InterMoor’s Dusan Curic discusses the decommissioning of Anadarko Petroleum’s Red Hawk spar in the US Gulf of Mexico, a job that involved several industry firsts.
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With holes cut into the hull, Red Hawk spar is being tipped underwater. Images from InterMoor. |
The world’s first, and so far only, cell spar is also the first spar upended and moored without using construction vessels or derrick barges. Anchor handling tug supply (AHTS) vessels were utilized instead of the conventional methodology. This is the first producing spar decommissioned and the first spar turned into an artificial reef or “reefed” FPS hull and the deepest floating platform decommissioned in the Gulf of Mexico. All these are synonyms for Red Hawk Spar, a former FPU installed by Kerr McGee, an Anadarko predecessor company, and now an artificial reef in Eugene Island block 384 offshore Louisiana.
Red Hawk’s productive years
Red Hawk spar began its first life in 2004 when it was installed in 5300ft of water in Garden Banks block 876 by horizontal wet towing from Ingleside, Texas, upended to a vertical position and moored using a chain-polyester-chain system with suction pile anchors. Anadarko found itself in a position as the lease holder on a field that was soon to expire. A decision on the platform’s decommissioning had to be made. When the time came to decommission the spar, it was determined that simply reversing the steps from the installation procedure–ballast the hull to a horizontal attitude and tow it back to shore–was not the best solution.
Decommissioning
A totally different approach was taken thanks to Anadarko’s experience with fixed platforms which had been included in the US Bureau of Safety and Environmental Enforcement (BSEE) Rigs-to-Reefs program. When a suitable location was found in 430ft deep Eugene Island block 384 and after BSEE and the Louisiana Department of Wildlife and Fisheries agreed that marine life would benefit from converting the Red Hawk hull into an artificial reef, the planning phase for the decommissioning was ready to begin.
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Red Hawk Spar was towed vertically to its reefing site. |
Anadarko teamed up with two main contractors, Versabar for the topsides lift and InterMoor, an Acteon company, for integrated decommissioning engineering and offshore execution. InterMoor’s scope included ballasting and deballasting of the Red Hawk hull, mooring disconnect and recovery, hull tow and finally, hull reefing at EI384. Ballasting and deballasting were performed remotely from McDermott’s Derrick Barge 50 (DB50), while Edison Chouest Offshore’s AHV Kirt Chouest was used for mooring disconnect and mooring recovery. InterMoor has not only supported multiple projects for Anadarko prior to Red Hawk, but also was involved, at the time as a division of Technip, in the installation of Red Hawk. Todd Veselis, general manager of Permanent Moorings for InterMoor, said that engineering manuals from Red Hawk installation were still shelved in his Houston office when the decommissioning engineering started. That knowledge and experience was certainly beneficial to the project.
Mooring disconnect
The mooring system on Red Hawk had only one windlass and one 800ft long work chain located on the topsides to service all six mooring legs. To reduce the loads during mooring disconnect the work chain had to be removed prior to the topsides lift. The work chain was cut in three pieces and paid out on three mooring legs, thus effectively lowering the tension in the entire mooring system. At that point the system was slacked to facilitate topsides removal. However, the platform still needed to remain storm safe throughout the topsides lift. High capacity C-link connectors were procured and used to connect work chain segments to top chains and additional analysis proved that the hull could sustain a 10-year return storm with or without topsides. Tugs were hooked up to provide station keeping while the mooring disconnect was completed.
Ballasting and deballasting
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Topsides being lifted off towing. |
Limited access to the spar during key activities required a remote ballast/deballast system for all operations. The variable ballast control system was located on the topsides. The connections between the hull and topsides had to be removed prior to topsides lift. The spar hull was 560ft long, 64ft in diameter and assembled with six outer cells connected to the center cell. There were void tanks on top of each cell with variable ballast tanks under them. Each had an air-over-water system that was open to the sea at the bottom. Pumping air pushed water out and reduced the draft, while venting air would let more water in to lower the spar. The hull first needed to be ballasted low enough for Versabar’s heavy lift vessel VB10000 to hook up and lift the topsides. After that, the hull needed to be deballasted to reach a tow draft of 400ft to enable the spar to enter the 430ft deep reefing site. To facilitate the required draft variations, InterMoor set up three different systems on DB50 and the spar: an air ballast/deballast system, a water ballast system and a water deballasting system.
Pumps, compressors and manifolds were staged on the DB50 and connected to the tanks in the spar hull through water and air hoses. Ballasting continued during the four-hour topsides lift operation to maintain the draft as the weight was gradually taken by VB10000. The lift required very calm seas as this was VB10000’s first topsides lift from a floating platform. Therefore, the motions of the two bodies had to be adequately managed. Free of its topsides, the hull was then deballasted in two stages. First, the water deballast system pumped water out of the provisional ballast tanks. Submersible pumps and deballast hoses were preinstalled into provisional ballast tanks, and a generator was transferred to the top of the hull after the topsides were removed to power the pumps. After mooring lines were disconnected and temporarily laid on the seabed, compressed air was then pumped into the variable ballast tanks from the DB50 to raise the hull to the 400ft draft. Since the windlass and work chain were no longer available, the DB50 assisted in the mooring disconnect by lowering each top chain through the chain stopper with its crane. Once the Kirt Chouest hooked into the top chain below the surface, an ROV cut a sacrificial sling and the AHTS vessel laid each line on the bottom for recovery after the hull was towed from the field.
Towing and reefing
The 70mi tow route from Garden Banks 876 to Eugene Island 384 was laid out to minimize pipeline crossings, maximize the hull’s distance from other platforms, and maintain adequate bottom depth clearances during the final approach to the proposed reefing site. The tow route was pre-surveyed several months ahead of the tow to verify information obtained from public databases as well as to account for any seabed objects or anomalies. With some convenient seas assisting from the stern, the tow lasted less than two days.
Once at the reefing site, the tugs reconfigured into their station keeping arrangement. The crew was transferred to the spar to vent the air from variable ballast tanks until the spar was sitting firmly on the seabed in a vertical position. A highly skilled rope access crew was then transferred to the spar to prepare the hull for the reefing operations. Once complete, all air valves for variable ballast tanks were left open and the crew transferred back to the DB50. One tug disconnected from the hull and the other two pulled in the planned reefing direction until the hull began to free flood. The hull gradually pivoted around the bottom edge of the two cells and in less than 10min it laid on the bottom in a horizontal position. During the sinking the tugs powered off and paid out work wires, which were then disconnected from tow rigging with ROV assistance.
This methodology sets precedence for other floating structures nearing the end of their design life. Valuable experience was gained, not only for the companies involved with the reefing of Red Hawk but for the offshore oil and gas industry as a whole. Paramount to the entire operation was the very high safety standard achieved.
Dusan Curic served as the project manager for InterMoor for four years and has more than 10 years’ experience in the oil and gas industry. Curic previously worked as an engineering project manager and assistant project manager where he was responsible for developing engineering plans, approving engineering deliverables and managing engineers and designers from various disciplines. Curic received a master’s degree in mechanical engineering from the University of Novi Sad, Serbia, and in ocean engineering from Florida Atlantic University.
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