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doi:10.2204/iodp.sp.322.2009

Risks and contingency

Contingency

Figure F12 shows the decision tree for contingencies if operations remain on schedule and days are not lost because of adverse weather, and so on. Our first priority will be to use the time for additional operation at proposed Site NT1-07A and/or to core and log proposed Site NT1-01A. On the other hand, substantial risks to full achievement of our scientific objectives include

1. Lack of data about hole conditions or formation properties,

2. Potential problems with the Kuroshio Current,

3. Unusually slow rates of penetration, and

4. Typhoon and bad sea state.
   

To mitigate these risks, we have devised a comprehensive contingency strategy. It is important to note that this contingency plan is based on our state of knowledge at the time of this writing and modification may be required as additional information becomes available, including recommendations from the NT-PMT. Within our primary operations plan, we have identified several potential contingency options in the case of hole problems and/or time constraints (Fig. F12). First, if coring operations at proposed Site NT1-07A require significantly more time than allocated, we will need to rebalance all of the remaining scientific objectives. Contingency options for typhoon evacuation are shown on Figure F12. If we must abandon a hole to escape bad weather, reentry will not be possible; a new hole will be drilled to the depth where coring stopped.

Optional operation at proposed Site NT1-07A

Once the interface between sediment and basalt is encountered (at ~1200 mbsf), and if sufficient time remains to pursue additional objectives, we will consider several options. The first is to deepen the hole to at least 40 m below the sediment/basalt interface. This will permit logging tools to extend completely into basement and record the character of the lowermost sedimentary strata. After logging, the second option is to core the upper 400 m of sediment using HPCS and ESCS coring systems, thereby recovering the entire stratigraphic column at proposed Site NT1-07A. The third option is to occupy a new site (proposed Site NT1-01A), as described below. Criteria for making this decision will include the number of operational days remaining and an assessment of the scientific results from proposed Site NT1-07A.

Alternate site

If hole conditions, strong currents, or mechanical problems preclude some of the operations at the proposed primary site, we will move to an alternate site. Alternate Site NT1-01A is described briefly below. For comprehensive site descriptions, refer to Proposals 603A-Full2, 603A-Add, and 603D-Full2. This site could also be drilled if a sufficient number of contingency days remain after operations at proposed Site NT1-07A have been completed.

Alternate proposed Site NT1-01A

Shikoku Basin above basement high

This site will be drilled near the crest of a prominent bathymetric knoll (Kashinosaki Knoll) that is underlain by a basement high. The site is located on IFREE Line 95 at a water depth of ~3610 m (Fig. F13). The crossing line is CDEX Line ODKM03-22 (Shotpoint 1685) (Fig. F14). Visual observations and sampling from a JAMSTEC submersible indicate that strata at the seafloor are moderately lithified (Fig. F15), perhaps because of winnowing by strong bottom currents and/or precipitation of carbonate crusts. Acoustic character within the sediment column is largely transparent, which indicates that the strata are composed of hemipelagic mud and mudstone, with few sand packets. The boundary between upper and lower Shikoku Basin facies is ambiguous but could be near a relatively strong reflector at ~160 mbsf. Depth to an unusually strong basement reflector is ~600 m. We plan to core at least 40 m of the basement section; as a longer term goal, extending this penetration to 200 m below the sediment/basalt interface would be optimal.

Risk and risk mitigation

In this section, we present some of the known risks to the successful expedition implementation and the steps being taken to minimize them. The risks include lack of formation information, the Kuroshio Current, and overall operations time available.

Lack of formation information

One of the primary risks to the overall expedition success is the lack of information available regarding the formation at the proposed sites. Potential impacts range from not being able to achieve the total depth objectives (e.g., because of unstable hole conditions) to longer-than-anticipated drilling times (e.g., because of inaccurate seismic velocity models and erroneous estimates of depth to basement). The operations plan (Table T1) is based on formations and depths inferred from earlier seismic and regional geological interpretations and revised depths calculated from recently acquired 3-D seismic data. In particular, we have used lithologic information from the analogous reference sites that were cored and logged during ODP Legs 190 (Moore, Taira, Klaus, et al., 2001) and 196 (Mikada, Becker, Moore, Klaus, et al., 2002).

Encountering serious hole stability problems at the proposed primary sites (e.g., because of overpressured uncemented sands), however, would be problematic for our expedition because time and budget allocations will not permit any installation of casing. Given the potential for moderately unstable formation conditions and the priority of downhole measurements, we plan to use weighted mud liberally. This strategy will build upon the demonstrated successes of IODP Expedition 308 (Expedition 308 Scientists, 2005). We plan to fill the hole with weighted mud prior to conducting pressure measurements and wireline logging.

Kuroshio Current

The Kuroshio Current is a swift western boundary current that presents substantial risk to all expedition operations. If we encounter the maximum Kuroshio Current strengths at any given drill site, drilling, coring, or logging operations may be impossible. The core of the current migrates and meanders significantly and unpredictably (Fig. F16), so efforts will be made to monitor its location and velocity using available online resources. Forecasting the current's general behavior a few months in advance is possible, but accurate and precise predictions are not. A strong Kuroshio Current will have adverse impacts on instrument deployments by inducing substantial amounts of drill string vibration. Previous work in the region demonstrated that such vibrations are potentially damaging to all hardware and tools deployed.

Expedition operations time

Unforeseen circumstances could result in insufficient time being available to complete the entire operations plan. Examples include collapse of a borehole because of difficult formation conditions (unstable sands), persistently adverse environmental conditions (e.g., Kuroshio Current), hazardous weather (typhoon), hardware failures, or unusually slow rates of penetration. In anticipation of challenging and fluctuating environmental conditions, we have included 11.5 days of contingency for the entire expedition in the operations plan and time estimate.

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