No, its not naive. Just technically and commercially very, very difficult. Why do it? - Ok a suface vessel is prone to storms, storms and waves that can occur as 1 in 50 years, 1 in 100 years and even 1 in 1000 years.

If you submerge a vessel, you have a whole bunch of other issues. Air, Air purity, Water Incursion, Pressure, etc etc. All of these require a new set of engineering solutions, exacting engineering standards, power generation, backup systems , backed up by more back up systems etc.

Why do it?. No point: law of diminishing returns and with increasing orders of complexity

Like the man says, it is cheaper to build a rig that can withstand a Cat 3, and hope a Cat 5 does not happen.

You must also perform a cost - benefit analysis: Cost of a complex new-build rig (eg Thunderhorse) compared with the benefit of oil in dollars recovered from the field.

As steel, yard costs, manpower costs all increase and as the cost of new deepwater rigs increases, sub sea rigs would be an untried and even more costly experiment.

The industry is faced with quite a few crunch points. Sure, oil is coming in at 70+, but the costs of sourcing rigs, people and essential material is keeping pace.

Finding and lifting costs for operators is becoming a serious issue. Projects are slowing, being delayed or subject to cost spiralling.

The whole industry just got caught on the hop.

Industry outsiders do not realise just how pared down the industry became between 1986 and 2004. The years of cheap oil almost killed off the oil industry. Few new built rigs, far too few deep water semis and drillships, Land rigs rusting in fields, Semis and jack ups cold stacked, oil industry personnel being shed at every opportunity.

We are ALL paying for it now.

All sections of the industry are on the critical list:

Pipelines: Who Would spend millions on pipeline maintainance with oil at 15 US / bbl? More than your jobs worth.

Refineries: Most were never close to capacity during this period and building a new one was fraught with exacting emissions and safety standards.

Rigs: Why build new ones when existing rigs were chartered at or below operating costs?

People: Why get up to your eyes in debt studying Geology, Geophys, Pet Eng when chances of secure employment were next to zero? There is a whole generation of missing specialists. In 2002, the US graduated 430 petroleum geologists and 43 000 lawyers. (Speech at Houston Oil Show IIRC)

And the worse is yet to come: There is a whole generation of oilfield specialists about to hit retirement.

This will impact every level. From rig electricians who joined a platform as a boy and know the platform intimately, to Exploration and Drilling Managers.
Even getting to work may be problematic: Chopper pilots and service crews have the same age profile.

The skills gap will worsen before it gets better. And who will join if we all know that we are at peak....

Watch out for Zimmer frame access on the new builds...