Record setting deep-draft caisson set for installation

Crude from Hoover; natural gas from Diana

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Final construction is underway on the topsides and the hull assembly for Exxon's Hoover/Diana project. Once installed, the Hoover/Diana vessel will be the deepest draft drilling and production system in the world.

The Hoover/Diana Development deep-draft caisson (DDCV) will be located in the Gulf of Mexico,160 miles east of Corpus Christi, Texas. The caisson looks much like a conventional spar hull, with the primary difference being oil storage. A spar accommodates oil storage, while the caisson does not. The Hoover/Diana caisson platform will process oil and gas from the Diana field (predominantly gas) and from the Hoover field (predominantly oil).

Diana was the first of the two fields discovered. Located over blocks 945, 946, 988, and 989 in the East Breaks region of the US Gulf of Mexico, the Diana discovery well was in 1990. This was followed by two appraisal wells drilled in 1992 and extensive 3D seismic mapping of the area. The field was fully appraised in 1997; however, Hoover forced a change of plans.

The Hoover field was discovered in March of 1997 in Alaminos Canyon Blocks 25 and 26, roughly 15 miles east of Diana. Exxon saw the economic advantage and design potential of producing the two promising fields from one structure. Because Diana is being drilled and completed with a mobile offshore drilling unit (MODU), it will be the first to begin production. This will allow early production to begin, while Hoover is still being drilled. Total recoverable reserves from the two fields have been estimated at 300 MMboe. Exxon is the operator and holds 66.67% interest. BP Amoco holds the remaining 33.33%.

In April of 1998, Exxon announced it would use a DDCV, located in Alaminos Canyon Block 25, over Hoover, to drill and produce the two prospects. Diana and Hoover will both be brought onstream in 2000.

Caisson hull

The DDCV is a cylindrical hull with a diameter of 122 ft and a length of 705 ft. The hull is constructed with air-filled compartments in the upper portion of the hull and ballasted with seawater and fixed ballast in the bottom compartment. This design offers:

  • Greater stability than a semisubmersible
  • Can be used for drilling, eliminating the need for a MODU
  • Enables use of surface production trees.

Aker Mantyluoto fabricated the hull in sections at its Pori, Finland, yard, where all of the spar-type hulls were built. The hull was initially fabricated in 20-50 ft long semi-circular sections. Each of these sections was 1/12 of the circumference of the hull. These sections were then assembled into two partial rings, one 5/12 and the other 7/12 of the circumference. Finally these sections were joined to create one of 21 rings.

The rings were then assembled to create the two sections of the hull. The lower section was fabricated first and is longer than the upper section. It was shipped to the Aker Gulf Marine yard in Corpus Christi, Texas on board the Dockwise Transshelf.

The second section is shorter, but contains the hard tanks for buoyancy as well as a variety of winches, piping, fairleads, and other equipment making it the heavier of the two. This section was transported on the back of the Mighty Servant 1.

The two sections are being joined as of this writing. Joining is occurring in two stages. The sections are floated dockside, and 60% of the seam is welded. Then the sections are rotated for the final 40% of the welding. Once the seam, and other miscellaneous work are completed, the sections will be towed to the Hoover site for installation in late October 1999.

Topsides

The Hoover/Diana project is so large that many large and small contractors are involved in some phase of the work. The topsides, designed by Mustang Engineering (facilities), and Aker Engineering (structural), are being fabricated by Halliburton's Brown & Root Energy Services at the Green's Bayou yard in Houston.

The three-level topsides (utility, production, and drilling deck) will include power generation, dehydration, separation, treatment, and compression facilities. The Ensco drilling package will be installed for the drilling and completion of the Hoover wells.

Following Hoover, drilling and completion work will conclude in 2001, and the rig, drilling quarters, and other drilling support facility will be removed. The topsides construction began in December of 1997, and is 93% complete, including loadout and precommissioning, according to Exxon Fabrication Manager Glenn Mannina.

The decks measure almost an acre each in size and incorporate more than 100 vessels and 170 total pieces of equipment. The two lower decks were assembled side-by-side at the yard. The pancake drilling deck was then lowered onto the upper module production deck in a single lift. This 1,900-ton lift took almost 8 hours to complete and required 11 cranes, including a Lampson 1,500-ton crane.

Hull-topsides mating

The topsides will loadout in time to coordinate offshore joining with the installed hull. The vessel will be anchored in 4,800 ft water depth by twelve, 7,100-ft. spiral strand wire ropes with chain leads. The wire rope will be connected to piles approximately 6,900 ft from the vessel with a lateral offset of 5,100 ft. These will be taut mooring lines combining 200 ft of 5 15/16-in. bottom chain, two segments of 3,300 ft of 5 5/8-in. wire rope, and another 800 ft. of chain up to the fairleads.

The suction pilings and mooring lines will be preinstalled by the Saipem S-7000. The pilings are being fabricated in Corpus Christi and measure 21 ft. in diameter and 105 ft long.

Following installation of the hull, the modules will be joined to the hull. This will be accomplished using the recently upgraded S-7000. The two lifts will each be about 8,000 tons, the largest offshore lifts in the Gulf of Mexico. Once this is completed, the S-7000 will install the flowlines and steel cantinary risers for the flowlines and export lines using a new J-Lay system.

The export lines will be laid by the Allseas Solitaire pipelay vessel. Gas will be exported to the High Island Offshore System via an 85-mile pipeline. Oil will be exported to Freeport, Texas through a 150-mile pipeline. Installation of both lines will be completed by year's end. Stolt Comex will install the electrical and control umbilicals for the subsea wells using the Condor vessel.

Field layout

Five wells at Diana will be drilled and completed by the Marine 700 and produced into two satellite subsea manifolds. Six wells in the Hoover field will be drilled and completed once the vessel is in place.

The five wells from Diana will be tied back from a subsea manifold to the vessel. Production is scheduled to begin in 2000.

Once installed, Hoover/Diana will set a depth record for a drilling and production platform with an associated subsea development. The project will use surface production trees similar to those found on the traditional platform design for Hoover.

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The illistration shows what the 1015 ft tall assembled caisson and deck will look like on location of the Hoover/Diana site.
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The valves and flowlines will be located in the 48 ft by 48 ft wellbay area that runs throughout the hull and lower deck modules of the vessel. The trees will be attached to tensioned risers that connect to the wells at the seafloor. Using surface trees allows Exxon to perform interventions and workovers with conventional technology. The system is designed to handle peak production of 100,000 b/d of oil and 325 MMcf/d of gas.

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