Risks and contingency

Three principal factors could affect the implementation of the drilling plan:

  1. Adverse hole conditions at the principal sites (e.g., encountering thick intervals of loose sediment/rock that can collapse into the hole).

  2. Weather conditions that can limit the ability to drill in shallow water by exceeding the water depth–dependent restrictions on allowable heave.

  3. Shallow hazards.

Hole conditions

Poor hole conditions at either site will be dealt with in the first instance by using frequent high-viscosity mud sweeps and or heavy mud to condition the holes. One possible remedial action if hole conditions prove to be problematic would be to case off the upper sections of a hole with a limited amount of casing (up to 600 m) which might be available if it is not used during one of the previous expeditions. However, casing, which also requires the installation of a reentry cone, will be employed only as a last resort because it will involve a significant time penalty that would almost certainly lead to the loss of one site from the planned drilling program. The advantages and disadvantages of casing use will have to be evaluated at sea should the need arise.

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. Station keeping turned out not to be a significant problem when shelf drilling was conducted on the New Jersey margin (ODP Leg 174A) and on Canterbury (IODP Expedition 317). However, 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.

Alternate sites

Seismic profiles of all proposed alternate sites (as well as all primary sites discussed above) are included in the "Site summaries."

Slope sites (CRIS-10A and CRIS-11A)

These sites serve as alternate sites for primary Sites CRIS-3B and CRIS-4A. If poor hole conditions or other operation difficulties encountered, we may move from primary sites to alternate sites if it is judged that better conditions may be encountered at such alternate sites.

Base of the slope site (CRIS-2B) and incoming plate site (CRIS-1A)

These two sites could be drilled as contingency sites if drilling at the two slope sites becomes impossible because of hole conditions or sufficient time remaining available. The base of the slope site (CRIS-2B) is the highest priority contingency site. The alternate for Site CRIS-2B is Site CRIS-9A, and the alternate for Site CRIS-1A is Site CRIS-7A.