Offshore wind turbine

Figure 1. Offshore wind turbines.

Offshore wind turbines operate by transforming the kinetic energy in wind over water into rotational kinetic energy which is used to generate electricity. Turbines can be installed both in ocean waters and inland lakes and are typically around 100 meters high with a rotor blade diameter of up to 80 meters. Installation is complex and power transmission is via undersea cables.

Offshore wind turbines are commonly larger and more efficient than onshore wind turbines because they are powered by stronger and more consistent winds over the open water. Oceans have a smoother surface than land does: trees, buildings, and complex topology create turbulence and drag which slow the wind down. The temperature differences that drive wind are also more stable over large bodies of water.

Types

Fixed-Bottom Turbines

Fixed-bottom turbines are similar in design to onshore wind turbines where the turbine tower is built up from the ground. Fixed-bottom turbines are limited by the depth of the water because the turbine tower has to be built into the sea-floor; fixed-bottom turbines are only viable in depths up to 60 m. Suitable construction sites are relatively limited and have the potential to interfere with other industries such as shipping or fishing.

Monopiles are the most common type of fixed-bottom offshore turbine, they consist of one large steel tube that goes into the seabed. Gravity-based foundations are different because rest on the seabed and are filled with ballast (heavy material) to be stable. Jacket foundations are lattice structures made of steel tubes. There are also other less frequently used foundation types such as the tripod and suction caisson. Overall, the choice of foundation depends on water depth, soil conditions, and cost.

Figure 2. Types of fixed-bottom offshore turbines and floating offshore wind turbines.

Floating Turbines

Floating Offshore Wind Turbines (FOWT) are of interest because sites are no longer limited to shallow waters. Floating turbines can be positioned in areas with the strongest winds without being as limited by water depth. Some areas, such as the Mediterranean and Arctic, that are too deep to allow fixed-bottom turbine construction can be suitable for development with FOWTs.

The challenge of floating turbines is to create structures that are capable of withstanding the forces of high wind speeds and open-ocean waves without a foundation built into the sea-floor. Technological development is focused on the substructure, the floating platform the tower is constructed on. The substructure must be capable to holding the tower steady and upright while remaining in the same location via a system of cables anchored to the sea floor. To date, several floating turbines have been designed but are still limited to a water depth of around 200 m because of the necessity for an anchoring system to hold the turbine in place.

There are 3 main types of floating turbines: spar, semisubmersible, and tension-leg platform. The spar achieves its stability with a weight below the buoyancy tank. The semisubmersible achieves stability by distributing its buoyancy over a wide area. The tension-leg platform (TLP) uses the tension of the mooring lines with a submerged buoyancy tank to achieve stability. Each method has its own unique challenges. All of them are tethered by "mooring lines" for cabling and to limit migration.

For Further Reading

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