Risks and contingency

Adverse weather conditions can cause reentry failures, and large swells may prevent safe operations. Coring was challenging during Leg 45. We expect that highly fractured volcanic and brecciated units may cause slow penetrations rates, poor core recovery, and poor hole conditions. Operations and time estimates were made after taking into consideration all previous drilling in North Pond. Poor weather and sea state can be problematic for borehole operations, in particular the installation of casing and CORK deployments. Experience from expeditions with similar drilling and instrumentation objectives on the Juan de Fuca Ridge flank (Expeditions 301 and 327) was also used in designing our operations and CORK observatory plans. We expect severe hole conditions in the uppermost basaltic crust; thus, we plan on drilling without coring through this region at each site (uppermost ~150 m of basement at Site NP-1; uppermost 20 m at Site NP-2) so that we can install 10¾ inch casing as quickly as possible. Poor hole conditions may require longer hole preparation than is currently planned, which may reduce the amount of time available for lower priority operations (e.g., coring). Poor hole conditions and rubbly basement may also prevent sealing the base of the 10¾ inch casing with cement, which is why we will use a mechanical casing seal system for use between the 16 and 10¾ inch casing strings. To the extent possible, we also intend to use a BHA that puts drill collars in the open hole to help prevent rubble from falling into the hole on top of the drill collars. Internal CORK experimental instrumentation will not extend outside the CORK casing to minimize problems related to poor hole conditions in deploying and retrieving them (Figs. F6, F7, F8, F9). Instead, we plan to deploy all CORK instrumentation so that it resides inside perforated 4½ inch fiberglass casing.