Risks and contingency

The biggest imponderable and probably greatest risk to the successful implementation of the drilling plan is the weather. Although at a latitude of 32°S, Atlantis Bank lies to the north of the edge of the Antarctic Circumpolar Current and Southern Ocean and can be affected by storms and poor sea states at any time of year. This has occasionally affected scientific operations during previous research cruises. As detailed in Operations plan, we have identified several contingency sites where we could drill single-bit bare-rock spud-in rotary-cored holes until the weather improves. These operations should be possible in sea states greater than the ≤1 m heave conditions required to set the hard-rock drill-in reentry system at the primary proposed Site AtBk6. It should be noted that these contingency sites are not envisaged as alternate deep-penetration sites. In marginal conditions there is a risk that the HRRS will not be drilled in perfectly vertically, which may cause problems for the setting of the casing strings and, ultimately, successful deep penetration during Expedition 360 and future drilling expeditions. A complete backup drill-in reentry system will be carried, as well as a less sophisticated drilling/casing system (Plan B), as outlined in Operations plan. It should be noted that once the reentry system and casing strings have been installed, coring operations should be much less vulnerable to adverse weather conditions.

Hole conditions and core recovery in Holes 735B and 1105A on Atlantis Bank have been exceptional (average 87% recovery over >1500 m in Hole 735B); accordingly, we do not anticipate the difficult hole conditions encountered at almost all other ODP/IODP ocean basement drill sites.

Overall, we have allowed 3.8 days for weather contingency. If this is not required during Expedition 360, we will simply drill deeper at the primary proposed Site AtBk6.