SOUTH CHINA SEA FIELD SEES UNIQUE PRODUCTION, FLOW LINE INSTALLATION

John F. Sheridan, Simon J. Edis
Clough Stena (Asia) Joint Venture
Nedlands, West Australia

Use of a unique configuration of vessels to install floating production and flow line systems in a South China Sea field indicates the arrangement can be used for deepwater installations.

The installation's 360-m water depth set a world record for a catenary anchor-leg mooring (CALM) SBM installation.

Terminal Installations Inc., a member of the SBM Group of Companies, and Alcorn (Production) Philippines developed the West Linapacan field with installation of a floating production, storage, and offloading (FPSO) production system and associated flow lines. The field lies 50 km northwest of Palawan Island in the Philippines (Fig. 1).

In January 1992, Terminal Installations and Alcorn awarded Clough Stena (Asia) joint Venture two contracts for development of the West Linapacan field. The contracts required installation of a six-point CALM system, tensioning of anchors, and hookup to the 127,000-dwt converted tanker, FPSO II.

The requirement of the flow line installation contract was to connect the FPSO 11 to three subsea wellhead completions.

Of particular interest was the installation method used by Clough Stena, which is a 50/50 joint venture between Clough Engineering Group Ltd. (Australia) and Stena Offshore Ltd. (U.K.). The construction spread was based on a dynamic-positioning vessel and a flat-top construction barge moored together for the duration of the installation.

Installation with this DP/barge system represents a workable alternative to moored derrick barge-based installation spreads for FPSO systems.

Installation of the flow lines incorporated the layaway technique for flow line initiation.

WEST LINAPACAN HELD

The field was discovered in October 1990 by Alcorn (Production) Philippines, with initial drilling results indicating a marginal field of up to 108 million bbl of oil.

The subsequent feasibility study indicated that the most viable development option was a production system based on three subsea wells with individual flow lines connected to a moored buoy and storage tanker. Expected production rate was approximately 18,000 b/d. Fig. 1 shows a conceptual arrangement of the field's production system.

The rapid development of the field was facilitated by the use of the existing FPSO system, FPSO 11, which had been recently disconnected from the depleted Cadlao field also located in the Philippines.

The FPSO II featured stern-fixed yoke arms permanently attached to a CALM buoy configured for a six-point mooring system. The vessel had previously been located in water depths of less than 100 m.

Locating the vessel in 360 m of water posed additional problems to the mooring system designers, SBM. The static catenary load of each mooring leg had to be matched to the buoyancy capacity of the CALM buoy.

To overcome this weight problem, the design incorporated 5-in. wire rope into each mooring leg replacing the normally used chain.

To keep down field development costs, existing chain was used from the Cadlao field along with SBM's surPlus supplies. As a result, chain availability dictated that three different sized chains be incorporated into each mooring leg.

The final mooring-leg configuration consisted of a single 25 metric ton Ste-,,shark anchor, 250 m x 5.25-in. chain, 27 m x 5.5-in. chain, 280 m x 6-in. chain, 360 m x 5-in. wire, and 27 m x 5.25-in. chain.

Each mooring leg weighed more than 330 metric tons (Fig. 2).

The flow line configuration consisted of three 5-in. flow lines and three control umbilicals connecting the FPSO II to three subsea wellhead completions.

CONSTRUCTION PHILOSOPHY

The 360 m water depth was significant in the selection of a diving service vessel (DSV)/cargo-barge construction spread.

This water depth is generally beyond the anchoring capabilities of typical Southeast Asia-based derrick barges.

The option of mobilizing a suitable deepwater-capable derrick barge was considered uneconomical.

The alternative was to base the construction spread on a vessel with dynamic positioning capability, an option that presented problems with cargo deck space and adequate craneage.

The vessel the contractor intended to use was the DP/DSV Essar Stena 1 which lacked both sufficient deck space and craneage for the deployment of the mooring-leg components.

The lack of deck space was overcome by incorporating a flat-top construction barge 300 ft x 90 ft, into the final spread configuration to provide the additional cargo space and to act as the deployment platform for each mooring leg.

The cargo barge was moored alongside the DSV for the duration of the mooring installation via a dynamically responsive mooring "springer" system. A total free deck space of 2,000 sq m (2,400 sq yd) was thus provided between the cargo barge and the DSV with almost unlimited loading capacity (Fig. 3).

DSV/CARGO BARGE

This construction spread configuration had previously been used on another FPSO installation and proved both reliable and economically competitive.

The mooring arrangement between the two vessels had been developed based on the thruster configuration and general characteristics of DP/DSV vessels, in particular the Essar Stena 1.

The depth of the Essar Stena l's forward and aft Azimuth thrusters, approximately 6.5 m below her water line, vectored thrust well below the cargo barge hull.

Past experience with this spread arrangement has shown that mooring the barge alongside the DSV does not affect the DP stationkeeping characteristics of the vessel.

The design of the barge/DSV mooring system as previously developed on other projects was refined for the West Linapacan project to account for locally expected sea conditions. The system was based on a bow/stern springer arrangement capable of safely mooring the two vessels together in 30 knots of wind and 4-m swells.

Once the two vessels were moored together, they remained connected as one spread at all times on DP and under changing vessel heading as dictated by the various installation activities. The most severe weather conditions experienced were winds up to 25 knots and sea swells up to 2 m.

The system performed as expected with no downtime because of either mooring line or DP problems.

The installation sequence for each anchor leg is outlined in an accompanying box.

DEPLOYMENT SYSTEM

The development of the mooring-leg deployment system was driven by the lack of craneage available on the DSV/cargo barge installation spread.

Installation loads of up to 220 metric tons could be expected under maximum dynamic loading conditions with available craneage limited to a single 75 metric ton pedestal crane on the DSV and a 200 metric ton crawler crane on the cargo barge.

The installation loads therefore precluded the use of cranes in deployment of the mooring-leg components. A winch-based deployment system was developed which launched chain and 5-in. wire rope over a stern-mounted "gypsy" wheel via down-deck stroking of a multipart rigging system (Fig. 4).

When chain was being deployed, two purpose-built chain claws were located in the deployment track: one claw was fixed at the barge stern cradle, the other in a mobile cradle which moved up and down the barge deck.

The hold back for deployment was provided by the multipart rigging attached to the mobile chain claw and passed back to a winch fastened to the barge deck.

The procedure for mooring-leg deployment was to load the mobile chain claw with the static weight of the chain and anchor already overboarded and to payout on the winch.

Once the down deck stroke was complete, the load was transferred into the fixed chain claw at the stern of the barge. The mobile claw was repositioned at the top of its stroke and the procedure repeated.

The stern-mounted gypsy wheel was designed to accommodate the 5-in. wire rope and three sizes of chain. The gypsy wheel eliminated horizontal bending of the chain links and prevented the combined bending and tensile stresses in the wire rope from exceeding allowable stress limits.

Two 500 metric ton-rated wire grippers were used to deploy the 5-in. wire rope, one located at the barge stern, the other connected to the front of the mobile chain claw. Wire-rope deployment involved the same stroking operation as for chain deployment.

The most difficult deployment operation was the overboarding of the 5-in. wire rope spelter socket. The problem was that no bending of the wire was allowed at the socket-termination point.

The contractor examined several options to overcome this problem. The one selected required a tri-plate to be located between the 6-in. chain and the 5-in. wire spelter socket. As the triplate approached the overboarding gypsy, it was rigged to a purpose-built A-frame located at the barge stern (Fig. 5).

The purpose of the A frame was to raise and lower the triplate as it passed over the overboarding gypsy

The witching load for the A-frame was via a secondary multipart system attached to a second deck-mounted winch.

With the triplate derigged, deployment of the wire commenced. Once fully operating, the deployment system was capable of deploying an anchor leg in approximately 24 hr.

POSITIONING CONTROL

Accurate positioning of the spread was required for the installation of the mooring-leg components. This was achieved by the preinstallation of a long baseline acoustic transponder array linked to the field's grid coordinate system.

The system was configured to give real-time positioning of the barge stem relative to the transponder array deployed on the seabed. The DP referencing system was completely independent, using a combination of Simrad and Artemis systems.

Verbal commands between surveyors operating the subsea transponder array and the operator controlling the DSV linked the two positioning systems. Typically, the system would operate with the surveyors positioning the construction spread over a preselected point such as an anchor-deployment position.

While stationary, the DP system would maintain position without surveyor support. Once an anchor was successfully positioned on the seabed and chain-wire was being deployed, the DSV was constantly moving at a rate similar to the chain-wire deployment rate.

Surveyors monitored the mooring-leg touchdown point through an ROY system linked into the acoustic array. The surveyors monitored the "lay back" and lateral deviations from the designed lay route and corrected the spread position with verbal commands to the DP operator.

Course and position correction were input manually into the DP control system.

Use of the DP vessel's Artemis positioning system, with its fixed microwave reference station located on the nearby semisubmersible drilling rr, improved deployment rates: No downtime was lost due to resetting of the Simrad acoustic transponder system common to DP systems.

ANCHOR TENSIONING

Several options were examined for tensioning the anchors. These options included subsea tensioning and several variations of "across deck" tensioning.

After consideration of temporary chain requirements, chain up-haul loads, execution durations, and restrictions on the bending capacity of the 5-in. wire rope, a tug-pull rather than across-deck tensioning was chosen.

Each anchor leg was recovered to the barge deck and the end length of chain locked off in the stern chain claw. Four load cells were located behind the chain claw and connected to a digital-analog recording device.

The four tugs were connected to the bow of the barge, with lateral positioning during tensioning being provided by the DSV which also provided more than 50 metric tons bollard pull load to the anchor.

With the successful tensioning of the anchors, the next operation was to remove a portion of chain from those mooring legs which dragged beyond a specified range.

Because of the anchor-leg configuration, the only section of the mooring leg which could be shortened was the 6-in. chain, which meant recovery of the 5-in. wire rope, spelter sockets, and triplates aboard the barge.

During the recovery of the wire rope and chain, it was noticed that a residual torque in the 5-in. wire rope was transferring back into the 6-in. chain causing it to twist. Each leg recovered suffered from this action which produced as many as 14 tight twists in the 6-in. chain.

The contractor faced the problem of undoing 14 twists in 6-in. chain loaded to 180 metric tons. The solution was to rotate the entire construction spread about the stem of the cargo barge 14 times.

This was done relatively easily because of the DP systems' stern-rotation capability.

Once this operation was complete, the mooring system was fully installed ready for hookup to the FPSO II.

FPSO II HOOKUP

The FPSO II was towed to site and brought into the mooring pattern stern first using three anchor-handling tugs. The top section of chain of each mooring leg was recovered to the barge deck where a segmented pennant was attached to the 5.25-in. chain links.

The tanker's CALM buoy was maneuvered to the stern of the barge where the segmented pennant was passed across to the FPSO buoy via the crawler crane on the barge deck.

A purpose-built A-frame located on the SBS buoy was used to stroke the segmented pennant line through the SBS buoy chain's hawser. Once the top 5.25-in. chain links of the mooring leg were stroked through the chain hawser, the pennant was disconnected and the procedure repeated for the remaining five anchor legs (Fig. 6).

With all the anchor legs in position, final pretensioning was completed with the A-frame, and the chains were cut above the chain hawser.

With acceptance of the FPSO II hookup to the mooring system, the cargo barge was demobilized from the field and the DSV commenced flow line installation.

REEL TRANSFER AT SEA

A further advantage of the cargo barge/DSV spread for this installation was that the flow line lay preparations were completed on the DSV before mobilization to the field.

Flow line installations commenced immediately upon completion of the mooring-system installation with no spread interface problems.

The reel-drive systems, consisting of upgraded under-roller drive bases, were fully installed and commissioned on the DSV deck before sail away. By utilizing the cargo barge's deck for the mooring installation operation and using spare DSV deck space for storage of mooring and flow line materials, the preinstalled reel-drive units did not interfere with the mooring installation program.

Typically, all materials for mooring and flow lines' installation would be loaded onto the DSV and cargo barge before mobilization to the field.

Scheduling delays, however, caused several major permanent items related to the flow line installation to be unavailable for the marine spread loadout. This necessitated the charter of a heavy-lift vessel (HLV) capable of transporting and lifting flow line reels with diameters exceeding 8.5 m and weighing up to 180 metric tons.

Transport of the reels and other permanent materials was timed to coincide with completion of the mooring system installation, thereby minimizing the hire costs of both HLV and DSV.

The reel-transfer operations were performed in sheltered waters at Ulugan Bay, approximately 50 nautical miles southeast of the site. This normally weather-sensitive and delicate operation was simplified by the design of the reel driving bases.

These bases work on an under-roller principle, whereby torque is transferred from hydraulically driven steel rollers to the outside flange of the reels through contact friction between the two surfaces. They were built to accept reel loading by offshore ship-to-ship transfer.

For this type of base, attaching the reel to the drive mechanism is simply a matter of lowering the reel onto the base's driver rollers.

LAYAWAY TECHNIQUE

The 360 m water depth at site removed a range of installation options normally available, the major limitation being the exclusion of saturation diving techniques.

As a result, an alternative method to the normally diver-intensive tasks had to be employed, such as connection o(flexible flow lines and umbilicals to the subsea tree and function testing of tree valves. In this case the lay-away method was used.

The layaway technique requires the drilling rig to be set up over the nearly completed well with running tool and tree guide wires in place. The flow line installation vessel must be capable of maintaining a given stand-off distance from the rig and keep station there for several hours.

With the tree in position on the rig work deck, a winch line is passed beneath the rig pontoon (in the case of a semisubmersible rig) to the installation vessel and attached to the tree end of the flexible flow line and control umbilical.

Under ROY observation and using premarked chain-age points to monitor the amount of line payed out, the flow line-umbilical ends are transferred to the rig work deck by alternating between recovering on the rig winch and paying out on the flow line-umbilical reels.

It is paramount that the amounts of winch wire and flow line cut at a given time be closely monitored. If the catenary is allowed to get too tight, the flow line may be damaged on the pontoon; if allowed to get too loose, the flow line minimum bend radius may be exceeded causing kinking of the line.

Once the lines are on the work deck, they are attached to the tree and tested to ensure line integrity, continuity, and functionality of tree vines. If all tests pass, the tree is ready to be lowered.

This consists of a calculated sequence of moves involving coordination between lowering of the tree on the completion riser, paving out of flow line-umbilical, and vessel movements all designed to minimize the horizontal load on the tree caused by the flow line-umbilical catenaries.

During this operation, the ROY monitors the position of the lowest point of the catenary belly, comparing it to precalculated values and flagging any discrepancies so that corrective action can be taken. This operation continues until the tree is seated and locked down in its final position on the wellhead.

From here, laying of flow line and umbilical to the CALM buoy can commence.

The layaway technique for flow line and umbilical connection to the tree was ideally suited to this field's layout and development. The three subsea wellheads are located in an area approximately 2 km from the CALM buoy and are each separated by a distance of only 35 ft.

Because of the fast track nature of the West Linapacan field development, drilling activities were still being completed during installation of the CALM buoy, enabling the use of the drill rig during flow line-system installation. The close proximity of the wellheads allowed the rig easily to move location to the next well without having to reset anchors.

LAYING FLOW LINES

Initially, plans for laying lines in this water depth were that it would be impossible without use of a tensioner to take the excessive catenary loads expected during laying operations. Calculations indicated that loads in excess of 15 metric tons would be common while laying. Several factors mitigated against use of a tensioner; these included deck space requirements on an already cramped deck layout, the potential for flow line damage between the powered reel and tensioner entry, and availability of suitable tensioners.

These factors led to a decision instead to upgrade the existing under roller bases.

Load tests conducted on the modified bases indicated more than adequate reeling tensions were achievable in both the payout and reel-in modes of operation. As added insurance during field laying, the heavier flow lines used in the dynamic part of the system were laid dry to reduce catenary loads further.

Accurate laving of the lines along a predetermined route was essential to ensure that the flow line configuration in the dynamic section of the system was installed according to the design. To achieve this positioning accuracy, the long baseline acoustic transponder array previously established and used for the mooring installation was utilized.

During laying operations, the ROY with an acoustic transponder attached would be positioned at marked chainage points along the flow line already in place on the seabed. The positional information provided by the ROY was then compared to the predetermined flow line route based on as-built flow line lengths, the position of wellheads, and the final as-installed mooring center of the CALM buoy.

Any deviations from the designed route were corrected by either inducing a slight curve in the route for the case of too much line out or reducing a curve if more fine was required. With this method, the final position of each line deviated no more than 2 m from the designed route.

WIDER APPLICATIONS

Successful installation of the West Linapacan field testifies to the ability of the DSV/cargo barge spread configuration's ability to develop FPSO-based production systems in a wide range of water depths and mooring-leg configurations.

The use of DP-based installation systems for FPSO field developments has proven to be both successful and economical for project scopes which include both mooring and flow line installations.

The shortfalls of a stand alone DSV-of inadequate craneage and deck cargo space-can be overcome with use of an inexpensive cargo barge and development of deployment systems which do not rely on heavy cranes. The water depth of 360 m, while presenting problems to the anchor-positioning systems on typical Southeast Asia derrick barges, was adequately handled by the DP vessel's dynamic positioning system.

The advantages of a DSV/cargo barge spread are its ability to undertake installation works without anchor-handling tug support, elimination of risk associated with the derrick barge anchor and wire damage to subsea facilities, and minimization of construction-spread interfaces between different installation operations. The flexible configuration of the DSV/cargo barge spread allows for a quick transformation from a mooring installation spread to a standalone flow line-installation vessel. The success of the spread configuration is largely due to the particular attributes of the DSV Essar Stena 1. The vessel's power output and its deep azimuthing thrusters are well suited to a "hipped up" configuration without affecting the vessel's station-keeping characteristics.

ACKNOWLEDGMENTS

The authors wish to thank Single Buoy Moorings Inc. and Alcorn (Production) Philippines Inc. for their contributions and permission to publish this article.