illustrate the capabilities by means of a number of examples."