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

On Friday 02 February 2007 20:18, Craig Jungers wrote:
 
> 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.

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 :-)!

(snip)

> 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 shipyard I worked for went from, in about ten years time ships, to one 
floating diock (the world's biggest), to jack-ups, to semisubmersibles: 
first a few accomodation platforms, then some drilling platforms, 
Balmoral, a big production platform, and the last two were diving 
platforms. 

I was often heliguard, even had a chopper crash upon me (at least that was
my impression at the time), with the chopper balancing on the edge of the
net, surrounding the helipad. The water 20 meters below was freezing so
they were damned lucky that the engine exploded just before touch down.
No-one was seriously hurt, but one guy had to take early retirement, and 
one had nightmares years after.

Anyway, semis (the submersible kind) work great as long as the hulls are 
under the water surface, or nearly so, and the platforms themselves over. 

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. 

Once during the a test trip with a Norwegian drilling platform, while we 
were anchored just a few miles from the shipyard, an unplanned
hurricane approached, rapidly. Due to the bad weather there was no 
way the smaller ships could get out to us, so a helicopter shuttle line 
was established, using Longrangers and Siouxs, but then after a few trips 
to shore, the wind proved to much for them too, so a lot got stranded 
onboard for the night. 

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.

I myself and a handful others got off the rig by being hoisted off the
rig, dangling over a foamy sea, onto a Norwegian offshore ship that
backed in under us as far as the skipper dared, but after just picking 
up a handful he called it quits - never seen bigger waves in my life.
and that was inshore - God knows how it would have been out in
the North Sea. Well, we got safely to shore, the rig didn't topple and
a hundred extras were forced to stay the night, the laundry with its
dirty linen became a popular place to snatch a few hours sleep -
some teams worked through the night, as they didn't find anywhere 
to sleep.

The semis that came to the shipyard were sometimes really beaten up,
usually by waves that had banged the bottoms of the platforms into
intersting new shapes. To fix their problems some needed extra legs to 
float higher, while others got their legs thickened. 

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.

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.

In short semisubmersibles are highly complex, and highly vulnerable to 
damage from colleages that has come adrift, or from supply ships that 
venture too close, but they are very seldom lost.

Fire, as usual with sea vessels of all kinds, is their worst enemy, by far!

Yours,

Tord
***************************************************************************
PaddleWise Paddling Mailing List - Any opinions or suggestions expressed
here are solely those of the writer(s). You must assume the entire
responsibility for reliance upon them. All postings copyright the author.
Submissions:     PaddleWise_at_PaddleWise.net
Subscriptions:   PaddleWise-request_at_PaddleWise.net
Website:         http://www.paddlewise.net/
***************************************************************************