A number are now on order and, with many new sites located further from shore with larger turbines, the industry is recognising the changing needs. Two installation vessels currently operating, the Kraken and Leviathan, have recently been modified to carry larger turbines. One of the new vessels, the 14,000 tonne self-propelled Seajacks Zaratan, will be able to operate in deeper and more distant waters and carry large turbines. It will also be equipped with a helideck — evidence of the increasing role of aviation, both land-based and maritime, as an adjunct to shipping.
As wind sites move further away from shore, maintenance access by boat is likely to become more difficult. While only a few offshore wind farms have been operational for more than three years, lessons are already being learned about access over their intended 20-year operational period. The risks became apparent early on in the Horns Rev 1 project off Denmark, when the transformers in all 80 turbines had to be replaced. Helicopter access was chosen to improve response time and allow access in rough seas.
Benefits of helicopters
In the UK helicopters will be used for the first time for maintenance of the 500MW Greater Gabbard wind farm, featuring 140 turbines off the east coast of England.
Maintenance trips are significantly shorter. It takes a vessel up to six hours to reach the giant Dogger Bank wind site in the North Sea, and that is with wave heights of less than 1.5 metres or winds less than 10 metres per second (m/s). A helicopter does it in 25 minutes.
A report from Newcastle Science City, in association with Newcastle University, suggests that significant wave heights will stop boats reaching turbines in the North Sea for up to 71 days a year. Yet, Clipper Windpower estimates that its Britannia turbines will require up to six maintenance visits per year.
Generally, small boats cannot operate beyond a significant wave height (a standard measurement of wave height) of 1.5 metres, or an average (one hour) wind speed of 10m/s. A larger jack-up vessel cannot operate beyond a 2.5 metre wave height or 20m/s wind speed. Helicopters, on the other hand, are not constrained by wave height and can operate in wind speed up to 20m/s.
Unlike for oil and gas installations, a helicopter’s role with offshore wind will involve carrying people and equipment, says Captain Andrew Miller of Bond Offshore Helicopters, and will require specialised knowledge from the crew. "It will require winching skills, and training will be needed to get people on and off a relatively small wind turbine," he says.
Sending a Windcat catamaran on a maintenance trip costs around £1,400 (€1,600) a day plus fuel. Sending a helicopter with two technicians 20 miles in ten minutes could cost £650 (€743) plus fuel for the return trip. For a wind farm 120 miles away, a larger aircraft could deliver 12 passengers for around £4,500 (€5,143) including fuel.
While the use of faster helicopters is likely to increase in the North Sea, boats are likely to remain the offshore workhorses despite their limitations in bad weather.
Vessel technology will take time
Vessel designs and transfer systems continue to be developed, including SWATH or small water plane twin hull, a tender vessel built by Abeking and Rasmussen, and the Ampelmann ship-based self-stabilising access platform, which can operate off the back of a ship. Both can withstand seas of 2.5 metres.
Jo Chan, an independent consultant, has statistically shown the cost-effectiveness of using helicopters in far offshore wind farms. He says: "We have researched the Round Three offshore zones specifically on wave height significance and conclude that current vessels offer a low operational window in accessing offshore turbines."
While new vessel technology is in the pipeline, Chan believes helicopters are the answer for now. "Given that developers will start constructing Round Three in 2015, it does not leave much time for this new technology to be researched and developed in time for rollout."