Even some nearer-shore projects such as Alpha Ventus off Germany and Horns Rev off Denmark are understood to be using helicopters for frequent operations-and-maintenance activities, and the Greater Gabbard project, currently under construction some 25 kilometres from England's shore, is planning to use them in the near future.
Good accessibility and quick response times offered by helicopters fits well with the relatively high-frequency, quick-to-fix failures that form a large proportion of wind turbine downtime, leaving vessels to attend to the less frequent, larger failures as well as the scheduled maintenance burden. Indeed, weather conditions that restrict helicopter flights - poor visibility and low cloud base - often occur during the more benign periods when boat access is possible. While operating costs increase with the use of helicopters, industry experience and projections suggest that this cost is minimal in relation to the revenue increase from the resulting improved turbine availability.
The land-based helicopter shuttling approach used for nearer-shore wind sites, when frequent trips are made with small aircraft carrying four to six passengers such as the Eurocopter EC135, becomes less attractive for far-shore wind projects. Increased transit times require larger helicopters with the capability for longer flying times, and the benefits from rapid response are reduced.
Of course, boat access to turbines is still a necessity even when helicopters are available at a site. Indeed, the rapid, wave-resistant benefits of a helicopter are complimented well by work boats with bulk-carrying capacity and an ability to operate in poor visibility. Therefore, as projects move further offshore the slow transit times offered by floating vessels will result in a radically different approach to turbine access, whether helicopters are included in a strategy or not. The manner in which helicopters might assist with access to far-shore sites is therefore unavoidably interlinked with the chosen approach to floating access.
Far-shore developments may require large numbers of technicians to live offshore in fixed or floating accommodation, which opens up two potential roles for helicopters: transporting crews between the onshore and offshore bases during shift changeovers; and shuttling crews between the offshore base and the turbines, delivering crews of two or three to many different wind turbines.
To transport crews from shore, larger helicopters with substantial technician-carrying capacity are likely to be most appropriate, such as the Sikorski S-92 or equivalent, carrying about 19 passengers. Between the offshore base and turbines, the role is much the same as that already used at near-shore projects, with one important exception; the aircraft must be stationed and refuelled offshore.
Safety is always a pre-eminent issue for developers when determining their approach to turbine access, and this is exacerbated with distance from shore due to the increased response times of land-based search-and-rescue teams as well as the more onerous conditions. The large area of the North Sea over which oil and gas platforms are installed led BP to introduce its Jigsaw search and rescue concept, comprising onshore and offshore-based helicopters as well as multiple vessels to minimise rescue times. While such large-scale solutions are unlikely in the offshore wind sector in the near future, it may be possible for operators to include a helicopter nominally allocated to operations and maintenance activity in their emergency response plans.
Ultimately, the access strategy at a far-shore project will be highly dependent on the site characteristics such as local conditions, project configuration and the nature and location of the shore-based support. There is little doubt that helicopters can add significant benefits to project economics when used in a manner that exploits their wave tolerance and fast transit times, but project developers and helicopter operators need to work together from an early stage to ensure these advantages can be realised effectively.
Robin Redfern is a project engineer for GL Garrad Hassan