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doi:10.2204/iodp.proc.339.105.2013

Operations

The vessel was offset in dynamic positioning mode 2.2 nmi from Site U1386, on a bearing of 128°, to Site U1387 (proposed Site GC-09A). During this move, accomplished in 3 h, maintenance was performed on the 480 V switchboard, which required shutting down the regulated power from 1800 to 1845 h. The vessel was positioning on Site U1387 at 2030 h, 8 December 2011.

Three holes were drilled at Site U1387 (Table T1). Hole U1387A was cored using the advanced piston corer (APC) to 47.7 meters below seafloor (mbsf) and then with the extended core barrel (XCB) to 352.4 mbsf. Hole U1387B was cored with the APC to 46.9 mbsf and then with the XCB to 338.3 mbsf. Hole U1387C was drilled without coring to 290 mbsf and then cored using the rotary core barrel (RCB) to the target depth of 870 mbsf. Downhole logging was carried out in Hole U1387C using the triple combination (triple combo), Formation MicroScanner (FMS)-sonic, and Versatile Sonic Imager (VSI) tool strings (see “Downhole logging at Site U1387”). Overall recovery at Site U1387 was 97.03 m (102.6% recovery) with the APC, 578.42 m (97% recovery) with the XCB, and 409.5 m (70.6% recovery) with the RCB. The total cored interval at Site U1387 was 1270.7 m, and total recovery was 1084.95 m (85.4%).

Hole U1387A

Prior to spudding Hole U1387A, a 2 h underwater camera survey of the seafloor was made during which many linear furrows on the seabed were observed. These furrows were presumed to be the result of fishing bottom trawls. The vessel had to be offset 10 m from the original position to avoid spudding into what appeared to be man-made debris (a spiral of loose wire or cable). Hole U1387A was spudded with the APC at 0325 h, 9 December. Seafloor depth calculated from the recovery of the first core was established at 570.5 meters below rig floor (mbrf) (559.1 meters below sea level [mbsl]). APC coring advanced to 47.7 mbsf, where very sticky, firm clay prevented further progress. The APC cored 47.7 m and recovered 48.83 m (102.4% recovery). Cores 339-U1387A-4H through 6H were oriented. The advanced piston corer temperature tool (APCT-3) was deployed on Core 4H (33.9 mbsf). Nonmagnetic core barrels were used to obtain all piston cores. XCB coring was initiated at 0915 h, 9 December, and deepened Hole U1387A from 47.7 mbsf to a final depth of 352.4 mbsf by 0545 h, 10 December. The XCB system cored 304.7 m with a 98.1% recovery. The total cored interval in Hole U1387A was 352.4 m, with a recovery of 98.7%.

Hole U1387B

The bit was pulled clear of the seafloor at 0715 h, 10 December, and the vessel offset 20 m east of Hole U1387A. The underwater camera was deployed to ensure no man-made debris was present on the seafloor prior to coring. Hole U1387B was spudded with the APC at 1050 h and established a water depth of 569.6 mbrf (558.2 mbsl). Piston coring advanced to 46.9 mbsf (102.8% recovery) before switching to the XCB. Cores 339-U1387B-3H through 5H were oriented. All cores were obtained with nonmagnetic core barrels.

XCB coring deepened Hole U1387B to a final depth of 338.3 mbsf. The average recovery for the XCB cored interval of 291.4 m was 95.9%. APC/XCB coring recovered 96.8% of the cored interval in Hole U1387B (338.3 m). Coring was terminated prior to the depth objective of 350 mbsf because the quality of the cores was compromised because of biscuiting disturbance, discovered when the deeper cores in Hole U1387A were examined in the ship’s core laboratory.

Hole U1387C

The drill string was recovered with the bit clearing the rotary table at 1305 h, 11 December. After the nonmagnetic drill collar and seal bore drill collar were laid out along with the APC/XCB polycrystalline diamond bit, a four-stand RCB bottom-hole assembly (BHA) with a new CC-3 Rock Bit International bit was made up and deployed along with the underwater camera. Once the video images provided by the underwater camera confirmed that the seafloor was clear of obstructions, the driller tagged the seabed with the RCB bit at 569.8 mbrf (558.4 mbsl) at 1640 h, 11 December. Following retrieval of the camera, Hole U1387C was spudded with the RCB at 1850 h. The hole was drilled with a wash barrel in place to 290.0 mbsf, where continuous RCB coring was initiated.

Rotary coring proceeded without incident to a final depth of 870 mbsf by 0930 h, 16 December. A total of 580 m was cored in Hole U1387C with an average recovery of 70.6%. The average rate of penetration for the cored interval was 11.6 m/h. The drilled portion of the hole was 290 m and the total penetration (cored plus drilled) was 870 m. While penetrating the cored interval, the drillers pumped eleven 20 bbl and two 30 bbl high-viscosity mud flushes to keep the hole clean of cuttings.

Downhole logging at Site U1387

After a wiper trip and hole conditioning, the bit was dropped at the bottom of Hole U1387C and the hole was displaced with 290 bbl of 10.5 ppg mud. The end of the pipe was positioned at 103.8 mbsf. During the wiper trip, the driller had to pick up the top drive and wash and ream a tight portion of the hole (ledge or bridge) from 649.0 to 653.0 mbsf. There was 28 m of soft fill at the bottom of the hole that was flushed clean with a 50 bbl Hi-Vis sweep.

The triple combo tool string was made up and deployed at 0230 h, 17 December. The tool could not advance past 650 mbsf, so the hole was logged from that point. The triple combo tool string was recovered at 0835 h. The VSI tool string was run in the pipe at 1105 h but could not be lowered deeper than 254 mbsf. The marine mammal protocol was observed while conducting the vertical seismic profile (VSP) experiment. The VSI was recovered at 1420 h. The last log was made with the FMS-sonic tool string, which was deployed at 1610 h and could not go deeper than 334 mbsf. The FMS-sonic tool string was recovered and disassembled by 2300 h. The end of the pipe was pulled clear of the seafloor at 2355 h, and the drill string was recovered. The drilling equipment was secured and the vessel departed for Site U1388 at 0230 h, 18 December. Total time on site was 9.3 days.

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