Dalgic, Yalcin and Lazakis, Iraklis and Turan, Osman (2013) Vessel charter rate estimation for offshore wind O&M activities. In: Developments in Maritime Transportation and Exploitation of Sea

Estimating the cost of vessels is an important factor for the operation & maintenance (O&M) costs of offshore wind farms. Accurate cost estimation is required during operation of the wind farm. This paper presents the development of the O&M charter rate modelling for offshore wind turbine jack-up vessels under different operational strategies. The main objectives of this modelling are to explore the differentiation of charter rates associated with the charter periods and with the capabilities of different vessels; as well as to allow offshore wind farm operators to plan their maintenance strategies through considering the share of vessels costs in the overall maintenance costs. In this respect, various jack-up vessels in the offshore wind industry are investigated and the most effective charter periods are identified. The seasonal effects on the charter rates are also considered to provide critical insight into the selection of the optimum charter rate scenario. section 3. In addition, the influence of seasonality on the charter rates will be highlighted. In the conclusion section, results of the study and the recommendations will be presented. 2 MAINTENANCE OF OFFSHORE WIND FARMS 2.1 Available vessels in the market The main tasks of the vessels in a maintenance operation are to provide accommodation for ship crew and technical personnel, loading, transporting and assembling failed turbine components in offshore environment. Due to the number of available vessels, their properties and the variety of benefits & drawbacks of the vessels have to be taken into account in the vessel selection process, which allow operators eliminating either insufficient or oversize and unnecessary vessels which will increase the cost of operations (Table 1). During the operational span of an offshore wind farm, a number of scheduled and unscheduled maintenance tasks have to be performed in order to keep the turbines operational and to sustain the power generation. In this respect, there are two main categories of O&M vessels in the offshore wind energy market: vessels for minor maintenance and vessels for major maintenance. Vessels for minor maintenance are used for the minor repairs and technical problems, which do not require heavy equipment transport or heavy crane operations. In the case of minor repairs such as; electrical system, electronic control, sensor and hydraulic system repairs, vessels for minor maintenance are utilised in the operations. Monohull boats, small catamaran vessels and Small Waterplane Area Twin Hull (SWATH) vessels are generally utilised in minor maintenance operations, which allow operators to keep the cost of minor maintenance operations at acceptable levels. Catamaran configurations are often the preferred choice but operations are restricted to relatively low wave heights. The most distinctive characteristics of these vessels are high speed, small deck spaces, small crane capacities and safe access to wind turbine structures that will allow operators to take quick actions in case of urgent repairs. Due to the higher charter rates, jack-ups, legstabilised and heavy lifting vessels are not considered appropriate for minor maintenance operations. The capabilities of these vessels are also above and beyond the scope of minor repairs. Table 1. Comparison of vessels currently employed in the offshore wind maintenance market Category Vessel type Benefits Drawbacks