Twelve months ago Kurt Thomsen wrote an article in Cranes Today describing the technical difficulties of erecting the massive offshore wind farms that are planned for European waters. How do you cope with high waves, strong currents and winds, as well as the massive logistical problems involved in getting 80 turbines, each weighing 250t, to the site in the right time and manner, he asked.
Since that time, Thomsen has not only come up with his own solution but he has set up a company called A2SEA A/S, secured design patents, bought equipment, and is nominated lifting subcontractor for turbine manufacturers Vestas of Denmark (the world’s biggest) and Nordex of Germany.
Thomsen, who is Danish, says: “I came up with the design much by chance, and the immediate response from the turbine manufacturer, to whom it was first demonstrated, was that this would be their preferred system if it proved feasible. The end result is that we are negotiating for more than 220MW (around 110 turbines) within the next two years. And in 2003 a further 60 to 80 turbines are to be erected.
“Other turbine manufacturers have been very receptive towards our system, and the first projects that went up, or are in process, have all been monitored by us to see what they are doing wrong so we learn from their mistakes.”
The starting signal for the wind farm revolution in Europe was when the Blyth offshore Consortium began erecting two Vestas wind turbines, each capable of generating an impressive 2MW of electricity. The turbines are more than 60m high and each weigh around 230t. They are mounted on a monopile foundation, driven into the ground using a large hydraulic hammer. The task was more difficult than expected. The piling and erection contractor Amec Seacore had its share of problems in terms of rig breakdown and bad weather. It took more than eight weeks to erect the two turbines, a record that did not impress the wind turbine industry.
Meanwhile, Hydrosoil Contractors of Belgium managed to erect seven Tacke Enron 1.6MW machines in the calmer waters of Kalmarsund in Sweden.
Muhibbah Marine’s rig JB-1 is engaged on the Middelgrunden project in the Øresund strait. Currently the largest offshore wind farm, it comprises 20 Bonus 2MW turbines. Although progress is now good as it approaches completion, the project has been far from smooth. Completion was initially due in August or September this year.
There are various reasons why contractors have struggled with offshore wind turbines. First, there is the overall approach to the problem. Offshore wind farms is a new industry that suppliers approach in the same way as they do the oil industry. Big rigs are used which are not easy to move. They have limited space for cargo and are susceptible to the weather. They can only jack up and down in calm waters, meaning waves should be no higher than 1m, and they are vulnerable to scouring – the phenomenon that occurs when a spud penetrates the seabed, and strong currents remove the seabed material from around the spud, thereby washing away the support below the jack-up rig. Danish piling contractor Per Aasleff A/S lost a rig on the Horns Rev off the west coast of Denmark, when within hours scouring removed the sand below one of the rig’s spuds.
Rigs and sheerleg cranes depend on finely tuned logistics to finish the job on time and therefore the supply of turbine parts must be continuous, but the fact that barges and tugs can only sail in calm waters is a problem in itself.
Solving problems
Thomsen says that his solution works around the specific problems relating to the erection of offshore wind farms. He set out to meet four demands, he explains.
5 Safety: offshore work is dangerous and because of the heave, pitch and roll of the vessel, it is hard to predict the motion of the load which is to be lifted. Stability is paramount to reduce or even eliminate these motions. The jack-up rig can do that, but it does not move to the next position independently of the weather conditions, so this is not the solution.
5 Availability: the ability to be on the jobsite, and all over the jobsite, regardless of waves, currents, wind etc. The method must allow safe movement around the site even in bad weather. When lifting a nacelle (the generator) to a height of 65m or 70m, the wind is the determining factor, and there is a relationship between waves and wind. At a wind speed of 12m/s, at an altitude of 85m (boom tip height), waves will be between about 2.5m and 3m high – too high for a jack-up rig to work, let alone a sheerleg crane.
5 Speed: to take maximum advantage of the limited weather window, movement to and from site must take no more than two hours. A barge and tug cannot supply that speed and reliability. Something faster is needed.
5 Price: erection cost is critical, especially as electricity prices have fallen 25% in a year. The onshore erection price of $30,000 per turbine must be beaten.
Secret solution
Thomsen is trying to keep details of his solution under wraps – at least until April when his vessel hits the sea and begins live trials. But he says that his equipment and methodology have been validated by computer and scale model tests based on data on the most severe offshore site known, the Horns Rev site (a project that Vestas is bidding for with A2SEA as its nominated lifting subcontractor), and has passed the test convincingly.
“We have employed the best naval architects, engineers and designers to develop my original idea, and so far the testing facilities, verification companies, etc. have all given the go ahead without hesitation.”
As for the equipment that A2SEA will use, Thomsen is confident it will work fine and at half the cost of currently used techniques. “But don’t take my word for it,” he adds. “Ask the turbine manufacturers. They have cast their vote in our favour, based on a safety, price and availability calculation giving our system the leading edge by a large margin.”
