When it comes to the differences between offshore and onshore wind farms, offshore wind farms have many more advantages over onshore wind farms as far as their efficiency, capacity and generation of higher levels of electricity are concerned. Moreover, they eliminate many of the disadvantages of onshore wind farms like noise and visual impact to the environment.
The bigger disadvantage of offshore wind farms is the complexity of the substructure that is used to support the offshore wind turbine, which may represent up to 20% of the total installation cost. The complexity of the substructures is also one of the limiting factors that prevents offshore wind turbines to be largely deployed in depths more than 165 ft (50 meters). There are different types of substructures that can be used and are summarized below:
Monopiles. These consist of a steel pile that is inserted into the sea bed. Most of the existing deployments in waters under 65 – 82 ft (20 -25 meters) consist of monopile foundations. Their main advantage is that they are relatively easy and cheaper to manufacture and are easy to install. Their major disadvantage is that it cannot be used in waters more than 82 ft (25 meters) deep as it becomes unstable.
Gravity Based Structures (GBS). Most of the remainder of today’s installations consist of GBS which are also relatively easy to manufacture. As opposed to the Monopiles GBS are not inserted into the seabed but rather sit on the seabed. Their structure is filled with sand, concrete rock and iron in order to keep it stable. GBS are most suitable for depths up to 100 ft (30 meters).
Space Frame Structures. Even though today these are not heavily used they are considered for deeper waters. In the future they are expected to be further developed and may be heavily used in future wind farms. Space Frame Structures come in three variations. First, the tripod, which is a standard three legged structure that have a central steel shaft that is attached to the turbine tower. Similar to the monopiles, each leg is inserted into the sea bed, but together they form a much stronger structure suitable for waters 65 – 165 feet (20 – 50 meters) deep. Second the jackets, have a similar concept to the tripod but they differ in the fact that they consist of a larger plan area through the majority of the structure, allowing the steel shaft to be positioned away from the centre of the axis. This design results in considerable savings of materials. Similar to the tripod, each leg is inserted into the sea bed using piles. Thirdly, the Tri-piles, consist of three foundation legs (piles), which are connected at the turbine tower with a transition piece located above the water level.
Floating Structures. Even though at experimental stage, floating structures are expected to evolve in the future particularly because there are many locations that it is not viable (technically and economically) to install fixed substructures (particularly in waters over 165 ft or 50 meters deep). Floating structures have great flexibility in the production and are easier to install. However, they pose a major challenge that have to do with the stability of the wind turbine.
Monopiles are expected to continue to dominate the market until they reach their design limitations with respect to the wind turbine size and water depth. GBS will also maintain a small portion of the market until a more cost effective design is promoted. For most other cases in deep waters the space frame structures will dominate the market. Development of these is expected to evolve rapidly particularly because many countries, including US, have aggressive plans for the deployment of offshore wind farms, in order to reach their goals.
Source: EWEA The coming of Europe’s offshore wind energy industry