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doi:10.2204/iodp.pr.341S.2013

SCIMPI installation plan

SCIMPI is designed so that after installation in a freshly drilled borehole, the borehole walls collapse around the tool, thus leaving the modules with a firm connection to the subseafloor formation. One of the challenges of installation is to drill a hole that remains stable enough for the installation phase when borehole circulation and drill string rotation cannot occur but then collapses after the drill string is pulled out of the hole. Premature collapse of the borehole could cause the drill string to be stuck in the formation.

The first SCIMPI installation procedures were tailored for Hole U1416A and included the following:

1. Complete slow drilling of the borehole to 260 mbsf;

2. With the drill string ~10 m off the bottom of the hole, cease rotation and circulation for ~3 h prior to installing SCIMPI to ensure that the borehole will remain stable for a long enough time period for installation;

3. Lower SCIMPI on the Lamont-Doherty Earth Observatory (LDEO) Borehole Research Group (BRG) wireline with the Multi-Function Telemetry Module (MFTM) and the Electronic RS (ERS) Overshot to the bottom of the hole;

4. Check that SCIMPI is on and collecting data;

5. Release SCIMPI using the wireline release system;

6. Pull the wireline up to the ship:

7. Trip the pipe out of the hole at a constant rate while monitoring the pipe with the vibration-isolated television (VIT) camera; and

8. Use the camera system near the seafloor to inspect the installation.

Actual installation in Hole U1416A followed this plan closely (see “Operations” for a detailed description of operations). A small diversion from the plan was that the first release approach was unsuccessful and SCIMPI returned to the surface with the wireline. The second wireline release was successful, resulting in the configuration shown in Figure F4. As this was the first operational deployment of the SCIMPI using the ERS and MFTM, bench tests were conducted before the expedition as well as on the ship. These tests and the modifications made to these tools are described in “Appendix A.”

A second hole (U1416B) was drilled ~40 m north of Hole U1416A to determine the baseline physical properties of the sediment and gas hydrates in Hole U1416B. Prior to selecting a location for the second hole, science party discussions included the rationale for drilling a second hole so close to the primary SCIMPI. A hole located close to the SCIMPI hole could contaminate the in situ fluid flow and pressure signals measured with SCIMPI. Alternatively, a closely spaced hole could serve as a baseline for interpreting SCIMPI observations and for hole-closure monitoring. A decision was made to drill a hole as far as possible from SCIMPI within the previously surveyed area as a baseline and for hole-closure monitoring. Hole-closure monitoring will be conducted in collaboration with NEPTUNE during their semiannual operations and maintenance cruises.

Logging of this hole was not completed because of the excess pressure and flow up the drill string that occurred during final logging preparations (see “Operations”). Following pressure and flow mitigation, the drill string was pulled from the seafloor. Because Hole U1416B exhibited fluid and gas flow out of the borehole, a scientific flow-monitoring program was put into place. Using the VIT sonar, surveys at the seafloor and 50, 100, and 150 m above the seafloor were conducted. These surveys detected the gas bubble plume from the borehole to a maximum depth of 150 m above the seafloor. However, this is a minimum depth for the rising gas plume, as the VIT sonar is relatively small and limited in range. Typical naturally occurring gas bubbles observed with ship-mounted sonar in the vicinity of Site U1416 prior to drilling were seen to rise as high as 500 m beneath the sea surface, which coincides with the upper limit of the gas hydrate stability field in the ocean. In addition, SHRIMPI was free-fall deployed ~4 m south of Hole U1416B to serve as a longer term observatory after the R/V JOIDES Resolution leaves the site. Installed with a command module and buried at ~1 mbsf, SHRIMPI will be recovered later in 2013 by Ocean Networks Canada’s NEPTUNE operations cruise.

Direct visual observations of Hole U1416B ~3 weeks following Expedition 341S during an Ocean Networks Canada’s NEPTUNE cruise documented that flow out of the hole had completely ceased.

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