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OFFSHORE E-NEWS
May 2000
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DECK
SPACE OF SEMISUBMERSIBLE INCREASED
WITH NOVEL DESIGN
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Increased
deck space on offshore structures often comes at a premium but
designers of a recent semisubmersible for Transocean/Sedco-Forex
found a novel solution. They relocated the engines, generators
and mud tanks into the pontoons, which lowered the center of
gravity, improving stability and increasing the speed at which
operators can drill. |
Issues that
had to be overcome to make this project a reality included concerns
about how regulatory guidelines applied to this new design, particularly
the provision of necessary electrical power, piping, engine exhaust
and fire protection for a system located below the waterline and
facing possible water ingress.
"From
a regulatory perspective, this project needed special attention
in terms of assessing how it fit in with existing statutes,"
said Bob Major, principal engineer, ABS Americas, who was a primary
ABS liaison bringing this project, the Cajun Express, into
class and regulatory compliance. Applying appropriate regulations
to innovative offshore designs can be one of the more difficult
aspects faced by project management teams spearheading offshore
advances.
With the Cajun
Express, the concept of moving weight down into the pontoons
meant adding equipment not usually associated with a conventional
rig and not specifically covered by regulatory statutes. Huge exhaust
trunks are needed to ventilate from the pontoons to the deck level
and the derrick itself is four times as large as a conventional
derrick, running the breadth of the deck.
Additionally,
with the new design came new strength and fatigue considerations.
ABS worked closely
with Transocean/Sedco-Forex from the early design stages through
to commissioning to ensure that the regulatory aspect of the project
went smoothly. "An innovative project like this is just the
kind of thing we, as engineers, truly enjoy working on," said
Major.
The design of
the Cajun Express, rated for drilling in up to 8,500 foot
water depths and upgradeable to 10,000 feet, used the expanded variable
deck load to vertically store drill pipe and riser joints.
Explained Major,
"With this design, the drill pipe and risers are carried very
high, making it possible for the operator to move the riser up into
the derrick and then down-hole rather than pulling from a traditional
piperack on deck. This increases the speed at which the well can
be drilled."
Another distinct
advantage of the design was the lowered center of gravity that improved
stability in rough waters.
When delivered,
the unit will be ABS classed as self-propelled with the propulsion
from the dynamic positioning thrusters. It is capable of dynamic
positioning during drilling operations in deep water. The Cajun
Express will receive ABS’ highest class designation for
a self-propelled semisubmersible drilling unit, +A1 Column Stabilized
Drilling Unit +AMS +DPS3.
The Cajun
Express has been chartered by Marathon for work in the Gulf
of Mexico. Two sister units, the Sedco Express and Sedco
Energy are also due for delivery during 2000.
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