Towards the end of 2009 many grouted connection joints connecting the transition piece and large diameter monopiles of wind turbines were failing. On closer inspection it was discovered that the transition piece had slipped down, the grout connection had become instable due to tension stresses. The reason for this is the repeated tower bending caused by waves and dynamic wind loads. It was concluded that an estimated 70 ％ of the wind turbines installed in Europe are affected. A joint venture developed three solutions in order to avoid the slipping of the transition piece: shear keys with grouted connections, conic steel tubes for the monopile and the transmission piece, and bolted flange connections. No agreement could be reached in terms of cost distribution up to now.
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