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doi:10.2204/iodp.proc.334.104.2012

Operations

Transit to Site U1379

After a 1.25 h transit from Site U1378 covering 6 nmi and averaging 4.8 kt, speed was reduced and thrusters and hydrophones were lowered. Dynamic positioning was initiated over Site U1379 at 0345 h (Universal Time Coordinated [UTC] – 6 h) on 20 March 2011. The position reference was a combination of GPS signals and a single acoustic beacon. The positioning beacon was deployed at 1109 h on 17 March and recovered at 0940 h on 2 April.

Site U1379

Three holes were drilled or cored at this site (Table T1). The first hole was drilled with a 8½ inch drill bit with logging-while-drilling (LWD) tools in the bottom-hole assembly (BHA). The next hole was drilled with the advanced piston corer (APC) system to recover a mudline core and one additional core. The third hole was drilled with the APC coring system to refusal at 91.2 mbsf followed by extended core barrel (XCB) coring to refusal at 949.0 mbsf (1087.8 meters below rig floor [mbrf]). The advanced piston corer temperature tool (APCT-3) was deployed six times, and usable data were recovered five times. The Flexit orientation tool was deployed on all APC cores in Hole U1379C, but data were lost from the first 10 cores when a critical computer was turned off during the first tool’s deployment, causing the tool to lose synchronization with the computer. All APC holes were cored with nonmagnetic core barrels. Recovery for Site U1379 was 100.3% with the APC system and 84.4% with the XCB system. Overall core recovery for Site U1379 was 85.9%. A total of 120 cores were recovered after coring 959.5 m. Total length of core recovered at this site was 824.17 m.

Hole U1379A

Rig floor operations commenced at 0345 h on 20 March 2011. The trip to the seafloor was uneventful in the very shallow water. The top drive was picked up, the drill string was spaced out, and the LWD hole was spudded at 0955 h on 20 March. After 73 h of drilling, Hole U1379A was terminated at 962.8 mbsf. After picking up off bottom, the hole was displaced with heavy drilling mud (10.5 ppg) and the drill string was tripped out of the hole to ~500 mbrf while experiencing high torque and significant overpull. The top drive was set back and the drill string was tripped the rest of the way to surface. The 6½ inch drill collars were laid out for offloading in Puntarenas, Costa Rica. Data were downloaded from the LWD tools as they were broken down at the rig floor. The LWD tools were then stored for transit and the rig floor was secured. Thrusters and hydrophones were pulled and the vessel was put into cruise mode, ending Hole U1379A at 2254 h on 23 March. The vessel transited to Puntarenas to offload the LWD tools and equipment.

Hole U1379B

After offloading the LWD tools in Puntarenas, the vessel made the 9 h transit back from Puntarenas, covering 97.2 nmi and averaging 11.8 kt. Speed was reduced and thrusters and hydrophones were lowered. Dynamic positioning was initiated over Site U1379 at 0530 h (UTC – 6 h) on 25 March. After offsetting the vessel 20 m west of Hole U1379A, Hole U1379B was spudded at 1030 h. The mudline core established seafloor depth at 138.5 mbrf. APC coring continued until 10.5 mbsf. Two cores were taken with a total recovery of 8.72 m for an overall recovery in Hole U1379B of 83.0%. The drill string was tripped to just above the mud line, clearing the seafloor at 1030 h on 25 March, ending Hole U1379B.

Hole U1379C

After offsetting the vessel 20 m north, Hole U1379C was spudded at 1255 h on 25 March and advanced with the APC coring system to 91.2 mbsf before refusal. All cores after the second core were partial strokes and the hole was advanced by recovery. The coring system was switched to the XCB coring system and coring continued from Core 334-U1379C-18X through Core 118X. Temperature measurements were taken with the APCT-3 on Cores 4H, 7H, 10H, 12H, 13H, and 16H. Microspheres were run on Cores 20X, 30X, 40X, 50X, 60X, 75X, 92X, 105X, and 107X. A total of 17 APC cores were taken with a total recovery of 91.92 m (100.8%). A total of 101 XCB cores were taken over a 857.8 m interval, recovering 723.64 m of core (84.4%).

After reviewing the LWD penetration rates, which remained high into the basement section of Hole U1379A, and given the high recovery rates of the XCB system, a decision was made to push the XCB system into the basement section of Hole U1379C. XCB core recovery continued to be excellent until basement was reached. After Core 334-U1379C-104X, the recovery dropped off as we experienced soft to hard to soft formation changes. Average recovery after Core 103X through 118X at 949 mbsf was 19.8%. Average XCB recovery prior to Core 104X was 89.2%.

The total cored interval in Hole U1379C was 949.0 m with 815.56 m of core recovered for an APC/XCB combined recovery of 85.9%. Last core on deck for Hole U1379C was at 0130 h on 2 April. The hole was displaced with 10.5 ppg mud, and the drill string was pulled back to 565 mbrf with the top drive in place. High torque (>500 A) and overpull were observed all the way to 565 mbrf. At 565 mbrf, the top drive was set back and the remainder of the drill pipe and BHA were tripped back to the surface. The seafloor was cleared at 0755 h, and the bit cleared the rotary table at 0910 h. The rig was secured for transit and the beacon was recovered at 0941 h, ending Hole U1379C at 0945 h on 2 April.

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