3

Technical Considerations

RECENT REMOVAL PATTERNS

The Minerals Management Service (MMS) requires that platforms be removed within one year after termination of a lease. Lease operators may also elect to remove a platform if the cost of upgrading or maintaining a structure to meet current design or safety standards exceeds reserve revenues or if the structure is obsolete or damaged.

Platform Removals by Water Depth

Figure 3-1 tracks all platform removals since 1985 by water depth. The majority of removals have been in less than 100 feet of water. The percentage of nonexplosive removals is highest in less than 25 feet of water (55 percent) and decreases with increasing water depth. It is easy to understand why nonexplosive technology is most efficient in shallow water. Divers can work more easily and safely in shallow water, and smaller, less expensive construction equipment is needed to lift platforms. Nonexplosive methods in shallow-water removals pose fewer financial and safety risks than they do in deep water.

Platform Removals by Year

Approximately 70 percent of platform removals since 1987 have involved the use of explosives (figure 3-2). Prior to 1987, no record of removal methods is available. Nonexplosive methods used for the remaining 30 percent include mechanical cutters, abrasive cutters, and cuts made by divers with cutting torches.

Platform Removal Trend

The number of platform removals is growing (figure 3-3). In 1992, for the first time, the number of removals exceeded the number of installations. A 1985 National Research Council report on the disposition of offshore platforms projected the increase in platform removals to continue almost unabated for 25 years. The ratio of deep-water to shallow-water structures will also increase as more platforms in deeper water reach the end of production. Because removal costs in deeper water are higher, the total cost of removals of platforms in the Gulf of Mexico will increase rapidly (NRC, 1985).

TYPICAL ABANDONMENT PROCESS

The complete abandonment of a platform well involves five steps: (1) securing permits—securing necessary approvals for plugging and abandoning wells and pipelines, structure removal, and site clearance procedures; (2) plug and abandon wells—setting various cement plugs in the wells; (3) decommission platform—cleaning and purging hydrocarbons from production equipment and abandoning the pipeline(s); (4) remove the platform—the focus of this study; and (5) clear the site—removing any remaining debris and verifying a clean site by trawling.

Removal Options

For a more complete assessment of present platform removal methods, the disposition options and various factors and requirements for selecting removal methods are described in the next two sections. Disposition options include leaving-in-place, partial removal (including toppling in place), and complete removal.

Leave-in-Place Option

Commercial and recreational fishermen, environmentalists, and others concerned with maintaining or expanding the habitat that platforms provide (and avoiding the damage they perceive from explosive platform removals) would prefer that platforms be left in place as artificial reefs, thus expanding the marine ecosystem by adding hard-bottom habitat. This scenario, however, raises significant problems. Federal law requires that nonoperating platforms be removed. But even if the law were changed, problems would still exist. Who would maintain the structures? Who would be liable for accidents, collisions, and other potential hazards? How would navigational problems be resolved? How would conflicts with other users of the ocean— such as shrimpers and commercial fishermen—be resolved?



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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES 3 Technical Considerations RECENT REMOVAL PATTERNS The Minerals Management Service (MMS) requires that platforms be removed within one year after termination of a lease. Lease operators may also elect to remove a platform if the cost of upgrading or maintaining a structure to meet current design or safety standards exceeds reserve revenues or if the structure is obsolete or damaged. Platform Removals by Water Depth Figure 3-1 tracks all platform removals since 1985 by water depth. The majority of removals have been in less than 100 feet of water. The percentage of nonexplosive removals is highest in less than 25 feet of water (55 percent) and decreases with increasing water depth. It is easy to understand why nonexplosive technology is most efficient in shallow water. Divers can work more easily and safely in shallow water, and smaller, less expensive construction equipment is needed to lift platforms. Nonexplosive methods in shallow-water removals pose fewer financial and safety risks than they do in deep water. Platform Removals by Year Approximately 70 percent of platform removals since 1987 have involved the use of explosives (figure 3-2). Prior to 1987, no record of removal methods is available. Nonexplosive methods used for the remaining 30 percent include mechanical cutters, abrasive cutters, and cuts made by divers with cutting torches. Platform Removal Trend The number of platform removals is growing (figure 3-3). In 1992, for the first time, the number of removals exceeded the number of installations. A 1985 National Research Council report on the disposition of offshore platforms projected the increase in platform removals to continue almost unabated for 25 years. The ratio of deep-water to shallow-water structures will also increase as more platforms in deeper water reach the end of production. Because removal costs in deeper water are higher, the total cost of removals of platforms in the Gulf of Mexico will increase rapidly (NRC, 1985). TYPICAL ABANDONMENT PROCESS The complete abandonment of a platform well involves five steps: (1) securing permits—securing necessary approvals for plugging and abandoning wells and pipelines, structure removal, and site clearance procedures; (2) plug and abandon wells—setting various cement plugs in the wells; (3) decommission platform—cleaning and purging hydrocarbons from production equipment and abandoning the pipeline(s); (4) remove the platform—the focus of this study; and (5) clear the site—removing any remaining debris and verifying a clean site by trawling. Removal Options For a more complete assessment of present platform removal methods, the disposition options and various factors and requirements for selecting removal methods are described in the next two sections. Disposition options include leaving-in-place, partial removal (including toppling in place), and complete removal. Leave-in-Place Option Commercial and recreational fishermen, environmentalists, and others concerned with maintaining or expanding the habitat that platforms provide (and avoiding the damage they perceive from explosive platform removals) would prefer that platforms be left in place as artificial reefs, thus expanding the marine ecosystem by adding hard-bottom habitat. This scenario, however, raises significant problems. Federal law requires that nonoperating platforms be removed. But even if the law were changed, problems would still exist. Who would maintain the structures? Who would be liable for accidents, collisions, and other potential hazards? How would navigational problems be resolved? How would conflicts with other users of the ocean— such as shrimpers and commercial fishermen—be resolved?

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES FIGURE 3-1 Number of platform removals, excluding caissons, by method and water depth (1985–1994). Source: Courtesy of Minerals Management Service. Given all of these problems, the leave-in-place option is probably not feasible now, except in a very few cases, such as when a structure has become a popular spot for recreational fishermen. Some way of handling the liability problem, such as an industry-financed fund, would have to be established to make leaving-in-place a viable option. Table 3-1 summarizes the positive and negative aspects of the leave-in-place option. Partial Removal Option The partial removal of platforms (in a manner that does not create hazards to navigation) provides less extensive habitat but reduces residual liability and maintenance costs for operators. Substantial savings, compared with complete removal, could be realized if this option were permitted, particularly for larger platforms located in deeper water. Shrimpers are the primary opponents of partial removals in waters shallower than 300 feet, because partially removed platforms could create obstructions that decrease trawlable waters. Partial removal involves removing the top sections of a platform to between 50 and 150 feet below the water surface. The exact depth depends on state and federal requirements. The U.S. Navy and Coast Guard were contacted by the committee and asked to comment on issues relevant to the navigation of commercial and navy ships and federal responsibilities under international agreements. Any modification of current regulations requiring removal to 15 feet below the seafloor must take into account the safety of navigation and the operational needs of the U.S. armed forces, particularly submarine passage. Any change in U.S. practice should minimize interference with the navigational rights and freedom of other states in U.S. territorial waters and the exclusive economic zone. Partial removals can be done by nonexplosive methods (or using smaller explosive charges) more easily than total removals. For example, divers can work fairly efficiently in water depths of less than 150 feet. Problems with partial removals remain, however. The wells still have to be severed and removed, and there are no efficient means of severing them other than with bulk charges. Also, structural members in addition to the legs and piles must be cut, and the risks increase if divers are used, especially for the last few cuts. Another problem is that on many platforms the annular space between the jacket leg and pile is filled with grout, and severing grouted annulars presents the same problems as severing wells.

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES FIGURE 3-2 Number of platform removals, including caissons, by method of removal by year (1/1/85–11/21/94). Source: Minerals Management Service. FIGURE 3-3 Number of platform removals, including caissons, Gulf of Mexico (1/1/53–11/21/94). Source: MMS (1994).

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES TABLE 3-1 Assessment of the Leave-in-Place Option Advantages Disadvantages No harm to marine life Maintains unnatural habitat Immediate cost savings Maintenance costs escalate with age requires protective coating above water requires cathodic protection under water requires navigation-aid lights and horns remains susceptible to storm damage Provides recreational fishing, diving habitat Continues conflicts with other users Provides emergency safe havens Potential liabilities unauthorized boarding collisions surface and subsurface navigation hazards Maintains status quo structure remains visible requires no research and development requires no site clearance provides migratory animal habitat (surface) provides reef habitat (subsurface) May require eventual removal with reduced structural integrity increased safety risk increased cost   Negatively affects construction/removal industry no recycling of steel   Requires changes in regulations and laws Partial removals would be less expensive for operators, especially in deeper waters. There would be some benefit to the marine environment because the portion of the structure left would continue to provide habitat for marine life. There would be some benefit to commercial fishermen, but not to shrimpers, because of the possibility of trawl nets catching on the remaining structures. The structures to which such an option would logically apply are located in deeper waters relatively far from shore, so sport fishermen would reap little or no direct benefit. They would, however, benefit overall from the existence of more fish habitats. This option is likely to be exercised in only a limited number of casesmostly in areas designated for the rigs-to-reef progam or in deeper water, where the economic and safety benefits to an operator may be significant and there is some benefit to the marine environment. In rigs-to-reef areas, there is little or no problem for shrimpers because of the water depth. Some nonexplosive methods probably can be used more efficiently for partial removals than for total removals because they would be in much shallower water. Adequate clearance for navigation would have to be maintained. Table 3-2 summarizes the positive and negative aspects of partial removal. Topple-in-Place Option An option similar to a partial removal is to topple a structure in place. The deck portion could be removed for reuse, scrapped, lowered to the sea bottom, or toppled with the jacket. The topple-in-place option involves severing the structure near the sea bottom and pulling it over on its side until it rests on the seafloor where it would become a habitat for marine life. This option would be less expensive for owners than total removal (no transportation charges) and would be of some benefit to the marine environment. But it would be of no benefit to shrimpers. Toppling-in-place would, however, benefit commercial and, particularly, sport fishermen if the structure is relatively close to shore (e.g., in a rigs-to-reef area) where the 85-foot clearance requirement allows for toppling shallow-water structures. If the wells are not severed but simply bent as the structure is toppled (assuming this is structurally feasible), the use of explosives could be minimized. Complete Removal Option This option requires removal to a sufficient depth below the mudline to eliminate any interference with other users of the site, including fishermen, shrimpers, ships, and naval operations. The area around the platform must be cleared of debris and verified clean by trawling. The obvious advantages of complete removal are that the site is returned to a natural condition, there is no interference with shrimping or navigation, and there is no maintenance or liability problem. The disadvantages include cost, possible harm to marine

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES TABLE 3-2 Assessment of the Partial Removal Option Advantages Disadvantages Potentially reduces harm to marine life during removal and maintains some reef habitat Does not return habitat to natural state Eliminates habitat structure in upper range of water column Potentially cost effective requires no maintenance requires no site clearance Must maintain buoys Useful only in water depths allowing sufficient clearance Potentially increases diver risk during removal May provide recreational fishing and diving habitat Decreases shrimping access Operators released from liability Liability attaches to regulatory agency court test inevitable creates navigational hazards (surface and subsurface) Encourages innovative removal methods Loss of resources eliminates surface habitat no recycling of steel life during removal, and the elimination of reef habitat if the platform is scrapped on shore. Towing all or part of a platform to a designated disposal site under the rigs-to-reef program lowers the cost of disposal for operators but requires either permission from the state managing the designated rigs-to-reef site or permission from the state to establish and maintain such a site. Current regulations require that structures be removed to 15 feet below the mudline. The committee was unable to determine the basis or origin of this requirement or its technical rationale. Changing the depth of removal to a depth that still meets shrimpers needs (i.e., preventing damage to trawlers) would be advantageous in several ways: using nonexplosive or less damaging explosive methods would be easier; the need for jetting the soil inside the pile, which is required for abrasive, mechanical, or diver internal cuts, would be lessened; jetting would be required outside the pile to vent oxygen build-up in cases where an internal diver cut is made, which would immediately reduce costs and safety risks. In cases using an external diver cut or diver placement of abrasive cutters, a considerably smaller excavation of the soil would be required, which is less expensive and safer because the danger of cave-ins would be reduced. Table 3-3 presents the positive and negative aspects of complete removal at both the present requirement of removal to 15 feet below the mudline and for a substantially shallower depth of less than 5 feet. An investigation of the displacement of sediment on the ocean floor by natural forces, called scour, shows that scouring to depths of more than 3 or 4 feet is extremely rare (see the discussion of soil strengths in the next section, “Factors in Selecting Removal Methods”). Moreover, regulations require that pipelines be buried to a depth of 3 feet in waters less than 200 feet deep. (There are no requirements for pipeline burial in water depths of more than 200 feet.) Thus there is a strong case to be made in favor of changing the 15-foot requirement to a 3-foot requirement and little or no discernible reason to maintain the existing regulation, which incurs extra costs and risks and encourages the use of bulk explosives. Factors in Selecting Removal Methods Factors to consider in selecting a method for each platform removal include the age of the platform, the water depth, the configuration and type of platform, the weight of the lifts, soil strength, weather conditions, and scour. The age of the platform is an obvious factor in selecting a removal method. The older the platform, the less likely it is that accurate records and drawings are available. If drawings and records are not accurate, the chosen removal method may prove to be inadequate. Another related factor is the condition of the platform. A badly corroded platform may not maintain its integrity during removal, which could create a dangerous situation. In the early days of platform installation, operators made field changes, such as increasing or decreasing the length or thickness of pile sections but made no record of these changes. Platform owners, therefore, tend to select removal methods that can be adapted to various conditions. Water depth is an important factor in the selection of a removal method. Nonexplosive methods were used 55 percent of the time in water depths of less than 25 feet. This may be because shallow-water construction equipment and activities cost much less than deep-water equipment. Consequently, nonexplosive methods carry much less financial and operational risk in shallow water than in deep water. Abrasive

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES TABLE 3-3 Assessment of the Complete Removal Option AT 15 FEET BELOW MUDLINE Advantages Disadvantages Meets shrimper requirements maintains clearance for trawlers Environmental impacts relocates or eliminates reef habitat fish kill from explosives Requires no changes in regulations or laws Expensive to operators explosives require NMFS observer program restricts use of explosives discourages development of nonexplosive techniques requires transportation to shore or reef site Poses no navigational hazards Requires site clearance may require backfill Eliminates liability and site maintenance Hazardous to divers Allows reuse and recycling Potential removal problems from soil skin friction at 15 feet below mudline AT SHALLOWER DEPTH BELOW MUDLINE (less than 5 feet) Immediate cost savings requires less jetting minimizes problems from soil skin friction Requires changes in regulations and laws Encourages use of nonexplosive methods less hazardous to divers easier to clean for access by mechanical or abrasive tools Explosives may still be necessary in some cases although advanced techniques using smaller charges could be used Meets shrimpers requirements nothing remains above mudline Site clearance required Poses no navigational hazards Requires no backfill Eliminates liability and site maintenance Reuse or recycling possible Environmental impact relocates or eliminates reef habitat requires disposal or mechanical cutters can be used to remove shallow-water caissons or small platforms using a lift-boat or a barge with a small crane. Cutting the wells and pilings of a small platform can sometimes be accomplished at the same time the wells are plugged and abandoned using the same equipment. Derrick barge costs increase as the water depth increases, as do diving costs; at the same time, the length of dives decreases. While divers are in the water, descending, working, ascending, or decompressing, no construction work can be safely undertaken topside. Increases in platform size and weight are also strongly correlated with water depth, adding to costs. The reliability of mechanical or abrasive cutters decreases as water depth increases. Problems can arise with the delivery of abrasives to the cutting nozzles of cutters driven by downhole motors. Both mechanical and abrasive cutters require cranes to position cutting equipment. The type and configuration of a given platform are important considerations in selecting a removal method. Abrasive and mechanical cutters have been used effectively on shallow-water platforms, such as caissons and small well-protector jackets. Larger caissons have been most effectively cut by divers. On larger platforms, however, especially platforms with wells, the preferred method has been

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES explosives. When severing wells, bulk explosives can be sized for unexpected field changes (such as unanticipated member sizes), give a clear indication the wells are cut, and are cost effective when compared with other methods. If the wells are not severed prior to the arrival of derrick barges, the cost increases tremendously because of the added time (while the barge is waiting) of severing the wells by nonexplosive means. According to a presentation to the committee by a supplier of abrasive cutting services, the time required to cut one 30-inch-diameter multistring well with an abrasive cutter is two hours, plus rig-up and rig-down time. Cutting six wells sequentially (with no problems) would take at least 12 to 18 hours, plus several hours rig-up and rig-down time. This would result in an increase of $50,000 to $70,000 for an 800-ton derrick barge. In comparison, it would take about two hours to load and sever all six wells using explosives. If casing strings are cemented together, mechanical cutters can not always cut more than two casing strings at a time. Casing strings that are not concentric, a common occurrence, also cause problems for mechanical cutters. Configured explosive charges, shaped charges, explosive cutting tape, and fracturing tape have not been used to sever conductors because inside access is limited by the smallest casing string (usually 7 inches in diameter). Divers cannot cut wells efficiently from the outside because of problems caused by the cement between casing strings and the huge crater in the soil required to reach 15 feet below the mudline (the side slopes of an excavation in weak soil must be gradual to ensure stability). To reach an inner casing string, the cement in the annulus must be chipped away by hand. If the operator chooses the more costly option of using a nonexplosive method to sever wells, the severing operation should be done in the decommissioning process in order to minimize construction costs. In many cases, the well and the wellhead are supported by the lower casing strings cemented to the foundation. So if the casing is severed, this support is lost and the weight above the cut must be transferred to the deck. This may require strengthening to reinforce the deck. A serious disadvantage of nonexplosive methods is that determining if wells have been completely severed often requires using a large crane to test the result. It is also possible that a severed deck may have to support a portion of the well load. Pilings in conventional platforms have been cut with explosives, mechanical cutters, and abrasive cutters, as well as divers, with varying degrees of success and at various costs. Mechanically or abrasively cutting piling through access windows cut in the deck legs or jacket legs could be done without a derrick barge on site, leaving the deck in place. If successful, questions of safety and liability arise because the platform may not be stable and could be overturned in a storm. In some cases, cutting a single pile could lead to a sudden displacement that could endanger a diver. If enough of the right piles are cut, the platform could fail under its own dead weight. Skirt-piled platforms are generally constructed in deep water (more than 200 feet) and are not good candidates for mechanical, abrasive, or diver cuts. Weight of the lifts and strength of soil are also factors to consider in selecting a removal method. Clay soils are prevalent in the Gulf of Mexico. The skin friction that soil exerts on pilings and wells near the mudline may vary from less than 100 pounds per square foot at the mouth of the Mississippi River to 2,000 pounds per square foot offshore western Louisiana and eastern Texas. In the high-shear-strength areas, stiff to very stiff clay can add considerably to the force required to pull pilings, caissons, conductors, or jackets out of the mud. For example, for each foot of soil penetration of a 48-inch-diameter pile or caisson, about 12 tons of added force are required to remove it from the soil. Removing a 48-inch-diameter pile or caisson cut 15 feet below the mudline in stiff clay can take 200 tons of force plus the weight of the pile or caisson. If the explosion from bulk explosives deforms the pile or well at the cut into a bell shape, the distortion can add to the soil removal forces. A platform that has four 48-inch-diameter piles grouted to the jacket legs with a combined buoyant weight of 400 tons may require as much as 800 tons of force to remove using bulk charges. Eight-hundred tons is the maximum capacity of several derrick barges in the Gulf of Mexico. One option is to use mechanical or abrasive cutters to avoid belling the piles. Another option has also been using “focusing” charges that shatter the piles rather than bell them. Figure 3-4 and Figure 3-5 show the number of existing platforms by soil type, shear strength, depth below the seafloor, and FIGURE 3-4 Number of platforms by soil type. Source: Courtesy of Fugro– McClelland.

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES FIGURE 3-5 Number of platforms by soil type and water depth. Source: Courtesy of Fugro–McClelland. water depth. Soil strength and varying water depth are considerations that clearly affect removal loads. The weather in the Gulf of Mexico can change quickly. A placid day with 2- to 4-foot seas can quickly deteriorate to 10-to 15-foot seas with 40-knot winds. Even under ideal conditions, it takes several days to remove a deck, cut a well and pile, lift the jacket off the seafloor, set it on a floating cargo barge, and secure it for safe transit. Minimizing vulnerability to adverse weather conditions is essential for protecting the safety of operating personnel, equipment, and the environment. Operating floating construction equipment in a hostile environment requires realistic and adequate planning, flexibility, and a reliable back-up plan. Scour and deposition rates are also factors that must be taken into account. MMS regulations for removing offshore structures require that “all casing, wellhead equipment, and piling shall be removed to a depth of a least 15 feet below the ocean floor, or to a depth approved by the district supervisor after a review of data on the ocean bottom conditions.” This requirement is intended to ensure that obstructions do not protrude above the seafloor and that subsequent erosion at the site will not result in a seafloor obstruction. A knowledge of deposition and erosion is necessary for recommending a reasonable depth of removal of all structural components that penetrate the seafloor. The geology of the continental shelf in the northern Gulf of Mexico has been studied extensively; several hundred thousand miles of high-resolution seismic profiles and more than 2,000 foundation bores on the outer continental shelf (OCS) have been collected and studied. Methods of assessing erosion and deposition processes on the OCS include: radiocarbon and other tests on cored material; examination of repeated bathymetric surveys; operator experience regarding exposure or burial of existing objects on the seafloor (such as pipelines and platform legs); and site-specific studies of deposition and erosion after storms or other high-energy events, such as hurricanes and earthquakes.

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES Geologic and geotechnical studies conducted on the Gulf of Mexico continental shelf (Coleman and Roberts, 1988) have shown that Holocene deposits (sediments generally less than 12,500 years old) vary in thickness across and along the shelf. During the Holocene epoch and during the last rise in sea level, the shelf was a net receiver of sediments. Generally, the largest deposits (6 inches per year) on the continental shelf occurred near the mouth of the Mississippi River (excluding mudslide areas); the smallest deposits (less than a fraction of an inch per decade) occurred off the coast of Texas and in deeper water near the shelf-edge break. Regional studies have shown that there has not been much erosion; however, these are long-term averages and do not account for short-term changes or localized effects. Nevertheless, one can conclude from these studies that most of the continental shelf does receive sediment and is not subject to long-term erosion. Detailed bathymetric surveys have been conducted within virtually every offshore block that has been leased, and in many instances repeated surveys of a block have been made. Geotechnical reports and examination of these surveys indicate that over a relatively short period of time (one to five years), there is no significant erosion except locally around structures. Erosion varies greatly and rarely exceeds 3 to 5 feet at any one site. Erosion usually occurs in water depths of less than 30 feet. No scouring of more than 2 feet in water depths of more than 30 feet has been observed. Thus, in a period of a few years, erosion appears to be confined to relatively shallow water (less than 30 feet) with magnitudes of less than a few feet (Tubman and Suhayda, 1976; McClelland Engineers, 1979). Scour around existing structures has been monitored by various operators, and to the committee's knowledge, localized scour of more than 3 feet has not been observed. In deeper water (more than 100 feet), no scour has been observed, even after major storms. One operator (Shell Offshore, Inc., 1995) indicates that no scour of more than 3 feet has been observed around any of the company's hundreds of platforms. Moreover, once a platform is removed, the major cause of scour is removed. In addition to existing platforms, there are nearly 17,000 miles of marine pipeline in OCS waters and another 5,000 miles in state waters. Regulations require that these pipelines be buried at least 3 feet under the seabed out to water depths of 200 feet. In 1991, the U.S. Department of Transportation Office of Pipeline Safety began requiring that operators resurvey pipelines in water depths of less than 15 feet and rebury pipelines with a foot or less of sediment cover to a depth of 3 feet. With more than 95 percent of the survey completed, only 24.4 miles of the 1,456 miles surveyed were found to have a foot or less of sediment cover. This survey was conducted in water depths where the greatest scour from storms occurs, but only 1.7 percent of the pipelines were exposed or scoured to a depth of 2 feet. Thus, scour around obstacles on the seafloor tends to be minimal (generally less than 3 feet). Even in extremely shallow water (less than 15 feet), repeated pipeline surveys show that scour is minimal (NRC, 1994). Studies of scour have rarely been conducted on the continental shelf, but analysis of data associated with meter moorings does provide some information on bottom current speeds and localized scour associated with major storms and hurricanes. A number of studies (Partheniades, 1971; Tubman and Suhayda, 1976; Young and Southard, 1978; Wells et al., 1979) document the effect of bottom currents on fine-grained sediment, which comprises the vast majority of the bottom soil on the continental shelf. All of these studies indicate that the fine-grained sediments are extremely resistant to scour by bottom currents generated by storms. Along the Florida coast, bottom currents during Hurricane Camille were as high as 5 feet/sec (Murray, 1970); off Texas, Hurricane Anita generated currents, some two to three times the normal speed (Wells et al., 1981). Even these abnormally swift currents did not cause significant scour of muddy sediments. Current meters placed during these measurement periods remain in place but have not caused significant scouring. Thus, although the number of studies is limited, all scientific data tend to indicate that scouring to depths of more than 3 or 4 feet is extremely rare on the Gulf of Mexico continental TABLE 3-4 Comparative Costs of Platform Removals Using Explosives (in dollars) Water Depth (ft) Caisson Well Protector 4-Pile Production Platform 8-Pile Drilling and Production 50 100,000 180,000 470,000 760,000 150 — — 780,000 1,030,000 250 — — 1,275,000 1,945,000 Source: Courtesy of Offshore Operators' Committee.

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES TABLE 3-5 Cost Comparison for Alternative Removal Methods (in dollars) Alternative Bulk Explosives Mechanical Cutting Abrasive Cutting Diver Cutting Rigs to Reef COST 4-Pile Production Platform • 50′ water depth 470,000 662,000 605,000 785,000 n/a • 100′ water depth 620,000 871,000 780,000 1,070,000 n/a • 150′ water depth 780,000 1,079,000 950,000 1,415,000 535,000 • 200′ water depth 960,000 1,295,000 1,185,000 n/a 610,000 • 250′ water depth 1,275,000 1,660,000 1,520,000 n/a 875,000 8-Pile Drill/Production • 50′ water depth 760,000 1,125,000 965,000 1,105,000 n/a • 100′ water depth 810,000 1,211,000 1,025,000 1,320,000 n/a • 150′ water depth 1,030,000 1,497,000 1,270,000 1,690,000 7 20,000 • 200′ water depth 1,350,000 1,910,000 1,625,000 n/a 950,000 • 250′ water depth 1,945,000 2,575,000 2,255,000 n/a 1,335,000 Remarks/Reliability The abandonment costs include severing piles and conductors with the method listed at the top of each column. Assumed each structure had six (6) conductors (30"/10.75"/7"). Site clearance costs are included. Very reliable High experience level Excellent safety record Lowest exposure time of all severing methods Prone to having problems Very labor intensive Safety concerns Not very reliable to date This assumes using very high pressure with low volume abrasive cutters Still in testing phase Guaranteed cut if can be done safely Diver safety concerns Very high risk of blowback due to grout and/or mud Very labor intensive Jacket and deck are toppled in place Piles and conductors cut by explosives COST Six 30" diameter conductors in 83′ of water 18,900 104,500 55,300 300,000 n/a Remarks/Reliability Conductor makeup = 30"/10.75"/7" All strings grouted. Conductor removal costs are already included in each of the above estimates of total abandoment costs. Very reliable Excellent safety record Lowest exposure time of all severing methods Prone to having problems Very labor intensive Not very reliable Safety concerns about high pressure hoses or piping Reliable Safety is a major concern Very high risk of blowback due to grout and/or mud   shelf, and no scour has been observed in water more than 30 feet deep. In some areas of the Gulf of Mexico, immediately off the Mississippi River delta, active mudslides cause localized removal of material to subbottom depths of more than 20 feet. These areas have been mapped (Coleman and Prior, 1988) and are readily identifiable, and offshore structures in these areas are designed to withstand such forces. None of these platforms is scheduled to be removed in the near term, but requirements for removing them should be given special consideration. TYPICAL PLATFORM REMOVAL COSTS It is impossible to provide a complete table of the costs for all removal methods for all types and sizes of platforms in all water depths and soil conditions. But table 3-4, table 3-5, and table 3-6, which were submitted to the committee by an offshore operator, provide a general idea of the order of magnitude of the costs. It is important to recognize that these figures do not represent the costs of abandonment or removal costs. Well-plugging and transportation costs are not included. Costs for

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES TABLE 3-6 Detailed Cost Comparison of Alternative Removal Methods (in dollars) Alternative/Criteria Bulk Explosives Mechanical Cutting Abrasive Cutting Rigs to Reef Cost—Operator/Consumer 8-Pile production/drill platform (150′ Water Depth, 6 Wells)   Decommissioning cost 100,000 150,000 140,000 100,000 Derrick barge removal cost 616,000 859,000 714,000 497,000 NMFS cost 13,000 n/a n/a 13,000 Direct severing cost 12,000 180,000 110,000 12,000 Pipeline abandonment cost 43,000 43,000 43,000 43,000 Site clearance cost 180,000 180,000 180,000 0 Positioning cost 16,000 20,000 18,000 10,000 Miscellaneous support cost (engineer, supervisor, etc.) 50,000 65,000 65,000 45,000 TOTAL COST 1,030,000 1,497,000 1,270,000 720,000 removing larger, complex platforms in the same water depths may be twice as high as the costs indicated. NOTES: 1. Severing cost includes only the direct cost from the severing method. Additional support costs such as additional decommissioning and derrick barge costs are included in their respective categories. 2. The above costs assume no major problems are encountered during the severing operations. 3. Rigs-to-reef alternative does not include payment (lump sum donation) to state agency for site/buoy maintenance. This payment is usually 50 percent of the difference of disposal onshore versus disposal at a reef site. 4. Cost estimates provided by Chevron U.S.A., Inc. Table 3-4 is a comparison of cost estimates for removing four types of platforms in 50, 150, and 250 feet of water using bulk explosives to sever piles and conductors. Removals of caisson and well-protector platforms in deep water were not included because they are relatively in small number and the removal costs vary widely. Table 3-5 compares cost estimates of removing a 4-pile production platform and an 8-pile drilling and production platform (with 6 wells) in 50 to 250 feet of water using present explosive and nonexplosive cutting techniques. The costs in each estimate assume a “trouble-free ” severing operation with successful cuts on the first try. Allowances for the actual costs of unanticipated field problems and safety risks must also be considered in cost projections. A further breakdown of the costs of removing an 8-pile drilling and production platform with 6 wells in 150 feet of water is shown in table 3-6. This table details the estimated cost of each phase of the typical abandonment process for a representative platform using various cutting techniques. The cost estimates are based on a trouble-free operation and successful first cuts. The data indicate that, at this time, explosive cutting is the most economical and safest (to personnel) removal method. REFERENCES Coleman, J.M., and D.B. Prior. 1988. Mass wasting on continental margins. Annual Review of Earth and Planetary Sciences 16: 101–119. Coleman, J.M., and H.H. Roberts. 1988. Sedimentary development of the Louisiana continental shelf related to sea level cycles. Geo-Marine Letters 8(2): 1–119. McClelland Engineers. 1979. Strength characteristics of near seafloor Continental Shelf deposits of North Central Gulf of Mexico. Report No. 0178–043. Houston, Texas.

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AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES MMS (Materials Management Service) 1994. Application for platform removal listing, December 15. U.S. Department of the Interior, Washington, D.C. Murray, S.P. 1970. Bottom currents near the coast during Hurricane Camille. Journal of Geophysical Research 75(24): 4579–4582. NRC (National Research Council). 1985. Disposal of Offshore Platforms. Marine Board. Washington, D.C.: National Academy Press. NRC. 1994. Improving the Safety of Marine Pipelines. Marine Board. Washington, D.C.: National Academy Press. Partheniades, E. 1971. Erosion and deposition of cohesive materials. Pp. 25–91 in River Mechanics, Vol. 2, H.W. Shen, ed. Boulder: Colorado State University Press. Shell Offshore, Inc. 1995. Letter from Kent Satterlee, Acting Chief, Technology Assessment and Research Branch, Minerals Management Service, U.S. Department of the Interior, March 2. Tubman, M.W., and N.J. Suhayda. 1976. Wave action and bottom movements in fine sediments. Pp. 1168–1183 in Proceedings of the 15th Conference on Coastal Engineering. New York: American Society of Civil Engineers. Wells, J.T., J.M. Coleman, and W.J. Wiseman, Jr. 1979. Suspension and transportation of fluid mud by solitarylike waves. Pp. 1932–1952 in Proceedings of the 16th Coastal Engineering Conference. New York: American Society of Civil Engineers. Wells, J.T., R.L. Crout, and G.P. Kemp. 1981. An assessment of coastal processes, dredged-sediment transport, and biological effects of dredging, Coast of Louisiana. Sea Grant Publication No. LSU-T-81-001 (Coastal Studies Institute Technical Report No. 314). Baton Rouge: Louisiana State University Press. Young, R.W., and J.B. Southard. 1978. Erosion of fine-grained marine sediments: sea-floor and laboratory experiments. Bulletin Geological Society of America 89: 663–672.