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

On 2/2/07, Tord Eriksson <tord_at_tord.nu> wrote:
>
>
> That must have been interesting, Craig! But Soviet subs do not grew on
> trees, so you'd have to have something fancy! Actually I saw something
> about the operation on Discovery - don't remember seeing you, though :-)!


I was playing table tennis under the helo deck at the after end of the ship
when some of those videos were shot. And just playing that game on a moving
ship is interesting.

During testing normally the two hulls are truely semi-submerged, so that
> the hulls can be used as wharves (sp?) for smaller supply ships and when
> transporting the shipyard crew back and forth, as there are normally too
> many people onboard during daytime to sleep onboard during testing.


All the offshore rigs I worked on were self-propelled and dynamically
positioned and not anchored. Unlike most semis (which are towed by
ocean-going tugs) we would bring the hulls up to the surface, aim all the
thrusters aft, and drive to the next location. However when the weather got
too bad we would have to ballast the hulls down to keep the rig stable. This
slowed our speed from about 10kts down to 2 or 3 knots.


> An added problem was that the electrics for the sensors involved in the
> trimming of hulls fused, so the rig got stuck in top position, and leaned
> a lot in the 80 knot gale. Scary, really scary when you saw the look of
> the really experienced North Sea guys - if they were worried things
> were quite bad, and it was.


The ballast control panel on the semis I worked aboard was located at the
bridge and often directly behind the dynamic positioning control panel.
About 20 years ago the Ocean Ranger took a wave directly into the bridge
which shorted out that panel and led to the loss of the rig and all aboard
her (off Newfoundland where I had been working only a few years previously).


So, while semisubmersibles stand a lot of seas, many a sailor will
> tell you that sleeping in accomodation platforms can be very scary,
> especially when the waves slap into the bottoms of the platforms,
> just as a ride in a catamaran with a deck between the hulls can be
> when the waves are of the wrong size and form.


I have several years of sleeping aboard semis with only a steel plate
separating my room from the waves (as opposed to most of the hull on a
ship-shape). As far as comfort goes the semi wins hands down. The greatest
disadvantage to a semi is that in cold weather the deck is very, very cold.
And one must watch stability closely.


> And it is these huge waves that sometimes wrench the platforms
> away from their eight (sometimes more!) gigantic anchors, and snap
> the man-thick, specially hardened, chains like they were made of thin
> plastic! The last mile, or so, is thick steel wire, and the winches are
> humongous, equipped with both disc brakes and band brakes, the latter
> having better locking power.


Dynamically positioned semis (and drill ships) do not anchor as the weight
of the chain and cable to anchor them in 8,000-plus feet of water would sink
them (especially the semi which has a very limited load-carrying ability).
Equipment (then made exclusively by Honeywell) would interpret signals from
beacons placed on the sea floor to control thrusters which would pivot and
thrust to drive the vessel in such a way as to counteract the effects of
current, wind and wave action and keep it over the well-head. I was an
officer (Chief Mate at one time) on the Discoverer Seven Seas (a ship-shape)
when we set a world record for drilling in deep water back in the 1980s.
Something over 8,000 feet. (I wonder where my plaque has gone.)

Interesting work and from what I read in the news it's getting active again
due to high oil prices.

Craig Jungers
Royal City, WA
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