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doi:10.2204/iodp.sp.350.2013

Risks and contingencies

Expected major risks

Borehole instability

The drill site is in the rear arc, which seems tectonically inactive as compared to the area from the subduction front to the back-arc rift; therefore, large differential horizontal stresses producing borehole breakouts are unlikely.

The formations to be drilled are mostly well stratified in seismic profiles and expected to consist of volcaniclastic sediment and hemipelagic mud from the Quaternary to the Pliocene downhole to 1196 mbsf. Below that depth, volcaniclastic and volcanic rocks of the Oligocene are expected downhole to total depth (2200–2400 mbsf). Similar formations have already been cored or drilled during previous ODP legs on the fore-arc side without significant borehole problems downhole to 1682 mbsf (ODP Site 793). As long as the hydrostatic pressure is balanced with the formation pore pressure, the borehole is expected to be stable beyond the previous deepest penetration depth.

No hydrocarbon occurrence is expected at the proposed rear-arc sites.

Submarine cables

Submarine cables exist in the vicinity of the proposed IBM-3 sites. Submarine cable companies generally recommend that any seafloor operation be kept clear of the cables by a safety distance three times that of the water depth. Given the water depth of 2114 m at proposed Site IBM-3C, that distance would be ~6.5 km. However, the actual distance of the nearest cable is ~3 km south of proposed Site IBM-3C. To mitigate the risk of damaging the submarine cable, a seafloor camera survey should be conducted prior to spud-in.

Mechanical failures and delay caused by metaocean conditions

Potentially rough sea conditions may cause operational delays not only caused by waiting-on-weather but also caused by mechanical problems in drilling and logging tools. The probability of a typhoon is highest during May and August to October (up to 2 per month), and November to March and June are months of relatively frequent rough sea conditions. If the Kuroshio Current passes the drill site, an event that has occurred five times since 1967, vortex-induced vibration may affect the drill string and cause mechanical failures to drilling equipment and logging tools.

Contingency plan

If the primary proposed Site IBM-3C presents unexpected challenges, operations will be carried out at one of the alternate proposed Sites IBM-3D, IBM-3E, or IBM-3F to the northwest and west of proposed Site IBM-3C.

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