OPERATIONS SYNOPSISTransit to Site 1187
The depth objective in this hole was to core into basement until time expired or the bit failed as a result of excessive wear. We spudded Hole 1187A with the RCB at 1920 hr on 29 October. The bit tagged the seafloor at 3803.6 mbsl. We drilled ahead with a wash barrel in place to a depth of 365.5 mbsf, where progress became very slow because of a change in formation. From our interpretation of the seismic profile, we expected to encounter basement at ~ 410 mbsf. However, we encountered the sediment/basalt interface in the first RCB core (Core 192-1187A-2R) and inferred it to be at 372.5 mbsf, based upon the change in drilling parameters noted by the driller. We continued rotary coring in basement to a depth of 508.3 mbsf, where the bit expired after 65 rotating hours. The failure was manifested by high, erratic torque, indicating that one or more cones had fallen off the bit. The average recovery in basement was 74.3%, and the average penetration rate was 2.4 m/hr. Total penetration into basaltic basement was 135.8 m.
After retrieving the drill string and BHA, we found that the bit was missing two cones and one cone shank. The two remaining cones had chipped and missing teeth in all rows. After retracting the hydrophones and thrusters and securing the drilling equipment, the crew put the vessel under way to Guam at 0330 hr on 3 November.
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