I know I'm hardly the first to propose this; studies for it go back to at least the early 1990s. However, in the wake of the BP oil disaster in the Gulf of Mexico and deep water drilling technology still fresh in the public's mind, this may be the most receptive environment for this idea in many years, even with the current economic turmoil.
OTEC stands for Ocean Thermal Energy Conversion. More information on the technology can be found HERE. Basically, it uses large differences in temperatures between surface water and the sea depths to drive a working fluid such as ammonia to turn electric turbines. A full-scale OTEC plant is estimated to be able to produce up to 250 megawatts under optimal conditions, about a quarter of the average fossil-fuel power plant. The technology is still officially in development, but seems poised to becoming a fully viable power generation resource in the near future.
When an oil drilling platform runs dry, its usually a fairly costly procedure to decommission and dismantle the platform itself. This is especially true of deep water rigs, and usually just the structures near the surface are removed.
One the biggest cost headaches in producing practical OTEC plants is the building and establishment of deep water platforms. Ideally these are envisioned as expansive, consolidated 'energy islands' that would combine OTEC with other types of power production, such as solar, wind, tidal, and wave generators. However, OTEC is always envisioned as the heavy lifter, producing three times as much as those other types of generators combined. So having an energy island with just an OTEC generator would still be potentially profitable. The only real limiting factor (for the US, at least) is that OTEC generators must be located in waters that are relatively warm year round, which for the US would mean the Gulf of Mexico and the waters off of Florida and Hawaii.
The proposal is straightforward: as deep water rigs have their wells run dry and are decommissioned, instead of spending money to have it dismantled, they can be sold or leased to alternative energy interests for conversion into OTEC facilities. This would mitigate one of the biggest upfront costs of an OTEC plant--creating the deep-ocean platform necessary for the technology to work. Major investment would still be necessary, especially laying the long power cables needed to transfer the current to shore along the seabed. However, once set up, the OTEC facility could produce power indefinitely, long beyond the handful of years the drilling platform would produce crude oil.
OTEC technology has proven itself experimentally and could be fully commercially developed in the near term with enough investment. But the biggest roadblock to this idea may be political rather than technical or economic. The large oil and energy companies that build the drilling platforms have traditionally been hostile to alternate energy technologies and may not want to give a promising new source of power for fear of the competition, even if they may mean additional revenue in place of a tapped out well.
I always thought this was a foolish and short-sighted position. The transition to renewable, alternative forms of energy is already underway, and while fossil fuels will be part of the energy landscape for many decades yet, their dominance will slowly fade. Rather than trying to block alternatives and only delaying the inevitable, the big energy companies should start wholly investing them, and get ahead of the curve. This way, they will not only be helping society through a very necessary and beneficial transition, they will ensure their own long-term solvency by adapting themselves to the changing technological landscape.