Well-placed sources say they know of ten offshore wind farms affected by the foundation problem (Windpower Monthly, June 2010). The first successful remedying action was completed in June at the 108MW Owez site off the Netherlands' North Sea coast, jointly owned by Royal Dutch Shell and Swedish utility Vattenfall.
The industry has been anxious for a solution. Figures presented during April's European Wind Energy Conference 2010 showed that, on a megawatt basis, monopile foundations comprised 88% of all offshore wind turbine installations in 2009.
Another statistic revealed that of the 830 turbines totalling 2GW operating in European waters at the end of 2009, 65% of the capacity was built on monopile foundations.
Monopile substructures frequently consist of a pile driven into the seabed and a transition piece (TP) that slides loosely over the top. The tower is mounted to this. The circular gap between the pile and TP is filled with a concrete grout, creating the bonding.
Corrective monopile concepts have been developed for two wind farms. The solution at one, the 60MW Scroby Sands in England, did not use a TP. Instead, a tower-mounting flange was welded to the pile top.
But, according to experts, the first wind farm where the problem has been successfully tackled is the Egmond aan Zee (Owez) project, where monopile slippage was first officially reported late last year. Here, the TP fits into the pile with grout connecting the two components.
The Dutch civil engineering contractor Ballast Nedam has filled all 36 foundations at Owez. Concrete was poured from inside the turbine towers and the steel bottoms of TPs were submerged into a small portion of the liquid concrete. After the curing process, adequate load transfer between TPs and piles was re-established.
The filling operation took several weeks and was completed by mid-June, says Shell spokesman Andre Romeyn. Experts are monitoring the results. The operation was conducted from a mobile vessel equipped with a self-stabilising Ampelmann access platform.
Jan van der Tempel, assistant professor of offshore engineering at Delft University of Technology in the Netherlands, is an expert in monopile foundations. "The Owez design enabled a relatively straightforward solution," he says. "But, a similar method cannot simply be repeated for monopile foundations comprising an external TP for structural reasons.
"Alternative remedying solutions involving welding will prove complex and costly. The grout can hamper such operations and the typical 50mm to 80mm steel wall thickness of the piles and TP provides a real challenge."