Pluto gas flowline repaired at water depth of 2,150 ft
Wasp, ROV units conduct seafloor repair
An in-line bolted flange spool-piece was installed to repair Mariner Energy's 8-in. Pluto gas flowline in 2,150 ft water depth in the US Gulf of Mexico, setting a depth record for a deepwater pipeline repair. The repair was conducted by Oceaneering.
The firm had established a short-lived water depth record of 1,000 ft for an in-line spool-piece flowline repair just six months earlier with the repair of dual 4-in. Dulcimer flowlines in Garden Banks 323, also for Mariner Energy.
Both repairs were performed by the firm's Oilfield Projects Group using WASP Atmospheric Diving Systems in conjunction with heavy work remotely operated vehicle (ROV) systems to remove the damaged pipe sections and install in-line bolted flange spool-pieces to repair the damaged flowlines. Both repairs were performed from the firm's dynamically positioned (DP-2) Ocean Intervention.
The Dulcimer flowlines were damaged by an 18-in. flowline bundle which was towed across the dual 4-in. flowlines and the double-armored Multiflex control umbilical, wearing large holes in the tops of the two flowlines and leaving the umbilical largely intact. The leak in the Pluto flowline was caused by a crack in a butt weld.
The Dulcimer flowlines were repaired entirely on bottom using Oceaneering's Smart Flange Plus connectors to connect the in-line spool-pieces to the flowlines, while the Pluto spool-piece was bolted to flanges welded on the flowline ends by the pipelay barge.
A leak was discovered in the 28-mile-long Pluto 8-in. gas flowline during hydrotest by the installation contractor, Global Industries, prior to start-up of production. An ROV/survey vessel was dispatched to inspect the flowline and the leak was located in Mississippi Canyon 541 in 2,150 ft water depths about three miles from the Pluto subsea well. Due to the success of the earlier Dulcimer repair, Oceaneering was contacted by Global Industries and asked if they could repair the flowline. Oceaneering agreed, since 2,150 ft water depth was within the depth range of the WASP Atmospheric Diving System (2,300 ft), and well within the depth range of the MSV Ocean Intervention and its built-in moonpool deployed Hydra Millennium 150-hp ROV system (10,000 ft). A repair project team was established with engineers from Global Industries, Mariner Energy, and Oceaneering's Oilfield Projects Group.
The flowline damage was delaying production start-up and deferring revenue from the Pluto project. Getting the flowline repaired and production started quickly was a project goal that drove the selection of equipment and repair procedures. There would be no time to design and fabricate ROV-friendly systems.
At 2,150 ft, the damage was well beyond the depth range of saturation divers. The repair had to be performed by atmospheric diving systems and/or remotely operated vehicles, neither of which work well on the seabed. The flowline would have to be elevated from the seabed where WASP and ROV units would have access to the work and their thrusters would not stir up sediment and impair visibility. A soft and sloping seabed at the repair location was also a factor.
At 9,400 psi (maximum allowable operating pressure), the Pluto flowline system would require a high-pressure repair solution. The Smart Flange Plus connectors used to repair the Dulcimer flowlines had no track record at this pressure rating. The 8.625 in. by 0.866 in. wall X-65 SMLS pipe was installed by Global Industries' pipelay reel barge Chickasaw, which leaves the installed pipe in a slight spiral under tension on the seabed. Two situations were expected:
- Residual tension would cause the flowline to separate and curl when it was severed to remove the damage.
- The heavy wall 8-in. pipe would be less flexible and would require a more substantial alignment system than was needed to align the 4-in. Dulcimer flowlines.
The flowline repair would occur in October, which in the Gulf of Mexico combines the end of hurricane season with the beginning of the winter cold fronts. These weather conditions make it difficult for a support vessel to hold station or launch and recover an ROV. The Ocean Intervention with its Simrad SDP-21 (DP-2) dynamic positioning system, two 1,000-HP tunnel bow thrusters and two 2,000-HP azimuthing stern thrusters, is designed to hold station in any weather conditions short of a named storm. The resident ROV with its dedicated moonpool and cursor deployment system is designed to be launched and recovered in any weather in which the vessel can hold station.
Due to the high pressure rating of the Pluto flowline and the lack of straight pipe on the seabed the following repair procedure was developed:
- Cut out the damaged section of pipe
- Recover the flowline ends to the surface
- Cut off 100 ft of curved pipe from each end
- Weld 100 ft of straight pipe to each end
- Weld flanges to the flowline ends
- Replace the flowline ends on the seabed
- Raise and align the flowline ends with each other
- Measure, fabricate, and install an in-line bolted flange spool-piece
- Lower the flowline back to the seabed, and hydrotest.
This was accomplished in three phases. As the cause of the leak was initially unknown, the Ocean Intervention was mobilized to recover the section of pipe containing the damage and return it for analysis. The WASP and ROV excavated a large bell hole at the site of the damage. A Wachs hydraulic guillotine saw was used to cut out a 10-ft length of pipe containing the damage.
It was anticipated that the pipe would separate during the initial cut due to the residual tension. When the saw blade was two-thirds of the way through the pipe, the pipe began to rotate. It separated, broke the saw, and damaged the ROV, which was supplying hydraulic power to the saw. The ROV and the saw were repaired. The second cut was made without incident, and phase one was completed in two days. Analysis of the damaged section of pipe revealed that the leak was caused by a crack in a butt weld.
For phase two, the pipelay reel barge Chickasaw returned to MC-541 and recovered the flowline ends with an ROV-friendly pipeline recovery tool. Curved pipe was removed from each of the flowline ends and 100 ft lengths of straight pipe and flanges were welded on. WASP friendly nut retainers were installed on the flanges. The flowlines were hydrotested in both directions to determine if there were additional leaks. Another leak was discovered at the platform riser but it had no impact on the deepwater repair. Pulling heads were installed, and the flowline ends were replaced on the seabed and aligned with each other.
During phase one and two Oceaneering performed the engineering for and prepared the repair procedures for phase three. Oceaneering also fabricated four pipeline alignment frames, one spool-piece deployment frame, and a taut-wire, spool piece metrology device.
To perform the third phase of repairs, the Ocean Intervention with WASP, ROV, and pipeline alignment and repair equipment returned to the site. The four pipeline alignment frames were overboarded and positioned over the flowline ends, 25-ft and 85-ft from the flanges. Pipe clamps on the alignment frames were attached to the flowline, and ROV-friendly winches on the pipe alignment frames were used to raise the flowline ends approximately 8 ft above the seabed where the flowline ends were leveled parallel to the sloping seabed and aligned with each other. The WASP, using hydraulic impact wrenches powered by the ROV and the taut-wire, removed the pulling heads. A spool piece metrology device was installed.
Final alignment adjustments were made, aligning the flowline ends as closely as possible. The gap between the flanges was measured (just over 22 ft) with the taut-wire system. The repair spool piece was fabricated and tested onboard the Ocean Intervention, rigged in the spool piece deployment frame, overboarded by the vessel crane, and lowered into position between the raised flowline ends.
The spool piece was maneuvered into position between the flowline flanges with ROV friendly winches on the deployment frame, WASP divers inserted bolts into the flanges, and the flowlines were lowered slightly to allow the flanges to come together as the bolts were run up using hydraulic impact wrenches powered by the ROV. The WASP, using Hytorc wrenches powered by the ROV, tensioned the flange bolts to specification. The flowline was lowered and repositioned on the seabed and the deployment frame and alignment frames were removed and recovered to the vessel. Phase three was completed in 6.5 days.
Pipelines can be repaired quickly and cost effectively in water depths down to 2,300 ft using atmospheric diving systems, and working from small (compared with pipelay vessels) dynamically positioned support vessels.
Pipelines can be repaired quickly and cost effectively in water depths exceeding 2,300 ft by remotely operated vehicles working from small dynamically positioned support vessels, provided that specialized pipe handling and pipe repair equipment has been designed and fabricated in advance.