Saga Petroleum and Aker Engineering have designed a triangular tension leg platform (TLP) that can potentially reduce development costs in deepwater by up to one third. For example, the two developers say a triangular unit designed for the Snorre field offshore Norway could be constructed for NKr400 million, rather than the NKr1.02 billion spent to build the conventional TLP structure.
Ironically, the Saga/Aker TLP design most resembles the original unit tested offshore California in the US in the mid-1960s. At that time, operators were trying to find a way to position a platform in depths over 600 ft. The developers say that all designs since then have been built on the four-column design popularized by semisubmersible drilling unit designer Gotaverken Arendal (GVA) of Sweden in the early 1980s.
The environmental parameters used for the design were those typical of the V-ring Basin off mid-Norway. Water depths in the area vary from 400 meters up to 1,500 meters.
The Saga/Aker triangular design is based on a topside load of nearly 40,000 tons. A hull made of steel was found to be very stable, more so than four-leg versions, design engineers point out.
Three-leg stability
One of the interesting points resulting from the design study conducted by the two firms is that the three-leg version is inherently more stable than the four-leg version. Removing one of the legs eliminates the need for equipment to adjust the tethers to each other, since the platform is more stable.
In addition, the design simplifies procedures and equipment needed for installing the tethers and platform in the field. All of this yields major savings for the owner operator, which is why a three-leg unit can reduce costs by as much as two-thirds.
The tethers for the triangular unit would probably be welded to the structure externally, whereas in four-leg units today, the tethers are attached mechanically inside the four columns. The developers say the tethering system then would be no more costly than a catenary mooring unit for a floating production system.
The designers also say the triangular unit can eliminate 3,000-4,000 tons of steel from the hull and 6,000 tons from the topside support frame.
The Saga/Aker design uses an open space frame for the deck support near the wave zone. Aker Engineering has also earmarked further weight and space gains for the topside framing and equipment components.
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