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    ABS highlights FPDSO design

    Vessel & ROV News // August 25, 2000
    When outfitted with a tension-leg deck (TLD) system, the Floating Production, Drilling, Storage & Offloading (FPDSO) platform can provide a high level of flexibility whilst substantially reducing or eliminating the need for subsea trees, manifolds and risers, says American classification society ABS.

    In the most recent issue of its Surveyor magazine, ABS addressed this emerging technology.

    In the changing world of FPSOs, D stands for 'drilling', and the FPDSO incorporates drilling capability through a moon pool in its ship-shaped hull.

    In the article, ABS says Brazilian national oil company Petrobras has begun studying this new technology in its PROCAP 3000 research and development programme.

    Petrobras is well known for its technical innovations, explains ABS. Its most recent innovations include development of synthetic mooring lines, development of the DICAS system mooring system (see Surveyor March 1998), and deployment of what is currentlythe deepest FPSO project, in 1,853m water depths in Brazil's Campos Basin.

    Petrobras has selected FPSOs for eight of its last ten development projects. The next four fields are planned for development with three FPSOs. As these projects are to begin in the near future, Petrobras is not considering an FPDSO, but the technology is still under study.

    In a paper delivered to the October 1999 Deep Oil Technology conference in Stavanger, Norway, Monaco-based Single Buoy Moorings (SBM) proposed an FPDSO unit incorporating an innovation called the tension-leg deck (TLD) system.

    In the FPDSO with TLD, drilling facilities are mounted above the moon pool on the FPDSO, which also includes process, oil storage, offloading equipment, and accommodations. Mounted in the moon pool, above the water line, would be the tension-leg deck, which is a flat deck holding dry production trees, the blowout protector and other equipment.

    The TLD is fixed to the seabed by tendons that restrain the deck against heave. Deck load is counterbalanced by a system of weights.

    Unlike for a TLP, tension on the tendons would not result from restrained hull buoyancy. The TLD system counters its deck load using weights that are hung over the side of the unit, attached to wire and chain that run through sheaves mounted on the FPDSOdeck. The weights are suspended approximately 100m below sea level, to avoid the effects of wave action and to damp pendulum motions. Telescopic and flexible joints would account for the relative motions between the FPDSO and the TLD.

    The TLD concept was originally proposed for development of a West African field, using a ULCC as the FPDSO hull. It sounds like a complex system, but according to SBM, will substantially reduce or eliminate the need for subsea trees, manifold and risers.

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