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doi:10.2204/iodp.proc.341.107.2014

Operations

Transit to Site U1421

After an 11.9 nmi transit from Site U1420 averaging 7.9 kt, the vessel arrived at the final expedition site. The vessel stabilized over Site U1421 at 0235 h (UTC – 8 h) on 22 July 2013, and the positioning beacon was deployed at 0245 h.

Site U1421

Site U1421 consists of three holes (Table T1). Hole U1421A was cored to 702.7 m drilling depth below seafloor (DSF) using a combination of the advanced piston corer (APC) and extended core barrel (XCB) systems. Hole U1421B was cored to 6.2 m DSF. Hole U1421C was cored to 38.2 m DSF. Total time spent on Site U1421 was 154.25 h (6.43 days).

A total of 92 cores were recorded for the site. The APC system was deployed 26 times. The XCB system was deployed 66 times. A total of 26 APC cores were collected over 140.8 m, with 114.47 m recovered (81%). A total of 66 XCB cores were drilled over 606.3 m, with 61.54 m recovered (10%). The overall recovery for Site U1421 was 23.6% (Fig. F4).

Hole U1421A

Hole U1421A was spudded at 0730 h on 22 July 2013. The mudline core recovered 6.82 m of sediment, and the seafloor was calculated to be at 718.5 meters below sea level (mbsl). All piston cores were tide corrected at the drill down before each core was shot. Nonmagnetic core barrels and the APC system were used for Cores 341-U1421A-1H through 8H, and FlexIT orientation was performed on Cores 2H through 8H. Cores 9H through 19H were recovered using the half APC coring system with steel core barrels. Partial APC strokes were recorded on Cores 6H, 8H through 10H, 13H through 16H, 18H, and 19H. The APC cores were affected by frequently encountered large clasts, several of which prevented recovery. The XCB system was deployed with a soft-formation cutting shoe for Cores 20X through 64X. On Core 65X, the cutting shoe was changed to the hard-formation carbide-tipped shoe. Coring continued through Core 85X (702.7 m DSF). Total depth was reached at 1825 h on 26 July.

A total of 360 bbl of high-viscosity mud was used to clean the hole during coring operations. The hole was displaced with 50 bbl of high-viscosity mud followed by 328 bbl of 11.8 ppg heavy mud for logging. After displacing the hole with heavy mud, the trip out of the hole was performed while fighting high rotational torque on the fifth single of drill pipe. Torque became minimal at 391.8 m DSF, and the top drive was set back. The drill string was then pulled back, and the end of the pipe was set at 96.6 m DSF. A modified logging tool string termed the Sonic-induction tool string, measuring gamma ray, borehole diameter, sonic velocity, and resistivity, was run to 699 m DSF, and two passes were made. The logging string was then pulled back to surface and rigged down at 0845 h on 27 July.

The Versatile Seismic Imager (VSI) tool string was rigged up and run into the hole. Protective Species Observation watches began at 0700 h. No protected marine species were observed during this period within the 1410 m diameter exclusion zone for this site, so the seismic source (a parallel cluster of two 250 inch³ Sercel G. Guns) was ramped up starting 1 h after the watches commenced. Watches continued throughout the VSI run, with guns fired manually as the tool string was run into the hole. Once the VSI tool string was in the hole, firing control was delivered to the Schlumberger engineer. After tagging bottom, the tool was positioned at the first station. Nine stations were attempted with successful measurements recorded at six stations (see “Downhole logging”). The tool was then pulled out of the hole to the surface and rigged down at 1630 h on 27 July. The guns were secured, and Protective Species Observation watches ceased after the last station was completed. At the conclusion of logging, the circulating head was removed, the knobbies were set back, and the drill string was pulled from the hole to just above the seafloor. The bit cleared the seafloor at 1715 h on 27 July, ending Hole U1421A.

A total of 19 APC cores were taken over a 96.4 m interval, with 79.18 m recovered (82%). A total of 66 XCB cores were cut over a 606.3 m interval, with 61.54 m recovered (10%). The overall core recovery for Hole U1421A was 140.72 m for the 702.7 m cored interval (20.0%). Total time spent on Hole U1421A was 134.75 h.

Hole U1421B

After clearing the seafloor, the vessel was moved 28.3 m southwest of Hole U1421A. Hole U1421B was spudded at 1925 h on 27 July 2013. The mudline core recovered 6.23 m of sediment, and seafloor was calculated to be 722.7 mbsl. Coring in Hole U1421B was terminated at a total depth of 6.2 m DSF at 1925 h on 27 July, after the stratigraphic correlators confirmed that the piston core shoot depth was incorrect. One piston core was taken over a 6.2 m interval, with 6.23 m recovered (101%). Total time spent on Hole U1421B was 2.25 h.

Hole U1421C

After clearing the seafloor, the vessel was offset 20 m west of Hole U1421B. Hole U1421C was spudded at 2005 h on 27 July 2013. The mudline core recovered 8.57 m of sediment, and the seafloor was calculated to be at 721.8 mbsl. All piston cores were tide corrected at the drill down before each core was shot. Nonmagnetic core barrels and the APC system were used for Cores 341-U1421C-1H through 4H. Cores 5H and 6H were recovered using the half APC coring system to 38.2 m DSF. Temperature measurements were taken with the advanced piston corer temperature tool (APCT-3) on Cores 4H and 6H. Coring in Hole U1421C was terminated when the core barrel for Core 6H became stuck in the bottom-hole assembly (BHA). After shearing off the core barrel, the sinker bars were retrieved and the drill string was tripped clear of the seafloor with the top drive installed. The top drive was then set back, and the drill string was tripped to surface. The BHA was set back in the derrick, and the core barrel was removed and laid out. The BHA was then tripped back into the hole, and each drill collar was pulled up and laid out in the forward drill collar racks. The rig floor was secured at 1230 h on 28 July. The thrusters were then raised, and the vessel began the transit to Valdez, Alaska (USA), at 1245 h, ending Site U1421. A total of six APC cores were taken over a 38.2 m interval, with 29.06 m recovered (76%). Total time spent on Hole U1421C was 18.0 h.

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