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doi:10.2204/iodp.sp.330.2010

Risks and contingency strategy

Three principal factors could affect the implementation of the drilling plan during Expedition 330: (1) adverse hole conditions, (2) weather conditions that limit our ability to continue coring or stay on station, and (3) time delays (arising from equipment breakdowns, inclement weather, or measures taken to respond to hole conditions).

Hole conditions

Because the Louisville seamounts have not been drilled or piston cored before, no previous data regarding the nature of the overlying sediments exist. Based on MCS and dredge expedition site survey data, a thin pelagic cover 10–40 m thick is expected to rest above a 50–110 m thick layered material assumed to consist of volcaniclastics, limestone(?), and/or minor intercalated late-stage (posterosional) lava flows. It is unclear how consolidated this material is, but the overlying sediments/clastics drilled during Leg 197 (Emperor seamounts) were stable during drilling. Nevertheless, basaltic debris repeatedly jammed the float valve and bit throats at three out of four sites during Leg 197, resulting in empty core barrels and requiring time-consuming pipe trips for inspection/cleaning.

The sediment surfaces on top of the Nintoku (Hole 1200A) and Koko (Hole 1206A) Emperor seamounts were apparently too consolidated to allow proper impingement of the FFF, so the FFF tipped over when the drill string was pulled out of the hole for bit replacement. The operators were able to reenter the open hole, but such success cannot always be expected. As a backup, a reentry cone and single-string casing (reaching down to the beginning of basement) could be deployed, but this would be time consuming, and availability of the required hardware on the ship cannot be guaranteed for this expedition.

In contrast, unusually adverse hole conditions are not expected when drilling into the igneous basement section at the Louisville seamounts. Hole conditions in igneous sections depend on the type and consolidation of the igneous rocks. It is common to encounter fractured friable zones while drilling lava-flow sections, usually between flows. Occasionally, rock pieces from the borehole wall cave into the hole after being knocked loose by the drill string. Loose rocks above the drill bit can cause the drill string to bind and may require clearing the hole. On rare occasions, such debris causes so much difficulty that the hole must be abandoned. Depending on time estimates and the importance of the site for achieving the overall objectives of this expedition, a new "B" hole may be drilled at the same site. To save valuable time, coring in the new hole will not start before reaching the maximum penetration depth of the abandoned "A" hole.

Weather

Another potential issue that may shorten operations time is weather. Expedition 330 will take place from December to February. The area of the northernmost sites (primary Site LOUI-1C and alternate Sites LOUI-1B and LOUI-6A) has a moderate cyclone risk from November to April. All other primary sites have a moderate cyclone risk from January to March. Therefore, it is possible that Expedition 330 could lose several operation days because of a nearby cyclone. If a hole must be abandoned because of an approaching storm, we may deploy an FFF if return to this site and reentry is deemed necessary.

Timing

If significant time is spent responding to poor hole conditions, slower than expected penetration rates, and/or weather-related delays, a primary site may be dropped from the schedule. Such a decision will only be made following consultation with the science party. In terms of meeting scientific objectives, the primary sites are currently prioritized in the following order: LOUI-1C, LOUI-2B, LOUI-4B, and LOUI-3B.

If additional contingency time is available (e.g., because of faster than expected average penetration rates), extra time may be allotted for advanced piston corer (APC) coring of a "B" hole to obtain a better recovery of the pelagic sediments or penetrate deeper into the volcanic basement at a later primary site. However, such a decision will be based on evaluation of the scientific value (and quantity) of the sediment recovered at the previous sites with the RCB (gravity-pushing) technique.

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