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doi:10.2204/iodp.proc.331.106.2011

Operations

Arrival at Site C0016

After we finished casing and capping operations at Hole C0014G, the D/V Chikyu moved to Site C0016, the top of NBC Mound (Fig. F1). Preparations began on the evening of 25 September 2010 and continued overnight into 26 September. After a seafloor survey with the remotely operated vehicle (ROV) in the early hours of 26 September, we were ready to drill into the mound (Table T1).

Hole C0016A

The desired location for the mound top site was a slight depression tagged at 1010.5 m drilling depth below seafloor (DSF) on 26 September 2010. A cloudy flow was observed from the hole almost immediately upon entry. Coring continued to 18 meters below seafloor (mbsf), often obscured by the strong cloudy outflow from the hole. The drill string was often observed to deviate from vertical by as much as 10°.

When penetration depth was reached and it was time to exit the hole, the outer barrel broke after a couple of spikes in overpull—up to 250 kN at the time of breakage! The flow from the drill hole was very strong at this time and flashed briefly, suggesting intense phase separation as the hot fluid poured out of the hole. The bit, split ring housing, extension sub, outer sleeve of the full closure catcher, and stabilizer were lost in the hole. The lower part of the inner barrel was dropped outside the hole but was later recovered by the ROV.

Hole C0016A was, needless to say, abandoned, with no recovered core.

Hole C0016B

After abandoning Hole C0016A, the Chikyu moved to the foot of NBC Mound. On the evening of 26 September 2010, the third guide base was placed at the foot of the mound, and we left to core at Site C0017 for a few days.

On 30 September, we returned to Site C0016 and entered the guide base in the early afternoon. Hole C0016B was spudded, and the first core was cut with the Baker Hughes INTEQ (BHI) system using a conventional friction core catcher. Coring went well, but upon tripping the 9 m barrel back to the ship, we discovered a total of only 71 cm of core, consisting of short segments of hard rock.

The second core at Site C0016 was taken on 1 October using an 18 m BHI barrel, again with the conventional friction core catcher. This time only 31 cm of core was recovered, comprising three quite hard pieces of different and interesting lithologies. These pieces fell out of the barrel when jostled slightly on deck, suggesting that much more core probably fell out during the retrieval trip. We decided to try one last 18 m core and strongly recommended that the full-closure core catcher be used to avoid losing more core.

The third and final core was taken on 2 October (yet again using the friction core catcher) on another 18 m barrel. Core 331-C0016B-3L recovered 72 cm of core consisting mostly of loose rubble. A total depth of 45 mbsf was achieved in this hole before we ended coring operations and capped the guide base. Rather than casing this hole, a 3 m length of pipe was added to the corrosion cap to guide instruments into the hole.

The cap was set by ROV on the evening of 2 October. By the morning of 3 October, the Chikyu had retrieved all transponders and was headed toward anchor point near Nakugusuku Port, Okinawa, Japan. Expedition 331 operations had come to an end, and the expedition officially ended when we arrived at port on 4 October.

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