Robert D. Pickard
Enserch Exploration Partners Ltd.
Houston
Production equipment and pipelines for a major U.S. Gulf of Mexico deepwater project are under construction or being fabricated this summer as it moves toward a 1995 startup.
Contractors for operator Enserch Exploration Partners Ltd.'s Garden Banks Block 388 project are nearly finished fabricating the subsea production template, floating production facility, and production riser.
The shallow-water jacket for the processing decks to receive produced oil and gas was installed in April; deck fabrication is nearing completion in Morgan City, La.
And, pipelay for fuel lines and transmission pipelines got under way last month.
Enserch Exploration is having the semisubmersible drilling rig Enserch Garden Banks, formerly Glomar Biscay I, converted to a floating production facility to perform simultaneous drilling and production operations.
The production facility will be permanently moored over a two-well template on Garden Banks 388 in 2,190 ft of water. A free-standing production riser will link the rig to the template.
Two pipelines will transport production 54 miles to a processing platform in 250 ft of water in Eugene Island Block 315.
A map of the area appears in Fig. 1a; the development plan, in Fig. lb.
Enserch Exploration Partners is a limited partnership owned 99.2% by Enserch Corp., Dallas, and it holds a 100% interest in Block 388 unit.
DRILLING; CONTRACTS
Garden Banks Block 388 is located approximately 250 miles southwest of New Orleans. Water depths ranging from 2,100 to 2,400 ft over the block precluded conventional development methods.
Several studies conducted by Enserch beginning in 1990 led to the choice of a floating production facility because such a development approach would allow initial production in the shortest amount of time.
The unit, consisting of four blocks, was first drilled in 1989 when Exxon Co. U.S.A. was the operator. The discovery well cut 250 ft of net oil pay in sandstone confirmed productive by well logs, cores, and wireline.
Enserch Exploration subsequently acquired all interest in the unit in 1992 and proceeded to drill the fifth development well on Block 388 (OGJ, June 29, 1992, p. 42).
Enserch has reported proved reserves at 27 million bbl and ultimate reserves of 40-50 million bbl, possibly as much as 100 million bbl.
Enserch Exploration purchased the Glomar Biscay I, renamed Enserch Garden Banks, and the free-standing production riser in December 1992. Contracts for the detailed engineering design were awarded beginning in January 1993.
PMB Engineering, Houston, was selected for the template design (OGJ, Apr. 12, 1993, p. 34), Aker Omega Inc., Houston, for the rig conversion, CBS Engineering, Houston, for the shallow water platform, and Brown & Root Marine, Houston, for the pipelines.
Cooper Oil Tools, Houston, was awarded the design and fabrication of the subsea trees and the free-standing production riser. FSSL, Houston, will design and manufacture the control system components.
The American Bureau of Shipping (ABS), Houston office, was awarded a contract to perform certified verification for the template, riser, rig modifications, and the rig mooring system.
FLOATING PRODUCTION
The contract for the shipyard work necessary to convert the Enserch Garden Banks to floating production was awarded to HAM Marine/PMB Engineering joint venture in January.
The modification work is being done in Pascagoula, Miss. The first large components, the pontoon extensions, were installed on the rig on May 26. Fabrication of the other components, such as the new quarters and production decks, is in progress. Structural strengthening throughout the rig as well as the crane installation have been completed.
The Enserch Garden Banks, shown in Fig. 2 before conversion work, is being modified to support the production operations while maintaining its full drilling capability. The original variable deck load has been maintained through the addition of floatation.
Major modifications to the rig are the addition of processing facilities, an upgrade of the mooring system to a 12-point system, and an increase of the quarters accommodations to 110 persons.
Also, part of the modifications are additions of sponsons and blisters, a top-drive unit, a third crane capable of handling the production riser joints, and porch structures on the pontoons to support the flexible pipes coming from the production riser.
The Garden Banks will retain its Panamanian flag status and be an ABS-classified rig.
Production facilities will be added to the forward portion of the rig on new decks designed for this purpose.
Full well stream production from the wells will be flowed to a manifold system on the rig. The production will go through three-stage separation with the oil being flashed down to 100 psi then pumped up to delivery pressure so that no gas is flashed from the oil on its way to the processing platform.
The flashed gas from the oil during pressure reduction will be compressed and returned to the gas stream then dehydrated to 4 lb H2O/1 MMscf of natural gas.
These measures are to prevent hydrates from forming in the gathering lines that connect the production facility to the processing platform.
The existing 8-point mooring system is being upgraded to a 12-point system designed to moor the vessel on location permanently. New mooring components will be located on four of the sponsons added to the rig.
The new mooring system will differ entirely from a conventional semisubmersible mooring system because the Enserch Garden Banks must maintain a tight watch circle for the production riser.
For normal operations, the offset will be no more than 6% of water depth. The maximum offset in hurricane conditions will be 10% of water depth. The system is designed to remain intact with the loss of one mooring leg in the most extreme storm conditions.
Additionally, the system allows the rig to be move over the template well locations and the three existing well locations.
Structural modifications to the rig include pontoon extensions to offset the production equipment and sponsons added to each of the eight exterior columns to maintain the variable deck load.
Two new production decks are being added to the forward portion of the rig. As a result of a global analysis and fatigue study, some strengthening of the existing structure will be required.
Additional personnel required for dual operations has resulted in the addition of quarters and office space which are being added on top of the existing quarters. A helideck is being included on top of the quarters addition.
The existing drilling system is being replaced or modified, as necessary, to permit drilling to 20,000 ft in this water depth. A drilling riser designed for this water depth is being fabricated by Cooper Oil Tools.
Other additions include a top-drive drilling unit and a new crane capable of handling the production riser joints and the subsea trees. All utility systems are being upgraded to accommodate the revised service requirements.
The firewater system in particular is being upgraded because of the addition of the new process area. In addition, the life-saving equipment is being upgraded and supplemented to handle the increased crew size of the vessel.
TEMPLATE, RISEP
PMB Engineering has finished construction of the 24-slot template and at the end of May completed an extensive systems integration test. It will be installed adjacent three existing wells on Garden Banks Block 388.
The first tree produced was landed on each well bay and operated by use of the subsea control system. All pipeline connectors were test fit.
A dummy ROV constructed by Oceaneering, Houston, was used to check for access to all areas of the template.
The template will be loaded out by month's end with installation by McDermott's DB-50 beginning early next month.
The three existing wells will be tied back to well bays on the exterior row of the template which is designed so that all well bays are identical.
There are two exterior rows of seven slots and two interior rows of five slots. The exterior well bays are designed to be used as either template or satellite wells with the interior bays designated as template well slots.
The template weighs approximately 1,200 tons and is 150 ft long x 20 ft tall x 85 ft wide. It contains piping for two flow lines per well and hydraulic control lines and electric power and signal lines for each well slot.
The template has base receptacles for the electro-hydraulic multiplexed control system. The export gathering pipelines originate on the template, each having one tie-in point for future developments.
Piping on the template has a subsea pig launcher in each gathering line that is used for launching cleaning pigs or intelligent pigs, as needed. Valves on the export lines can be actuated by the control system from the production facility.
The template will be supported on eight pilings and leveled on four pilings by use of Hydra-Loc equipment.
The largest component of the template will be the production riser base fabricated by Cooper Oil Tools. Four of the pilings will be centered on the riser base itself.
Enserch Exploration purchased the Green Canyon 29 riser and is modifying it to meet the water depth at the Garden Banks Block 388 location. The existing riser joints are being disassembled, inspected, and now reassembled. Part of the process is shown in Fig. 3.
All existing components are either refurbished or replaced to meet the requirements of this application. The major modification to the existing riser is the addition of new joints for the greater water depth.
Other changes are increased size of the production flow lines, modification of the upper connector due to the larger diameter of the flow lines, and replacement of the straps used to secure the syntactic foam modules as a result of corrosion of the original straps.
A complete analysis of the riser has been done including extensive testing in the model basin at Texas A&M University, College Station. How the production riser will affect the rig and its mooring system was studied as well as the riser response to design environmental conditions.
A major concern studied was the interaction of the drilling and production risers.
The riser is designed to remain connected to the rig for all conditions including a loop current. A leash system from tensioners on the rig to the riser top maintains the proper relation of the riser to the production facility.
The system has the capability of being disconnected at the riser connector package on the riser top which will be approximately 180 ft below the water surface. A disconnection will be a planned operation performed in favorable weather.
The riser has air cans on the top four joints that provide buoyancy to keep the riser in tension. Additional buoyancy is provided by foam modules on each joint. The modules provide guidance for the 48, 3-in. flow lines from the template.
The flow line tubing strings terminate at the riser top some 180 ft below the water surface. The flow lines are linked to the rig by flexible lines to porches on the pontoons of the rig.
The main riser structure is provided by an internal tube that has flanges on each end of the 50-ft joints (Fig. 3). One export gathering line runs through the center of the joint while the other is on the exterior of the joint as are the flow lines.
The riser will be instrumented to measure stress as well as inclination. Environmental conditions, such as current and wave height, are also monitored by the instrumentation package.
Six tensioners are used in the riser leash system to maintain the riser top in the proper relation to the rig. Turndown sheaves centralize the wire rope to the top of the riser. Slipping winches will allow the working sections of the wire to be replaced during operation.
Cooper Oil Tools has completed inspection of all existing riser joints. Reassembly of these joints is under way with two thirds of the joints having been completed at the end of May.
The structural cans for the new riser joints were received on May 18. Fabrication of the new joints is currently under way at Victoria Machine Works, Victoria, Tex.
TREES; SUBSEA CONTROL
The first tree was delivered in March and has been used for the system integration testing as mentioned previously. Manufacture of the remaining spool trees continues on schedule (Fig. 4).
Production trees for the Garden Banks 388 project will be horizontal flow trees supplied by Cooper Oil Tools. These trees were selected because of the nature of the field and because there is no need of a separate completion riser.
There are stacked pay zones in many of the proposed locations that could require multiple recompletions.
Elimination of the step of pulling the tree for a recompletion will be more cost effective. Less rig time would be required because the completions are made through the tree body using the drilling riser.
The subsea trees will be a 10,000-psi design with single master and wing valves on both production and annulus bores.
The trees will have a crossover valve so that the flow lines can be pigged in a loop from the production facility. This feature also allows the use of both flow lines to produce the wells should flow conditions warrant.
The trees will be controlled by an electro-hydraulic multiplexed system and set up for chemical injection both downhole and at the wing valve.
Temperature and pressure will be monitored at the tree by the control system.
All the template-mounted control components have been installed and tested.
The subsea control system is an electro-hydraulic, multiplexed system that will control the tree valves, the chemical injection, and the pipeline block valves on the template. The system will monitor pressure, temperature, and the chemical injection rate at the wellhead. The rate will be set at the tree by use of an ROV-adjusted valve.
The master control station on the production facility will be used to monitor the process and safety systems on the vessel as well.
Major components aboard the production facility supplied by FSSL are an hydraulic power unit, a chemical-injection unit, and a master control station.
Subsea equipment includes dual control umbilicals incorporated into the free-standing production riser, hydraulic distribution modules, manifold hydraulic modules, and control modules for each tree.
Subsea distribution pods are landed on template-mounted receiver plates to connect with the control lines routed to each well-bay insert. The subsea control modules are mounted on the trees but can be run and retrieved separately from the trees themselves.
The flow line mandrels have the couplers that mate with the tree to complete the control circuit. Complete redundancy is achieved through the hydraulic distribution assemblies.
An additional feature of the system permits hydraulic programming, from the surface to reallocate hydraulic controls to alternate template slots.
The electrical portion of the system can be rerouted by use of flying leads installed by an ROV to a limited extent. Two control umbilicals will be installed, each of which can operate the entire system.
IN SHALLOW WATER
Jacket fabrication and installation for the shallow-water processing platform on Eugene Island Block 315 have been completed by McDermott.
Jacket installation was the first offshore activity for the Garden Banks 388 project and was completed Apr. 26. Deck fabrication also is nearing completion at McDermott.
This platform, 54 miles from the production facility, will perform the final processing to specification product for sales. It will be connected to existing gas and oil pipeline networks by sales lines.
The four-column jacket, to be set in 245 ft water, will utilize skirt piles. The six-column deck section is tied to the four jacket legs.
The main processing on the shallow-water platform will improve the crude-oil stream to sales quality. The inlet stream will be heated and flashed to the proper vapor pressure, and the water will be removed and treated.
The gas flashed from the oil stream will be compressed and dehydrated then combined with the gas stream arriving from the production facility.
The platform will contain slug catchers on each arriving stream from the production facility. The gas stream will go through the slug catcher and be measured before leaving the platform to sales.
Provisions are made for future addition of compression to boost the gas to salesline pressure.
PIPELINES' DESIGN
Pipeline construction began May 16 when McDermott's DB-28 mobilized to install the oil and gas sales lines. Installation of the 54-mile gas-gathering pipeline began May 22.
Two 12-in. OD pipelines will carry the production from the template 54 miles to the shallow-water platform. The route is shown in Fig. 1a. Both pipelines have a design pressure of 2,160 psi.
The lines originate in a water depth of 2,190 ft and reach a maximum depth of 2,300 ft en route to the shallow-water platform. The lines are designed with sufficient wall thickness to prevent buckle propagation.
Design flow rates are 40,000 b/d for the oil line and 120 MMscfd for the gas line. The wall thickness will be a maximum of 0.688 in. which steps down to 0.562 in. then to 0.500 in. in the shallower water.
The gas-gathering pipeline will be concrete coated for the 0.500 in. W.T. section.
Both lines will have a corrosion coating of 14-16 mils of fusion-bonded epoxy. Standard bracelet anodes will be installed on the lines to give a 20-year design life.
The original development concept called for a single multiphase pipeline until the gas rate exceeded line capacity, at which time a second line would be installed.
During the detailed design, it was determined that the probability for hydrate formation existed. The main concern was that if the line was shut in for a period of time, the gas could contact water in low spots in the pipeline causing hydrates.
Should hydrates ever form in the liquids line, the pressure from the hydrostatic head would remain greater than the hydrate formation pressure so that a hydrate plug could not be dissolved by pressure reduction.
It was determined that space and weight limitations on the production facility would prevent total removal of water from the oil stream.
The most efficient way to deal with the potential hydrate problem was installation of two pipelines so that the gas phase could be dehydrated and the oil phase maintained at greater than the bubble point in the pipeline.
As mentioned earlier, the process scheme is to flash the oil to approximately 100 psi, then pump the oil to a discharge pressure sufficient to keep gas from flashing out of the oil phase as it is shipped to the shallow-water facility.
PIPELAY CONSIDERATIONS
One of the first tasks of the detailed design was to select the installation method. Several were compared including the bottom tow, conventional S lay, J lay , reel barge, and various combinations of these.
According to the study, either a combination J lay/S lay or the S lay would be acceptable technically and commercially. The final selection by Enserch Exploration was to use the S lay.
A contract to install the pipelines was awarded to McDermott (OGJ, Oct. 18, 1993, p. 36).
A conventional barge, the DB 28, was modified to lay the pipelines. The mooring system was upgraded for the deeper water depths. The method of running anchors also was altered to allow for the longer lines required.
Another major modification is the construction of a new stinger for this project which will be the deepest S lay of a pipeline in the Gulf of Mexico to date.
The pipeline route contains some rugged sea bed terrain. The survey requirements were set to gather data so that the number of spans would be limited.
Survey requirements called for a detailed swath bathymetry with a deeptowed fish along with a conventional set of survey instruments.
Requirements for on-site data reduction were also specified to reduce the amount of resurvey needed to find an acceptable route, should any problem areas be encountered.
During the course of the offshore survey conducted by Oceaneering, Houston, the pipeline was in fact rerouted due to a rough area. The reroute limited the number of spans to two, both of which are acceptable without any corrective postlay measures.
The pipelines will be connected to the template piping by jumper spool pieces installed by the lay barge. The lines will be positioned in a target area near the template as they are installed. Both lines are being laid from the shallow-water facility to the template.
Once the lines are positioned, ROV-deployed tools measure the offset length and angle from the pipeline hub to the template receptacle. The jumper spool is then fabricated on board the lay barge. The spool is designed with a weak link, or breakaway device, to prevent damage to the template should one of the pipelines be snagged by an anchor.
The pipeline sled, contains a check valve to limit any discharge from the lines.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
