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doi:10.2204/iodp.sp.321T.2009

Risks and contingencies

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

Poor weather/sea state: Cementing operations using the drill string in very close lateral proximity to the observatory as well as having to lower through the ROV platforms into the reentry cone will require reasonably good environmental conditions (weather, sea state, currents, etc.). The opening in the Hole U1301B ROV platform was successfully entered once during Expedition 301 with nearly the exact same systems. One difference is that our Expedition 321T work will be done without active heave compensation. The passive heave compensator was completely refurbished and is expected to allow us to accomplish this operation.

Inability to reenter small targets in Hole U1301A ROV platform (12 inch screened hole): Being able to stabilize the end of the drill string over and/or penetrating the much smaller (12 inch) circular penetrations in the Hole U1301A ROV platform is clearly going to be a challenge. In addition, the wellhead at Hole U1301A was determined to be loose ("wobbly") during 2007 submersible operations, but it is not known if this will present additional challenges.

Damaging the wellhead: Care must be taken to avoid damaging the wellhead as part of planned operations. There will be particular risks when the BHA is positioned close to the wellhead, either immediately before or immediately after reentry. The ship will need to be offset slightly away from the wellhead just prior to removing the BHA from the cone, so that it does not "swing" into the wellhead when it is free of the cone. It may be difficult to see the platform and wellhead clearly during these operations. In addition, it may be challenging to avoid having the BHA shift toward the wellhead when weight is put down on it against the (sloped) base of the cone. We will have to watch these operations carefully with the subsea camera system.

Cement deployed but does not seal holes: If this occurs, extra cement is available for deployment. There should be enough cement onboard to fill each reentry cone at least twice. Approximately 24 h of contingency time has been allocated at each site to allow for the risks listed above or other problems that arise.

The expedition could end early if operations go exceptionally smoothly or if there are difficulties that make completion of planned operations impossible. Researchers will return to Holes U1301A and U1301B in August 2009 with the submersible Alvin to download data and exchange sampler systems, and we should be able to assess the quality of the cement seals around the borehole observatories at that time.

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