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doi:10.2204/iodp.proc.344.105.2013

Operations

First transit to Site U1412

After a 7.0 nmi transit from Site U1380, the vessel stabilized over Site U1412 (proposed Site CRIS-9A) at 0200 h on 14 November 2012. The position reference used for dynamic positioning (DP) was a combination of GPS signals and an acoustic beacon, which was deployed at 0217 h on 14 November.

Hole U1412A

Upon arrival at Site U1412, an advanced piston corer (APC)/extended core barrel (XCB) bottom-hole assembly (BHA) was assembled and lowered to 1876 m water depth. After three unsuccessful attempts at recovering the mudline, Hole U1412A (8°29.3294′N, 84°7.6686′W) was spudded at 1520 h on 14 November 2012 at a water depth of 1921 m. Nonmagnetic core barrels and the FlexIT orientation tool were used with Cores 344-U1412A-1H through 15H. Advanced piston corer temperature (APCT-3) tool formation temperature measurements were taken with Cores 3H and 5H–8H. All cores after Core 3H were partial stroke cores, and APC refusal was reached at 108.2 mbsf with Core 15H. After APC refusal was reached, the XCB coring system was deployed and coring continued through Core 25X to a final depth of 200.3 mbsf. At that time, the hole had become too unstable to continue coring. The hole was plugged with 49 bbl of heavy mud and abandoned. The drill string was pulled from the hole, clearing the seafloor at 1410 h on 16 November and ending Hole U1412A.

Fifteen APC cores and ten XCB cores were recovered from Hole U1412A (Table T1). The APC cored interval was 108.2 m with 108.8 m recovered. The XCB cored interval was 92.1 m with 61.3 m recovered. Overall recovery for Hole U1412A was 85%.

Hole U1412B

After clearing the seafloor, we attempted to recover the last XCB core barrel that had been deployed. The first attempt resulted in the aft coring line parting above the core barrel. Two wireline runs were made before eventually bringing the core barrel and sinker bars back to the surface. The vessel was offset 350 m south-southwest in the hope that the formation would be more suitable for a deeper hole. Hole U1412B (8°29.1599′N, 84°7.7512′W) was spudded at 2350 h on 16 November 2012 at 1965 m water depth. Recovery of the mudline core was slightly problematic, as remnants of wireline from the previous wireline failure obstructed the overshot. After successfully clearing the debris, an XCB core barrel with a center bit was dropped and Hole U1412B was drilled from 6.7 to 155.8 mbsf. The center bit was replaced with an XCB core barrel, and coring began at 1700 h on 17 November. Cores 344-U1412B-3X through 20X were recovered to 304.3 mbsf. Coring over the last 75 m was punctuated by erratic and high torque. The drill string first became stuck at 233.4 mbsf. After working the pipe free with 14,000 lb of overpull and 700 A of torque, the hole was cleaned and coring resumed. Coring continued until 2245 h on 19 November, when the drill string became firmly stuck for the second time. This time the string was freed from the formation with 90,000 lb of overpull and 800 A of torque. When we were unable to wash back down to 304.3 mbsf, the decision was made to terminate coring and prepare the hole for logging operations.

With the bit at 289.4 mbsf, a 50 bbl high-viscosity mud sweep was pumped to clean the hole. The drill string was then pulled to 231.3 mbsf. Four mud sweeps (total = 160 bbl) were pumped to try to keep the hole open. The wiper trip continued to 85.6 mbsf. Over the next 5.75 h, we attempted to trip and wash back to bottom, eventually reaching a total depth of 289.4 mbsf. Another 50 bbl high-viscosity mud sweep was pumped, a go-devil was dropped to open the lockable float valve, and the hole was displaced with 130 bbl of heavy logging mud. We then raised the drill pipe to 91.0 mbsf, and the logging tools were rigged up. The first logging run with the triple combination (triple combo) string started at 1040 h on 20 November. However, the tools were unable to exit the BHA and the drill string became stuck. The tools were pulled from the hole and were laid out at 1600 h. The drill string was so firmly stuck that we were unable to lower the elevators the 1 m required to land them on the elevator stool, so the vessel had to be offset by 50 m to lower the elevators. Once the elevators had landed on the elevator stool, the top drive was picked up. With the top drive installed, the mud pumps were turned on and circulation was reestablished. The drill pipe was slowly pulled from the hole, and the vessel moved back over the hole. The remainder of the drill string was pulled to the surface, and the bit cleared the rotary table at 2245 h on 20 November, ending Hole U1412B.

One APC core was taken over a 6.7 m interval with 6.74 m recovered (101%) (Table T1). Eighteen XCB cores were taken over a 148.5 m interval with 21.52 m recovered (15%). Total recovery in Hole U1412B was 28.26 m (18%). Total time spent on Hole U1412B was 104.5 h.

Hole U1412C

After the drill string cleared the rig floor, the vessel was offset 20 m north-northeast. The rotary core barrel (RCB) coring BHA was picked up and run to just above the seafloor. Seafloor depth for Hole U1412C was set at 1965.0 m from the offset depth at the previous hole. Hole U1412C (8°29.1700′N, 84°7.7467′W) was spudded at 0730 h on 22 November 2012 and was drilled with a center bit to 300 mbsf in 20 h. The hole was then cleaned with a 40 bbl high-viscosity mud sweep. Coring continued with Cores 344-U1412C-2R through 10R to a final depth of 387.0 mbsf. Excessive torque (>800 A) was present while cutting the final core, and in order to pull the final core, the bit had to be pulled back to 326 mbsf. However, high torque was still present at 326 mbsf, and the drill string had to be pulled back all the way to 239 mbsf. We then attempted to wash back to bottom to resume coring. After several hours of working to clean the hole, we reached 307 mbsf. High and erratic torque was still present. Because of the lack of progress and difficult drilling conditions, the decision was made to abandon Hole U1412C. The drill string was tripped to the surface, and the bit cleared the rotary table at 2115 h. The acoustic beacon was recovered at 1815 h, and the rig floor was secured for transit at 2200 h on 23 November, ending Hole U1412C.

Nine RCB cores were taken over an 87.0 m interval with 36.6 m recovered (42%) (Table T1). Total time spent on Hole U1412C was 71.25 h. The vessel proceeded to Site U1413 before returning.

Second transit to Site U1412

After a 15.0 nmi transit from Site U1413, the vessel stabilized over Site U1412 at 0330 h on 1 December 2012. The initial position was the official location of Hole U1412B, and the vessel was then offset 50 m south-southwest in the hope that the décollement could be reached at a shallower depth. The position reference used for DP was a combination of GPS signals and an acoustic beacon, which was deployed at 0418 h on 1 December.

Hole U1412D

An RCB BHA with a used 9⅞ inch bit was made up and lowered to just above the seafloor. Hole U1412D (8°29.1402′N, 84°7.7793′W) was spudded at 0850 h on 1 December 2012 at 1973 m water depth. Drilling without recovery continued with a center bit for 19.75 h to 350.4 mbsf. The hole was cleaned with a 40 bbl high-viscosity mud sweep, and the center bit was recovered with the wireline. Coring continued with nonmagnetic RCB core barrels for Cores 344-U1412D-2R and 3R to a final depth of 369.2 mbsf. Excessive torque (>500 A) was present while cutting the final core. After adding another single joint of drill pipe and dropping the next core barrel, we were unable to advance the hole. Constant high torque (1000 A) kept stalling the top drive, and we had to offset the vessel 60 m from the hole location in order to free the stuck drill string. All efforts to condition the hole and continue coring failed, and the decision was made to abandon the hole. The seafloor was cleared at 1630 h, and the beacon was recovered at 1735 h on 2 December. After recovering the beacon, the vessel began a slow move using the DP system to Site U1414. During the transit to the next site, the remainder of the drill string was recovered. The bit cleared the rotary table at 2105 h on 2 December, ending Hole U1412D and Site U1412.

Two RCB cores were taken over an 18.8 m interval with 12.3 m recovered (65%) (Table T1). Total time spent on Hole U1412D was 41.5 h. Total time spent on Site U1412 was 277.5 h, or 11.56 days.

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