RIASSUNTO
ABSTRACT
An underwater environmental monitoring program was conducted in June 1976 to assess the effects of removal of the drilling rig George F. Ferris from the waters and sea bed of Kachemak Bay, Cook Inlet, Alaska. The program included the assessment of blast shields and a bubble curtain to limit the effects of the blast on the local biota, the measurement of the blast propagation out to a distance of 1,000ft, and the installation of fish cages at several locations to examine the effects upon a variety of fish species. While peak pressures were considerably higher than those predicted, caged fish survived positive peak pressures approximately 40 times greater than those previously considered lethal. Blast attenuation techniques were found to be moderately effective.
INTRODUCTION
This paper presents the results of an environmental monitoring program to assess the effects of removal of the drilling rig George F. Ferris from the waters and sea bed of Kachemak Bay, Alaska. Measures to mitigate and monitor the impact of underwater explosives were carried out in compliance with stipulations established by the Alaska Dept. of Fish and Game (ADF&G).
Raymond Internation , Inc. (RII), through its subsidiary, Offshore Constructors, Inc. (OCI), and the firm of Jet Research Center, Inc. (JRC) , provided for the design and implementation of the requisite blast modification shields and bubble curtain believed necessary to satisfy the above stipulations. RII contracted with Dames and Moore (D&M), Anchorage, to coordinate the environmental monitoring program with OCI and ADF&G, to conduct the blast measurement program, and to participate in all surface and underwater environmental surveys associated with the project. Bolt, Beranek, and Newman, Inc. (BBN), under subcontract to Dames & Moore, conducted the pressure-monitoring program at several distances from the. Ferris. Surface and bottom pressures resulting from the blast were recorded and used in conjunction with ADF&G's fish-cage experiments at the same locations.
Post-blast inspections by ADF&G and D&M included the surface collection and inspection of injured fish; bottom surveys with underwater television and sidescan sonar; diver inspections of the caged fish; and subsequent laboratory examination of fish 48 hours after exposure to the blast.
The 6,000-ton jack-up drilling rig, George F. Ferris, was first positioned at its site approximately 1t miles offshore in Kachemak Bay during Jan. 1975. Located in 10 fathoms of water (at mean lower low tide), with 82 ft of its legs penetrating the bottom sediments, the Ferris remained onsite for nearly 16 months.
While preparing to move to a new location, the Ferris was jacked down into the water to provide buoyancy for lifting its legs out of the bottom sediments. As the barge was jacked deeper into the water, the legs failed to break free of the bottom, and a malfunction of the jack-up system occurred.
Since the barge could not be raised following the jacking mechanism malfunction, incoming tides (typically 20 to 30 ft) completely engulfed the lower levels, forcing the evacuation of crewmen and causing extensive destruction to electrical systems, equipment, and supplies.