Gras Dowr joins world's FPSO fleet

The Gras Dowr FPSO is shown docked at Tees Offshore Base in the U.K.

Oil production from the fields is scheduled to begin this month. One estimate is that the fields, discovered in 1994, contain about 20-30 million bbl of recoverable oil.

The Gras Dowr, anchored in about 90 m of water, lies between the Durward (U.K. Block 21/16) and Dauntless (U.K. Block 21/11) fields, about 3.5 km from the subsea wellhead locations (Fig. 1 [41157 bytes]).

The Gras Dowr's main functions, according to Bluewater Offshore Production Systems Ltd., are to:

• Receive fluids from well risers

• Process incoming fluids to separate the fluid into crude, water, and gas

• Store dry crude oil and maintain the required temperature

• Treat effluent to allow for water discharge to the sea

• Compress gas for gas lift as a future option

• Provide chemical injection skid for process chemical injection

• Use a part of the produced gas for fuel gas, and flare excess gas

• Inject treated seawater into the injection wells

• House power generation for process and offloading operations and utilities

• Offload to a tandem moored shuttle tanker including receiving liquid fuel from the same tanker.

• Provide accommodations for operating and maintenance crews

• Allow helicopters landings and takeoffs

• Allow handling and storage of goods transported by supply vessels

• Moor a shuttle tanker

• Control the subsea wells.

FPSO design

Bluewater Offshore Production Systems Ltd. was awarded the contract for the Durward/Dauntless FPSO in December 1995. Bluewater provided the riser bases, production risers and umbilicals, overall subsea control system, and FPSO. It also has a shuttle tanker service option.

Conversion of a newly built tanker to an FPSO included:

• Decommissioning of redundant ship equipment and fabrication and installation of new ship structures such as moonpool, helideck, and turret deck house

• Turret fabrication, including a chain spider and turntable complete with manifolds and emergency shutdown valves

• Four process and utility plants weighing a total 3,500 metric tons.

  The anchoring layout provides large, interference-free anchor-leg sectors in which to install flexible risers and umbilicals.

  Bluewater describes the mooring system as an internal turret design. The turret is located close to the bow and has the following main components:

• Nine anchors to connect the nine individual mooring legs to the seafloor

• Nine composite (chain/wire) mooring legs (each about 1,050 m long) grouped in three times three mooring legs equally spaced, 120° apart

• An internal turret with spider for connecting the nine mooring legs by means of chain stoppers. The turret also carries J-tubes through which risers can be pulled in (Fig. 3 [45685 bytes]). The mooring system is totally passive and does not require any thruster assistance.

• A bearing arrangement connecting the turret with the vessel hull. This enables the tanker to weather vane 360°, taking a position with the least resistance to wind, waves and current.

• Risers for produced fluids, water injection, and umbilicals through the turret to the turntable at the turret's topdeck level

• A swivel stack for product, test product, water injection, chemical injection, and hydraulics, as well as a slip ring for powering and signal transmission in the turret.

Bluewater says the Gras Dowr is designed for a 20-year life, allowing for normal maintenance and wear and tear repairs, and is capable of retaining certificates without leaving the field for 8 years.

Bluewater expects the annual average production uptime to be in excess of 95% because the design includes sufficient redundancy for reconditioning equipment.

The Gras Dowr base was a newly built existing tanker. The FPSO's turret mooring system and the flare boom are in the forward part of the vessel (Fig. 2 [49972 bytes]).

Additional equipment for power generation and processing of crude and other fluids is located in large modules, placed on main deck supports. An offloading system is at the stern.

Bluewater selected a highly automated and flexible control and monitoring system based on digital process system technology. Essential alarms of the vessel's existing control system are integrated into the new control system. A central control room has been arranged with control and display terminals and data management system.

The FPSO requires seawater for cooling and for well injection. Also, most heat exchangers are directly cooled by seawater. Fresh cooling water closed loops are provided for diesel engines.

Two electrically driven feed pumps supply the seawater. Part of the cooling water will be discharged overboard, and part, after being boosted in pressure, will be injected into the reservoir.

Table 1 [70328 bytes] lists the vessel specifications.

Processing systems

The topside processing systems consist of three major components: crude oil separation equipment, water injection facility, and gas lift compression as a future option (Fig. 4 [60899 bytes]).

After crude oil is separated into crude, gas, and water, the crude will flow to storage tanks, and gas will be used in the power plant for fuel and as pilot and purge gas with the balance being flared. Water will be cleaned prior to discharge overboard.

The water injection facility components include filtration, deaeration, chemical injection, and high-pressure water injection pumps.

The design provides for a future gas lift facility that will include a treatment plant, compressors, knockout drums, and gas coolers to recycle the lift gas between the process plant, compressors, oil supply wells, and reservoir.

All processing and injection equipment is served by a chemical injection skid, consisting of tanks and pumps.

Power generation

Bluewater, in addition to an existing power generation plant in the engine room, installed a topside electrical power generation plant.

The plant has one diesel generator set (DGS) and three gas-turbine generator sets (GGS). The power module is on the main deck. The existing vessel fuel systems provide the diesel or heavy fuel oil to run the DGS and the fuel gas or gas oil to run the GGS.

The existing steam boiler and a new topside steam boiler unit provides steam generation. Both systems run on low-pressure fuel gas and/or liquid fuel.

Safety

Bluewater says the Gras Dowr design complies with the design and construction regulations that meet the goalsetting safety regime required by U.K. legislation such as The Safety Case Regulations. The design and construction has been verified by an independent and competent authority.

To the existing tanker safety systems and equipment, Bluewater has upgraded them and has added new systems such as firefighting, and life saving equipment to minimize the risks to a level known as "As Low As Reasonably Practicable (Alarp)."

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