SUBSEA TECHNOLOGY: 'Daisy chaining' subsea tiebacks pioneered on US Gulf facility

Process taps very small fields

While Shell and others have been exploiting excess capacity on production platforms to bring new subsea fields on line at lower cost, the Einset project in the US Gulf of Mexico takes this process to a new level by using excess subsea capacity as well. - Shell's Einset is a single well subsea tieback that ties into the subsea manifold of Southeast Tahoe, located in the Gulf of Mexico.

Large jackets and deepwater floating prod-uction facilities are very capital intensive. They require large, prolific fields to pay off the initial investment. Once the investment is recovered and the field has run its course, oil companies are finding these assets can be very useful for developing smaller fields nearby. These fields can be developed subsea and then tied back to the production facilities for processing and export. This approach has become so successful that some of the newer production systems, such as Shell's Brutus, are being designed with such future use in mind.

Daisy chaining

This hub and spoke approach, in which several subsea fields tieback to a centralized host, is not new. What is new is daisy chaining production back to a host. In this case, a new subsea tieback is linked to an existing tieback, using excess flowline capacity to reach the production platform.

Such an approach not only makes use of flowline and manifold equipment already installed and paid for, but saves on the capital expenses tied to the new field. For Shell in the Gulf of Mexico, Einset is the first example of a subsea field tying back to another subsea field, according to Steve Sears, Subsea Development Manager for Shell E&P Co. (SEPCo.).

Einset is a single-well subsea tieback, located on Viosca Knoll Blocks 872 and 873 in the Gulf of Mexico. The well is in about 3,500-ft water depth. This single well is tied back to the subsea manifold of Shell's Southeast Tahoe project, five miles away in Viosca Knoll 784. "We were able to use the infrastructure already in place," Sears said.

Production from the Einset well will tie into an available slot on the Southeast Tahoe manifold, then travel about 12 miles to Shell's Bud Lite platform, located on Main Pass 252, where it will be processed and exported. The subsea controls for the new well will extend from Bud Lite, through the Tahoe umbilical termination structure, and then along a 7-mile umbilical connected to Einset.

Pigging situation

Sears said the plan is feasible mainly because Shell is using a lot of standardized components in these projects. The wellheads on Einset and Southeast Tahoe are compatible, so there is no problem tying the one field into the other's manifold. The same goes for the umbilical terminations.

Southeast Tahoe initially flowed back to Bud Lite on two lines, but the field has gradually depleted, making one of the lines available. This second line may still be needed for pigging of the Southeast Tahoe tieback. That means whenever the line has to be pigged, both Tahoe and Einset would have to be shut in. Sears said this is a small cost, compared to the price of laying an additional flowline from Einset all the way to Bud Lite.

When pigging of the Einset to Southeast Tahoe portion of this tieback is required, new technology, in the form of a subsea pig launcher, will allow for a single flowline to be laid. This is a good example of how smaller fields rely on a cost-conscious application of new technology.

Sears said SEPCo is already designing its new subsea field solutions in anticipation of future tie-ins. While these systems are not completely built out for such connections, there are low-cost design decisions that can be made initially. These decisions make it possible to tie-in a field later. Such simple things as installing a manifold with spare slots or, in the case of a floating production solution, adding riser baskets ease the process. "We spend enough money to give us the option in the future," he said.

Future expansion

Completely outfitting a subsea development for future expansion makes sense. For one thing, the details of the future field dictate, to a large extent, how these accommodations are designed. There are questions of flow composition and rates, quality of production, and control system requirements that would dramatically affect the design of the expansion system.

Even if these variables were known, it does not make economic sense to tie down the capital budget of the host facility with a lot of equipment and capacity that would not be needed until a satellite is brought onstream. It makes more sense to tie such costs to the capital budget of the satellite field.

Initially, these potential host facilities are assessed on their own merits. These projects, such as Bud Lite, have to offer an attractive return on investment independent of future developments. Bud Lite is a good example. Sears said that since when the Tahoe and Southeast Tahoe fields were brought onstream to Bud Lite, the Einset lease had not yet been acquired, much less produced. Only as production to Bud Lite began to deplete was the idea of tying back Einset considered.

When Einset was acquired in 1998, the plan for Bud Lite was to tie Tahoe and Southeast Tahoe back. It was thought this would use the emerging excess capacity of the platform. It was fortunate that the quality of the production from Einset was such that it could be processed on Bud Lite.

At the same time, there was excess capacity in the SouthEast Tahoe system that allowed Einset to be tied in. With the addition of controls and metering equipment, Bud Lite required only minor modification to handle the additional prod-uction stream.

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