An offshore windfarm is an array of power-generating wind turbines usually built in the shallow water close to the coastline. Offshore wind power has three main advantages over land-based wind power: the wind directly offshore is usually stronger and steadier than it would be inland, the windfarm can be placed very close to the urban area that will be using the power, and there is less “not-in-my-backyard” opposition from local residents, since the local residents are mainly fish. The primary disadvantage of offshore wind farms is that they are much more expensive to build and maintain than other forms of low-carbon power generation.
The European Union is by far the world leader in offshore wind generation, with about 10 gigawatts (GW) of capacity. China and Canada both have modest windfarm operations and India is working on windfarm development, while the United States will open its first operational windfarm in late 2016 off Block Island in Rhode Island. Despite the high costs of offshore wind power development, projections of installed capacity continue to grow; the European Union expects to have between 20 and 40 GW of capacity online by 2020, and China perhaps optimistically expects to have 30 GW by the same year.
The primary component of an offshore windfarm is the wind turbine - the huge set of blades set atop an enormous pole, which spin as the wind blows. To achieve maximum effect, turbines need to be high off the surface of the water, so the pole must be very long - more than 200 meters.
Such a large structure must be anchored extremely securely to the seabed, and there are a number of ways in which this is done. Regardless of the exact nature of the structural engineering, the underwater structures of offshore wind turbines need to be inspected on a regular basis.
In the United States, offshore windfarm inspections will be under the oversight of the Department of the Interior’s Bureau of Safety and Environmental Enforcement (BSEE). BSEE’s draft standards draw heavily on the experience of the United Kingdom, and lay out an inspection regime that concentrates a great deal of attention on the turbines themselves, naturally enough. However, the underwater bases of the turbines come in for their share of attention as well.
BSEE draft standards will likely require that windfarms be inspected on an annual basis, with 20% of a farm’s individual turbines and foundations inspected in each annual cycle. Critical components should be evaluated annually, while less critical areas can be inspected as infrequently as every five years. Foundations need to be checked for structural health, bioaccumulation, scouring, spalling, and corrosion. Subsea cables need to be inspected for damage due to sand, marine animals, or anchoring.
BSEE suggests that general visual inspections can be carried out by ROVs, while close visual inspections need to be done by human divers.
BSEE standards are likely to recommend the use of ROVs specifically in order to reduce risks to human divers. Because there are often powerful tidal currents around the foundations of structures like wind turbine foundations, this can be a dangerous dive environment for humans. There is a great deal of general visual inspection work that needs to be done, but where an extremely close view is simply not required; the inspection needs to check the general condition of the pilings and caissons, for example. In this work, putting a human diver into the water isn’t necessary, as an ROV can provide the needed visual check.
(Primary source: http://www.bsee.gov/uploadedFiles/BSEE/Technology_and_Research/Technology_Assessment_Programs/Reports/700-799/747AA.pdf)