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Operations summary

Hole U1311A

After poor drilling conditions prevented our attempt at deep penetration at the primary site on the hanging wall, we selected an alternate site based on seismic and bathymetry data ~1 nmi west of Site U1310. This site was interpreted to be near the westernmost edge of the presumed hanging wall outcrop, where a single-bit hole might penetrate the detachment fault. A subsea camera survey revealed a smooth, sedimented surface with rare accumulations of angular to subrounded, submeter-sized boulders. The seabed at this location slopes to the south, and during our survey we crossed two small (2–3 m) east-facing scarps with rubble-strewn surfaces (Fig. F1). We traversed upslope (north), noting several outcrops of angular blocks interpreted to be probable talus, and at the end of the traverse encountered a steep south-facing cliff at least 20 m high. The subrounded morphology of outcrop at the base of the cliff suggested pillow basalt flows, so we traversed back to a location ~70 m south of the cliff face to attempt coring in an area devoid of visible rubble.

Hole U1311A was initiated with a rotary core barrel (RCB) bottom hole-assembly (BHA) at 2040 h on 18 December 2004. Owing to the spacing of BHA components, our first coring interval was 12 m. High and erratic torque was constant during the 25 h required to core this interval, and rock fallen into the hole had to be redrilled several times. Running the bit back to bottom after recovering Core 304-U1311A-1R (Table T1), the driller determined that there was at least 3 m of fill in the 12 m hole. After several hours attempting to clear the hole proved unsuccessful, Hole U1311A was abandoned. With more than half the potential bit life expended during this first coring interval, we elected to recover the BHA and replace the bit before attempting another hole.

Hole U1311B

For our second coring attempt at Site U1311, we conducted another subsea camera survey starting ~100 m northwest of Hole U1311A, above the cliff face observed during our first camera survey at this site. During our second survey, we saw a broad expanse of smooth sediment with rare rounded and sediment-covered rocks. Given the poor drilling conditions we had encountered during all our attempts to drill on the hanging wall, we had little confidence that RCB coring from the seafloor would allow deep enough penetration to meet our objectives before bit life was exhausted. Our engineers suggested the alternative strategy of drilling a large-diameter hole without coring and then attempting a reentry into that hole with a conventional RCB BHA. We hoped that the large-diameter bit would be more efficient at clearing the rubble falling into the hole and we could begin coring with additional weight on bit to allow faster and deeper penetration. Because drilling with any rotary system was likely to encounter the same poor drilling conditions we suffered with the RCB, we decided to attempt to drill the large-diameter hole with a wing-style reaming hammer bit. Although we were not convinced the hammer system with a wing-style bit could set casing in this environment, we had seen it open a hole and felt this strategy had a higher chance of success than using a large-diameter rotary bit.

However, after 10 m of penetration, downward motion with the bit ceased while we attempted to hammer through rubble falling into the hole. After 19 h, we were not able to advance the bit to the bottom of the hole, so the attempt was abandoned.

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