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    Semi-sub design gets more deck space

    Vessel & ROV News // August 11, 2000
    Increased deck space on offshore structures often comes at a premium but designers of a recent semi-submersible for Transocean Sedco Forex found a novel solution, as classification society ABS explains.

    The owners of the semi-sub relocated the engines, generators and mud tanks into the pontoons, which lowered the centre 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 systemlocated 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.

    With the new design came new strength and fatigue considerations, so classification society 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.

    The design of the Cajun Express, rated for drilling in up to 8,500ft of water, used the expanded variable deck load to vertically store drill pipe and riser joints.

    "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 pipe rack on deck. This increases the speed at which the well can be drilled," explained ABS.

    Another distinct advantage of the design was the lowered centre 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 semi-submersible drilling unit, +A1 Column Stabilised 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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