OCEAN SUPPLY VESSELS COVERING ALL AREAS OF SUPPLIES FOR OFFSHORE INSTALLATIONS

Wind speed

As a general rule, wind generators are practical where the average wind speed is 10 mph (16 km/h or 4.5 m/s) or greater. An 'ideal' location would have a near constant flow of non-turbulent wind throughout the year with a minimum likelihood of sudden powerful bursts of wind. A vitally important factor of turbine siting is also access to local demand or transmission capacity.

Usually sites are preselected on basis of a wind atlas, and validated with wind measurements. Meteorological wind data alone is usually not sufficient for accurate siting of a large wind power project. Collection of site specific data for wind speed and direction is crucial to determining site potential. To collect wind data a meteorological tower is installed with instruments at various heights along the tower. All towers include anemometers to determine the wind speed and wind vanes to determine the direction. The towers generally vary in height from 30 to 60 meters. The towers primarily are guyed steel-pipe structures which are left to collect data for one to two years and then disassembled. Data is collected by a data logging device which stores and transmits data for analysis. Great attention must be paid to the exact positions of the turbines (a process known as micro-siting) because a difference of 30 m can sometimes double energy production.

Altitude

The wind blows faster at higher altitudes because of the reduced influence of drag of the surface and lower air viscosity. The increase in velocity with altitude is most dramatic near the surface and is affected by topography, surface roughness, and upwind obstacles such as trees or buildings. Typically, the increase of wind speeds with increasing height follows a wind profile power law, which predicts that wind speed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34%.

Wind park effect

Wind farms have many turbines and each extracts some of the energy of the wind. Where land area is sufficient, turbines are spaced three to five rotor diameters apart perpendicular to the prevailing wind, and five to ten rotor diameters apart in the direction of the prevailing wind, to minimize efficiency loss. The "wind park effect" loss can be as low as 2% of the combined nameplate rating of the turbines.

Environmental and esthetic impacts

See Wind power#environmental effects for a discussion of environmental impacts of wind power. Near-shore and certain inland wind sites may have significant esthetic impact, since the turbines are visible for great distances. Wind farm siting must also consider impacts on wildlife, including migratory animals. Wind project proponents may face opposition from area residents concerned about sound level, light flicker, appearance, and the other impacts of wind turbine placement.

Types

Onshore

Onshore turbine installations in hilly or mountainous regions tend to be on ridgelines generally three kilometers or more inland from the nearest shoreline. This is done to exploit the so-called topographic acceleration as the wind accelerates over a ridge. The additional wind speeds gained in this way make large differences to the amount of energy that is produced. Great attention must be paid to the exact positions of the turbines (a process known as micro-siting) because a difference of 30 m can sometimes mean a doubling in output. Local winds are often monitored for a year or more with anemometers and detailed wind maps constructed before wind generators are installed.

For smaller installations where such data collection is too expensive or time consuming, the normal way of prospecting for wind-power sites is to directly look for trees or vegetation that are permanently "cast" or deformed by the prevailing winds. Another way is to use a wind-speed survey map, or historical data from a nearby meteorological station, although these methods are less reliable.

Wind farm siting can sometimes be highly controversial, particularly when sites are picturesque or environmentally sensitive (for instance, having substantial bird life)

Near-Shore

Near-Shore turbine installations are on land within three kilometers of a shoreline or on water within ten kilometers of land. These areas are good sites for turbine installation, because of wind produced by convection due to differential heating of land and sea each day. Wind speeds in these zones share the characteristics of both onshore and offshore wind, depending on the prevailing wind direction.

Common issues that are shared within near-shore wind development zones are bird migration and nesting, aquatic habitat, transportation (including shipping and boating) and visual aesthetics. Residents near some sites have strongly opposed the installation of wind farms due to these concerns.

Offshore

Offshore wind development zones are generally considered to be ten kilometers or more from land. Offshore wind turbines are less obtrusive than turbines on land, as their apparent size and noise can be mitigated by distance. Because water has less surface roughness than land (especially deeper water), the average wind speed is usually considerably higher over open water. Capacity factors (utilisation rates) are considerably higher than for onshore and near-shore locations.

In stormy areas with extended shallow continental shelves, turbines are practical to install. Denmark, for example, has many offshore windfarms.[4]

The United Kingdom plans to use offshore wind turbines to generate enough power to light every home in the U.K. by 2020.[5]

Locations have begun to be developed in the Great Lakes — with one project by Trillium Power approximately 20 km from shore and over 700 MW in size. Ontario, Canada is pursuing several proposed near-shore locations in fresh water and one on the Pacific west coast [7].

Offshore installation is more expensive than onshore but this depends on the attributes of the site. Offshore towers are generally taller than onshore towers once the submerged height is included. Offshore foundations may be more expensive to build. Power transmission from offshore turbines is through undersea cable. Offshore installations may use high voltage direct current operation if significant distance is to be covered. Offshore saltwater environments can also raise maintenance costs by corroding the towers, but fresh-water locations such as the Great Lakes do not. Repairs and maintenance are usually more costly than on onshore turbines. Offshore saltwater wind turbines are outfitted with extensive corrosion protection measures like coatings and cathodic protection, which may not be required in fresh water locations.

Offshore wind turbines will probably continue to be the largest turbines in operation, since the high fixed costs of the installation are spread over more energy production, reducing the average cost. Offshore wind farms tend to be quite large, often involving over 100 turbines.