Shell advancing gulf deepwater developments

April 28, 1997
Guntis Moritis Production Editor Mensa main components include a 200-ton, 94-ft diameter, 12.5-ft high template base and 50-ton, 16-ft diameter, 16-ft high manifold (top photo at left) and subsea trees that are 4-in. by 2-in., 10,000-psi composite block, guidelineless design with a vertical piping connection mandrel for mating with the well jumper (photo above). The template base is shown being towed out to location in the Gulf of Mexico (bottom photo at left). Photos courtesy of Shell
Guntis Moritis
Production Editor
Mensa main components include a 200-ton, 94-ft diameter, 12.5-ft high template base and 50-ton, 16-ft diameter, 16-ft high manifold (top photo at left) and subsea trees that are 4-in. by 2-in., 10,000-psi composite block, guidelineless design with a vertical piping connection mandrel for mating with the well jumper (photo above). The template base is shown being towed out to location in the Gulf of Mexico (bottom photo at left). Photos courtesy of Shell Deepwater Development Inc.
The deepwater Mensa project, nearing completion, is the latest in a flurry of subsea well projects that Shell Deepwater Development Inc. (SDDI) has on tap for the Gulf of Mexico.

By yearend 2005, Shell may have installed about 110 subsea completions in the Gulf of Mexico, up from only 14 it expects to have at yearend 1997. Of the 110, as many as 50 may be in place by yearend 2000.

SDDI was formed at yearend 1996 and is part of Shell Oil Co.'s New Orleans-based exploration and production group.

Shell Deepwater Production Inc., another member of the group, will be the operating company during the production phase of Shell's deepwater projects.

Subsea trend

The ratio of surface to subsea projects in Shell's gulf deepwater portfolio will change, said D.K. Peart, Shell project manager, subsea systems/major projects.

For currently committed projects, that ratio is about 67:37, while future deepwater gulf projects are more likely to have a ratio of 50:50.

Peart says that future deepwater projects will include many smaller prospects, completed subsea, that will tie into the existing deepwater infrastructure such as the four tension-leg platforms (Auger, Mars, Ram-Powell, and Ursa) that Shell has installed or will have installed by 1999.

In addition, Shell contends that most of its future deepwater subsea completions will be producing oil. Currently, most produce gas. Of the 110 subsea completions projected by yearend 2005, Shell expects more than 80 will be oil producers.

By the end of 1997, Shell's subsea gulf completions will include:

  • Tahoe, gas, 1,500 ft water depth (WD). One producing well was installed in 1994.

  • Popeye, gas, 2,000 ft WD. Two producing wells were installed in 1996.

  • Rocky, clustered oil, 1,785 ft WD. One producing well was installed in 1996.

  • Mars satellites, oil, 3,300 ft WD. One producing well was installed in 1996.

  • Tahoe II/S.E. Tahoe, gas. Two producing wells-five total-were installed in 1996.

  • Mensa, gas, 5,300 ft WD. Three wells were installed, with first production in mid-1997.

  • Popeye II, gas. One well is under design, scheduled for installation in 1997.

Shell also has interest in the Troika subsea development, in 2,800 ft WD, operated by BP Exploration.

Mensa project

Mensa, on Mississippi Canyon Blocks 686, 687, 730, and 731, is nearing completion.

Offshore installation of subsea equipment started in September 1996.

Gas production from the first well in the three-well project is scheduled to start in June. Shell expects the second well to be completed by September, followed by the third in March 1998. Transocean Offshore Inc.'s Richardson semisubmersible is drilling and completing the wells.

Shell holds a 100% working interest in the project. Shell acquired Blocks 730 and 731 in Sale 98 in May 1985 for bonuses totaling $12.6 million. In June 1993, it traded with Pennzoil Co., Amoco Corp., and ARCO for Blocks 686 and 687 in a deal valued at about $2.3 million.

The project, about 140 miles south of New Orleans in 5,300 ft of water, sets a world water-depth record. But other projects, in the Gulf of Mexico and Brazil, may soon top this record.

During a 7-9 year life, Shell estimates Mensa's three wells will recover 720 bcf of gas at a peak production rate of 300,000 MMscfd. Each well is planned to have a producing capacity of 150,000 MMscfd.

All three producing wells will be on Mississippi Canyon Block 687. Two will be standard extended-reach vertical completions, but one will have a horizontal lateral section.

The discovery well, drilled in 1987, was followed by an appraisal well in 1988. Neither of these wells will be completed.

The producing reservoir is an upper Miocene sand at a depth of about 15,500 ft. Average net thickness is about 100 ft. The reservoir has a pressure of 10,040 psi and a temperature of 176° F. Shell expects to be producing very dry gas, 99.5% methane, with no paraffin or H2S, and only a trace, 0.1 mole %, of CO2.

Shell expects a shut-in wellhead pressure of 8,970 psi and initial wellhead flowing pressures of 4,000-5,000 psi.

Development plans call for the three wells to be connected to a subsea manifold 5 miles away, which then is tied back with a 63-mile, 6,000 psi, 12-in. flow line to the West Delta Block 143 platform.

Shell claims the 68-mile tieback is the longest in the world and that this is the first time that a section of a tieback has a design pressure less than the shut-in wellhead pressure.

Dennis McLaughlin, Shell's Mensa project leader, says the 5-mile distance from the wells to the manifold was chosen as an added precaution to prevent hydrate formation. The distance, he said, allows the gas temperature to decrease to that of the surrounding water, helping to minimize the likelihood of hydrate formation. To ensure the cooling effect, the flow lines between the wells and manifold are elevated above the seafloor to help dissipate heat, also a first for a subsea completion system, according to McLaughlin.

In addition to these precautions, Shell plans to inject glycol for preventing hydrate formation and for internal corrosion protection.

But in case hydrates do form, Mensa's design allows a surface vessel to tie into the subsea equipment to lower the pressure on each side of the hydrate plug, so that the hydrate plug dissipates.

Mensa components

Mensa's main components include a subsea manifold with a template base, an electrical distribution structure, subsea trees, flow lines, umbilicals, and a master control station.

The manifold/template base has four well receiver slots and eight utility service slots, including hydraulic umbilicals, glycol injection, and hydrate remediation.

The 200-ton, 94-ft diameter, 12.5-ft high template base sits on the seafloor. No piles secure it to the seafloor. Shell used a suction leveling system for the installation.

The manifold assembly is separate from the template and can be recovered independently. It weighs 50 tons and has a 16-ft diameter and 16-ft height.

The purpose of the electric distribution structure, located near the manifold, is to receive electric power and communications signals from West Delta Block 143 platform, amplify the signals-which have decreased over the 63-mile distance-and distribute the power and signals to the subsea wells 5 miles away.

The three subsea trees are 4-in. by 2-in., 10,000 psi composite block, guidelineless design with a vertical piping connection mandrel for mating with the well jumper. Shell has equipped the trees with non-intrusive sand erosion monitors; continuous, large-pressure drop subsea chokes; a multiplexed electrohydraulic workover control system; monobore tubing hangers; and the deepest-set surface-controlled subsurface safety valves.

The subsea control system has a guidelineless retrievable repeater pod and a local hydraulic emergency shut-down dump valve.

The three, 5-mile, 6-in. flow lines from the trees to the manifold are carbon steel and are connected to the manifold with a stab and hingeover termination and to the tree with a laydown sled and rigid jumper.

The 12-in., 63-mile flow line from the manifold to West Delta Block 143 platform is also carbon steel. It is connected to the manifold by a sled and jumpers and to West Delta 143 platform with risers.

The larger-diameter, helically bundled steel-tube hydraulic umbilicals are carbon steel with a zinc coating. They supply hydraulic fluid and glycol for injection. Three 5-mile, seven-line umbilicals connect the wells to the manifold, and one 63-mile, three-line umbilical connects the manifold to West Delta 143 platform.

The electric umbilicals are double-armored cable and transmit electrical power and signals between the master control station on West Delta 143 platform and the electric distribution structure. Three 5-mile umbilicals connect the wells to the manifold, and one 63-mile umbilical connects the manifold to West Delta 143 platform.

A master control station on West Delta 143 platform is a computer-based system that monitors the operational status of wells and other subsea equipment and can open and shut in wells.

Alliance

FMC Wellhead Equipment Division in Houston manufactured the subsea well equipment, and Kvaerner FSSL in Sugar Land, Tex., manufactured the subsea production control system.

Both FMC and Kvaerner FSSL are in an alliance with Shell and will continue to develop systems for Shell's future deepwater subsea completions.

Shell estimates Mensa's total cost, including drilling, subsea system, completion, topsides, flow lines, and umbilicals at $280 million.

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