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Collaboration creates ultra-deep opportunities

It is ironic that one of the largest and most complex development projects undertaken in the US Gulf of Mexico (GoM) has been labelled the Independence Hub, writes Anne Feltus

The initiative is based on an intricate, interdependent relationship between five producers, a marine contractor, a midstream energy-services firm and its pipeline subsidiary, all sharing an interest in exploiting reserves in the ultra-deep-water GoM.

The project involves the construction and installation of a regional hub that will process up to 0.85bn cubic feet a day of natural gas and 4,250 barrels of condensate, produced from at least six fields, and transport it by pipeline to shore. The subsea production systems for the fields are as far as 30 miles from the platform in water depths ranging from 7,800 to 9,000 feet – some of the deepest discoveries made in the GoM. The fields are owned wholly or in part by Anadarko Petroleum, Kerr-McGee, Dominion Exploration & Production, Spinnaker Exploration and Devon Energy.

Although operators typically own the facilities that process their production, the Independence Hub will belong to a joint venture between EPP and Cal Dive International and will be operated for the owners by Anadarko, its primary tenant. In return for paying a fee, each producer will have the right to use part of the platform and will have access to its common facilities. The operators will share equally in the management of the platform and will each have one representative on its steering committee.

For each participant, there are benefits that could not be derived without the involvement of at least one of the others. For the energy firms, which joined forces to form Atwater Valley Producers Group (AVPG), the project will enable the development of gas discoveries that, individually, are too small and too deep to be produced economically. Most are far from existing platforms and pipelines in the virtually unexplored eastern GoM, where drilling was banned until a few years ago.

Enterprise Products Partners (EPP) will design, construct and install the $385m platform and its subsidiary, GulfTerra Energy Partners, will construct, own and operate the $280m pipeline to carry production to shore. However, their financial risk will be dramatically diminished by the long-term contracts signed by the producers, the acreage the producers have committed to the project and the likelihood that the hub will take on more production as additional discoveries are made in the area.

That risk was reduced even further when EPP sold 20% of its stake in the platform to Cal Dive International late last year. Cal-Dive, a marine contractor and operator of offshore oil and gas properties and production facilities, saw opportunities not only to deploy its vessels in the construction of the platform and pipeline, but also to utilise the hub to promote its goal of acquiring and developing proved undeveloped reservoirs.

Technological achievements

The hub's structure is also unique. When in position, in late 2006, and permanently moored in over 8,000 feet of water on Mississippi Canyon Block 920, the semi-submersible drilling rig will be the world's deepest floating production unit by some 1,500 feet. It will also be the first ultra-deep-water structure designed to minimise the stress on risers created by vortex-induced vibrations.

Strong waves and currents in the GoM batter the steel catenary risers through which production flows to the platform, causing the risers to vibrate and, eventually, to fail prematurely. Typically, fin-like strakes are wrapped in a helical pattern around the risers to deflect the currents. However, the strakes draw heat from the risers that is needed to help prevent the formation of ice-like solids, called hydrates, in the gas that can clog flow lines. The strakes on these risers are positioned in a way that minimises heat reduction while breaking up the currents. The hub will have slots for 17 risers, enabling it to receive output from the six anchor fields and from others.

With a displacement of about 37,000 tonnes, the Independence Hub will be significantly smaller than existing semi-submersible drilling rigs. Its hull will consist of four columns set on a ring pontoon and will have a draft of 105 feet – 15 feet deeper than the draft on its larger counterparts. Reducing the platform's vertical motion also reduces stress on the risers. To increase its cargo capacity, the 232 square feet topsides of the platform will have two decks instead of the typical one. The modularised platform design will allow its decks and hull to be assembled at dockside and towed to its destination.

The project also represents other industry breakthroughs. The platform will be tethered to suction piles on the seafloor with polyester mooring lines, which are lighter than steel moorings. Because the operators do not have to store heavy chains on deck, they can dedicate more space to more lucrative cargo. The steel-tube umbilicals through which the subsea wells will be remotely operated will be reinforced with carbon-fibre rods, which are almost as strong as steel, but only a fraction of the weight. The Independence Hub will also feature the largest mono ethylene glycol (MEG) reclamation unit in the GoM. MEG, which is added to the gas flow at the wellhead to help prevent hydrates formation, is separated from the gas, cleaned and returned to the wellhead for reuse.

Independence Trail, the deepest pipeline to be built to date in the GoM, will carry production from the platform to a facility to be built at West Delta 68, about 134 miles away in the shallow-water Gulf. Two pipelines owned by Tennessee Gas Pipeline will link the facility to Louisiana's southeast coast.

Plans call for the Independence Hub to begin operating by year-end 2006, with first production from the anchor fields expected in early 2007. Activity at the facility is expected to grow as members of AVPG fulfil their pledge to process future output from undrilled blocks nearby. Anadarko has already made discoveries on its Cheyenne and Mondo prospects in the Lloyd Ridge area, as well as satellites to its Atlas field. Spinnaker Exploration and Total have announced a find on the Calisto prospect nearby, in 7,800 feet of water in Mississippi Canyon. In addition, several producers plan to tie discoveries back to the hub.

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