Operations plan/drilling strategy

Expedition 359 aims to achieve an ambitious coring program that prioritizes 7 primary sites and 1 alternate site in 380–2400 m of water depth (Table T2). Six of the sites are in the Maldives archipelago and one is west of the Indian peninsula in the Arabian Sea (Figure F1). The final operations plan and number of sites to be cored (Table T3) is contingent upon the JOIDES Resolution operations schedule, operational risks (see Risks and contingency), and the outcome of requests for territorial permission to occupy these sites. Of particular relevance is the planned ~14 day transit from Darwin, Australia, prior to beginning coring operations in the Maldives. Should ship speed be less than the estimated average of 10.5 kt, the drilling schedule could be significantly impacted. The current planned operations schedule also includes two half-day moorings for required ship clearance and immigration formalities, one at the arrival to and one at the exit from the Maldives.

The planned sequence of drill sites, coring/downhole measurements, and time estimates is provided in Tables T3 and T4. For operational efficiency, the sites will be occupied in the following sequence: MAL-07A, MAL-05A, MAL-03A, MAL-02A, MAL-01A, MAL-04B, and KK-03B. The coring strategy will consist of triple advanced piston corer (APC) coring using nonmagnetic core barrels to ~200 meters below seafloor (mbsf) or to APC refusal at Sites MAL-07A, MAL-03A, and KK-03B. Coring using the extended core barrel (XCB) system will be used to advance the holes to total depths at Sites MAL-07A (642 mbsf), MAL-03A (435 mbsf), MAL-02A (325 mbsf), MAL-04B (554 mbsf), and KK-03B (450 mbsf). Coring with the rotary core barrel (RCB) system will be used to advance to total depth at Sites MAL-05A (420 mbsf), MAL-01A (1060 mbsf), and MAL-02A (560 mbsf). As described below and pending further discussions, at this point only cores from Hole A at each site will be oriented.

For planning purposes, APC refusal depth is estimated at 200 mbsf, although we anticipate that this may be exceeded at some of the more mud rich sites with target depths greater than 200 mbsf. On the other hand, we estimate that APC refusal may be reached at a shallower depth at Sites MAL-05A, MAL-01A, and MAL-04B because of the presence of lithified carbonate sediments (e.g., limestones). APC refusal is conventionally defined in two ways: (1) a complete stroke (as determined from the standpipe pressure after the shot) is not achieved because the formation is too hard or (2) excess force (>100,000 lb) is required to pull the core barrel out of the formation because the sediment is too cohesive or “sticky.” In cases where a significant stroke can be achieved but excessive force cannot retrieve the barrel, the core barrel can be “drilled over” (i.e., after the inner core barrel is successfully shot into the formation, the bit is advanced to some depth to free the APC barrel). When APC refusal occurs in a hole before the target depth is reached, the half-length APC system may be used to advance the hole before switching to the XCB technique (see Tables T3 and T4 for operations details per site). All target depths in the current operations plan for sites within the Maldives archipelago have been approved by the IODP Environmental Protection and Safety Panel and the Texas A&M University Safety Panel, but a request to extend the penetration depth at Site MAL-02A to 1060 mbsf will be submitted after publication of this Scientific Prospectus. EPSP approval also is pending for the drill sites in the Kerala-Konkan Basin (proposed Sites KK-03A and KK-03B) and will be requested at the same time as the depth penetration extension. Triple APC holes will allow us to build a composite stratigraphic section at each site for the upper ~200 mbsf and deeper at Site KK-03B, where double XCB will be used below APC refusal.

According to the current operations plan, Expedition 359 will core ~5400 m of sediment and potentially recover ~4100 m of core. The estimate of the amount of core recovered is based on 100% recovery with the APC system, 65% recovery with the XCB system, and 50% with the RCB system. Considering the significant transit time at the beginning of the expedition (~2 weeks), this coring schedule within the remaining 6 weeks of the expedition is indeed ambitious and will require tight operational planning and flexibility.