Ice road used to build offshore wind turbine
by T Lahti, Lumituuli Oy, Finland

To minimise costs, an ice road has been used in the construction of Finland's first offshore wind turbine, which was completed in March 1999.

Construction of the artificial island.

Introduction

The ice road was built to assist in the construction of an artificial island and the foundations for a new offshore wind turbine situated in the Gulf of Bothnia. The Gulf has very cold winters and lower seawater salinity than normal ­ both of these factors assist the construction of an ice road. Using the ice road made the project very economical.

Background

An ice road is a thick plate of ice floating on water. As soon as the ice cover forms and becomes thick enough to carry light machinery, a plough is used to keep it clear of snow, which would otherwise insulate it from the cold air. In temperatures of -20C or colder, this is sufficient to allow the ice to gain in thickness. However, if the temperature is slightly higher, between -10C and -20C, water is pumped onto the ice. This is done using a normal agricultural tractor carrying a drill. The drill is pushed through the ice and kept rotating to pump the water. Eventually, the lanes of the road form to about 30 m wide, being thicker around the centreline and thinning towards the edges. This procedure produces good quality "steel ice" with a high carrying capacity.

The project

To provide a back-up facility, several ice road lanes were built, as cracks and chasms in the ice could make a lane redundant. This was feasible because the construction process is cheap. The completed ice road was around 800 m long, comprising four lanes about 30­40 m apart. Because the water is relatively shallow, about one third of the road was ice right through to the seabed, especially around the artificial island. This made construction of the island much easier and cheaper.

Three lanes were used in the construction of the island, one lane for traffic in each direction and one spare; the fourth lane was built specially to carry a crane. The lanes used by normal construction machinery were about 0.7­1 m thick, which is sufficient to carry loads of 30 tonnes safely. However, the crane weighs nearly 80 tonnes and this lane had to be made thicker. The project's insurers demanded a high margin of error, so a target thickness of 1.35 m was set ­ enough to carry loads of up to 130 tonnes. The road was inspected by the Finnish Road Administration before being used.

Constructing the island and a concrete foundation for the power plant took about two months. The shape of the island is a modified horseshoe. It features small breakwaters and a pier to provide easy access, even in bad weather. The island is protected from erosion by moving ice sheets and waves.

The wind turbine

The Vestas V-660/47 wind turbine is rated at 660 kW, has a tower height of 50 m, and has a three-bladed rotor with a diameter of 47 m. Estimated power production is 1.8­2 GWh/year and the turbine is designed to operate in temperatures as low as -30C.

Economics

The total cost of the project to build the off-shore wind power plant was about FIM 6 million (where FIM is the Finnish markka). The total construction costs of the ice road, artificial island and turbine foundation were about FIM 1.6 million.

For more information contact Lumituuli Oy, Vironkatu 5, 00170 Helsinki, Finland. Tel: +358 9 41100778; Fax: +358 9 62200780;E-mail: info@lumituuli.fi

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