Design, Fabrication, And Loadout Of Auger Hull

Abstract The Auger Tension Leg Platform (W) hull is a 73,000 st displacement structure weighing 21,500 st. It consists of four stiffened cylindrical columns connected by four stiffened rectangular pontoons and operates at a mean draft of 94 ft. The structure was designed for in-place conditions using both time domain analyses to capture non-linear wave load effects and linearized frequency domain analyses. Extensive use was made of finite element analyses (FEA) to design structural components. The hull was fabricated by Belleli S.p.A. in Italy on Iandskidways, Ioaded onto two submersible barges, and floated off in preparation for a wet transport across the Atlantic Ocean. Throughout the fabrication, Shell and Belleli engineers and site construction personnel worked closely together to insure that the final product was a cost effective and fit-for-purpose structure. As a result of the work performed on the Auger hull a number of important lessons were learned that will be applicable to the next TLP hull and should result in an even more cost effective structure. Design Configuration The Auger TLP hull is a 73,000 st displacement structure supporting an operating payload of 17,800 st, including deck steel, topsides facilities, and riser tension, and tendon pretension of 12,000 st. It consists of four cylindrical columns connected by four rectangular pontoons and is held on location by 12 tendons and 8 mooring lines. The columns are 74 ft in diameter, stiffened longitudinally and circumferentially, with a 34 ft diameter inner shell, stiffened circumferentially. The pontoons are 28 ft tall by 35 ft wide boxes, made up of orthogonally stiffened flat plates. In the short direction the column centerlines are spaced at 214 ft, while in the long direction the spacing is 264 ft. The hull will operate at a nominal draft of 94 ft although an additional 5 ft of draft can be accommodated for possible subsidence due o reservoir compaction. Figure 1 summarizes the principle hull dimensions. The total weight of the hull is approximately 21,500 St. The hull is subdivided into compartments by horizontal flats in the columns and vertical bulkheads in the columns and pontoons. Pontoon compartments are permanent ballast tanks, as are the perimeter column compartments at pontoon level, All other compartments are void tanks, Compartment size is selected to prevent tendons going slack under the reduced extreme environmental condition (1 year return period hurricane or 10 year winter storm) with compartment damage and compensating ballast to even out the tendon loads, Damage is defined as two compartments flooded at the waterline or one compartment flooded elsewhere, The keel inside the inner shell is protected by a double bottom; consequently, the inner shell compartment is not considered susceptible to damage. A pumproom is located at the keel inside the inner shell at each column, with a 7 ft diameter access tunnel connecting each pair of pumprooms through the short pontoons.