No idea on the compression and reconversion loss, although intuitively 30% seems high.

I would assume a higher load factor than 30% for far offshore wind. Near-shore wind sites in the UK (North Hoyle, Scrobie Sands, London Array, et al) are assumed to have average load factor of, I think, 38%. Further offshore I think the factor would be higher. I have made a few visits offshore and even on the most pleasant summer day there's still a stiff breeze 300 feet above sea level even just 20 miles out in the North Sea. The same is true of ocean swells for wave generation (bigger and more consistent offshore than nearshore). I'd guess a 30% average load factor is very conservative.

Remember that wind farms are not sited that far out due to the water depth (ditto wave farms where the problem is to anchor them) and the consequent negative economic effects of "planting" them - using an exisiting platform would resolve this.

Each offshore jacket would be unlikely to be able to accomodate more than one or two wind turbines, though the potential for wave farm anchoring is significantly higher in my opinion. I'd be more inclined to go with point absorbers (eg AquaEnergy or Ocean Power Technology) than the Pelamis which needs to face into oncomming waves to "ride" them (difficult to predict wave direction offshore). Out of the two, I'd be inclined to go with the OPT Power Buoy, as it has discrete centralised generation, whereas the Aqua Buoy has a Pelton Turbine within the buoy itself. Obviously an offshore platform can be used to house a central generator which otherwise OPT envisage placing either on the ocean floor or onshore.

Another thought that strikes me is the with lateral mooring to an offshore platform, as opposed to vertical mooring to the seabed, there is further opportunity to generate electricity from lateral movements in the same way as the technology is designed to benefit from vertical movements.

As to air compression within existing pipelines, I agree that there is a question of infrastructure age and the question of leaking. I assume that both oil and gas pipelines are designed to operate at relatively high pressure (presumably considerably higher for gas than for oil) which would be accomodative to the concept. The upside being, clearly, that an air-leak is not going to be at all environmentally damaging.

I came up with the idea of air compression within the pipelines to overcome what I saw as a potential issue of power transmission loss from far offshore platforms. The ability to store power as compressed air and generate at peak times was an additional benefit. However, if the energy loss through compression and reconversion were significantly higher than the simple transmisison loss, it would probably not be worth it.

Your last point is the most significant - decomissioning costs sit as a huge liability on E&P balance sheets. The present value of even simply deferring these costs for 10 years is of significant value to these companies. I am pretty sure that these companies would accept a breakeven on wind/wave projects just for the benefit of the deferred costs, so the question simply becomes one of how much wind/wave you can load on to each platform, and what is the most efficient way of transferrring that energy to shore.

As a side issue, I imagine that companies like http://www.excelerateenergy.com/ will also bein the frame for deferment of decomissiong costs in the future for similar reasons.