The relocation of the approx 5,000 stones at the surface of the seabed, which may cause problems to the wind turbine and cable installation, is well underway. However, a number of planned wind turbine positions are presently being screened with advanced sonar equipment to unveil any critical stones in the seabed.
At a number of positions, there are geological deposits which could mean that there is a great risk of large stones at a critical depth. The positions are screened by means of special equipment to find out if there are any deep-lying stones. The measuring equipment is located at the seabed, and sonar reflections indicating the occurrence of any large stones at the specific position are measured over a 24-hour period.
The screening equipment surveys a number of specially selected positions for deep-lying stones. If the screening shows large stones – or groups of stones – at critical depths, the wind turbine position will be relocated.
Critical depth
In connection with driving of the foundation piles, there is a so-called critical depth.
The up to 460-tonne heavy monopiles are equipped with trunions making it possible to pull out the monopile again if large stones are found in the seabed. However, this is possible only down to a certain depth.
If the monopile runs into a stone at the critical depth, the monopile is at a depth where it cannot be pulled out again, but at the same time, it has not been driven far enough to stand safely. In that case, the installation vessel will have to hold the monopile while stones and solid ground are drilled away. If this is not possible, the monopile will have to be cut.
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Source: dongenergy, November 02, 2011