Light vessel used to pull off cut jacket sections
Alton Perry III
Amoco E & P
J. E. (Jim) Snyder
R. C. (Bob) Byrd
Twachtman Snyder & Byrd
- The large tug-supply vessel Seacor Vision attaches to a platform jacket section in preparation to pulling it away and dropping it to the seafloor [20,300 bytes].
- A diver cuts a platform member on Eugene Island 367 "A" in preparation for sectioning the structure below the required water depth [22,868 bytes].
- The top illustration shows the platform structure with planned sectional cuts. The bottom shows the cut and toppled sections, leaving the jacket top below the water depth required to accommodate shipping traffic [44,293 bytes].
Derrick barge commercial rates are higher today than they have ever been, and platforms are still required to be decommissioned one year after production has ended.
As a result, many operators are searching for alternative platform removal methods to reduce their overall decommissioning expenditures. Platform owners need to determine which method of removal is best for each decommissioning project.
Eugene Island 367 "A" structure, an 8-pile, 4-skirt pile platform owned by Amoco E&P's Offshore Business Unit (Amoco OBU), was removed in the latter part of 1997. The decommissioning project was unique in that Amoco OBU sought different methods for removing and disposing of both the deck and jacket. The EI 367 "A" removal implemented a new lifting system for the deck removal and an innovative jacket toppling methodology.
Deck removalPhase I of the EI 367 decommissioning project consisted of removing the deck using the Versatruss alternative lifting system. Versatruss utilizes two cargo barges outfitted with a series of A-frame booms which are connected to lifting lugs that are mounted under the cellar deck, attached directly to the deck legs. The deck leg-to-pile splices are cut and the barges are winched together, lifting the deck away from the jacket. The deck, when lifted, is secured in a stable catamaran configuration that can then be towed to shore.
The EI 367 deck lift, about 1,300 tons, was the largest of its type to date using Versatruss. Phase I was a success and the data provided from this portion of the removal can be used in later removals in different locales.
The initial plan for the disposal of the jacket was to donate it as an artificial reef. The EI 367 location, with a water depth of 349 ft, was fortunately situated in a designated rigs-to-reefs planning area and the jacket was permitted to be toppled in place (in situ) as a reef site by the state of Louisiana in 1996.
Amoco OBU planned to topple the entire jacket where it stood, severing the piles, skirt piles, and conductors at 15 ft below the mudline and pulling the structure over on its side using a derrick barge. Bid responses were received from contractors in 1996, but the producer and the project management company, Twachtman Snyder & Byrd (TSB) decided that the prices to complete this project were too high. Removal preparation took place in early 1997, and the project was re-bid, with the same results.
The producer decided to devise a new platform removal methodology. Amoco Worldwide Engineering & Construction (Amoco WE&C) suggested implementing the Versatruss alternative lifting system to remove the deck. Amoco WE&C believed that Versatruss could be applied to future decommissioning projects in the North Sea, and this Gulf of Mexico removal was an ideal way to prove that Versatruss would work on much larger platforms.
Amoco OBU agreed to WE&C's plan, and said they also wanted to apply the Versatruss lift method to the EI 367 "A" jacket removal. Discussions with Versatruss representatives established that only the top portion of the jacket would be able to be removed. The Louisiana Artificial Reef Program was willing to modify their reef permit for this location because this type of reef donation, a partial removal, is biologically more effective than a complete toppling.
The marine environment benefits more from a partial removal because the top of the reef is higher in the water column. General US Coast Guard requirements are that the highest point in a reef must be at least 85 ft below the waterline so that ships passing over the structure can do so without any interference. The US Minerals Management Service approved the partial removal concept for the EI 367 "A" jacket.
In designing the Versatruss lift method to remove the top portion of the jacket and place it on the seafloor, the original plan called for a lifting frame to be built and suspended from the Versatruss A-frame beams, from which the jacket could be lifted. As the project developed, concerns about the frame initiated a change in the plan.
This revision called for the EI 367 "A" deck to have all its facilities removed after being towed to shore. The bare deck would then be taken back offshore on the Versatruss barges and used as the lifting frame from which the upper jacket section was to be lifted. However, due to time constraints and the complexity of this lift, Amoco OBU decided to search for yet another way.
To avoid using an expensive derrick barge to remove the top portion of the jacket (or topple the entire jacket in place), Amoco OBU decided to investigate cutting the jacket legs with divers and use large anchor-handling towing supply boats to topple the top portion of the jacket.
Using these types of vessels to topple jackets is not the standard method in the US Gulf of Mexico, but calculation of the required toppling force indicated that it would be feasible. It was determined that two large tugs, one of 8,000 hp and one of 12,000 hp, could achieve the required pull. The logistical problems and associated expense of getting two such vessels at the same time prompted Amoco to devise another innovation for toppling the upper portion of the jacket.
InnovationAlton Perry, project manager for Amoco, suggested cutting the top portion of the 8-pile jacket into two separate 4-pile sections. This would reduce the force required to topple the jacket sections by approximately one-half, and therefore only one large tug would be necessary. Toppling force calculations verified this assumption; a single tug could generate the required pull.
Amoco Deep Water Group had a vessel of this type on long-term charter, which reduced ed the logistical difficulties. TSB began developing the jacket preparation procedures and rigging design. The vessel chosen for this project was the Seacor Vision. On charter to Amoco, the Seacor Vision is a 12,280 hp large anchor-handling towing supply boat, more than adequate for this particular project.
Amoco OBU and TSB reduced the on-site time of the tug by segmenting the jacket and pre-severing the jacket legs before the tug arrived on location. In addition, the rigging was designed so that it would release when the top of the jacket turned over.
The plan was to have the Seacor Vision back up to the jacket, pick up the rope attached to the rigging cables, secure the rigging to the tug's tow line and pull away from the structure to topple the first section. This procedure would then be repeated on the second jacket section and, once the jacket was completely toppled, the tug would leave the site.
Leg severingIn addition to the Seacor Vision, the M/V Ocean Diver V, a 155-ft long dive boat, and the M/V Tony "G", a 110-foot utility vessel, were used to prepare the jacket and rigging.
The jacket severing consisted of cutting the jacket off above the -92-ft bracing level. A dive crew from Oceaneering International verified the completeness of each cut by running a blunt knife through each incision. The four corner legs of the jacket were only partially cut, leaving a 6-in. metal strip on the outboard side of the leg to hold the jacket top in place and to act as a hinge in the toppling process.
Welding necessary to seal the jacket legs (in order to ballast them) took more time than was expected. This was due to the fact that the closure plates were prefabricated from the platform drawings. The plates' round shape did not precisely fit into the piles because they were not perfectly round. Future projects of this type might benefit from having a representative of the fabrication facility travel to the structure to take precise measurements. Even though this portion of the project presented a minor obstacle, this did not significantly affect the overall operation.
TopplingThe dive crew was used to install the rigging, which performed and released according to design. The fact that the rigging released as each section of the jacket turned over considerably reduced the amount of time the Seacor Vision had to remain on-site.
The Seacor Vision hooked up its towline to the slings and applied the pulling force. The outboard uncut metal strips on the legs acted as hinges and the top of the jacket rolled over and the slings slipped off. As the jacket top continued to roll over, the weight was enough to break the hinge and the jacket top fell to the bottom. The other half of the jacket top was pulled over in the same manner.
Although the Seacor Vision had to stand by for daylight and changing weather conditions for about 12 hours, the actual on-site working duration for the tug was less than 2 hours.
After the jacket sections were toppled and the tug left the area, divers were sent down to verify the final elevations of each leg of the jacket. The highest elevation was the B-3 leg, which was severed at 88 ft below the waterline. All of the other legs and the conductors were severed at or below -90 ft, and the submerged jacket was therefore in accordance with regulatory requirements.
Once again, Amoco has donated another steel jacket as an artificial reef providing habitat for marine life. No explosives were required to sever the skirt piles in this method, eliminating any potential harm to turtles, fish or other marine life and subsequently eliminating the need for any required monitoring by the National Marine Fisheries Service.
The key to the success of this project was planning and preparation of the jacket. Amoco OBU evaluated all of their available options, allowing the company to devise a removal methodology which best suited the situation.
This idea presents operators with a new way to efficiently remove the top of large jackets using a single large anchor-handling towing vessel. The partial jacket abandonment concept should be very cost effective in the decommissioning of larger structures in deeper waters (greater than 300 ft water depth) in the Gulf of Mexico. The EI 367 "A" platform was fortunately situated in one of the few offshore blocks that is designated as a rigs-to-reef planning area.
AuthorsAlton Perry III is a Senior Petroleum Engineer for the Offshore Business Unit of Amoco E&P (New Orleans). He holds a BS in Petroleum Engineering from Louisiana State University and an MBA from Tulane University.
J. E. (Jim) Snyder is a principal of Twachtman Snyder & Byrd (Houston). He holds a BS in Civil Engineering from Lamar University.
R. C. (Bob) Byrd is a principal of Twachtman Snyder & Byrd (Houston). He holds a Ph.D. in Civil Engineering, and is a Registered P.E. in Texas.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.