Risks and contingency

There are a number of risks to achieving the objectives of this program.

Hole conditions

Poor hole conditions (e.g., encountering thick intervals of loose sediment that can collapse into the hole) will be dealt with by using frequent high-viscosity mud sweeps and/or heavy mud to condition the holes. In addition, the plan at the mid-slope site is to case off the upper section of the hole, which requires the installation of a reentry cone, and to cement selected intervals to stabilize the hole.

Weather conditions

Drilling in shallow water is more challenging than drilling in deep water. In part, this is because the ship must maintain position to within a maximum 8% of water depth. Drilling in shallow water also involves restrictions on the amount of heave that can be tolerated by the heave compensator. The amount of allowable heave increases with water depth within three depth ranges: 76–300 m, 301–650 m, and >651 m. For example, coring will stop if heave exceeds 1 m in water <301 m. Therefore, we must be prepared to modify the drilling strategy in response to changing weather conditions.

Marine mammals

The check shot experiment with the VSI tool can only be performed during daylight hours and with the absence of any marine mammals. If marine mammals are sighted during the experiment, then operations will be suspended as required by IODP guidelines.

Alternate sites

A series of alternate sites are available for contingency operations (Table T2). In addition, Site CRIS-20A has been proposed as a contingency site to further characterize the sediments and correlate them with the 3-D seismic survey in case time allows. Seismic profiles of all proposed alternate sites are included in the “Site summaries.”