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doi:10.2204/iodp.proc.318.104.2011

Operations

Transit to Site U1356

We departed from Site U1355 at 0600 h on 22 January 2010. The short voyage to Site U1356 was without incident except for spotting a lone iceberg both on radar and visually. The vessel was positioning at Site U1356 at 1345 h on 22 January. The 82 nmi journey was made at an average speed of 10.7 kt. All times in this section are given in local ship time, which was Universal Time Coordinated + 11 h.

Site U1356

Because of the problems starting a hole with advanced piston coring system at the previous site, we decided to start coring with the rotary core barrel (RCB) coring system. The drill string was lowered to the seafloor and the RCB bit tagged it at 4003 meters below rig floor (mbrf). This was 21.5 m deeper than the corrected precision depth recorder depth of 3981.5 mbrf.

RCB coring in Hole U1356A started at 0025 h on 23 January 2010. RCB Cores 318-U1356A-1R through 106R penetrated to 1006.4 mbsf and recovered 350.13 m of core (35%). The presence of glacial erratics below the first two cores adversely affected recovery. Recovery varied considerably from 0% to 99% but improved with depth. For example, Cores 66R through 92R penetrated 254.3 m of formation (622.3–876.6 mbsf) and recovered 140.58 m (55%) (Table T2).

No significant hole problems were encountered. We started frequently circulating 20 bbl of mud beginning at 296.7 mbsf. This was increased to every other core starting with Core 318-U1356A-69R (660.6 mbsf). A single 40 bbl sweep was circulated at 631.9 mbsf and then 20 bbl mud sweeps were circulated at 814, 834, 852, 872, and 877 mbsf. Below this depth we circulated 20 bbl of mud after each core.

Minor disruptions to operations occurred for 1 h on 27 January when the ball valve and remote actuator required servicing and when we had to make an extra wireline run to recover Core 318-U1356A-97R after the shear pin parted during the initial retrieval attempt.

The total target depth of the hole was revised as we progressed to ensure full characterization of the greenhouse to icehouse transition across the inferred Eocene/Oligocene boundary reflectors. Ultimately, coring reached 1006.4 mbsf. As soon as coring was finished, we started to prepare the hole for logging by sweeping the hole with 50 bbl of sepiolite and then raising the drill bit back up to 103 mbsf. A tight spot was encountered at 409 mbsf. While lowering the bit back down into the hole to 476 mbsf, a ledge was encountered at 400 mbsf. The weather had deteriorated (up to 20 ft swells; 35 kt winds) to the point that we had to abandon the hole. Based on weather forecasts, we also thought conditions would not improve substantially for long enough to allow a reasonable chance to log the hole. We retrieved the drill string and by the time the drill string was recovered, the weather system had passed and the sea conditions were relatively mild. After the beacon was recovered, we began the transit to Site U1357 at 0615 h on 2 February.

The only ice seen during operations at Site U1356 occurred on 29 January when a pinnacle-shaped iceberg was detected 12 nmi to the southeast. It was estimated to be 40 m high, 110 m long, and moving haphazardly at ~0.5 kt. Two growlers were also observed visually in the general area of the iceberg. The iceberg came no closer than 10 nmi, did not impact operations, and drifted off radar at 0430 h on 30 January.

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