Toppled platform in-place creates reef in US gulf

Nov. 6, 2000
To abandon the Main Pass (MP) Block 254 platform in 280-ft water, Spirit Energy 76 cut the jacket at -100 ft subsea and toppled the upper portion, next to the remaining structure.
Toppling took only 37 sec from first motion to complete submergence of the top portion of the MP 254 platform (Fig. 1).
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To abandon the Main Pass (MP) Block 254 platform in 280-ft water, Spirit Energy 76 cut the jacket at -100 ft subsea and toppled the upper portion, next to the remaining structure.

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This was the company's first in-place toppling of a platform without removing the deck section and possibly also a first for the industry.

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Spirit Energy 76 commenced this project for creating an artificial reef on site after working with numerous governmental agencies, including the Alabama Department of Conservation & Natural Resources and the US Army Corps of Engineers.

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The platform was toppled (Fig. 1) and the reef created on Aug. 2, 2000.

Because of abundant marine life at the location, specifically sea turtles, Spirit Energy 76 decided not to use explosives for the abandonment.

Platform description

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The platform, installed in 1975, includes a 14-pile jacket with four skirt piles, a single level drilling and production deck, and associated production equipment (Figs. 2 and 3).

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No pipelines were attached to the structure. Production from the platform, recovered during testing operations, was routed to two deck-mounted tanks and then barged away.

Six wells had been drilled from the structure.

The major components weighed an estimated 3,100 tons for the jacket, 970 tons for the deck, and 1,100 tons for the pilings.

Spirit Energy 76 acquired the platform as part of a late 1980s acquisition, and its engineers and geologists saw insufficient value to support further lease development. Following a long history of partner and governmental issues, the US Mineral Management Service (MMS) required that the platform be abandoned and removed.

Company engineers began looking at alternative methods for abandoning the structure after initial cost estimates for conventional removal and disposal proved excessive. The engineers made detailed studies on toppling the platform in place, and following several internal discussions and consultation with outside firms, Spirit Energy 76 determined that toppling was feasible.

Toppling a platform in place without removing the deck section had not been attempted previously by Spirit Energy 76 or other known operators in the Gulf of Mexico.

Engineering

Because of the significant cost and equipment required to remove the jacket and deck conventionally, Spirit Energy 76 engineering and construction contracted an engineering firm to model the structure with SACS software. Numerous assumptions had to be made and input in the analysis because of the lack of piling make-up data.

This analysis determined that structurally the deck could transmit the toppling force to the upper portion of the jacket. It indicated that the required toppling force would be about 450 tons with flooded subsea drill-water tanks. A load of 500 tons was then used to design the rigging and other equipment for the toppling process.

Consultation with engineering and diving contractors determined the underwater cutting methods for ensuring diver safety, severing structural members completely, and ensuring a safe freestanding structure after toppling.

The analysis was rechecked after finalizing the member-cutting sequence and recovering subsea coupons of the hinge-piling material. It indicated that the upper section would topple freely after a 17° rotation for positioning the center of gravity of the upper section outboard of the hinge point.

Because the integrity of the tanks was unknown, the analysis required the toppling force to be calculated under various drill-water-tank ballasting scenarios. With all tanks flooded, the analysis provided a maximum toppling force of about 450 tons.

The analysis, however, did not include the weight, about 70 tons, of the field-installed structural steel on the deck.

The pad eyes for the toppling were designed and fabricated based on a 500-ton load.

Decommissioning

The decommissioning and deck preparation phase required installation of leased, self-elevating cranes.

After the cranes were installed, the condition of the existing deck required that a major portion of the deck be covered with temporary decking for personnel safety. Temporary living quarters and generators were then installed to support decommissioning operations.

Crews decommissioned and removed the production equipment and piping. All fluids used during this phase of the work were recovered and sent to shore for proper disposal. A third-party inspection verified that the deck and all attached equipment were free of hydrocarbons.

In the next phase, well-abandonment equipment was mobilized to the platform and the wells were abandoned according to MMS regulations. After well-abandonment equipment was removed, crews prepared the deck for installation of the rigging to topple the upper portion of the platform.

The platform had to be attended to at all times until the toppling process was completed because aids to navigation had been removed.

Rigging

A joint effort among the marine equipment contractor, consulting engineering contractor, and Spirit Energy 76 designed the rigging for the toppling. Each of the two pad eyes was designed to transmit 500 tons of toppling force.

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A section of each pad eye was cut out and ground smooth so that the rigging would disconnect from the pad eyes automatically after the top section rotated 90°. The pad eyes were welded to the top deck at Row B above the main deck columns on the north end of the deck (Fig. 4). Ultrasonic testing ensured integrity of all pad-eye welds to the deck section.

A 3.75-in. wire-rope bridle connected the 600-ton shackles, installed in each pad eye, to the main toppling wire. The single main toppling wire ran over a tower, installed over the hinge point on the southern part of the deck.

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This tower prevented the wire, under load, from contacting the deck beams, thereby, preventing damage to the wire while also providing upward force at Row B. The end of the main wire was lowered into the water on the south side of the platform and equipped with a buoy for easy recovery and connection by the marine toppling equipment (Fig. 5).

Underwater cutting

To save time and maximize diver safety, Spirit Energy 76 mobilized a small dive spread to prepare the appropriate surfaces for underwater cutting. This operation was conducted concurrently with well abandonment operations and provided invaluable diagnostics on the existing condition of underwater members and joints.

After removing marine growth from the members, the divers marked the surfaces, measured for depth from the sea surface, and videotaped to document each cut. For structural reasons, a minimum depth "hard line" for all cuts was established at -97 ft subsea.

The divers removed the launch-skid framing from the hinge piling and recovered hinge-piling coupons. All members to be cut were vented during this mobilization, and three of the five drill-water tanks were tested for integrity and prepared for deballasting, if required. A hose and valves were installed between the tanks and a surface manifold to allow the tanks to be deballasted remotely.

Spirit Energy 76 and the diving contractor developed a detailed procedure and cutting sequence after demobilizing this phase of the work.

After all equipment from the deck had been removal, Spirit Energy 76 mobilized a larger dive spread, and underwater cutting commenced following a weather review by all concerned parties.

The contractors cut all conductors housing wells so as to remove a section of the entire conductor and internal casing strings. MMS required this procedure so that during the toppling process, stresses would not be transmitted to the surface cement plugs below the mudline.

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All conductors and vertical diagonals were cut in sequence. The two conductors that served as hinges (Figs. 5 and 6) were cut to create a weak point that would allow a member to tear through the first 20° of rotation and then fail in shear or tension.

The diving operation then moved to the north side of the structure and removed a section of the B1 and B2 jacket legs. This exposed the main piling, which the divers single-line completely cut except for a 12-in. wide section on the outboard (north) side of the piling.

Not cutting this section prevented any movement of the upper piece relative to the bottom piece during the final rig up of the marine toppling equipment and weather related events. The divers passed a knife through all single-line cuts to ensure that the member was thoroughly severed.

After crews rigged up the marine toppling equipment, the divers cut the remaining sections and removed them from the water.

The diving operation for the cuts consisted of 176 dives and 205 hr of dive time, expended over 26 days. This work consumed 2,600 underwater burning rods and was performed accident free.

Toppling equipment

An anchor-handling vessel, equipped with two, 500-ton winches, provided the horizontal force necessary to topple the platform.

The vessel deployed a 15-ton Stevpris anchor and pull tested the anchor with 180 tons of bollard pull. After installing a buoy on the anchor, the vessel disconnected from the anchor and recovered the main topple wire from the platform.

The vessel then spooled out about 5,000 ft of work wire and reconnected to the anchor with the other winch. The vessel maintained position dynamically awaiting orders to commence the toppling sequence. Hand-held VHF on company frequency provided the communications links between all marine vessels on location.

During the first two attempts to topple the platform on Aug. 1, 2000, mechanical failure within the rigging system occurred in two components, the anchor chain and work wire. The cause of these failures was attributed to point loading on the anchor chain and overstress by bending on the work wire.

These attempts also failed because all of the deck weight was not included in the toppling analysis.

After replacing the failed rigging components, Spirit Energy 76 decided to deballast the subsea drill-water tanks that were flooded during the first two attempts. This action provided about 200 tons of buoyancy on Row B of the structure.

At 9:40 on Aug. 2, 2000, the platform was successfully toppled with a 460-ton horizontal pull. Only 37 sec were required from first motion to complete submergence of the toppled section.

Post topple survey

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Shortly after the toppling was completed, divers surveyed the reef site and found that the toppled section had landed deck down and was about 20 ft south of the bottom section of the jacket (Fig. 7). No rotation or movement from the north-south vertical plane of rotation was noted.

The entire post-toppling dive was videotaped. The survey indicated all points of the reef were a minimum -100 ft subsea, with the highest points of the toppled piece being a minimum -105 ft subsea.

The survey also found that the conductors, which served as hinges, as being torn in the horizontal plane. They had failed partially in compression after some rotation, and failed fully and finally in tension after tearing slightly down the vertical south face of the conductor.

Estimates are that the final hinge-material failure occurred between 90 and 110° of rotation.

Spirit Energy 76 installed a buoy to mark the site of the newly created reef.

The author

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Chris Whitney is superintendent of engineering and construction for Spirit Energy 76, Lafayette, La. He previously held various positions in production and reservoir engineering both onshore and in the Gulf of Mexico. Whitney has a BS in civil engineering from Louisiana State University. He is a member of SPE and is a registered professional engineer in Louisiana.