Risks and contingency

There are a number of challenges associated with drilling operations in deep water that could impact the drilling and coring strategy of this expedition. Primary among these is stress on the very long drill string (>6000 m), resulting from the deep water and our goal of drilling to at least 1450 mbsf. One factor that will influence the drill string stress will be the on-site weather/sea state and the resulting heave behavior of the ship. Expedition 351 has been scheduled to take place during the boreal summer. Severe weather, such as typhoons, may occur and could adversely affect operational efficiency. Although we are not sailing during the peak typhoon season (late August–September), the expedition could experience some waiting on weather delays depending on conditions during critical operations, such as casing/reentry deployment, tripping long drill strings, and/or RCB basement coring at depth.

Furthermore, the nature of the sediment may present risks that could affect core/logging operations, core recovery and quality, rate of penetration, and hole stability. However, the sediment is presumably well stratified, and as previously stated, sections older than the Oligocene (~600 mbsf) are likely to be well lithified.

To address the inherent risks, we have several contingency plans that are mainly constrained by the time available to accomplish operations:

  1. If we have to stop operations at Site IBM-1 relatively early into the expedition, we intend to transit and drill one of the similar alternate sites (IBM-1A or IBM-1B) (Fig. AF1, AF3). Operations would then proceed at the alternate site (Table T3) following a similar strategy to the primary site. A total depth of 1600 mbsf for alternate Sites IBM-1A and IBM-1B has been approved by the EPSP.

  2. If we have to end operations at Site IBM-1 far enough into the expedition that drilling either alternate Site IBM-1A or IBM-1B would not allow us to reach the lower sedimentary section and/or basement, we have two alternatives. In the first scenario, we would return to the RCB pilot hole (Hole C) and attempt to reenter the hole via the FFF. If we are not able to continue coring in Hole C (e.g., the hole has collapsed), we intend to move to our third alternate site (IBM-1C; pending EPSP approval). Although the entire sedimentary sequence is still present at this site, the sediment thickness is ~70% of Site IBM-1 (Fig. AF3). Furthermore, as another time-saving measure, we are seeking EPSP approval to drill through the upper sediment without coring until the depth at which we had to cease coring at Site IBM-1. This would allow us to reach the lower sediment column and basement much more quickly (Table T3). The rationale is that the upper sedimentary section has been cored at the primary site, which is only 7.3 km away, so there is not much to be learned by additional sediment coring.

  3. Finally, there is a request to drill a geotechnical hole at Site IBM-4GT (Table T3; Fig. AF4) to aid in preparations for a future riser expedition on the D/V Chikyu. Operations at this site will only be considered if Expedition 350 does not manage to drill here, sufficient time is available, and/or further drilling at the primary and alternate sites is not possible.

The overall proposed strategy should allow enough flexibility during the expedition to enable us to fulfill the scientific objectives.