[Paddlewise] Semi-submersible Drill Rigs - and Reserve Buoyancy

Looking at the photos of a drill rig under construction reminded me of my
days in the 70s working offshore. In the 1970s I was assigned to work on the
Hughes Glomar Explorer which was (ahem!) to explore the ocean bottom looking
for nodulese mangles or some such. At any rate, the only mangle it pulled up
was (so the story goes... I'm not admitting to anything) was a sunken Soviet
submarine.

I parlayed that experience into several jobs working for various offshore
drilling companies (back then there was a plethora of them). My first job
was on a semi-submersible called the Sedco 709. Pretty romantic name, huh?
The 709 was not the 709th rig but was, back then, state-of-the-art for deep
water drilling. The 709 was designed to drill in water that was up to 6,000
feet deep.

But the most interesting thing about semi-submersibles is their ability to
remain relatively stable in a very heavy seaway. In fact, I was aboard the
709 in the North Atlantic with sea heights of 70-feet. It may interest some
of you to learn how they do it.

A semi looks to most people like a platform because it rests on several (six
or more) columns that appear to the unitiated to be "legs" are connected to
hulls 40 (or more) feet below the water. So a semi is actually a form of
catamaran!

The reason for this design is that someone took a look at why a boat has a
lot of motion in a seaway and decided that the main reason is something
called "reserve buoyancy". This is buoyancy that is over and above that
required to merely keep the vessel barely afloat. In effect, it's anything
hollow and watertight above the waterline. The theory goes that a hull with
zero reserve buoyance will not rise to a sea but will, instead, let the sea
wash over it. Now there is bound to be some rise, of course, because the
water particles are rising; seems like we may have covered this in the
recent past....

Anyway, a hull that has waves washing over it might be something less than
useful; reserve buoyancy is pretty much needed for safety in everything but
a submarine. But what if you designed a semi-submarine?

So the idea behind a semi-submersible is to have two hulls provide the major
buoyancy and put them well below the action of waves. Then build a platform
over those that sits high enough out of the water to do the actual work.

The real genius in this plan is that the circular columns (or legs) of a
semi provides the reserve buoyancy. The legs have a lot of volume but
they're vertical not horizontal so the reserve buoyancy isn't very
effective. A large wave which would immediately raise the bow of a ship or
boat doesn't affect the hulls of the semi-submersible (40 feet below the
surface) much at all and the reserve buoyancy provided by the circular
columns is not enough to make the vessel move up and down much. Of course,
you have to put the platform waaaaay up there because a large sea could just
sweep over anything too low.

Which, by the way, is what the 70-foot monsters did to us in the N. Atlantic
back in 1976. They were breaking against the living quarters just below the
helicopter deck. But it makes a great story.


Craig Jungers
Royal City, WA
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