Previous   |    Next

doi:10.2204/iodp.proc.330.104.2012

Operations

The vessel arrived at Site U1373 on Rigil Guyot at 1730 h on 31 December 2010 after a 146 nmi voyage from Site U1372 that was accomplished at an average speed of 10.1 kt (all times are New Zealand Daylight Time, Universal Time Coordinated [UTC] + 13 h). A new bottom-hole assembly with a Type C-4 rotary core barrel bit and a mechanical bit release was made up and deployed. The corrected precision depth recorder depth for this site was 1455 meters below rig floor (mbrf). The vibration-isolated television (VIT) frame was deployed with the drill string, and by 0200 h on 1 January 2011 a seafloor strewn with large boulders and outcrops of hard rock was displayed on the monitor. From 0230 to 0445 h a VIT survey was made around the periphery of the site until a clear area was found that appeared to be able to support a free-fall funnel (FFF) deployment. After the driller tagged the seafloor at 1458.0 mbrf (1447.0 meters below sea level [mbsl]), the top drive was picked up and Hole U1373A was spudded at 0700 h on 1 January (Fig. F3).

The hole was routinely advanced to 65.7 mbsf, by which point the bit had accumulated 69.7 rotating hours and required replacement (Table T1). Basaltic basement was encountered at 33.9 mbsf. When the bit was replaced, penetration into basement was 31.8 m, with an average penetration rate of 0.9 m/h. The strategy of pulling half-cores helped to increase the average basement recovery to 91%. Average recovery for the entire hole at this point was 72%. The hole was flushed with 20 bbl mud sweeps prior to recovering each core to keep the hole clean of cuttings. An additional 40 bbl mud sweep was circulated before initiating the bit trip.

A FFF was made up and deployed at 1845 h on 4 January. The VIT frame was launched and the FFF was monitored as the bit was withdrawn from the hole. The bit cleared the lip of the funnel at 2005 h and was on deck by 1210 h on 5 January. The used bit was found to be in excellent condition (slight cone wear, no missing inserts, and tight bearings) and was ⅛ inch undergauge in spite of having accumulated 70 rotating hours. A new bit was made up to the bottom-hole assembly and deployed along with an additional stand of drill collars.

From 0630 to 0845 h the driller attempted a reentry into the FFF, but the bit appeared to bind about 1 ft into the throat of the funnel and could not be advanced any farther. The mud pump flow was increased on the chance that any obstruction would be hydraulically dislodged, but the end result was that the FFF tipped over on its side. It was surmised that the 2.7 m FFF casing was not lodged firmly in the hole when the old bit was withdrawn and that the only element that was holding the FFF vertical was the pile of cuttings.

Although the open hole was not visible on the camera, the driller attempted a blind stab into the hole by lowering the bit into the sediment cover around the periphery of the FFF. This course of action was terminated after 2.25 h, and the decision was made to offset the ship to a recently approved alternate site (prospectus Site LOUI-6B) located on the other side of the seamount’s summit. After the drill string was picked up to 1111 mbrf, the vessel was offset in dynamic positioning mode to Site U1374 in the afternoon hours of 5 January.

Top of page   |    Previous   |    Next