INNOVATIVE PRODUCTION SYSTEM GOES IN OFF IVORY COAST

John Barnes
Paragon Engineering Services Inc. and UMC Petroleum Corp.
Houston

The phased field development of the Lion and Panthere fields, offshore the Ivory Coast, includes a small floating production, storage, and offloading (FPSO) tanker with minimal processing capability as an early oil production system (EPS).

For the long-term production scheme, the FPSO will be replaced by a converted jack up mobile offshore production system (MOPS) with full process equipment.

The development also includes guyed-caisson well platforms, pipeline export for natural gas to fuel an onshore power plant, and a floating storage and offloading (FSO) tanker for oil export. Pipeline export for oil is a future possibility.

This array of innovative strategies and techniques seldom has been brought together in a single project.

DEVELOPMENT PLAN

Faced with economic constraints and a demanding schedule for first gas delivery, operator UMC Petroleum Corp. established a fast-track project plan that relies on strong contractor performance for its execution.

The development includes the Panthere gas field in 270 ft of water and the Lion oil field in 235 ft of water, both in Block CI-11 about 60 miles west of Abidjan and 8.5 miles offshore.

Oil,was first found in the Lion field by Phillips Petroleum Co. in 1980, but the company abandoned it as uneconomical. UMC discovered gas at Panthere in December 1993 and revived Lion oil prospects with a successful well in March 1994.

UMC contracted Paragon Engineering Services Inc. to coordinate overall field development design work. Oceaneering Production Systems (OPS) is providing the jack up MOPS unit and is the project manager for its reactivation, overhaul, conversion, tow to West Africa, and installation.

OPS production personnel have been responsible for field start-up and are currently in the field performing day-to-day operations.

The CI-11 location is remote from existing oil field infrastructure, the nearest facilities being in Nigeria about 700 miles away. The cost to mobilize and operate heavy marine construction equipment in the area is high. It thus became a project priority to avoid this sort of equipment, if possible, a decision that influenced the selection of production facilities.

PRODUCTION FLOW

The development plan is based on a jack up MOPS central production facility (CPF) and three caisson well protector platforms: Lion A, Lion B, and Panthere A (Fig. 1) (41642 bytes). All can be installed without a large derrick barge.

The CPF will be adjacent to the Lion A platform. A connecting bridge will support a flow line from Lion A to the CPF and also provide personnel access.

A 1-mile subsea flow line will move additional oil to the CPF from the remote Lion B platform. Gas from the remote Panthere A platform will be transported to the CPF through a 2.5-mile subsea flow line.

Processed oil will be pumped 0.6 miles through a 6-in. subsea flexible flow line to an FSO tanker moored to a catenary anchor leg mooring (CALM) buoy. Shuttle tankers will periodically offload oil from the FSO for export, using either tandem or STS (ship-to-side) loading methods. Pipeline export to a shore terminal remains a future possibility for CI-11 oil.

Treated gas will flow through a 14-in. pipeline from the CPF to the Abidjan electrical generating plant. The pipeline will consist of an 8.5-mile subsea segment from the CPF to a shore pigging station and a 54-mile onshore segment paralleling the beach about 300 yards inland to the generating plant.

FAST TRACK SCHEDULE

The extremely fast-track schedule (Fig. 2 (69531 bytes)) was necessary because of a competing gas production project that had started 2 years before UMC decided to develop CI-11.

UMC had first to ascertain, and then convince Ivory Coast authorities, that it could complete CI-11 development and begin gas deliveries within the same time frame as its competitor and also supply the gas at a lower price. A target date of October 1995 was established for the start of gas deliveries.

To meet this tight schedule, procurement of long-lead equipment began with the start of design work on May 1, 1994. Construction of the large equipment modules and the awarding of the contract for structural steel occurred in late August, when the design was only 90% complete. The pace quickened further with the beginning of the jack up MOPS conversion in February 1995.

EARLY OIL

At each discovery well, the drilling rig installed a guyed-caisson well protector structure. These permanent structures are designed for 100-year environmental conditions. The caissons were shipped to Abidjan in 80-ft sections aboard cargo ships and then welded together after arrival. This avoided the cost of leasing a barge and tug to tow a complete 400 ft long caisson to West Africa from the U.S. Gulf Coast or Europe.

The assembled caisson was floated using its own buoyancy and towed to the field for installation by a cantilever jack up drilling rig. The three guy cables stabilizing each caisson were installed from a four-point moored or dynamically positioned (DP) workboat using vibratory hammers to drive anchor piles.

Three guyed-caisson platforms are currently in place and others are planned.

With the Lion A caisson in place, UMC decided in January 1995 to start early oil production prior to installation of the CPF. The elements of the early production system (EPS) include:

• Separation equipment and a gas flare on the Lion A platform

• CALM buoy

• Minimal FPSO

• Flexible flow line between Lion A and the FPSO.

Production started in April 1995 at an initial rate of 10,000 bo/d. Heavy marine equipment was not needed for installing the early production system.

For the FPSO, UMC selected the 58,000-dwt tanker Red Teal owned by CARE Offshore. Because the vessel was unsuited to handle crude oil with a Reid vapor pressure (Rvp) in excess of 11 psia, it was necessary to install oil stabilization equipment on the tanker deck.

This equipment includes heat exchangers to warm the incoming oil to 1750 F., a flash separator, a flare scrubber, and a gas flare. All equipment was purchased used in the U.S. and shipped to Lisbon, Portugal, where it was installed aboard the Red Teal. Det Norske Veritas (DnV) conducted the design review to reclass the tanker as an FPSO.

A CALM buoy previously used by Elf offshore Cameroon was purchased and refurbished in an Abidjan shipyard for installation in 200 ft of water. The buoy's seals and piping were modified to handle crude oil with some dissolved gas.

The flow line between the Lion A platform and the buoy is a used length of Coflexip 6 in. and was installed with a dynamic positioned (DP) workboat.

JACK UP MOPS

A jack up MOPS meets two principal CI-11 project requirements: the MOPS fits UMC's fast-track schedule and it can be installed without heavy marine equipment.

To avoid slipping the schedule, design and construction of the production equipment for the MOPS was started before final rig selection. The design placed the equipment and much of the interconnecting piping on two large modules sized for a wide variety of jackup rigs.

To facilitate placement, the module design allows either lifting or skiding. The larger module has two levels and is 67 x 87 ft, with a height of 35 ft and a 750 ton lift weight. The smaller module is single level, measuring 50 x 87 ft and weighing 275 tons.

Module construction started in October 1994 when only major structural steel drawings were available and piping drawings were under development.

The production facilities are designed to handle 100 MMscfd of gas, 30,000 bo/d, and 16,000 bw/d. Space is provided for future installation of water injection and gas lift equipment.

The oil production equipment includes two separators and a 10 x 45 ft electrostatic heater treater. Ballast tanks built into the hull of the MOPS have been converted to oil surge tanks for pumping export oil.

Water-treating equipment includes a water-skimmer vessel, hydrocyclone, two CPI (corrugated plate interceptor) units, and a sump tank.

Gas production equipment consists of a high-pressure separator, two 50-MMscfd glycol dehydration trains, a 1,100-hp reciprocating low-pressure compressor, and a 5,000-hp Solar Centaur intermediate-pressure gas compressor.

JACK UP SELECTION

In January 1995, UMC's search for a suitable and economical jack up focused on the Gulftide, an independent-leg drilling unit available for lease-purchase from OPS. This four-leg rig had the deck space and load capacity to accommodate the required production equipment. Also, its independent-leg design was more suitable to the seabed conditions at CI-11 than several available smaller mat-supported jack ups.

The Gulftide, built in 1967, served as the world's first MOPS for Phillips' early production system at Ekofisk, producing the North Sea's first oil in 1971. At that time, the Gulftide could handle up to 44,000 bo/d from four subsea completions.

In 1976, the Gulftide re-entered the drilling fleet after being replaced by a permanent platform at Ekofisk. The rig was wet-towed to Newfoundland in 1979 and to the Gulf of Mexico in 1980, where it remained stacked since 1987.

Prior to a final determination on the Gulftide for CI-11, a thorough analysis was made, including:

• Detailed on-site structural inspection using non-destructive testing (NDT)

• Underwater inspection by divers

• Review of the preliminary analysis by the American Bureau of Shipping (ABS) to determine any barriers to MOPU reclassification

• Methods for transporting the unit from the Gulf Coast to West Africa.

The Gulftide required redesign of only minor equipment components, such as piping spools and riser supports.

CONVERSION SCOPE

Conversion of the Gulftide began in March at HAM Marine Inc.'s yard, Pascagoula, Miss., with OPS providing project management. Shipyard crews worked around the clock through the end of June to complete the job.

Major tasks in the conversion process included:

• Clearing drilling equipment from the deck

• Renovating the quarters and associated utilities, such as HVAC (heating, ventilating, and air conditioning).

• Relocating hull hatches and providing access to accommodate production modules and skids

• Converting hull liquid storage areas to clean oil storage tanks, wet storage tanks, and methanol storage tanks

• Adding instrumentation, piping, and pumping units to the converted tanks.

The hull was reinforced, renewed where necessary, and painted. A new hull ventilation system was designed to maintain positive pressure in the hull.

The legs were thoroughly inspected, refurbished, and painted. The spud cans were elevated out of the water for inspection and repairs.

To reduce work on site at Lion field, risers were installed in the rig legs. The risers include a 14-in. gas export line, an 8-in. oil export line, two 8-in. well-fluid flow lines, one 2-in. methanol injection line, one 2-in. gas lift line, and two fire-water pump suction lines.

CONVERSION WORK

Reactivation of the Gulftide began on Feb. 15 when a 10-man start-up crew proceeded to the rig with one workboat and a lift boat. Offshore tasks included making the hull water-tight, commissioning the jacking system, and readying the unit for a tow to the yard, with the objective of obtaining a "Permit to Proceed" from the U.S. Coast Guard.

At the same time, bids for the conversion work were solicited from six Gulf Coast shipyards. With a fast-turnaround request for bid (RFB), two yards were shortlisted, and HAM Marine was selected 13 days after the Gulftide was boarded. On the 17th day the rig was under tow, arriving on Mar. 6, and 8 hr later the derrick had been removed.

At the yard, removal of the additional drilling equipment and substructure continued in preparation for the overhaul of the rig and installation of production equipment. The aft end of the living quarters was removed to make room for the production modules. About 550 tons of steel and equipment were eventually removed.

Equipment such as mud pumps, dc generators, and centrifugal pumps was not removed, either because it would be used aboard the MOPS or because its resale value was lower than removal costs, and its removal was not required for space or weight considerations.

Classification society certification is a key factor in MOPS conversion projects. The Gulftide had been classed MODU + IA and is being reclassed as a "site-specific production unit-structure only," for a 10-year design life and rated to withstand a 100-year storm at the CI-11 location.

In addition to the reclassification, the unit had to undergo its sixth special survey, have its load line renewed, obtain a tonnage certificate, and obtain Liberian registration. The rig had not been registered previously.

The shipyard schedule was driven by a target date of Aug. 15 for completion of installation at the Lion A site in CI-11 a date which required departure from the yard no later than the fourth week of June, depending on the transport method selected.

The yard work was performed in two phases. The first involved recommissioning, repair, overhaul, and preparation of the rig for production equipment installation. Phase 2 included installation of the two large production modules, additional off-skid equipment, interconnecting piping, instrumentation and electrical controls, and the various risers and flare lines.

During March and most of April, inspection and repair of the legs and hull dominated the schedule. Over 8,500 ultrasonic thickness gauge readings were taken on the legs, deck, and the hull side and bottom plating. Sixty leg members and/or joints were replaced or repaired, but none of the main chords.

Other than inserts to replace deck and cargo hatches, ventilation outlets, booby hatches, piping penetrations, etc., little deck or side shell replacement was required. This was largely due to the strong original North Sea design of the rig. About 1,600 sq ft of bottom shell plate was replaced because of severe pitting.

Every void tank and compartment was completely cleaned and inspected, with little repair required. The four legs were inspected, repaired, and painted simultaneously for schedule reasons, at the same time risers were installed.

A cribbing barge was placed under the rig to allow spud cans to be raised out of the water one at a time for inspection and repair. The jackhouse structure was inspected at six different levels. Numerous small cracks in the uppermost and lowest levels were readily repaired. A new heliport was added to accept a Bell 212 or equivalent.

A 10-year anode system was installed to protect the legs, and the splash zone from 225 to 275 ft on the legs was given a three-coat protective coating system. The main deck, superstructure, hull sides, and bottom were painted. The hull interior was in remarkably good condition and required only touch-up.

The quarters, originally sized for 83 people, were reduced to accommodate 20 people plus hospital and were completely stripped and re-outfitted.

An A-60 bulkhead was placed as part of the aft portion of the refurbished quarters. A large portion of the second quarters deck was converted to a motor control center (MCC) for the production instrumentation and electrical controls. The redone quarters also included a fire and gas monitoring system and a new sprinkler system.

Two covered lifeboats and their davits were totally reconditioned and upgraded to current standards.

Because the rig will have to support field activities for 6-8 weeks upon arrival prior to start-up of gas production, the entire marine system was overhauled and repaired. Two of the three prime movers and the ac and dc generators were reconditioned. The dc generators are required to drive the mud pumps if it becomes necessary to jet the legs.

Switchboards and MCC units required for rig operation, the emergency generator, and below-deck lighting were overhauled and reconditioned. A new ventilation system was installed that will result in positive pressure throughout the lower hull. A new dual deep-well saltwater system was installed in the port forward leg. Two new hydraulic cranes were installed to replace the mechanical cranes that were removed.

A major challenge was the overhaul and commissioning of the jacking system to the high degree of reliability felt to be necessary for installation adjacent to the Lion A platform at the CI-11 location, where high currents and large swells are common. The positioning tolerance for the installation is tight, and the operation will have to be performed quickly without reservations about jacking system problems.

In addition, the weight of the new MOPS had increased the jacking system working pressure requirement. All 24 large jacking cylinders plus two spares were pulled, along with all pin cylinders, and most of the control valves were replaced or overhauled. The large volume of hydraulic fluid was centrifuged and cleaned up. The electrical jacking control system was completely rewired, with replacement or overhaul of all 74 solenoids and 128 relays. The entire system was function tested and the jacking system exercised extensively prior to loadout.

Safety systems were expanded with addition of a CO2 engine shutdown and engine room fire fighting system. A new fire and abandonment plan was created and appropriate equipment was installed in general compliance with Solas (safety on life at sea) and Gulf of Mexico standards.

Phase 1 activities continued up to loadout at the end of June. Phase 2 work began in April with installation of high-pressure riser pipes in the starboard forward leg. This work continued to loadout.

Phase 2 activity increased with the arrival of the two large production modules aboard a transport barge. Both modules were skidded onto the rig and secured in a 13.5-hr operation on April 28 (Fig. 3).

With the modules in place, off-skid equipment, interconnecting piping, instrumentation, and electrical systems were installed. Four hull tanks were converted to surge tanks for wet and dry oil and methanol. About 1,300 tons of new equipment were added to the rig, considerably increasing the lightship weight.

The overall conversion plan called for completion of all Phase I work and the major portion (85%) of Phase 2 work before loadout of the rig for the ocean transport. This aim was accomplished on June 21.

TRANSPORT AND INSTALLATION

Three options were evaluated for transporting the new MOPS to West Africa:

1. Wet tow by a tug

2. Dry tow aboard a 100 ft x 400 ft barge

3. Dry tow aboard a heavy-lift ship.

The wet tow was discarded because it would require 60 days and would expose the production equipment to a high risk of damage from waves and swell.

The barge tow offered no advantage in speed because the unusual shape and dimensions of the Gulftide made it impossible to load the rig aboard a barge with the spud cans out of the water. The drag created by the spud cans lengthened the tow again to 60 days.

Dry tow aboard a heavy-lift ship would require only 24 days and would offer greater safety during the hurricane season than the other two methods.

The Ivory Coast offers no protected area offshore that will permit offloading the MOPS unit from the heavy-lift ship. Thus, an offloading site at Bioko Island, Equatorial Guinea, was selected. This will leave a 700-mile wet tow by two large tugs to the Ivory Coast.

The jack up MOPS must be installed at CI-11, 10 ft from the Lion A platform with a tolerance of 2 ft (Fig. 4 (34066 bytes)). Achieving this precision will be challenging given that the area experiences persistent heavy swell and currents up to 3.5 knots. The jack up must also be positioned in relation to the three guy cables extending out from the platform caisson (Fig. 5 (25634 bytes)). Leg penetration of about 18 ft is anticipated at the site.

Prior to installation, bathymetry, magnetometer, and sidescan surveys will provide precise seafloor profiles and locations for all in-place facilities.

Four tugs will hold and position the Gulftide during installation operations. Multiple position reference systems will be used during the installation to provide redundancy. Global positioning system (GPS) satellite references will provide primary position information aboard the tugs and rig until the rig is 200-300 ft from the Lion A platform.

For the final precise positioning, a long-baseline underwater acoustic system will serve as the primary reference, backed up by a sector scanning sonar system that scans from the rig to the platform and vice versa.

The Gulftide conversion presents a fine example of the fast-track scheduling and execution that typify MOPS projects in the mid-1990s. Engineering, procurement, construction, and equipment installation occurred simultaneously or overlapped throughout the entire project. All schedule milestones were met, thanks in large measure to the shared understanding of project objectives and the close and cooperative relations among the field operator, the rig owner, and production contractor; and the engineering firms, equipment suppliers, and fabrication yard.

Copyright 1995 Oil & Gas Journal. All Rights Reserved.