The 1210-kt transit from Guam to proposed Site ENT-01A (Site 1173) was accomplished at an average speed of 11.8 kt in 4.25 days. During the transit, the drilling and coring equipment were prepared. A variety of repairs to the accommodations were made during the transit, including replacement of some of the ceiling tiles in the hotel hallways, replacement of sinks and toilets, painting, and so forth. The clocks were turned back 1 hr at midnight on the night of 27 May so the ship would be on Japan local time.
When approaching the site, the ship slowed from transit speed to ~5 kt to conduct a seismic reflection survey that covered 48 nmi and lasted 9.8 hr (Figs. F1, F2). Once the survey passed over proposed Site ENT-01A (Site 1173), the seismic gear was retrieved and the vessel moved to the Global Positioning System (GPS) coordinates of the site. Once over the GPS coordinates, a seafloor positioning beacon was dropped at 1202 hr on 28 May, establishing Site 1173.
After the hydrophones and thrusters were lowered and the vessel settled on location, the precision depth recorder indicated a water depth of 4794.4 meters below rig floor (mbrf). An eight-collar APC/XCB bottom-hole assembly (BHA) was made up with a 9.875-in polycrystalline diamond compact bit, lockable float valve (LFV), seal bore drill collar, landing saver sub, top sub, head sub, nonmagnetic drill collar, five 8.25-in drill collars, one tapered drill collar, six joints of 5.5-in drill pipe, and a crossover sub to 5-in drill pipe. Once the BHA was made up it was run down to the seafloor. Running the drill string to the seafloor took longer than usual because of the measuring and internal clearance inspection (strapping and rabbiting) that is routinely performed during the first deployment of the drill string during a leg. The pipe trip down to the seafloor was also interrupted for an hour while the active heave compensator (AHC) hydraulic umbilical sheath was repaired.
Once the bit was near the seafloor at 4780 mbrf and before we circulated any seawater through the drill string, the WSTP was deployed to obtain a water sample near the seafloor. Because the WSTP could not be pumped to bottom, ~3 hr was required to obtain the water sample.
After successfully obtaining a seafloor water sample, seawater was circulated through the drill string and a "pig" pumped through it to clean the interior of the drill pipe. The bit was placed at 4778.8 meters below sea level (mbsl) (4790 mbrf) and an APC was shot but did not recover any sediment. The bit was lowered to 4788.34 mbsl (4799.54 mbrf), the APC was shot again, and Hole 1173A was spudded at 1125 hr on 29 May. Core 1H recovered 7.14 m of core, indicating that the seafloor depth as determined by drill-pipe measurement was 4790.7 mbsl (4801.9 mbrf) (Tables T1, T2). APC coring continued to 225.6 mbsf (5027.5 mbrf), recovering 222.35 m of core (99%). Once the APC pull-out force reached 60,000 lbs, we switched to coring with the XCB.
We cored with the XCB from 225.6 to 734.3 mbsf (5027.5 to 5536.2 mbrf). A total of 53 XCB cores were taken over the 508.7-m interval, recovering 436.01 m of core for an overall XCB recovery of 86%. Although the XCB recovery was very good, the last four XCB cores recovered only 1.79 m (5% recovery). In addition, while we cored the last 7 m of the hole, the rate of penetration dropped to <5 m/hr and high erratic torque was experienced. When we lifted the bit off bottom, there was virtually no torque and the XCB cutting shoe was recovered in good condition; therefore, we presumed that the bit might have failed. At this point, we decided to stop coring and to log the hole. Subsequently, we found small pieces of basalt in the bottom of the last core, so we infer that the high and erratic torque may have been caused by rotating the bit on basaltic basement.
We circulated a 50-bbl pill of sepiolite mud to clean the hole of any cuttings and then filled it with sepiolite mud. The bit was raised to 95.12 mbsf (4897.0 mbrf), and the logging tools were rigged up. The triple combination tool (triple combo) was lowered in the hole but could not pass a bridge at 344.1 mbsf (5146 mbrf); therefore, we logged up from that point to the bit.
The logging tools were laid out, the drill string was lowered to 444.7 mbsf (5246.4 mbrf), and the drill string did not encounter any resistance when passing through the bridge. The bit was raised to 364.8 mbsf (5166.8 mbrf), and the logging tools were rigged up again. The triple combo was lowered down the drill string, and after having some trouble getting through the bit, it was able to reach 438.1 mbsf (5240.0 mbrf), where yet another bridge prevented the tool from passing any farther. Once again, the hole was logged from that point up to the bit. The logging tool had to be worked through the bit into the drill string. Once back inside the drill string, the logging tools were recovered and laid out.
The drill string was then lowered in an effort to clean out the hole. When the drill string was picked up off the slips, 70,000 lb of overpull was required to free the bit, which had become stuck in the hole. The drill string had to be worked with as much as 20,000 lb of overpull and set down weight to get to 637.1 mbsf (5439 mbrf), where the hole starting packing off. The drill string was then pulled to 628.1 mbsf (5430 mbrf), where 226 bbl of 10.5 ppg weighted mud was pumped to displace the hole. The drill string was then raised to 95 mbsf (4879 mbrf), and the logging tools were rigged up once more.
This final logging run included the sonic and FMS tools. This time the tools encountered a bridge at 378 mbsf (5180 mbrf), and we logged up from there. After the logging tools were recovered and laid out, we tripped the drill string back to the ship. Hole 1173A officially ended at 0445 hr on June 7 when the bit reached the rig floor.