Previous   |    Next

doi:10.14379/iodp.pr.351.2015

Operations

Expedition 351 departed Yokohama, Japan, at 0552 h (UTC + 9 h) on 4 June 2014 and arrived at Site U1438 at 0245 h on 7 June. Prior to commencing rig floor operations, a visual seafloor survey (50 m box pattern) was conducted using the subsea camera system to confirm that there were no subsea cables at the site. Operations at Site U1438 (4700 m water depth) encompassed the entire operating time of Expedition 351. Six holes were occupied during the expedition with a total of 189 cores recovered utilizing each of the available coring systems. Logging operations occurred in three of the six holes.

Hole U1438A (27°23.0108′N, 134°19.1020′E) was spudded on 8 June, and three APC cores were recovered to 26.5 mbsf before terminating the hole. Recovery for the entire hole was 94% (24.9 m recovered). This hole was intended only to gain additional material for higher resolution sampling of the upper 20 m for postexpedition research, so no discrete samples were taken for shipboard analyses.

Hole U1438B (27°23.0111′N, 134°19.1087′E), offset 10 east of Hole U1438A, was spudded at 1510 h on 8 June and occupied until 0525 h on 10 June. Six successful temperature measurements were made using the APCT-3 tool when collecting the upper 9 cores recovered with the APC system. Oriented APC coring continued in this hole to 168.9 mbsf, at which point we switched to the half-length APC system. The half-length APC system was used to 180.6 mbsf, where we then used the extended core barrel (XCB) to core to our total depth of 257.3 mbsf. In total, 19 full-length APC cores were recovered (168.9 m penetration; 159.2 m recovered; 94.2% recovery), three cores were collected with the half-length (4.7 m core barrels) APC (11.7 m penetration; 11.96 m recovered; 102% recovery), and eight cores were recovered (76.6 m penetration; 55.9 m recovered; 73% recovery) with the extended core barrel (XCB) system. Overall recovery for the hole was 227.04 m (88%).

Hole U1438C (27°22.9963′N, 134°19.0883′E), 10 m west of Hole U1438A, consisted of a reentry cone jet-in test that took place 10–11 June. Controlled jetting reached 65 mbsf in ~2.5 h.

Hole U1438D (27°23.0218′N, 134°19.1023′E) is offset 20 m north of Hole U1438A and was our pilot hole for a subsequent reentry hole. It was spudded at 1330 h on 11 June and drilled without coring to 219 mbsf. Coring with the rotary core barrel (RCB) system began with nonmagnetic core barrels to 286.7 mbsf, when coring halted due to bad weather at 0400 h on 12 June. RCB coring resumed at 1045 h and continued until 17 June, reaching a total depth 897.8 mbsf. In all, 71 cores were collected over a cored interval of 678.9 m, totaling 523.2 m of material (77% recovery).

Hole conditions during operations were good enough that a single casing string (10.75 inches) was deemed necessary for the reentry hole (Hole U1438E). The hole was then swept and displaced with heavy mud in preparation for wireline logging. Before logging, however, the drill string was raised to 304 mbsf and a free fall funnel (FFF) was deployed as a contingency plan. After releasing the bit on the seafloor on at 2135 h on 18 June, the FFF was reentered and the end of the drill string was positioned at logging depth (95 mbsf). The triple combination (triple combo) tool string run, measuring temperature, resistivity, density, porosity, and natural gamma, started at 0633 h on 19 June and reached 303 mbsf, where the tool string encountered a bridge. An uplog began at 0823 h, and the tool was recovered at 1200 h. As the depth of the obstruction corresponded to the depth of the pipe during deployment of the FFF, it was concluded that the pipe itself created the bridge. An attempt to pursue further logging operations was organized. The pipe was lowered to 328.15 mbsf (below the bridge), and the triple combo tool string was run back into the hole at 1525 h and reached the end of the pipe by 1800 h. Unfortunately, another bridge was encountered at 362 mbsf. Due to the difficulty of passing through the bridge and the shallow depth, it was decided to halt logging operations for Hole U1438D. No uplog was recorded for the second phase of logging. The drill string was recovered, and Hole U1438D ended at 1130 h on 20 June.

Hole U1438E (27°23.0153′N, 134°19.0898′E), offset 10 m south of Hole U1438A, was our deep reentry hole. It was spudded at 1415 h on 21 June when the reentry cone and casing assembly was jetted in (no rotation) to 60.1 mbsf. We then drilled a 14.75 inch hole to a total depth of 613.0 mbsf, which concluded at 1445 h on 24 June. The hole was swept multiple times with high-viscosity sepiolite mud, and one wiper trip was conducted due to the good borehole condition. No overpull or drag was present during the wiper trip, and there was no fill identified on the bottom. On 25 June, preparations began for making up and deploying the 10.75 inch casing string. At 0145 h on 27 June, the 10.75 inch casing string was lowered into the hole escorted by a mud motor-powered underreamer and 9.875 inch tricone pilot bit to 605 mbsf. Drilling then continued to 867.3 mbsf with the RCB center bit.

At 0900 h on 30 June, RCB coring began in Hole U1438E. Coring continued to 1319.6 mbsf. Coring stopped at 1825 h on 5 July due to impending severe weather. A typhoon necessitated our transit to a safe standby location east of Site U1438. At 0600 h on 7 July, the ship was positioned 144 nmi east of Site U1438. Upon arriving back on site at 0830 h on 10 July, we continued to wait for sea conditions to moderate enough to resume operations. At 1315 h on 11 July, we deployed the subsea camera system and Hole U1438E was reentered at 1655 h. RCB coring resumed with nonmagnetic core barrels on 12 July. Coring continued to 1460.9 mbsf, where the sediment/basement contact was identified (Core 351-U1438E-69R). RCB coring continued through basaltic basement rocks to a total depth for the hole, and site, of 1611 mbsf. Eighty-eight RCB cores were recovered from this hole; of a total of 743.7 m cored, 450.73 m was recovered (61%). More specifically, 407 m of sediment was recovered from 593.6 m cored (69%) and 44 m of basement was recovered from 150 m cored (29%).

In general, coring conditions were good, but there were several times when remedial hole conditioning was required. The sediment/basement contact was the depth at which the drill string had the most trouble. As a result, a wiper trip was conducted prior to wireline logging operations. Heavy mud was not used, as it was believed to have affected conditions in Hole U1438D. The drill pipe was pulled to 190.2 mbsf (inside the casing), and rigup of the triple combo began at 1511 h on 21 July. Due to the difficult hole conditions, the radioactive source was removed and a “hole finder” was attached at the bottom of the tool string to help pass the sediment/basement contact. The triple combo was deployed at 1656 h and reached 1186 mbsf, where it encountered a bridge and was unable to pass after several attempts. During the uplog, caliper measurements indicated that the hole diameter was >18 inches the entire logged depth. This precluded the use of the vertical seismic imager (VSI) and FMS-sonic tool strings. However, the Göttingen Borehole Magnetometer (GBM) is not affected by borehole diameter. The GBM was deployed and reached the same total depth as the triple combo (1186 mbsf). Logging operations were completed by 1320 h on 22 July.

Hole U1438F (27°23.0167′N, 134°19.0905′E), offset 10 m north of Hole U1438C and 20 m west of Hole U1438A, was a dedicated wireline logging hole. The hole was drilled to 700 mbsf, and logging operations began on 25 July. Hole preparation for logging included a wiper trip and circulation with seawater, after which the pipe was pulled to 95 mbsf. The triple combo began a downlog at 2117 h on 25 July. The tool string reached ~700 mbsf at 2151 h and began an uplog at 2250 h. The triple combo was recovered at 0224 h on July 26, after which the VSI was run to take advantage of full daylight hours. The VSI was rigged up by 0355 h and deployed. Protected species observation began at sunrise (~0500 h), and the ramp-up of the air guns began 1 h later, as no protected species were observed in the 940 m diameter exclusion zone for this site. The air guns fired every 5–10 min. The VSI took measurements at 9 stations on the uplog. The last deployment was the FMS-sonic tool string to 689 mbsf. Logging operations were completed at 2209 h on 26 July. With the recovery of the drill string on 27 July, operations for Expedition 351 concluded.

Top of page   |    Previous   |    Next