JOLLIET PROJECT PROVES FLEXIBLE PIPE FOR DEEPWATER DEVELOPMENT

William S. Tillinghast
Conoco Inc.
Houston

Hubert Lecomte, Lawnie A. Sturdevant, Philippe Cranham
Coflexip & Services Inc.
Houston

The flexible-pipe export system for Conoco Inc.'s Jolliet project in the Gulf of Mexico is the deepest flexible pipe and riser ever installed.

The project saw, as well, the first use of flexible pipe risers with a tension-leg well platform (TLWP) or a tension-leg platform (TLP).

Also, during the installation, large-diameter flexible pipe was abandoned in deepwater under threat of an approaching hurricane. The pipe was later retrieved and installation completed,

The deepwater pipeline system on Conoco's Jolliet field in the Gulf of Mexico consists of one process-gas line and one oil-water emulsion line. Flexible pipe was used for a large portion of both lines.

The flexible pipe was installed from the TLWP to approximately 6 miles away where subsea connections were made to conventional rigid steel pipelines. These flexible pipes were installed in water depths ranging from 930 to 1,760 ft in rough sea bed conditions. Flexible pipe risers were suspended from the TLWP in a simple catenary configuration.

DEEPWATER GULF OF MEXICO

The Jolliet project is a full-field development for extracting oil and gas from Green Canyon Block 184, about 170 miles southwest of New Orleans.

Twenty production wells are located on the TLWP. Processing facilities on the TLWP allow associated gas to be separated from the produced oil and water.

The produced oil-water emulsion and the gas are exported through the oil-water line and the gas-sales line to Conoco's central production platform (CPP) some 12 miles away in Green Canyon Block 52 (Figs. 1-3).

Production began in November 1989.1

The TLWP was installed in 1,760 ft of water during the summer of 1989. The hull has a displacement of 18,400 short tons with four vertical columns.

The bottoms of the columns are interconnected by pontoons providing the required platform buoyancy.

Two "I" tubes, 24 in. in diameter, are mounted externally on one of the columns through which the flexible pipe risers were pulled. A bell mouth is fitted at the lower end of each "I" tube.

The TLWP is moored to a foundation template by twelve 24 in. OD, neutrally buoyant tendons (three pipe tendons per corner). Flexible-pipe riser configurations were calculated to avoid any possible interference between the flexible pipe risers, the columns and pontoons of the TLWP, the tendons of the mooring system, and the tensioned steel production-well risers.

The CPP, located in 616 ft of water, consists of a conventional steel jacket supporting the processing facilities. Oil and gas produced at the TLWP are conveyed through the TLWP export pipelines to the CPP where the fluids are further processed and then exported through other pipelines outside the Green Canyon area.

EXPORT, FIELD SYSTEM

The flexible pipeline system consists of two flexible pipelines from the TLWP to a subsea connection point where there is a transition to a conventional steel pipeline.

A unified connection skid (UCS) is situated midway along the flexible pipeline length and is fitted with a "Y" fitting and valves and piping necessary to connect future pipelines underwater.

The "Y" fitting is symmetrical and can pass a scraper pig for removing paraffin from a dual pipeline. (This piggable Y fitting and its associated functional testing are described in Reference 2.) The steel pipeline system consists of two steel pipelines originating at the subsea connection point to the flexible pipeline system and terminating at the CPP. The steel and flexible pipelines were connected with ANSI Class 900 flanges.

The UCS (Fig. 4) is a self-contained structure with all pipe spools, connectors, and valves required for future lateral tie-ins of one gas and one oil-water pipeline.

The UCS was installed in 1,020 ft of water before the flexible pipelines were laid. The flexible pipelines were connected on both sides of the UCS with ANSI Class 900 flanges.

Produced oil and water flow through a 10-in. ID, flexible-pipe riser and on-bottom pipeline, about 6 miles long, to a point where the pipeline is connected with a 14-in. OD steel pipeline. The latter then runs to the CPP 6.2 miles away.

Gas flows from the TLWP through an 8-in. ID flexible riser and a 6-mile on-bottom pipeline to a connection with a 10.75-in. OD steel pipeline running to the CPP 6.2 miles away.

Pigs were specially configured for both pipelines to accommodate the diameter variations between steel and flexible lines.

Figs. 1 and 2 show the routing of the pipeline system. Fig. 3 shows the pipeline system through Green Canyon Blocks 184, 140, 139, 95, 96, and 52.3

As mentioned earlier, the flexible pipelines run from the TLWP to the UCS, then to the end of the steel pipelines. Extensive surveys, which used a fathometer, a subbottom profiler, and a side-scan sonar, were conducted during the early stages of the project to indicate the best routes and to allow calculation of the required lengths for the flexible lines.

Several areas of possible landsliding and complex faulting were found in Blocks 184 and 140, between the UCS and the CPP. Large areas of carbonate outcrops, some more than 50 ft high were also found in these areas.4

The pipelines therefore required careful routing within these areas. Additional extensive side-scan sonar and ROV surveys were performed a year before pipe installation to establish the final pipeline routes in this extremely rough area.

The flexible pipeline route in this rough area had to be laid in curved routes with horizontal radii as low as 100 ft and in corridors less than 40 ft wide.

FLEXIBLE-PIPELINE DESIGN

The flexible pipelines were designed and manufactured for the deepwater environment of the Jolliet project. Following are the design criteria:

Provide production service (oil and gas) at a 2,220 psig design pressure and 150 F.
Provide lifetime for more than 20 years for the flexible-pipe dynamic risers at the TLWP
Be installed with conventional flexible pipe-laying equipment.

Expected installation loads and hydrostatic-collapse resistance of the flexible pipe governed these criteria.

The major design constraints were collapse resistance when subjected to laying tension, recovery of the pipe full of water, and resistance to hydrostatic collapse when the pipe was filled only with air at atmospheric conditions.

The successive layers of the pipe structure, including material type and thicknesses, were carefully selected to meet these constraints. From inside to outside (Fig. 5), these layers are the following:

AISI 304 stainless steel interlocked carcass to provide resistance to external hydrostatic pressure
A specific type of Nylon 11 pressure sheath
Steel "Zeta" layer to provide resistance to hoop stress
Polyethylene anti-friction sheath
Double layer of cross-wound steel armors to provide resistance to the end cap effect and to accommodate laying tensions
Polyethylene sheath for external protection.

The riser sections of the flexible pipes were designed to sustain dynamic loads caused by environmental conditions and dynamic motions of the TLWP.

An additional thermoplastic layer was extruded between the Zeta layer and the armor layer of the dynamic risers. This thermoplastic layer reduces abrasion and wear within the pipe structure, thus providing an endurance life greater than the required 20 years.

The main properties of the selected pipe are summarized in Table 1.

A simple catenary configuration was selected for the riser portion of the flexible pipe. This type of riser configuration was chosen because of the relatively limited motions (in proportion to the water depth), especially in heave, of the TLWP.

Coflexip & Services Inc. performed an extensive dynamic analysis, involving specific in-house finite-element programs, which verified that a catenary configuration was suitable.

The configuration geometry of each riser was optimized. The analysis included numerous load cases of various sets of environmental conditions and TLWP offset.

The flexible lines were installed with Coflexip's dynamically positioned vessel MN Flexservice 3. The flexible pipelines were stored on reels 17 ft wide and 26 ft OD.

The 10-in. oil-water emulsion line was loaded on eight reels, and the 8-in. gas line was loaded on six.

The Flexservice 3 left the manufacturing plant in Europe loaded with seven reels (Fig. 6) required to lay the 8 and 10-in. lines from the rigid steel pipe ends to the UCS. The remaining reels were shipped via ocean freight to a support base on the Houston Ship Channel, where the Flexservice 3 was later reloaded.

Upon completion of the pipe-laying operations, the deck arrangement of the Flexservice 3 was modified to accommodate a saturation diving spread for performing six subsea flanged connections.

Two of the subsea connections were made to the rigid pipe ends, and the remaining four connections were made to the UCS.

Flexservice 3 arrived on site July 14, 1989. Pipelaying lasted until Aug. 17. Laying was interrupted by a planned return to Houston to reload pipe reels and by hurricane Chantal.

Diving operations were completed Sept. 7, and the pipelines were hydrostatically pressure tested 3 days later.

INSTALLATION

Standard flexible pipe laying techniques were adapted to meet challenges posed by this project; these challenges included the following:

A water depth of 1,760 ft leading to significant axial tensions in the flexible pipe at the pipelay vessel deck
A rough seabed terrain with narrow and curved corridors with local obstructions
Tie-in flanged pipeline connections to be performed by divers in water approximately 1,080 ft deep
Possible tropical storms and hurricanes during the installation window necessitating temporary abandonment of the flexible pipelines on the seafloor.

Both sections of the flexible pipe were laid toward the ucs.

Laying of the northern section of the flexible lines commenced at the connection point to the rigid steel pipelines. The second section of the flexible lines was installed beginning at the TLWP.

Laying operations for each pipe and each section differed only at the beginning of the pipelaying.

Flexservice 3 was dynamically positioned with two dedicated and independent "range-range" Syledis systems specifically installed for this project. Two separate Syledis networks, composed of emitters and receivers, provided complete redundancy for dynamic positioning system of the vessel.

The flexible pipe was laid in a J-lay catenary configuration. The touchdown point of the pipe, in spite of the water depth, was therefore within range of the ROV deployed from the vessel.

The ROV was positioned relative to the vessel with a short-baseline acoustic system. Two complete ROV systems were used on the vessel to provide redundancy.

The actual pipe touchdown point was tracked by the ROV when needed (for example, during laying in rough seabed areas). Visual monitoring by ROV verified that the flexible pipe was laid on its preplanned route.

Flexservice 3 could simply pick up any portion of pipe which was not satisfactorily laid, adjust its position, and then re-lay the pipe. This method, J-lay and ROV monitoring, allowed the two flexible pipes to be placed accurately within the narrow and highly curved corridors in deep water.

The pipe's flexibility and the precise accuracy to which it could be laid allowed selection of corridors in difficult seabed conditions such as rock outcrops approaching 50 ft high.

In practice, routing the flexible lines in these corridors was often adjusted during laying by a short length being picked up and re-laid along a different route.

Pipelay operations were interrupted during the passage of hurricane Chantal. With planned contingency procedures, the flexible pipe section was quickly abandoned on the seabed when the Flexservice 3's ROV disconnected the abandonment-recovery cable from the pipe pulling head.

Flexservice 3 returned to site after the hurricane passed. After the vessel located the abandoned pipe, the ROV attached the abandonment-recovery cable to the end of the flexible pipe.

The pipe was then recovered on board and welded to the next pipe section to be laid. Laying operations resumed immediately.

TO THE UCS; AT TLWP

Laying from the steel pipelines began when the flexible pipe end with its connector was lowered to an abandonment point on the seabed near the end of the steel pipe.

This was done by a clump weight being attached at the end of the flexible pipe, along with an acoustic transponder. The end connection was abandoned a few feet past the steel pipe end, thus providing a predetermined excess of flexible line near the connection point.

This excess allowed the divers later to pull in the flexible pipe for connection to the steel pipe.

Upon arrival at the UCS, the end of the pipe was abandoned in a similar manner. ROV monitoring enabled this second end to be abandoned near the UCS for later pull-in and connection by divers.

Actual pipelay was so accurate that the divers accomplished the pulls and connections with little effort.

Of the six connections made, the farthest from the flexible pipe connector to the mating flange was about 10 ft and in one case this distance was only 3 ft.

The southern sections of the flexible pipelines were laid from the TLWP to the UCS. The operation began with installation of the flexible pipe risers to conform with the design catenary configuration (Fig. 7).

With the Flexservice 3 about 200 ft from the TLWP, a messenger cable that had been preinstalled through its "I" tube on the TLWP was passed to the Flexservice 3.

The messenger line was then attached to the first end of the riser on board the Flexservice 3. A small pull-in winch on board the TLWP transferred the flexible-pipe-riser top end to the TLWP and up through the "I" tube, while the Flexservice 3 paid out the proper length of flexible pipe.

The riser transfer operation was completed when a suspension collar was attached around a groove in the riser's top-end connector and then on top of the "I" tube.

At this time, the Flexservice 3 remained dynamically positioned approximately 200 ft from the TLWP; the flexible pipe riser was simply suspended in a predetermined double catenary configuration between Flexservice 3 and the TLWP.

The Flexservice 3 then moved away from the platform while paying out the riser at a predetermined rate. Monitoring a known point on the flexible pipe in relation to transponders located on the template determined the riser's touchdown in the preselected target area.

The target area was preselected to provide the required riser-catenary configuration for the TLWP position at time of installation.

Laying of the flexible line continued towards the UCS. Upon arrival there, the pipe was abandoned for later connection to the UCS by divers.

Installation of the second riser and its associated flexible pipeline to the UCS was conducted similarly.

DECK ARRANGEMENT

Laying flexible pipes is usually carried out in the simplest way: The flexible pipe is paid out from a powered reel, passed over the stern of the vessel through an overboarding chute, and maintained in a catenary.

By continuous control of the position of the vessel and the pipe length that has been paid out, the catenary during laying can be kept in a satisfactory configuration.

Flexible pipelines are normally laid full of water for better stability of the catenaries. This method is limited, however, to shallow water where developed tensions are insignificant (for example, 10-15 tons). At such tension levels, resulting torque on the laying winch is acceptable, and the reels are not crushed in the squeeze exerted by the flexible pipe.

Because of the water depth of the Jolliet field, the laying tensions approached 30 tons. As a result, the usual laying method was invalid because the tension on the flexible pipe could not be taken by a powered reel.

The solution adopted for the Jolliet project basically provided for the tension on the pipe to be taken by two specifically built three-belt linear tensioners, each tensioner with a 30-ton capacity.

To reduce laying tensions, the lines were laid empty. In addition, the pipe was overboarded at the stern of the vessel with an 18-ft diameter sheave instead of a chute, thus significantly reducing sliding friction (Fig. 8).

Another major adjustment was made to the standard laying procedures to allow overboarding of the welded connections performed on the deck of Flexservice 3 between two adjacent flexible-pipe sections.

Connections (whether flanges, hubs, or welded) make a rigid assembly. Thus, to avoid high bending stresses at the connector contact points, connections require special handling when being passed over the stern sheave and entering the water.

With lower tensions in shallower water, this transition is managed by the pipe being held with a Kellum grip behind the connection. It is then lifted with a deck crane to relieve tension in the connection to assist its passage into the water.

The large expected pipe tensions encountered in laying the flexible pipelines on the Jolliet project required that a new overboarding method be designed. The solution involved hydraulic clamps being mounted onto a small A-frame that would rotate around the stern sheave axis.

In practice, the A-frame picked up the welded connection in the clamps which were allowed to rotate freely. In this manner, the connection was supported above the stern sheave.

The connection then rotated smoothly through an arc from horizontal to vertical without being exposed to the full bending stress created by the weight of the suspended pipe. A secondary frame prevented overbending of the flexible pipe above the connection.

As the connection reached vertical, the clamps were released and the connection entered the water. This overboarding method proved to be safe, speedy, and satisfactory.

Seven reels were installed on the deck of Flexservice 3 upon departure from France, and seven were shipped to the Houston support base. Upon completion of laying the lines from the rigid steel pipelines to the UCS, Flexservice 3 sailed back to Houston where the seven empty reels were replaced by seven full ones.

Upon return to site, pipelay operations were completed.

Flexservice 3 returned a second time to Houston where the deck was rearranged to accommodate the deepwater saturation diving system.

With completion of the six subsea connections of the flexible pipelines to the UCS and to the rigid steel pipelines, the diving system was demobilized and the deck fitted with the equipment for the next project.

CONNECTING FLEXIBLE PIPE

Six diver connections were performed when pipelay operations were completed. Four connections were made to connect the flexible pipe to the UCS in 1,020 ft of water, and two were made to connect them to the end of the rigid steel pipelines in 1,080 ft of water.

All flanges were ANSI Class 900 ring type joints. Rotating flanges were used on both ends to facilitate the divers' work.

The diving operations were performed by Global Divers & Contractors working from the Flexservice 3 in a dynamically positioned mode. Six Global divers began a 37-hr compression and stabilization period upon departure of the vessel to the field.

Operational diving started on Aug. 26, 1989.

Apart from the great depth which forced divers to work more slowly, the six connections were made in almost a routine fashion in 8 days. The divers worked in three two-man shifts during the operation.

The flexible pipe ends were pulled into position at their respective mating flanges by the divers who used only air bags and handoperated "come-alongs." Hydraulic stud tensioners were used to make up the studs.

When the connections were completed, the divers were decompressed during a period of 10 days.

When all connections were completed, the pipeline system-both steel and flexible-were satisfactorily pigged, gauged, and tested at 125% of the design pressure for 8 hr.

Note that the flexible pipe had already been subjected to a factory acceptance pressure test for 24 hr at 150% of design pressure at Coflexip's Le Trait, France, plant. Each steel pipeline had been previously hydrostatically tested for 24 hr by its installation contractor.

ACKNOWLEDGMENT

The authors acknowledge Conoco Inc. and its joint-interest owners in the Jolliet project, OXY USA Inc., a subsidiary of the Occidental Petroleum Corp., and Four Star Oil & Gas Co., a subsidiary of Texaco Inc., for their approval of the publication of this article.

REFERENCES

Langewis, C., and Koon, J. R., "Jolliet-The Project," OTC Paper No. 6359, 1990.
Decker, L., and Tillinghast, W.S., "Development of a 10 Inch Piggable 'Y' Fitting for the Jolliet Project," OTC Paper No. 6415, 1990.
Tillinghast, W. S., "The Deepwater Pipeline System On The Jolliet Project," OTC Paper No. 6403, 1990.
Campbell, K.J., Tillinghast, W. S., Roulstone, J. A., and Hoffman, J.A., "Geohazards Surveying and Complex Seafloor Conditions Along Deepwater Jolliet Pipeline Routes," OTC Paper No. 6370, 1990.