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12/08/2010
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Grout injected during the erection of offshore wind farms is breaking up, leading to concerns over their structural integrity, according to engineers at Scottish & Southern Energy.
 Industry figures believe 600 turbines at 13 wind farms could currently be affected, leading to repair bills estimated to be around the £25 million mark.
For the time being, they are recommending monitoring of any movement and installing steel blocks on the T-piece brackets that support the structure.
Meanwhile, the grout, which was initially classified with a safety factor of 3 has been reclassified for this type of design to 1 or below – with the proviso that the larger the design structure, the less safe it is.
As for the costs, Jim Smith, director of offshore operations, Scottish and Southern Energy has quoted £4—5 million as the likely bill for the 140 turbines at Greater Gabbard, should they all require a retrofit.
The problem arises with current offshore wind farm designs that use a monopile construction, involving a hollow steel rod (typically 4m diameter, 45mm thick and weighing 300 tonnes for a 2MW turbine) driven into the sea bed to depths of 30 to 40 metres.
A tapered transition piece (4.3m tapered to 3.4m in diameter, with a wall thickness 57—37mm and weighing 115 tonnes) is then placed on top of the monopole. Once grouted in place, the towers are placed on top.
However, to correct the monopile angle, hydraulic jacks, set on the monopile, level the transition piece, before it is bolted in place.
Grout is then injected into the gap between the T-piece and the monopile, and secured by rubber seals, before the jacks are removed.
The grout transfers axial loads from the T-piece to the monopile, and is critical to prevent movement of the T-piece either downwards or out of alignment.
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Author
Brian Tinham
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