Offshore Wind Turbines – How do you install a wind turbine out at sea?
In 2015, 46% of the UK’s electricity was generated by low-carbon sources and renewable energy accounted for 8.3% of total energy consumption (heating, transport and power etc.) (1). The UK has just over 7,000 wind turbines which produce 38 million MWh/p.a., powering an equivalent of 9.6 million homes (2). In 2016, wind power generated 11.5% of the UK’s total energy output, over taking coal power for the first time which only generated 9.2% (3). However, the majority of wind power electricity generated in the UK is from turbines located onshore. There are 1,088 operational onshore projects accounting for 79% of total turbines, whereas there are only 27 operational offshore projects with 1,465 turbines (21% of total) (2).
Offshore wind power may prove to be an area where significant increase in energy generation can be made. One example is the new Rampion Wind Farm currently being constructed off the Sussex coast near Brighton by E.ON, which will consist of 116 turbines with a total generating capacity of 400MW (4). Rampion will supply power to 290,000 homes, which equates to producing the total domestic household energy consumption for more than 4 in every 10 households in the whole of Sussex (5).
The question I wanted to know the answer to is how do you install a wind turbine out in the open ocean? The engineering challenges faced by trying to install a turbine at sea have produced some specialist items of machinery and innovative construction techniques.
The wind turbine itself is constructed on shore in a kit form. The turbine is constructed in separate sections, split into its main components which include: the base (seat), tower sections, nacelle (which holds the generator) and turbine blades. These components, like a model kit, can be assembled together out at sea to build the turbine.
These turbine components are transported and assembled out at sea on special seajacking ships. These are purpose-built with the ability to jack themselves out of the water to provide a stable platform where precision lifting can occur. As well as a high performance crane, the ships also carry hydraulic rams which are used for the installation of the turbine foundations.
In the case of the Rampion Wind Farm, a monopile is used to secure the turbine to the sea bed. A monopile is a steel cylindrical tube with diameters ranging up to 6m and 150mm thick steel skin. Monopiles are one of the most common foundation designs in offshore wind construction due to their ease of installation in shallow to medium depths of water. The steel cylinder is piled into the sea floor by a specialist hydraulic ram.
Once the monopile has been set in the sea bed, a transition piece (someone times called chairs) is then fitted on top. The transition piece is slowly lowered into position and secured carefully as it has the important job of connecting the turbine and the monopile together. The transitional piece is usual brightly coloured and includes a boat mooring stage, where a ladder takes technicians up to the work platform.
The next stage is assembling the turbine tower which is craned into position and bolted together. Once all the tower pieces are connected, the nacelle is attached to the top and the generator connected. The final step is attaching each turbine blade to the hub on the nacelle. When fully assembled, the blades pitch angle and yaw can be adjusted to optimise the performance of the turbine. Each wind turbine is then connected to the offshore substation which feeds in to the National Grid.