Offshore wind energy technologies harness kinetic energy from the wind to generate energy and transport that energy back to shore via a subsea export cable. The main advantage of offshore wind energy is access to stronger and more consistent winds, allowing for the use of larger turbines. The development of offshore wind in Europe and Asia has preceded development activities in other parts of the world; however, several projects in the United States are currently underway. There are different types of offshore wind turbine foundation on which the turbine can be installed, depending on the depth and substrate.
Monopile foundations are used at shallow depths (0-30 m) and consist of a pile that is driven into the seabed. Monopiles are typically less costly than other foundation types and are the most commonly used.
Tripod fixed bottom foundations are used at transitional depths (20-80m) and consist of three legs connecting to a central shaft that supports the turbine base. Each leg has a pile driven into the seabed, which creates a wide foundation that allows for the piles to be placed at a shallower depth in the seabed than monopile foundations.
Gravity foundations are also used at shallow depths (0-30m) and consist of large, steel or concrete bases that rest on the seabed.
Jacket foundations are also used at transitional depths (20-80m) and feature a lattice framework that comprises three to four anchoring points driven into the seafloor.
Floating foundations are used at deep depths (40->1000m) and consist of a balanced floating substructure moored to the seabed with fixed cables. The substructure may be stabilized using buoyancy, mooring lines, or a ballast.
The environmental concerns associated with offshore wind farms vary with foundation type. Monopiles, for example, require pile-driving, which produces incredibly loud noises that tend to propagate far in the water, even after mitigation strategies such as bubble shields, slow start, and acoustic cladding are employed. Alternatively, floating foundation’s mooring lines may cause minor scouring or pose a risk of collision or entrapment, but the turbine foundation is mostly located in the upper layer of the water column where there tend to be less organisms. More generally, collision risk with birds and bats is a major concern, though the impact is more difficult to quantify offshore because carcasses that provide evidence of collisions become lost at sea. While the footprint of offshore wind energy is relatively small, it may still cause scouring and changes to sediment transport. As with all electricity generation, there is a slight concern that electromagnetic fields generated by power cables may affect animals that use Earth's natural magnetic field for orientation, navigation, and hunting. Artificial reefing is likely to occur around wind farms as the new hard substrate provides protection and attracts colonizing invertebrates and fish.