Eastern GoM hub frees independent deepwater fields

Six fields are tied back to Independence Hub. The Independence Trail export line connects offshore to the Tennessee Gas Pipeline near the Louisiana coast.

Independence Hub appropriately describes the nature of the project, as independent gas producers and a third-party independent midstream company work to bring new production to market. The platform will process 850 MMcf/d of gas and 4,250 b/d of condensate. Enterprise will own the infrastructure, platform, and pipeline, which allow the producers to invest capital in their core business of drilling exploration, appraisal, and development wells. Anadarko will operate Independence Hub, as it owns the largest percentage and working interest in the anchor fields.

Project plans

In December 2003, the six independent companies awarded three front-end engineering and design (FEED) studies to Atlantia Offshore Ltd., Aker Kværner, and Technip. Technip proposed a spar platform, and Atlantia and Aker Kværner proposed a semisubmersible. Based on the contractors’ proposals, the AVP Group and Enterprise awarded Atlantia the hull and mooring system design, fabrication, construction, and dry transportation contract. Enterprise issued a letter of intent to Atlantia in early July 2004, but did not sign a final deal with the AVP Group until early November. Along with the contracts Enterprise awarded to Atlantia, the company also awarded Kiewit Offshore Services of Ingleside, Texas, the topside fabrication and installation contract, Alliance Engineering the topsides engineering contract, Allseas USA the pipeline installation contract, and Heerema Marine Contractors the hull and mooring systems transportation and installation contract.

“The contractors displayed a lot of confidence that this project was going to happen,” Bart Heijermans, senior vice president of Enterprise, says. “Some of the contractors had other opportunities for their vessels, but kept the work window available for our project. This commitment from our contractors was instrumental in maintaining the project schedule while finishing our commercial negotiations.”

Independence Hub is in ultra-deepwater in the Eastern GoM, but water depth isn’t the biggest challenge. The Eastern GoM is notorious for its severe Loop currents. “Most of the Gulf has Loop currents that start in May and run through October,” Heijermans says. “But the Eastern Gulf sees Loop currents almost 12 months of the year.”

The advantage of a semisubmersible is that the deck and topsides facilities can be installed at the quayside in a controlled environment. “Contracting with Kiewit to lift the almost 9,000-ton deck onto the semisubmersible hull was a significant step in reducing exposure for the Loop currents and reducing the risk associated with an offshore heavy lift,” Heijermans says. “The key to the design of a floater in ultra-deepwater is the design of risers. Once you have designed the risers and determined the maximum motions they can withstand, you can start designing the platform.”

Atlantia studied the most extreme motions before designing the hull. Atlantia subcontractor Jurong Fabrication Yard will build the hull in Singapore. The platform will be secured with 12 mooring lines. Each 2.4-mi line is made of 9 5/8-in. diameter polyester with 5-in. diameter chain at the top and bottom connected to the suction piles and the semisubmersible hull.

“We have contracted with Heerema to use theirBalderinstallation vessel,” Heijermans says. “It is one of the biggest vessels out there.”

In fact, the HeeremaBalderfeatures DP Class III, a large work deck area for storage of mooring components and other equipment, dual cranes, a J-lay tower with a hoisting capacity of 1,050 million tons, and the largest mooring line deployment winch in the world.

“Some operators use anchor handling vessels to install suction piles and mooring lines,” Heijermans says, “but anchor handling vessels are more exposed to the elements. TheBalderis less sensitive to the sea state conditions and therefore should be more productive, hopefully resulting in less weather downtime days.”

TheBalderwill first install the suction piles, followed by the bottom chain section of the mooring lines, then the polyester section, and the top chain section.

“We will hang each mooring line off a separate buoy while we finish all 12 of them,” Heijermans says. “Then we will bring in the platform and pull in the mooring line chain sections with the winches on the platform.”

The platform is then moored and ready to support the flowline and export pipeline steel catenary risers that were pre-installed. The risers will be recovered and connected to the platform, and after the hook-up is completed, the platform will be pre-commissioned at the quayside in Ingleside. Enterprise does not expect a long commissioning program at the offshore location.

Enterprise will also own, install, and operate 140 mi of 24-in. pipeline with capacity in excess of 850 MMcf/d. Independence Trail will deliver production from Independence Hub to the Tennessee Gas Pipeline in West Delta block 68. The company has oversized its capacity to allow for additional development from nearby undiscovered fields.

Enterprise has contracted Allseas’SolitaireDP vessel, which is the largest pipe lay vessel in its class, to lay the export pipeline.

“The export pipeline is 140 mi long, so there will always be sections where there is no Loop current exposure,” Heijermans says. “In all likelihood, we will start in the middle, and if the Loop currents are more severe in the deep section, we’ll go to the other end and start laying in the opposite direction. That way, the strong Loop currents will be less likely to delay the work.”

Hub, spoke, trail

The Independence Hub platform is a combination of a natural gas processing platform and a compressor station. The condensate produced with the gas will be re-injected into the gas pipeline. It will not process crude oil. The platform has more than 62,000 hp compression and requires 3,000-psig pressure to move 850 MMcf/d of gas from the platform through the export pipeline.

The platform also will host the largest mono ethylene glycol (MEG) reclamation system in the Gulf. The MEG that is required to prevent hydrate formation will be pumped into the umbilicals and injected downhole and/or into the flowlines at the well location. Initially six fields will tie back to Independence Hub using six riser slots on Independence Hub. The fields will produce from 15 to 18 wells, but Enterprise designed the semisubmersible for 17 risers so that additional fields could be tied back in the future.

Independence Hub will be a semisubmersible platform with 12 mooring lines and room for 17 risers.

“Based on existing gas tiebacks in the Gulf of Mexico, fields can be tied back over distances of up to 70 mi, creating a very large supply basin of more than six million acres for the platform,” Heijermans says. “We have made provisions for this with payload, space, receptacles, and I-tubes reserved for 10 future risers and eight future umbilicals. We also expect that many fields will be tied back to the anchor field manifolds, in-line sleds, and future manifolds and in-line sleds. This allows the development of smaller fields that so far have not been on the producer’s radar screen due to the lack of infrastructure in the area.”

The six anchor fields will tie back to the hub through producer-owned subsea flowlines. The field’s water depths range from 7,800-9,000 ft, and the longest tieback will be 30 mi from Independence Hub.

“We have an interest in owning some of these flowline systems that bring supply to Independence Hub in the future. And given the opportunity, we would oversize the flowline systems and add future connections points in a manner that is similar to how we have designed our oil and gas pipelines,” Heijermans says. “This would provide capacity and lower connection costs for future fields. In the case of Atwater Valley, the producers wanted the flowlines to serve their field developments and own the flowlines themselves. We are satisfied with our investment in the platform and the gas export pipeline.”

Enterprise believes that producers and midstream companies working together to develop future infrastructure is the most expedient and economical way to develop the deepwater and will establish the facilities required to make smaller deepwater fields commercial.

“A large platform like Independence Hub in the middle of a group of fields will make all the fields economical to develop. In the Atwater Valley area, some of these fields are 250-bcf fields, so they are pretty big. But with a gas field 141 mi from the shore and in 8,000 ft of water, far away from existing pipeline and platform infrastructure, 250 bcf is not enough to develop,” Heijermans says.

By aggregating several fields into one hub, marginal or uneconomic fields in the area can become commercial. Enterprise believes there will be more developments similar to Atwater Valley in all areas of the GoM.

“This is our fifth platform development and our third deepwater platform development, and we believe there are more opportunities for this kind of project,” Heijermans says.