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.