The problem is in the grouting used at the top of the monopile, where a "transition piece" slides over it. This supports the tower by holding the monopole foundations in place and is beginning to fail as a result of the high stress levels imposed by the turbine. Investigations revealed the foundations at a number of offshore wind farms have shifted.
Speaking to Windpower Monthly, Dong said the largest displacement among it own developments had occurred in the Horns Rev 1 wind farm, which it owns jointly with Vattenfall. The project was built in 2001. The issue does not affect older wind farms, which were built using a different design.
According to Dong, companies affected by the problem have formed a group to find a solution. It is headed up by Norske Veritas (DNV), which originally certified the design.
Dong said its Burbo Bank and Horns Rev 2 wind farms were also affected. A spokesman said the company was looking at number of possible solutions and that the issue could be resolved by September.
In all Dong said 164 of its turbines in Denmark and England are affected, although it is an industry-wide problem. There is no immediate danger of a turbine collapsing as a result of the problem.