Retractable column floater could shift deepwater development

Primary separation for marginal fields

Aug 1st, 1999

Marginal field production in deep water is the goal of a newly designed, patent pending, floating satellite separation platform (FSSP) developed by OPE. The platform actually combines many familiar features of existing solutions into what the developers claim is an inexpensive and versatile design.

Marginal fields in deepwater are a strong motivator for novel, minimal floating platform designs. The fields often have great potential, but distance from shore, or more specifically, the distance from the pipeline grid, renders the field difficult to develop economically. Whether the development calls for dry trees or wet trees, it is very expensive to install a taut or semi-taut floating platform in deepwater to process and transport gas and oil. It is also very expensive to install a stand alone subsea system that accomplishes the same goal.

Development costs can be offset when the field is very large or the prospects are close enough together to be pooled. The Spar, TLP, and mini-TLP designs have offered some alternatives for these high costs, but each of these solutions requires just the right combination of production rates, water depth, and composition to be profitable.

Marginal opportunities

OPE says the FSSP could open up a new class of marginal fields to production, because it is a small, floating platform and can be constructed and installed quickly. The FSSP acts as a gathering platform for subsea wells and provides first stage separation of satellite fields, and can handle as many as six satellite wells. OPE Vice President Rick Haun said OPE designed the FSSP to take advantage of this niche in marginal deepwater field development.

The initial design of the platform shows a hexagonal deck 120-ft in width. The six deck sides alternate for either mooring or cantenary risers and umbilicals providing large lateral separation between mooring lines and risers.

This design avoids interference between the mooring system and the steel cantenary risers (SCRs) tied back to the floating platform. The FSSP can be strategically located so that lengthy pipeline tiebacks will not be required and results in greater flow assurances. The FSSP is also designed to accommodate pigging units and chemical injection systems to keep the flowlines clear.

Below the three-level deck, the hull tapers and then spreads out below the waterline. This forms a skirt to dampen motion. Six

vertical separators, effectively insensitive to motion, occupy the center of the FSSP. The separators are part of a 250-ft center column and take advantage of the column's length to overcome the long retention times often associated with vertical sep arators. Each separator can process up to 50MMcf/d of gas or 30,000 b/d of production. Because each of the wells has its own separation system, the separators can serve the dual function of being test separators and production separators. The center column itself has a mat at its base that dampens heave. The column can be retracted or extended to accommodate shallow dock sides and also to fine tune the heave, once the FSSP is anchored in place.

Retractable column

For transport, the column is fully retracted and extends above the FSSP deck. This feature allows the FSSP to be constructed at a fabrication or shipyard with a minimum of 35 ft of draft dockside. The design integrates the deck and hull so there is no need to mate topsides and hull offshore, simplifying the installation.

The FSSP is designed to operate with SCRs, which are far less expensive than flexible flow lines. The problem is these rigid risers don't accept motions readily. One of the challenges OPE had to overcome was developing a connector that could be used to tieback the SCR and accommodate the motion of the FSSP. While there are other connections in the market place that accommodate some motions, Jim Adams, Manager of Business Development for OPE said his company set out to create and patent its own design capable of handling greater motions as well as the specific needs of the FSSP and other floaters in operation today.

The OPE designed SCR Connector itself is the subject of a separate joint industry project (JIP) OPE is involved in with DNV and ABB Vetco Gray, who is scheduled to construct the connector. While the FSSP is ready for tank tests and certification, OPE is looking for an oil company to become involved in the testing, certification, and construction of the first FSSP. OPE President Gary Quenan said once a buyer is lined up, the FSSP could be constructed in 12-14 months. It is possible the testing and certification may take the form of a JIP, but Quenan said he would prefer to simply test, construct, and install the FSSP for a customer.

Installation

Using a conventional mooring system, the FSSP can be towed out and hooked up, as well as relocated in the future, using a tow vessel and anchorhandling boats. The initial design is for a GOM-type environment up to 8,000 ft water depth. It would handle 3-6 satellite wells and

perform the initial or full three-stage separation. The water would be treated and discharged, while the oil and gas would be exported to a shore-based facility or a host platform. Initial applications will be in the Gulf of Mexico or offshore West Africa. With modifications, OPE said the FSSP could handle more wells and operate in the North Sea or other harsh environments.

The FSSP also can be configured as a well-gathering platform, a utility work platform, remote power or communication transmission hub platform, relay platform, or early phase development.

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