Key offshore wind turbine points
More power
Wind speed is affected by the surface over which it passes. On land, there are usually hills, rocks, buildings and such which all contribute to reducing the wind speed.
Out to sea, the surface is perfectly flat and there's much less friction, so you get faster winds.
An offshore wind turbine in the Thames Estuary. Photo courtesy of Phil Hollman
Aesthetics
Even if you like the look of them, you probably wouldn't want to live in wind farm. Offshore though, who cares? That said, there have been complaints of the ugly cables coming to shore to transmit the electricity.
Space
Unless you can find land that has no other use, chances are that filling up land with wind turbines will take the space away from other applications. It could be argued, though, that since they are spaced widely, you can still use the land.
Differences between offshore and land bases turbines
A row of wind turbines off shore in Denmark. Photo Courtesy of m.prinke
Size
Wind turbines can be built larger offshore than their land based counterparts. According to the BWEA (link opens pdf file), they are likely to be up to 120m tall, with 40m blades. Bigger turbine = more power.
Farm size
Because there are no physical or human obstacles, offshore farms can be much larger than those based on land. This means that one single offshore farm could produce significantly more energy as a single unit (saving money on operating costs) than a land wind farm.
Return to alternative energy sources from offshore wind turbines