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    Twin hull tanker will decommission rigs

    News // April 21, 2000
    A Dutch company backed by well-known offshore contractors Heerema has developed a unique vessel with which it plans to target the burgeoning market for decommissioning offshore oil and gas platforms.

    Excalibur Engineering, of Delft in the Netherlands, says its concept - a twin hulled vessel in which a pair of 125,000dwt tankers are combined to form a huge 'catamaran' - provides a safe and cost effective approach to the removal of large oil and gas offshoreplatforms.

    The company decided to use tanker hulls because they could be used to produce the huge unit required at acceptable cost.

    The 'slot' in the catamaran is 90m long and 43m wide, and has been dimensioned to accept jackets of the type that are due to be removed first, although the connecting structure between the hulls can be widening in future in order to fit even the largestjackets. The hulls can also be separated at regular intervals, in order to allow the hulls to be inspected during dry-docking.

    The Pieter Schelte, as the vessel is known, will be equipped with a 16-point spread anchor mooring system in order to maintain her in position. Whilst in transit, the main propulsion systems in the tankers will be complemented by azimuthing bow thrusters, which will help ensure that she maintains course.

    The company says it plans to commission a dedicated vessel based on the design in 2002.

    According to Excalibur Engineering's Managing Director Josso van Boxtel, the Pieter Schelte design is capable of decommissioning platform topsides of up to 42,000 tonne and jackets of up to 25,000 tonne in a single lift operation. He says detailed engineering has already started, and suitable tankers will be acquired in due course.

    "The Pieter Schelte is designed to decommission topsides and jackets in a safe, environmentally responsible, and cost effective manner", says Josso van Boxtel, "and bring them ashore in one piece".

    Its lift capacity - 42,000 tonnes for topsides and 25,000 tonnes for jackets - is sufficient to serve the entire range of jacket based platforms throughout the world, says the company, and still be economic on platforms where the lifts involved are somewhat smaller, in the 5,000 tonne range.

    When lifting a complete topsides facility, the lifting equipment on the Pieter Schelte will be attached to the legs of the module support frame on the topsides structure. Jackets will be removed in their entirety by lifting and tilting them onto the deckof the Pieter Schelte.

    Having dismantled the structure, the Pieter Schelte will set sail for a shore-based facility and deliver the load to a dismantling site for scrapping, or to another platform so that it can be re-used.

    To lift topsides facilities from a platform and to support them during transportation, Excalibur Engineering has devised a flexible structure, which is dimensioned to fit the platform. The system engages the legs of the module support frame, or in its absence, the top part of the jacket legs, and thus lifts and carries the topside facilities using their existing support structure.

    "The lifting system on the Pieter Schelte uses a unique mechanical and hydraulic arrangement", says van Boxtel, "which enables the lifting equipment to be connected to the platform legs while the vessel remains free to move". Pendulum rods and hydraulicconstant tension cylinders guarantee that little or no loads are transferred from the vessel to platform in the hook-up phase.

    With the system connected to the platform legs, pre-tension in the lift system is gradually increased to transfer the weight of the topsides from the jacket to the vessel.

    In the final stage, rapid lift-off eliminates the risk of impact between topsides and jacket. After lift-off, the vessel moves away from the platform, and the hydraulic pressure in the topsides lift system will is bled off and the load comes to rest in the mechanical and structural components of the lift system.

    In contrast to topsides lifts, larger, heavier jackets would be recovered onto the Pieter Schelte over the side of the vessel using twin-frame lifting gear which attaches to the top of the jacket using slings.

    The jacket would be severed at a depth required by the individual project, usually at the mud line, and jacket legs and braces would be punctured to ensure that water drains out of the structure as the lift proceeds. All cutting would be carried out before the decommissioning vessel arrived, with the exception of a few piles, which would be left in place in order to provide the jacket with sufficient stability.

    Having been lifted to a point at which a large part of the jacket structure was above water, the jacket would then be tilted onto the deck of the Pieter Schelte and skidded into a position suitable for transport and sea fastened.

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