A major component of the world's first concrete tension leg platform (TLP) is in place for development of the Conoco Norway Inc. group's Heidrun oil field in the Norwegian Sea frontier.
Norwegian Contractors AS, a member of the Aker group, Oslo, employed a new method to install four concrete structures on the seabed during an 18 day operation, including tow-out. The structures make up the foundation for the platform, due on stream Aug. 1, 1995.
Heidrun lies in 345 m of water, mostly in Block 6507/7 and extending into Block 6507/8, on Haltenbanken off mid-Norway. Only about 100 miles south of the Arctic Circle, it is the northernmost field development site off northern Europe (see related story, p. 62).
Norske Shell AS opened Haltenbanken hydrocarbon production with start-up of Draugen oil field last Oct. 19 (OGJ, Nov. 1, 1993, p. 23). Draugen is the only field on stream in the Norwegian Sea.
INSTALLATION METHOD
Norwegian Contractors' foundation installation method cut time and cost by avoiding the use of divers and heavy lift crane barges. The company said the method can emplace very large seabed structures with a high degree of accuracy. There are virtually no water depth limits.
In brief, the structure is towed to the installation site on the surface, then gradually submerged by adding water ballast. In this case, the lower part of the structure penetrated 5 m into the seabed.
Final preparation in the field began 2 days before the first foundation structure arrived on site. The Amethyst surface support vessel performed a seabed survey and installed and adjusted its navigation system. Then the positioning system was installed, including hook-up of lines and guides to docking piles.
Upon arrival on site, foundation structures were connected in sequence to the main descent wire, hold-back line, pull-in lines, and an umbilical. All systems were then tested.
The umbilical allowed the Amethyst's main control room to remotely control the structure's mechanical equipment module (MEM). The MEM, with all functions operated from the surface, controlled ballasting, positioning, and seabed penetration.
The MEM carried hydraulic pumps powered by electricity from the surface.
Remotely operated vehicles, four tugs, and the surface support vessel conducted the first stage of the installation. Water ballast was pumped into the structure, and after 8-10 hr the entire structure was submerged.
Preparations for final positioning and seabed penetration began after another 1-1/2 hr, when the structure approached the seabed.
Final positioning of the foundation took place against two preinstalled docking piles designed to resist the weight of the 21,000 metric ton concrete structure at a top speed of 0.1 m/sec. The foundation, each structure with 19 concrete cells, was pulled toward the piles with winch wires connected to the pile head. The other end of the wire was connected through guide frames on the foundation to winches onboard the Amethyst.
With the foundation docked on the piles, Norwegian Contractors pumped ballast water from the structure's skirt compartments 'to allow final descent and seabed penetration. The 5 m penetration below the mudline was deemed sufficient to hold each foundation structure in place, so solid ballast was not required.
Norwegian Contractors finished the TLP foundation installation July 26 as part of a $360 million contract with Conoco.
The foundation structures were built by Kvaerner Concrete Construction.
Norwegian Contractors plans to use the method next summer to install a 105,000 metric ton concrete gravity tank in British Petroleum plc's Harding field in the U.K. North Sea.
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