FLOATING OIL PRODUCTION UNIT SLATED IN SMALL FIELD OFF GABON

The first U.S. tanker converted to a floating production, storage, and offloading (FPSO) unit will take up station in Gombe-Beta field off Gabon by Dec. 1. FPSO Ocean Producer will work under a 3 year, day rate contract let late in 1990 by Amoco-Gabon Gombe Marin Co., a unit of Amoco Production Co. (OGJ, Dec. 24,1990, p. 27). Gombe-Beta field is in the Atlantic Ocean about 70 miles south of Port Gentil, Gabon.

Oct. 14, 1991

7 min read

The first U.S. tanker converted to a floating production, storage, and offloading (FPSO) unit will take up station in Gombe-Beta field off Gabon by Dec. 1.

FPSO Ocean Producer will work under a 3 year, day rate contract let late in 1990 by Amoco-Gabon Gombe Marin Co., a unit of Amoco Production Co. (OGJ, Dec. 24,1990, p. 27).

Gombe-Beta field is in the Atlantic Ocean about 70 miles south of Port Gentil, Gabon.

Ocean Producer will be moored in 50 ft of water 3.7 miles off Gabon, with Gombe-Beta's unmanned production platform about 820 ft astern. The vessel will be held in position by a disconnectable, asymmetric, six point, spread mooring system.

It is owned and operated by Oceaneering International Services Ltd. (OISL). Affiliate Oceaneering Production Systems (OPS) converted the 78,061 dwt oil tanker MT Baltimore Sea at a capital cost of $25 million at Gulf Copper Manufacturing Corp.'s Port Arthur, Tex., shipyard. Both companies are units of Oceaneering International Inc., Houston.

OPS said Ocean Producer's use in Gombe-Beta field is the shallowest water FPSO application in the world.

Amoco-Gabon chose an FPSO production system for Gombe-Beta because it expects the remote field to have a short economic life, and the oil requires extensive processing.

Ocean Producer is scheduled to sail in early October for Lisbon, where it is to be painted in about 1 0 days. It is expected to depart about mid-October for Africa and arrive on station by mid-November.

The ship is certified by the American Bureau of Shipping (ABS) as a floating production system/offloading system and an oil carrier. The 789 ft FPSO is registered in Liberia.

MT Baltimore Sea was a working oil carrier in February 1991 when OISL bought it from Amoco.

PRODUCTION PLAN

Two 6 in. flexible flow lines will run 500 ft from Gombe-Beta's unmanned production platform to subsea anchor points, then 320 ft to the FPSO. From anchor point to FPSO, each flow line will float in a double lazy S shape to provide enough slack to offset FPSO motion without threatening the integrity of flexible risers connected to permanent piping feeding the process plant inlet manifold.

Ocean Producer's cargo capacity is 524,500 bbl. However, to keep from exceeding an operating draft limit of 35 ft, OISL will limit oil storage to 364,400 bbl. At 24 day intervals, a shuttle tanker will moor bow to bow with the FPSO to offload 350,000 bbl of export oil.

The offloading flow line from FPSO to shuttle tanker will be a 1,005 ft, 16 in. floating hose composed of 29 sections. On the FPSO, the offloading flow line will be fixed to the export pipe on the port bow, upper deck. At the shuttle tanker, the flow line will end with a 25 ft barbell tanker rail hose. When not in use, the shuttle tanker end of the flow line will be tied off to the recovery marker buoy on FPSO mooring leg No. 1.

PROCESSING HIGHLIGHTS

Ocean Producer will flow Gombe-Beta oil through a three phase processing system to remove gas, water, and salt.

Gombe-Beta export oil must have an Rvp of less than 13 psi, salt content less than 25 lb/1,000 bbl, and bs&w content less than 1%. Oil content of produced water will be reduced to less than 50 ppm before discharge.

Separated gas not used for fuel on the FPSO will be piped to a 6 MMcfd flare atop a 60 ft tower on the starboard main deck, forward of amidship.

Ocean Producer's processing system is designed to handle as much as 15,000 b/d Of produced liquids and 6 MMcfd of gas. To meet ABS requirements, process equipment was placed on a 156 ft by 47 ft platform 10 ft above the starboard main deck.

Wet oil will leave Ocean Producer's inlet manifold at about 90 F. and 75 psig, then pass through the first of several heat exchangers. After being warmed to about 100 F., the crude will be fed through production or test separators where removal of gas, salt, and water will begin, then heated again to more than 200 F. At that temperature, it will be directed through a surge suction drum operating at 20 psi to remove most gas remaining in solution.

Degassed oil will be repressured to 100 psi and directed to two electrostatic coalescers. The first electrostatic treater will remove excess water from the production stream. Fresh water will be combined with coalesced oil and brine from the first treater, then be directed to the second electrostatic treater where dry oil will be separated from the diluted brine.

Dry oil from the second electrostatic coalescer will go through a heat exchanger to warm incoming crude oil, then enter storage aboard the FPSO to await offloading.

Diluted brine from the second electrostatic coalescer will be combined with produced water from the production and test separators in a surge drum, where additional fuel gas will be separated. Water from the surge drum will be pumped to a Vortoil centrifugal separator, from which clean water will be discharged overboard and wet oil sent to storage for reprocessing.

It will be possible technically for Ocean Producer's processing system to reduce discharge water contaminants to as low as 15 ppm.

The Vortoil unit is the only piece of process equipment that will not be affected by FPSO motion. All other separators are baffled to prevent ship motion from reducing process efficiency.

Mustang Engineering Inc., Houston, handled process mechanical design, procurement, inspection, and construction management services on developing the FPSO processing system.

SPREAD MOORING DESIGN

OPS said Ocean Producer will be positioned so prevailing currents and swell tend to push the craft away from Gombe-Beta's production platform.

Because of the shallow water depth at the field site, Ocean Producer's spread mooring system will act as a taut wire system rather than a catenary mooring. Also because of shallow water, the FPSO will react more as a breakwater than a floating vessel.

Ocean Producer's mooring system is equipped with quick disconnect hardware to allow the FPSO to detach quickly to sail to deeper water if threatened by storm or grounding. A mooring status monitoring system will sense and report to a console on the bridge the tension on each mooring leg and water depth under the keel.

Four mooring points are starboard, so the FPSO can take constant coastal currents of 1.5 knots from 155 south-southeast. Two forward starboard mooring lines will lead out at 57.5 and 62.5 angles and two aft at similar angles. Mooring lines will lead at 60 angles from points on the port bow and stern.

Each of Ocean Producer's six mooring lines will be about 1 mile long. They will consist of 945 ft of 3 in. ABS Grade 3 stud link anchor chain leading from a quick release mechanism on the FPSO to 4,352 ft of 31/4 in. independent wire rope core (IWRC) wire rope coupled to 150 ft of 4 in. IWRC wire rope which ends at an 11 ton anchor. A lighted recovery buoy of 4,000) lb buoyancy will be affixed at each point where an anchor chain joins the wire rope.

Shuttle tankers will be tandem moored at a 30 angle, with bows 200 ft off Ocean Producer's port bow. A tanker's bow will be attached to the FPSO by a 200 ft grommet 18 in. in circumference with 30 ft of 3 in. chaffing chain at each end. A tanker stern will be secured with tanker supplied mooring lines to two permanent, lighted buoys of 4,000 lb buoyancy, connected to 9 ton anchors by 650 ft of 23/4 in. stud link chain.