full cuts. Bulk explosives require minimal engineering, planning, and scheduling and (according to an explosives contractor who presented a report to the committee [Kenny, 1995]) result in a 95 percent success rate when sized properly. Increased water depth has no adverse impact on the success rate of bulk explosive cuts. If a bulk charge does not completely sever the piles or conductor, a back-up charge can be deployed quickly. The cost of bulk explosive cutting services is the lowest of all available alternatives (see table 2-2 and table 2-3). In addition to the environmental impact, the explosive force sometimes “bells” out piles and wells so piles cannot be pulled out through jacket legs. In these cases, the jacket must be lifted with the piles and the “belled” portion cut off.

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.

Ring Charges. Made from the same explosive material as bulk charges (Comp B or C-4), ring charges are formed into doughnut-shaped rings, which concentrates the explosive closer to the inside of the pile wall, thus making it more effective. Using this technique, the total weight of explosive charges can be reduced by approximately 10 to 15 percent.

“Focusing” Charges. These explosives are configured with steel tamping plates above and below the charge. The tamping plates have the effect of delivering more of the force horizontally, which allows reductions in explosive weight comparable to ring charges, with the added benefit of reducing or eliminating “belling.” The concept is proprietary and patented by one explosives contractor.

Both of the configured charges must be prefabricated and are sized to fit each application. There is enough size variation allowance built into each configuration to allow for small miscalculations of inside pile diameters or the dimensions of obstructions. Both types can be lowered into the pile from above, thus eliminating the need for placement by divers. Configured charges, however, cannot be used to severe wells because the diameter of the inner casing is too small to accommodate the charge.

Cutting charges include linear-shaped charges, which have been available for several years, and the more recently introduced cutting tape.

Linear-Shaped Charges. These charges use high-velocity explosive energy to accelerate a v-shaped liner material (usually copper) into a high-velocity jet (Welch, 1995) that can penetrate and cut the steel. A linear-shaped charge can be housed in a specially manufactured ring-shaped container made to fit around the outside of a pile, or it can be used with a running tool and an articulated device for making an inside cut. When accurately positioned to a precisely calculated stand-off distance between the charge and the target, smooth cuts can be obtained. The stand-off distance is a function of the thickness of the steel.

There are several limitations to using linear-shaped charges:

• If an external charge is used to cut a pile, there is no attenuation of the explosive energy afforded by the soil.

• To pass by stabbing guides in the pile, an internal-shaped charge must be the articulated type.

• If the thickness of the pile section is unknown (possible in older structures), if the pile is out of round, if the charge is not placed directly against the target, or if a stabbing guide is at the proposed cut elevation, a successful cut may not be obtained.

• Shaped charges require long lead times (several weeks) to fabricate the containers and articulated devices and cost about four or five times more than bulk charges.

• Divers can place the shaped charges, but safety and cost considerations must be taken in consideration.

• Performance of a shaped charge depends on the presence of an air gap between the liner of the charge and the target (pile or caisson). Water infiltration between the charge and the pile greatly diminishes performance (Welch, 1995).

A refinement of rigid, linear-shaped charges that may be useful in the future is explosive cutting tape. Explosive cutting tape is a flexible version of a linear-shaped charge. The explosive and the liner are extruded into a shaped charge housed in a flexible jacket that allows the tape to contact the pile and maintain a proper stand-off distance. Although the new type charge is flexible, variations in the shape and dimensions of the liner may cause problems, and the jacket may compress in the high ambient pressures of deeper water (more than 300 feet). Divers would be required to place the charges. According to the manufacturer, who prepared a committee presentation, explosive tape is not as efficient as linear-shaped charges and may not perform well in deep water.

Front Matter (R1-R8)

Contents (R9-R9)

Figures and Tables (R10-R10)

Executive Summary (1-4)

Overview of Existing Offshore Structures and Removal Regulations (5-15)

Assessment of Cutting Techniques (16-29)

Technical Considerations (30-41)

Environmental Assessment of Present Removal Techniques (42-49)

Summary Assessment of Explosive and Nonexplosive Technologies (50-54)

Conclusions and Recommendations (55-58)

Appendices (59-60)

A Biographical Sketches of Committee Members (61-62)

B Federal Register Notice and List of Respondents (63-66)

C Participants in Committee Meetings (67-68)

D Regulations Governing Removal of Offshore Structures (69-76)