Wind Systems Magazine, Jul. 2014, by James Dobbins, David Quarton, Paul Reynolds, and Joe Phillips.
http://www.windsystemsmag.com/article/detail/684/reducing-offshore-costs-through-integrated-design

"Compared to its land-based cousin, offshore wind is a new energy technology. This relative immaturity as well as the technical challenge of offshore wind means that it is currently around 50 percent more expensive to produce a unit of energy offshore than it is onshore. But the stronger, more consistent wind at sea means that there are real opportunities to narrow the gap and there are several government and industry-sponsored programs aimed at doing just that in the UK, Germany and Denmark — all leading countries in the creation of offshore wind sectors."