OLD concept. When I first heard about this (I think 40 years ago), it was called "FLIP" (though I forget what the acronym stood for). I have always wondered when someone would couple it with windmills. A bigger problem would seem to be the cost of transmission from 150 miles offshore.
Thanks be to Google:
http://history.nasa.gov/HHR-32/ch18.htm
"At the request of OTDA, GSFC issued 8 Request for Proposal on December 8, 1964, for a comprehensive feasibility study of SOP's as instrumentation facilities compared to conventional ships. The work statement for the study called for examination of two sizes of platform: a "small" type, generally associated with the concept of the FLIP ship then being operated by the Scripps Institute of Oceanography; and a "large" type, generally associated with the multi-leg floating platform concept as typified by the MOHOLE platform then being designed for the National Science Foundation (NSF). GSFC's evaluation of the eleven industry proposals received was presented to the Source Evaluation Board (SEB) in NASA Headquarters on May 4, 1965."
There would be an offshore substation to collect the power and one main transmission line to connect it to the onshore grid. The idea is that the increased power generated from the offshore winds, plus the reduced installation costs compared to sinking giant monopiles in the ocean floor, would more than offset the increased transmission costs.