An Assessment of Techniques for Removing Offshore Structures (1996) / Chapter Skim
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Assessment of Cutting Techniques
Assessment of Cutting Techniques
Pages 16-29
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From page 16... ...
Fracturing charges and cutting tape, such as contact and refraction tape, may have future applications. Available nonexplosive cutting techniques include mechanical cutters using hydraulically driven revolving blades; abrasive cutters using sand or slag with high-volume, low-pressure water pumps; abrasive cutters using low-volume, high-pressure water pumps with garnet injected at the nozzle; and diver cuts using oxy-arc torches.
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From page 17... ...
charges: Configured Bulk Charges Improvements in the configuration of explosives such as ring charges built to collide or "focus" the explosive detonation front have been effective in localizing pile belling and reducing the weight of charges. Ungrouted piling, which may need to be removed separately from the jacket to reduce lift weight, can be removed by collision charges.
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From page 18... ...
This type of explosive may reduce charge weight compared with bulk explosives, but has not yet been used to remove a standing structure. Another type of fracturing charge being developed is called a shock refraction charge, which is the size of a plaster charge and is shaped to resemble shaped charges and cutting tape.
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From page 19... ...
The time, scheduling, and expense of coordinating the Minerals Management Service and NMFS observers during explosive removals, and the restrictions on using explosives encourages operators to consider alternative methods when factors such as water depth, platform age, type, and configuration make alternative methods feasible. Nonexplosive methods presently used include mechanical cutters, abrasive slurry cutters that use high volumes of sand 19 or slag at relatively low pressure, and abrasive slurry cutters that use low volumes of garnet or other materials at high pressure.
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From page 20... ...
. · no serious Curies recorded Reliability · accepted by field operators and contractors · no moving parts · predictable results Flexibility · can be used for all platform types · bulk explosives can be molded to fit field conditions · easily deployed · can be used on all piles and conductors Cost · instantaneous severing · requires minimal engineering, planning, and scheduling · not affected by weather · not affected by water depth · lowest initial and overall cost Environment · kills fish · requires protection of turtles and marine mammals Flexibility · shaped charges applicable only to piles, difficult to prepare, deploy, place · performance depends on air gap, stand-off distance · requires long lead time Cost · costs 4 to 5 times as much as bulk explosives Regulation · requires special regulation · Minerals Management Service and NMFS observers required · no night shots allowed · aerial surveys and/or diver searches required · requires U.S.
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From page 21... ...
because mechanical cuts can take several hours each and the rig-up and rig-down times may also be considerable (Herbs", 19861. Abrasive Cutters Mechanisms that inject cutting materials into a water jet and abrasively wear away steel are called abrasive cutters (also called sand cutters, abrasive jet cutters, or abrasive slurry cutters)
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From page 22... ...
Wells with fully cemented casing strings are easier to cut with an abrasive jet cutter than wells without a cemented annulus. Fully cemented wells maintain the jet stream focus; but partially cemented wells may cause the cutting jet stream to be diffused, which slows or stops the cut.
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From page 23... ...
The goal of developers is to produce shears that can cut tubular elements up to 42 inches in diameter and 1.5 inches thick. Diamond Wire Cutter Diamond wire cutters use a steel wire with small beads embedded with diamond particles mounted on the wire at regular intervals.
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From page 24... ...
Source: Courtesy of Association of Diving Contractors. The cost, reliability, safety, and routine application of diamond wire cutters for removals in the Gulf of Mexico cannot yet be determined because, given the risks and high cost of failure, companies have been unwilling to test them in actual field operations.
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From page 25... ...
Chemical Cutters The chemical cutter technique uses highly corrosive liquids such as hydrofluoric compounds that are squirted from chemical flasks pressurized by a pyrotechnic mixture. Multiple jets are used to produce a series of closely packed perforations inside the casing.
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From page 26... ...
But there is little doubt that when ROVs can be configured to perform AN ASSESSMENT OF TECHNIQUES FOR REMOVING OFFSHORE STRUCTURES tasks that divers are unable to do because of water depth they will be widely used in platform removals. Bubble Curtains Bubble curtains have been used with some success in shallow, protected waters to attenuate the blast pressure of detonations (Regalbuto et al., 1977~.
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From page 27... ...
This is the first step in designing platforms for more efficient and less environmentally damaging removal in the future. The advantages and disadvantages of various nonexplosive cutting techniques are summarized in table 2-4.
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From page 28... ...
Coast Guard permit required · platform recertification less stringent Safety · risks to divers when cutting below mudline · risks to divers when cutting under mud mats · risks to divers when deploying external cutter · risks to divers if retrieval of cutter is required · risks to divers when hand jetting · risks to divers in poor visibility in shallow water or turbid conditions Reliability · depends on operator/diver skills · difficult to redeploy cutters to same cut · abrasive cutters require cleaner surface · complicated machinery · no clear indication of successful cut · explosives must be used in case of failure Flexibility · requires custom fit for each application · sensitive to platform type · sensitive to water depth · requires extensive planning and scheduling Cost · service costs more · time-consuming deployment and cutting operation · weather sensitive because of length of operation · multiple cuts must be done in sequence rather then simultaneously · requires more personnel and increases risk of personal injury
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From page 29... ...
Technical Assessment and Operations Support Section, Field Operations. Minerals Management Service, Unpublished 29 report, December, U.S.
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