An Assessment of Techniques for Removing Offshore Structures (1996) / Chapter Skim
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Technical Considerations
Technical Considerations
Pages 30-41
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From page 30... ...
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)
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From page 31... ...
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.
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From page 32... ...
n As 50 1 ~1 , ~ ,_,,, .,,, .,,,,,, .., .,,, . 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 94 Year FIGURE 3-3 Number of platform removals, including caissons, Gulf of Mexico ( l/l/53-l l/2 l/94)
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From page 33... ...
Maintains unnatural habitat Maintenance costs escalate with age · requires protective coating above water · requires cathodic protection under water · requires navigaiion-aid lights and horns · remains susceptible to storm damage Continues conflicts with other users Potential liabilities · unauthorized boarding · collisions · surface and subsurface navigation hazards May require eventual removal with · reduced structural integrity · increased safety risk · increased cost Negatively affects construciion/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.
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From page 34... ...
. · requires no maintenance · requires no site clearance May provide recreational fishing and diving habitat Operators released from liability Encourages innovative removal methods Loss of resources Does not return habitat to natural state Eliminates habitat structure in upper range of water column Must maintain buoys Useful only in water depths allowing sufficient clearance Potentially increases diver risk during removal Decreases shrimping access Liability attaches to regulatory agency · court test inevitable · creates navigational hazards (surface and subsurface)
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From page 35... ...
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.
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From page 36... ...
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.
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From page 37... ...
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 37 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.
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From page 38... ...
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)
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From page 39... ...
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 tables 3-4, 3-5, and 3-6, which were submitted to the committee by an offshore operator, provide a general idea of the order of magnitude of the costs.
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From page 40... ...
removing larger, complex platforms in the same water depths may be twice as high as the costs indicated. 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.
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From page 41... ...
1970. Bottom currents near the coast during Hurricane Camille.
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