Load levels from waves -- not wind -- are proving to be one of the most critical mystery issues for design and operation of offshore wind turbines. Close behind come the challenges of cost-effective accessibility and operations and maintenance (O&M). More ocean-specific issues are also coming to the forefront as wind researchers are finding just how different wind turbines are to other offshore structures. Meanwhile researchers, manufacturers and developers alike are watching one small offshore plant in particular for measurement data that is turning previous assumptions on their head.
These were some of the main findings discussed at Offshore Wind Energy in the Mediterranean and Other European Seas (OWEMES) in Naples, Italy, in mid April. Over two and a half long days, OWEMES was split about 70/30 among hard research papers and more practically oriented presentations from experience in the field.
The intimate conference of some 140 delegates has been held every third year since 1994 in Italy, each time with its now-expected organisational gaffes (the conference hall was an hour's transport from Naples and it was under construction, with occasional hammering and builders' noises drowning out presentations), charming hosts, awkward prizes (this year: a blue-velvet box containing a heavy stone with a hurriedly painted scene of the Bay of Naples, given to most anyone who was still around by the end of the third morning), and superb food and coffee (possibly one of the main reasons OWEMES has such a dedicated following). It has proven itself to be a barometer of the offshore sector over the last decade, giving a strong reference point for where research is focused and just how fast things are moving in offshore wind.
This year's gathering suggested that the sector has outgrown the event. The few presentations on the Mediterranean area offered a dismal view of any serious progress there compared to the real projects in the "other European seas." Furthermore, only a few of the presentations generated any questions or discussion -- as compared especially to the intriguing 2000 gathering -- and this was probably because OWEMES must now compete for "talent" among several other offshore wind events (including a well-attended one in the UK a month prior to Naples).
Overall, OWEMES 2003 was less a look into the future than a kind of "Where are we now?" status report, providing newcomers with a solid overview of the area -- there were a number of first-timers from the United States, offshore engineering firms and research areas like satellite weather forecasting and oceanography -- while the veterans were more or less pleasantly bored. Jens Bonefeld of TechWise, the Danish company of utility Elsam that has provided technical support for building the 160 MW Horns Reef wind station off Danish Jutland's west coast in the North Sea, said he was "not really" learning anything. That is not to say there was nothing to learn.
Loads a major issue
Horns Reef, with its 80, Vestas 2 MW Vestas turbines, or Blyth, the near-shore UK plant with 2 Vestas 2 MW units, were referred to in nearly every presentation -- Horns Reef for its size and accessibility issues (it lies 14-20 kilometres from shore), Blyth for its harsh marine environment.
Of the offshore wind plant installed so far, Blyth's turbines feel the highest loads from waves and water currents. A publicly funded research project called Design Methods for Offshore Wind Turbines at Exposed Sites (OWTES) has been the first chance for a full-scale measuring program at an offshore turbine in such conditions, said David Quarton of UK consultants Garrad Hassan, which is co-ordinating OWTES. The turbines are installed in about six metres of water and the wave height varies another six metres on top of that. One of the Blyth turbines is mounted head to toe with sensors to monitor wind, waves and currents, as well as loading and response. A video camera mounted on the other turbine provides a simultaneous visual record of wave data. "I'd like to get my hands on that database," was a comment overheard more than once after the OWTES presentations.
While OWTES measurements are scheduled to end this month, the data gathered can have large implications on future design issues. Andrew Henderson of the Dutch Technical University of Delft, one of OWTES' partners, said wind loads have been previously assumed to be more critical than wave loads for turbine design issues. "But we found that many of the highest loads came when turbine was turned off," says Henderson.
Quarton told about similar work -- also involving Garrad Hassan -- to develop an international design standard for offshore wind turbines. He said the group started with a visit to major oil and energy companies. "They told us: Look, we've been developing deep oil and gas platforms for decades. This should be no problem. But offshore wind turbines are quite different," Quarton said. "If you look at Blyth, you see things like breaking waves -- this is a major design issue of wind turbines that oil and gas platforms don't have to face. They are in 50-100 metres depth, and they don't get breaking waves." Existing offshore engineering technology is helpful in experience of dealing with the marine environment. "But a distinct factor that needs development that is not understood by the turbine or offshore community is how to calculate loads," says Quarton. "This is requiring new developments and new thinking."
It is an oil company, Shell, that has given access to some of the most valuable offshore data to date in its NEXT/North European Storm Study (NESS) database, said Gerard van Bussel in an operations and maintenance (O&M) presentation from the Dutch DOWEC offshore research project. The data spans 30 years, with nine consecutive years of wind speed and wave height. Data like this can help companies to generate weather "windows" -- periods during the year least likely to give storm weather and downtime in installation or access. These can have a far-reaching impact on costs, as pointed out by more than one speaker, though the details are still mainly lacking.
"I think the key things for offshore will be access and maintenance," said Andrew Garrad of Garrad Hassan. "Usually if you say [wind plant] availability is 95% instead of 96% you get a moan, but if you say 80% instead of 95%, everybody is going to be upset. That's going to be the big issue. It's a very relevant part of this conference."
One of the few non-academic presentations at OWEMES was by Kurt Thomsen of A2SEA, the Danish offshore contracting firm that handled transportation and erection of the Horns Reef turbines. Thomsen invented the concept of a ship with legs -- allowing a ship to sail to an installation site with its load as usual, then lower four legs onto the sea bed as a jack-up crane does, for stability during installation.
"One of the issues we didn't think about was the problem of good weather," said Thomsen. "High pressures rolling in from the east caused thunderstorms to lie over the turbines every day. It was only an hour or so but annoying downtime we didn't need. It's necessary to account for that type of thing when you're making plans," he told delegates, with other advice that included how to be prepared for the horrendous smell of bird droppings on waiting foundations. He concluded, "Things will break down. Things will go wrong. You cannot cut the planned time schedule."
Luk Vandenbulcke of the Belgian Hydro Soil Services, talked about experience gained on foundations and installation at the Swedish Utgrunden plant from 2000 and the new Danish Samsø station (Windpower Monthly, February 2003). "Technology is evolving so fast that the installation method we used in Utgrunden -- along with turbine and foundation weights and sizes, installation time, and so on -- evolved quickly by the time we began working on Samsø project." The monopile towers used at Utgrunden were three metres in diameter and 120 tonnes each. At Samsø, the monopiles had grown to 4.5 metres and 310 tonnes. Hydro Soil hammered the towers into the sea bed, providing a new means for the offshore contractor to progress know-how. "The technology in offshore wind turbines is bringing the hammering concept further," said Vandenbulcke. "Already it's state of the art, and then we're breaking records in this field."
Vandenbulcke also refered to factors governing the cost of offshore wind farm foundations. Water depth alone impacts costs steeply, he said; a project installed in 15 metres of water can cost twice that of one in eight metres.
More stories from the field came from Jens Bonefeld, who gave a status report of Horns Reef, which was scheduled to finish commissioning last month. Bonefeld advised other companies, "You must take this challenge seriously. Already before you go out into field, get a good hand on what the contractor is preparing and ready with. You should not just let a contractor say it's a piece of cake for such a little turbine. For the foundations, we were promised a big barge from big contractors -- but it was never ready." Similarly, cable laying was a disaster in the beginning. "Our contractor relied on the wrong material -- and this was a contractor with offshore experience -- and had to change equipment and then make new procedures for the equipment," Bonefeld said. "Authorities must be involved as well -- it's also new for them," he continued. "You need about four years to go through all stages with such a project -- and don't think you can do such a project in less than four years."
By the end of the conference, some delegates griped that they were missing more information on the environmental impact of offshore wind plant -- basically the only presentation was from the stringent environmental work being done for the 160 MW Nysted wind farm currently being installed in the Baltic Sea by Danish Energi E2/SEAS. Several delegates also mentioned the lack of attention at OWEMES to grid issues offshore as well as financing. Rüdiger Wolf of German offshore developer OWP said, "I personally missed the opportunities to mingle with the investors and financiers and get to know their view. It seems that British offshore conferences attract more money people."