Figure 34 Monopile foundations, courtesy of EEW SPC, © Andreas Duerst, STUDIO 301.
Function
The primary function of a monopile is to support the static and dynamic loads of the wind turbine through anchoring it firmly to the sea bed using the embedded part of the monopile. Secondary functions include supporting the wave loads on the monopile itself and enabling cable entry.
What it costs*
About £266 million for the monopiles for a 1 GW wind farm.
Who supplies them
Cambridge Vacuum Engineering, CS Wind, Dillinger Hütte, Dajin Heavy Industries, EEW, Haizea Windgroup, SeAH Wind Limited, Sif and Riggs Distler.
Key facts
Monopiles are the most commonly used foundation type to date and are considered to be a proven technology by the offshore wind industry in water depths up to approximately 50 m and this is expected to increase to closer to 60 m by the end of the decade.
The monopile is a cylindrical steel tube made up of several welded steel cans that is usually driven into the sea bed. The embedded section of the monopile is of constant diameter to allow entry into the sea bed. Along the rest of its length the monopile will taper from the widest section at the bottom to its narrowest section at the top.
Monopiles are made in highly automated factories with little work on top of rolling and welding of parallel cans. Two thirds of the cost is steel. They do not generally have a surface finish to resist corrosion and have Corrosion protection (B2.4) applied to suit the installed conditions.
Monopiles need to withstand the impact of pile driving into the sea bed. The pile needs to be designed to account for these impact loads, which will use up a percentage of its fatigue life. As it is a simple cylindrical structure it is relatively easy to transport and move into its vertical orientation. Increasing sizes of monopiles are leading to vessel obsoletion and the need for larger vessels for transport and installation.
For a 15 MW turbine in 60 m water depth, the dimensions will be up to 11.5 m diameter, 120 m overall length and 2,700 t. More typically, for a 15 MW turbine in 40 m water depth the dimensions will be up to 10 m diameter, 90 m overall length and 1,850 t. A number of suppliers have the capability to mass-produce this type of monopiles.
Although monopiles are considered an established technology, innovation continues. Fabrication capabilities in terms of diameter and wall thickness continue to increase to achieve the requirements of longer monopiles. Novel concepts such as tethered monopiles suggest that installation depths of 100 m can be reached, and alternatives to driving which is currently limited by available equipment and comes with an increased buckling risk for larger monopiles are being investigated.
A monopile will normally have a transition piece (TP) between it and the bottom of the turbine tower. These are typically separate pieces bolted to the monopile, limitations on bolted connections may lead to alternative connection types or the advancement of TP-less monopiles – see B.2.3 Transition piece.
What’s in it
- Steel plates
- Flange
- Joints