Semisubmersibles Lighten The Load

    Transocean Sedco Forex (Houston, Texas, U.S.A.) is working on a new design for its ultra-deepwater semisubmersible rig, the Sedco Express. A major offshore contractor specializing in deepwater drilling, Transocean has 166 MODUs and inland drilling barges in operation around the world. Semisubmersibles are mobile rigs that are supported on pontoons filled with water and are held in place — or repositioned — by dynamic positioning thrusters. Built to International Maritime Organization (IMO) guidelines, the Express-class rig is designed for use in 8,500 ft/2,600m of water and has a maximum drilling depth of 25,000 feet/7,620m.

    To achieve its goal — an ergonomically advanced rig with a total maximum deck load of 6,000 tons/5,454 tonnes — Transocean took full advantage of composites to reduce overall weight and maintenance costs, says its manager of corporate communications Guy Cantwell. Strongwell provided cable trays and phenolic gratings, while Ebo Systems (Obernai, France) provided handrails, ladders with safety cages, and gratings made with polyester and acrylic resins.

    According to Bernard Titreville, senior design engineer for Transocean in Montrouge, France, use of FRP topside outfittings (walkways, handrails, wind walls, and access ladders) eliminated 30 tons from the combined weight of the main structure and the living quarters. The derrick outfittings included about 4,300 ft²/ 400m² of fire resistant, anti-slip FRP gratings. More than 5,380 ft²/ 500m² of wind walls, about 650 ft/200m of vertical ladders and 2,600 ft/800m of handrails were installed, using a design that was approved by French governmental authorities. A new product, the handrails required recertification by ABS, due to initial problems experienced with the method of attachment to the structure. Reinforcements at the walkway attachments and additional gussets at the bolted fixture points solved the problem.

    In the sack storage area, 13,988 ft²/1,300m² of anti-skid composite surface plating was installed, with a uniform loading specified at 615 lb per ft²/3 tonnes per m² and concentrated loading specified at 922 lb per ft²/4.5 tonnes per m² to support forklift wheels.

    Floor installations revealed that bolting the FRP to the steel structure remains a challenge. Titreville explains: “The joining solution needs to be more deeply engineered with the FRP supplier, especially for flooring which supports high loads such as fork lifts.” Sealing of the floor panels was a problem, and the floor eventually had to be replaced with steel. Titreville believes that an installation technique that uses clips would be successful, but he stipulates that the clips would have to incorporated into the design at project conception.