Offshore wind power
Offshore wind power or offshore wind energy is the generation of electricity through wind farms in bodies of water, usually at sea. There are higher wind speeds offshore than on land, so offshore farms generate more electricity per amount of capacity installed.[1] Offshore wind farms are also less controversial[2] than those on land, as they have less impact on people and the landscape.
"Offshore wind" redirects here. For the meteorological phenomenon, see Sea breeze.
Unlike the typical use of the term "offshore" in the marine industry, offshore wind power includes inshore water areas such as lakes, fjords and sheltered coastal areas as well as deeper-water areas. Most offshore wind farms employ fixed-foundation wind turbines in relatively shallow water. Floating wind turbines for deeper waters are in an earlier phase of development and deployment.
As of 2022, the total worldwide offshore wind power nameplate capacity was 64.3 gigawatt (GW).[3] China (49%), the United Kingdom (22%), and Germany (13%) account for more than 75% of the global installed capacity.[3] The 1.4 GW Hornsea Project Two in the United Kingdom was the world's largest offshore wind farm. Other projects in the planning stage include Dogger Bank in the United Kingdom at 4.8 GW, and Greater Changhua in Taiwan at 2.4 GW.[4]
The cost of offshore has historically been higher than that of onshore,[5] but costs decreased to $78/MWh in 2019.[6] Offshore wind power in Europe became price-competitive with conventional power sources in 2017.[7] Offshore wind generation grew at over 30 percent per year in the 2010s. As of 2020, offshore wind power had become a significant part of northern Europe power generation, though it remained less than 1 percent of overall world electricity generation.[8] A big advantage of offshore wind power compared to onshore wind power is the higher capacity factor meaning that an installation of given nameplate capacity will produce more electricity at a site with more consistent and stronger wind which is usually found offshore and only at very few specific points onshore.
It is necessary to obtain several types of information in order to plan the commissioning of an offshore wind farm. These include:
Existing hardware for measurements includes Light Detection and Ranging (LIDAR), Sonic Detection and Ranging (SODAR), radar, autonomous underwater vehicles (AUV), and remote satellite sensing, although these technologies should be assessed and refined, according to a report from a coalition of researchers from universities, industry, and government, supported by the Atkinson Center for a Sustainable Future.[70]
Because of the many factors involved, one of the biggest difficulties with offshore wind farms is the ability to predict loads. Analysis must account for the dynamic coupling between translational (surge, sway, and heave) and rotational (roll, pitch, and yaw) platform motions and turbine motions, as well as the dynamic characterization of mooring lines for floating systems. Foundations and substructures make up a large fraction of offshore wind systems, and must take into account every single one of these factors.[70]
Load transfer in the grout between tower and foundation may stress the grout, and elastomeric bearings are used in several British sea turbines.[71]
Corrosion is also a serious problem and requires detailed design considerations. The prospect of remote monitoring of corrosion looks very promising, using expertise utilised by the offshore oil/gas industry and other large industrial plants.
Moreover, as power generation efficiency of wind farms downwind of offshore wind farms was found to decrease, strategic decision-making may need to consider – cross-national – limits and potentials for optimization.[72][73]
Some of the guidelines for designing offshore wind farms are set out in IEC 61400-3,[74][75][76] but in the US several other standards are necessary.[77]
In the European Union (EU), different national standards are to be streamlined into more cohesive guidelines to lower costs.[78] The standards require that a loads analysis is based on site-specific external conditions such as wind, wave and currents.[79]
The planning and permitting phase can cost more than $10 million, take 5–7 years and have an uncertain outcome. The industry is putting pressure on governments to improve the processes.[80][81]
In Denmark, many of these phases have been deliberately streamlined by authorities in order to minimize hurdles,[82] and this policy has been extended for coastal wind farms with a concept called ’one-stop-shop’.[83]
The United States introduced a similar model called "Smart from the Start" in 2012.[84]
In the EU, the revised Renewable Energy Directive of 2018 has simplified the permitting process to help initiate wind projects.[30]
Legal framework[edit]
The installation and operation of offshore wind turbines are regulated in both national and international law. The relevant international legal framework is UNCLOS (United Nations Convention on the Law of the Sea) which regulates the rights and responsibilities of the States in regard to the use of the oceans.[85] The maritime zone in which the offshore wind turbines are located determines which regulatory rules apply.
In the territorial waters (up to 12 nautical miles from the baseline of the coast) the coastal State has full sovereignty[85] and therefore, the regulation of offshore wind turbines are fully under national jurisdiction.
The exclusive economic zone (up to 200 nautical miles off the baseline) is not part of the State's territory but is subject to the coastal State's exclusive jurisdiction and control for selected purposes, one of which is the production of energy from winds.[85] This means that within this zone, the coastal State has the right to install and operate offshore wind farms and to establish safety zones around them that must be respected by all ships, as long as due notice of the installation has been given. Also, neither installations nor safety zones can interfere with sea lanes that are considered essential for international navigation.[85]
Beyond the exclusive economic zones are the high seas, or the international waters.[85] Within this zone the purpose of producing energy is not explicitly mentioned as a high seas freedom, and the legal status of offshore wind facilities is therefore unclear. In academia, it has been argued that the uncertainty of the legal status of offshore wind facilities on the high seas could become an object of interstate disputes over the rights of use.[86]
As a solution, it has been suggested that offshore wind facilities could be incorporated as a high seas freedom by being considered as ships or artificial islands, installations and structures.[86]
As of 2020, energy production from winds on the high seas is not yet technically feasible due to the difficulties that follow from deeper water.[87] However, the advancing technology of floating wind turbines is a step towards the realization of deepwater wind projects.[87]
Turbine construction materials considerations[edit]
Since offshore wind turbines are located in oceans and large lakes, the materials used for the turbines have to be modified from the materials used for land based wind turbines and optimized for corrosion resistance to salt water and the new loading forces experienced by the tower being partially submerged in water. With one of the main reasons for interest in offshore wind power being the higher wind speeds, some of the loading differences will come from higher shearing forces between the top and bottom of the wind turbine due to differences in wind speeds. There should also be considerations for the buffeting loads that will be experienced by the waves around the base of the tower, which converges on the use of steel tubular towers for offshore wind applications.[99]
Since offshore wind turbines are constantly exposed to salt and water, the steel used for the monopile and turbine tower must be treated for corrosion resistance, especially at the base of the tower in the “splash zone” for waves breaking against the tower and in the monopile. Two techniques that can be used include cathodic protection and the use of coatings to reduce corrosion pitting, which is a common source for hydrogen induced stress cracking.[100] For cathodic protection, galvanized anodes are attached to the monopile and have enough of a potential difference with the steel to be preferentially corroded over the steel used in the monopile. Some coatings that have been applied to offshore wind turbines include hot dip zinc coatings and 2-3 epoxy coatings with a polyurethane topcoat.[100]
Decommissioning[edit]
As the first offshore wind farms reach their end of life, a demolition industry develops to recycle them at a cost of DKK 2-4 million ($300,000-600,000 USD) roughly per MW, to be guaranteed by the owner.[121] The first offshore wind farm to be decommissioned was Yttre Stengrund in Sweden in November 2015, followed by Vindeby in 2017 and Blyth in 2019.