Leg 142

Engineering Tests: East Pacific Rise

Leg 142 represented the third phase in a series of cruises designed to refine hardware and techniques required to meet future endeavors. Previous efforts to conduct operations at young crustal centers had been problematic and Leg 142's primary objective was to further test the developmental capabilities of the HRB (hard rock guide base), the DI-BHA (drill-in bottom hole assembly), and the DCS (diamond coring system). The first and second phases of tests had been conducted during Legs 124E and 132. During Leg 142, one site (Site 864) was drilled on a flat, relatively unfissured lava flow that floors the axial summit caldera of the East Pacific Rise.

During Leg 142, the HRB system performed excellently through two deployments, one seafloor move, the initiation of three bore-holes, and a record thirty-five reentries into an 8-ft-diameter reentry cone. The system is now routinely employed.

The "nested" configuration and 6 3/4-in. drill collars on the DI-BHA functioned well but bit life proved to be the systemÍs primary weakness. This system must be improved before it can be considered fully operational.

The DCS is a wireline coring system capable of drilling and coring through highly-fractured, poorly cemented, crystalline rock. It was designed to provide flexibility in core-catcher selection, permit formation sampling options, allow a determination of the bit design best able to penetrate and core while maintaining minimum core diameter, allow development of friction reducer and mud programs for diamond drilling in fractured rock, provide a slim-hole drill-rod/tubing string capable of fitting within the drill pipe, while maintaining adequate annular flow area and withstanding rigorous high-speed operation with long pipe lengths. Problems with the secondary heave compensation system prevented successful DCS drilling during Leg 142. A fourth engineering test for the DCS (Phase II) is tentatively scheduled for 1995 when sea-tests will be made on modifications undertaken during 1992-1993. Development, testing, and proving of the secondary heave compensation system is critical to the future of the DCS, as it is impossible to slim-hole core offshore without effectively removing heave motion at the core bit.