The most obvious way to develop Nkossa field off Congo would have been to install two large drilling and production platforms with offloading into oil and liquid petroleum gas storage tankers.
Instead, Elf Aquitaine SA is using two wellhead platforms with semi-submersible rigs to drill development wells in a tender assisted operation. All oil and gas processing equipment is on a separate concrete barge.
This barge concept gives Elf Congo more space than a normal, crowded platform topsides. A visit to the Nkossa barge construction yard near Marseilles shows how many platform design problems have been overcome by this extra room.
For example, the barge is 220 m long by 45 m wide with five modules mounted in a line on top. It is 75 m from the LPG unit, potentially the most dangerous, to the quarters module. This compares with a typical 25 m from process plant to quarters on a modern, post-Piper Alpha platform in the North Sea.
CRANE AND RAIL SYSTEM
Recent risk assessments have shown that one of the major hazards on many production units is a load dropped by a crane onto a process unit. On the Nkossa barge, load handling by cranes is limited to one area.
Loads can be lifted aboard by crane to two main levels for passage along the length of the barge on small rail trucks and across the barge to another rail line by overhead hoists. This halves the risk of dropped loads and significantly limits potential damage.
There is financial benefit, too. Bernard Astier, Elf Congo's Nkossa project manager, said the barge approach allowed Elf to reduce development cost by 1 billion francs ($205 million) to 9.4 billion francs ($1.94 billion).
"Sixty percent of this cost reduction is due to the barge concept," Astier said. "It allows a higher loading capacity than a platform, and concrete is cheaper than steel.
"The other 40% is due to the high amount of onshore hookup and commissioning work the combination of barge and modules allows. A million man hours of integration work, which Nkossa's barge requires, is much cheaper in a Marseilles dock than off Congo."
WHY CONCRETE?
Elf's barge weighs 70,000 metric tons and carries a total 30,000 metric tons of topsides. There is room enough on board to install production equipment for satellite fields.
Nkossa has estimated reserves of 400 million bbl of oil. Astier said field life is expected to be 25-30 years, with a total 24 production wells during development. First oil is anticipated by June 1996.
Production is to reach 120,000 b/d of oil and 1,300 metric tons/day of LPG. The oil tanker will store as much as 270,000 metric tons. LPG tanker capacity will be 80,000 cu m.
Astier said "The Nkossa barge concept is unique-at least on this scale. A barge is ideal for waters off Congo and would suit any sea where there is a long swell, but not a place like the North Sea.
"We chose concrete mainly because it is light and rigid. An equivalent steel barge would have been heavier, and flexion in the sea's swell would lead to metal fatigue within the field's life."
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