GULF OF MEXICO GB388 template leveled on US Gulf deepwater slope

This image of the GB388 template shows the leveling tools positioned above several template positions.

• A plan layout of the GB388 template shows the arrangement of the piles.

In August 1994, Enserch Exploration Partners installed a 1,280-ton drilling and production template in 2,200 ft of water as part of the Garden Banks 388 development project, off Louisiana.

The 24-well-slot template, which was secured to the seafloor by eight 42-in. diameter foundation piles, is the largest template to be installed in water depths greater than 2,000 ft. When installed in early 1995, the Enserch Garden Banks Floating Production Facility (FPF) will be permanently moored over the template to facilitate simultaneous drilling and production operations.

Production will be through a free-standing, buoyant riser that will be connected to the FPF by flexible flowlines and to the template by a rigid titanium stress joint. Drilling and completion operations will be conducted through a separate drilling riser connected to a flex joint on top of the subsea BOP stack.

Levelness critical

Early in the template design stage, Enserch determined that levelness was an important consideration because of the need to maintain a reasonable drilling riser ball joint angle and to minimize bending in the production riser stress joint as the FPF moves in response to wind, waves and current.

While an absolutely level template would provide the FPF optimum range of movement, the allowable tilt was calculated to be 0.5°. Any additional angle, however, would narrow the window of movement. Enserch weighed the consequences of a smaller operating watch circle against the cost of leveling, and decided to incorporate leveling provisions into the template design.This decision was further justified as more definitive seafloor contour data became available, revealing a 3° slope at the site. Because of this slope, leveling would be an absolute necessity.

Enserch selected the Latch-Lok Leveling System, developed by Hunting MOS. The Latch-Lok pile-based system was introduced in 1992 as a more reliable alternative to conventional mud-mat jacks.

Since then, Latch-Lok leveling provisions have been incorporated on 22 subsea templates, including Conoco's 2,200-ton Heidrun drilling template in the North Sea. The system will also be part of Shell's Green Canyon Block 116 Popeye subsea template (2,000 ft water depth) and BP's Viosca Knoll Block 9B9 Phase II Pompano subsea template (1,860 ft water depth), and six jacket structures in 1995.

Latch-Lok leveling

The Latch-Lok leveling device, developed by Hunting MCS, is shown being lowered to a trial position over one corner of the Enserch Exploration Partners' GB388 template.

The Hunting MCS Latch-Lok Leveling Tool is a hydraulically actuated lifting device used for leveling templates and jackets. It works at any water depth and requires no guidelines or diver support. The tool is deployed after piles are driven and is supported by the driven pile. Its hydraulic cylinders are actuated to extend the latch frame downward until the latches lock into a groove in the temporary pile gripper (TPG), or latch ring. When the hydraulic cylinders are retracted, the tool reacts against the pile, lifting the template smoothly and precisely.

In developing a leveling plan, the minimum jacking capacity needed is generally taken as the full on-bottom weight of the template, even though a template is typically brought to level by lifting one corner at a time, resulting in a static lift equal to about one-half the structure's weight. Reserve lifting capacity is used to overcome mud suction.

In the early stages of the Enserch project, leveling requirements were expected to be minimal, so Hunting MOS proposed a leveling plan that included two 500-ton Latch-Lok leveling tools with a combined maximum capacity of 1300 tons, equal to the weight of the template.

When the later seafloor data became available, Enserch recognized that leveling would be much more difficult and requested a third Latch-Lok tool. This third tool provided an ample reserve of lift capacity and minimized the need to move Latch-Lok tools from pile to pile. It also maximized system redundancy by serving as a back-up means to hold the template level in the event a TPG is rendered inoperable.

Temporary grippers

The leveling plan also included four temporary pile grippers. The TPGs, which are welded onto the template pile sleeves during template fabrication, provide a means of temporarily securing the structure at level while the permanent structure-to-pile connections are made using the Hydra-Lok pile-swaging tool. Also, they can be actuated to hold the template in an intermediate position should one or more Latch-Lok tools have to be relocated from one pile to another, and they allow the template's levelness to be fine-tuned before the permanent connections were made. Each TPG has a latch ring that mates with the Latch-Lok tool. The TPGs used for the Garden Banks template were hydraulically actuated and provided a weight-energized, fail-safe grip.

Although there were eight piles on the Enserch template, it was determined that only four TPGs were required, each with a gripping capacity of 600 tons. The TPGs were located relative to the template's center of gravity (Figure 1). To maximize leveling flexibility, Hunting MOS recommended that latch rings also be provided on the stabbing cones at the other four pile sleeves so that the Latch-Lok tools could be deployed to any pile.

Pile-swaging tool

Hunting MCS was also the contractor for Hydra-Lok pile swaging, which uses hydraulic force to form immediate-full-strength structural connections between structure pile sleeves and foundation piles. This technique has replaced grouting as the preferred method for securing subsea templates, and is also used for jackets with skirt piles.

Like the Latch-Lok tool, the Hydra-Lok tool works at any depth and requires no guidelines or diver support. It forms a structural connection by hydraulically swaging the foundation pile outward into fabricated grooves inside the structure's pile sleeve. As of the fall of 1994, Hydra-Lok pile swaging had been used for more than 400 connections on subsea templates, subsea manifolds, and platform jackets.

Leveling operation

The template was lowered to the seafloor by McDermott's Derrick Barge 50. It landed on an incline of about 3-4° and would not remain stationary. While the template was supported from the derrick barge, six of the eight piles (A, C, D, E, G, and H) were spudded to self-support penetration, allowing the template to be positioned and released. Interference with the spudded piles held it at an angle of less than 3° from level.

Next, the D and H piles were driven to grade. Then two Hunting MCS Latch-Lok leveling tools were deployed to these piles to attempt leveling. The second of these was the master tool, which carried hydraulic jumper umbilicals that would be taken by an R0V to the two slave Latch-Lok tools.

Although none of the piles at the TPGs were driven to grade, this attempt at leveling was made only to permit the remaining two piles to be installed in the vertical. Access to these two sleeves while the template was held by the barge was obstructed by rigging.

This initial attempt at leveling was unsuccessful. A combination of factors, including binding of free-standing piles with the sleeves, position of the Latch-Lok tools, and mud suction were thought to be factors. The two remaining piles were then installed, and selected piles were driven to grade to enable the third Latch-Lok tool to be deployed (to Pile B) and to prepare for actuation of the TPGs.

With the third Latch-Lok tool in place, leveling operations began. The operation was coordinated by the ROV team, which was visually monitoring the tools, Hunting MCS personnel, and the surveyors, who monitored levelness using a Micronav level-detection system.

The third Latch-Lok tool at Pile B was actuated first. It lifted the template until significant resistance was encountered. Then all three Latch-Lok tools were used in alternating fashion to walk the template up the piles.

It took about three hours to bring the template to within 0.5° of level. By this time, the leveling operation was proceeding smoothly, and the decision was made to continue. In another 30 minutes, the template was leveled to better than 0.25°. When leveling was complete, the mud mat on one end of the template had been lifted about 36-48 in. above the seafloor.

At this point, the TPGs were actuated to secure the template, the Latch-Lok tools were released and retrieved, and the remaining piles were driven to grade.

Pile swaging operations

With the TPGs holding the template in position, the Hydra-Lok pile-swaging tool was deployed to Pile H, and swaging operations began. All eight piles were swaged in a total elapsed time of only 21 hours.

Each connection was verified using both pressure/volume charts and the tool's onboard ultrasonic inspection system. As each connection was completed, profile data was collected and plotted at the surface to confirm the swaged geometry.

Authors

Michael Blincow is senior project engineer for Enserch with responsibility for the design, fabrication, and installation of the Garden Banks 388 subsea systems. He previously various production and reservoir engineering assignments with onshore and offshore divisions. He holds a BS in petroleum engineering from Texas A & M University.

David Snell is a project manager for Hunting MCS, Inc. with responsibility for Hunting's contributions to the Garden Banks 388 project. He was previously a design engineer with McDermott International. He holds BS and MS degrees in divil engineering from Louisiana Tech University.

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