Special Report - Total climbs to the top with Matterhorn

The Matterhorn field includes multi-layer sands, with five reservoirs stacked from 6,000 to 11,000 ft below sea level. Cornelis van der Linden, Total E&P USA Inc. Matterhorn project surface facilities manager, explained that, because the developers were aware of the stacked reserves, they knew that workovers would be necessary. The need to access and workover wells in the future was a driving factor in the selection of the dry tree design.

(Picture above) Atlantia modified its SeaStar design for the Matterhorn TLP. It is the first to use dry trees.

Due to the limited extension of the reservoir structure and the fact it was possible to drill all development wells from a single location, project leaders chose the surface-piercing structure after intensive screening of 12 proposals. Through competitive bidding, Atlantia's SeaStar TLP design proved to be the best choice both from technical and commercial points of view. The TLP design was deemed ideal for dry trees because the platform and production risers move only a few feet, allowing for an inexpensive tensioning system. Production risers pass through the central moonpool to the trees on the production deck.

The wells were initially drilled with the Ensco 7500 semisubmersible and suspended at the mudline prior to TLP installation. Total is using a Nabors workover/completion rig installed on the TLP to tieback and complete seven wells. Once the completion campaign is finished, the rig will be de-mobilized. If it is necessary to workover wells in the future, a 750- or 1,000-hp rig can be remobilized. The dry tree design allows easy access to the well bore for wireline or coiled tubing operations. Each of the production wells was completed in two or three reservoirs, and wireline intervention will be necessary to shift sleeves to change zones. The dry tree concept also leads to decreased flow assurance risks. The Matterhorn TLP will accommodate subsea wells, which will be connected to the TLP through catenary/flexible flowline risers.

The Matterhorn is Atlantia's fourth TLP design using a single, central column with three pontoons. Atlantia's three TLPs before Matterhorn – the Morpeth, Allegheny, and Typhoon, all have produced from subsea trees only.

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Hull construction

After the award of the engineering, procurement, installation, and construction (EPIC) contract from Total in July 2001, Atlantia built the Matterhorn hull at the Keppel FELS yard in Singapore between January 2002 and January 2003. It arrived in the GoM in April 2003. The hull is almost twice the size of the previous three TLP designs – 84 ft in diameter, 125 ft high, with a 179-ft radius, and weighing 5,500 tons. It is capable of supporting approximately twice the payload of the previous three SeaStar TLPs.

The 36-ft-diameter moonpool has capacity for risers from nine wells, but only well slots 1, 2, 5, 6, 7, 8, and 9 are or will be active in August 2004. One direct vertical access well riser measures 9 7/8 in.; three are 9 5/8 in., and three are 10 3/4 in. The wellheads are spaced 25 ft apart on the seafloor. A subsea injector well will be tied back to the platform this month if pressure measurements indicate good reservoir connectivity.

Matterhorn has two export pipelines. Technip's Deep Blue installed an 8-in. oil export pipeline in April 2003 and a 10-in. gas export pipeline the following month. These operations were completed after hull installation but prior to setting the deck. Both export lines are connected to the hull by steel catenary risers. The oil pipeline ties into the main Chevron line in South Pass block 50, 15 mi. from Matterhorn. The gas pipeline, owned by Total, ties into the El Paso system in Mississippi Canyon block 64. At latest report, Matterhorn was producing 23,000 b/d of oil and 35 MMcf/d of gas, but is designed to produce up to 33,000 b/d of oil and 55 MMcf/d of gas.

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Current delay

The hull supports the 6,000-ton topsides, which was completed by Gulf Marine Fabricators in Corpus Christi, Texas, in July 2003. The topsides module was transported to the Matterhorn field on July 27, 2003.

"The topsides is of typical construction," Van der Linden said, "called pancake construction. Before floating a new deck on top of the lower one, we made sure all the equipment was installed. It was very efficient." The facility's three decks include a cellar, production, and drill deck. The topsides are designed for 30,000 b/d of water injection capacity and 20,000 b/d of produced water handling.

Six 32-in. steel tubular tendons anchor Matterhorn. The tendons are connected to six piles that are 96 in. in diameter, 418 ft long, and weigh a record 400 tons each. Heerema's Hermod vessel installed the piles beginning in March 2003. The tendons were attached in June 2003. Work proceeded on schedule until eddy currents exceeding 2 knots and two storms (Claudette and Bill) delayed work for 34 days.

Two tendons had been placed when the eddy currents disrupted activity by causing the tendons to move close to touching. A spacer had to be installed to prevent tendon contact or excessive bending. Des-pite the setback, Total still managed to meet the deadline.

The Matterhorn TLP generally moves very little, due to the nature of its design, but the platform is equipped with a full set of instruments to measure met-ocean data, wind speed, direction, and wave height. It has an Acoustic Doppler Current Profiler (ADCP) to measure the current direction and strength down to 2,200 ft below the sea surface.

"We have instrument packages in the hull to measure all platform motion," Simon Hare, offshore production manager, said. "The TLP is equipped with differential and standard global position system equipment, tendon tension monitoring systems, ballast measurement systems, and tri-axial accelerometers, all of which record platform motion history so we can go back and match the predicted global performance of the hull against the actual performance."

First oil flowed on Nov. 11, 2003, and four of seven wells are complete to date, with the rest to follow by August. But Matterhorn 's capabilities will not be achieved at that time. Future expansion is planned, and the equipment is standing by.

"Matterhorn carries spare risers for future subsea satellites and has room for four additional 6-in. hydrocarbon risers and one additional water injection riser," Hare said. "Matterhorn also has the capacity to hang off three umbilicals."

Total saves money, shares cost

Total will share both a supply boat and a helicopter among the company's Matterhorn (Misissippi Canyon block 243), Virgo (Vioska Knoll block 823), and Canyon Express assets once the drilling is complete on Matterhorn. Total expects to realize significant savings this way, reducing LOE costs for each asset.

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The Seacor Master, a 145-ft dynamically positioned supply vessel, is primarily used to supply methanol to Canyon Express. Due to the proximity of Total's GoM assets, the vessel will also be used to ship supplies to the Matterhorn and Virgo installations. Matterhorn is conveniently en-route between Fourchon and Williams' MP261 platform (host to Canyon Express), and Virgo is only 8 mi from MP261.

Total currently leases two helicopters, one ERA Bell 412, which holds 11-12 passengers, and one Bocal 105, which holds four passengers, to service the platforms. Later this year, they may replace the current helicopters with a new Augusta 109, which holds six to seven passengers. By running only one medium-size helicopter, the company will reduce leasing expenditures and spread the expense among the platforms it serves.