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

Operations

Overview

The operational objective of Expedition 321T is to conduct cementing operations, which will entail pumping cement into the reentry cones of Holes U1301A and U1301B in an attempt to achieve a hydraulic seal around the observatories.

Port call and transit to Hole U1301B

After a 1 day port call in San Diego, the ship will transit for an estimated 5.3 days (~1134 nmi) before arriving at Hole U1301B (Table T1). Since this transit will be heading into the southward-flowing California Current and into the prevailing wind direction, we have assumed a transit speed of 9 nmi/h.

Operations in Hole U1301B

Once we arrive at the coordinates for Hole U1301B, we will prepare the drill string for cementing operations and lower it to just above the seafloor. Once we locate the seafloor observatory we will lower the end of the drill string with a 5-1/2 ft cementing stinger attached to it through the ROV platform. The ROV platform at Hole U1301B has a sector that was cut out before it was deployed on Expedition 301 (Fig. F3B). The cementing stinger has two to three 3 inch diameter holes in the side within the lower 6 ft of the sub and has a mule shoe–style cutoff at the bottom angled at ~45° for penetration. Cement will be pumped into the reentry cone through the cutout in the ROV platform. Lowering the drill string through the cutout has to be done with care to ensure the drill string does not strike the CORK head situated above the ROV platform that contains sensors and data loggers. After the cement has been deployed, we will raise the drill string out of the reentry cone and offset the drillship to the Hole U1301A observatory without retrieving the drill string.

Operations in Hole U1301A

The operations in Hole U1301A will be very similar to those at Hole U1301B. One difference is that the ROV platform has no cutout (Fig. F3A). The ROV platform is mostly solid metal with 12 inch circular penetrations with a metal mesh welded beneath. Once the drill string has been stabilized over the ROV platform, one of the 12 inch holes will be penetrated using the cementing stinger and some of the weight of the bottom-hole assembly (BHA), and cement will be pumped into the reentry cone. After the cement has been pumped, the drill string will be pulled clear of the seafloor and the ship will offset away from Site U1301 to flush any remaining cement from the drill string. Once this is completed, we will return, inspect the cement job at both Holes U1301A and U1301B using the subsea camera, and then retrieve the drill string.

Transit to Victoria

Once the drill string is secured, we will transit to Victoria. The transit from Site U1301 to Victoria is ~0.7 days (181 nmi) at 10.5 nmi/h.

Cement mix

Based on the cement mix test results provided by BJ Services Company, both seawater and freshwater formed good slurry rheology and mixability. For cementing operations at Holes U1301A and U1301B we intend to use the seawater mix as it thickens slightly faster (6.5 h) than freshwater (7.5 h) and will result in a higher compressive strength for the hardened cement (513 versus 125 psi at 40°C). Cement will be mixed to the optimum 15.9 ppg slurry density using seawater (freshwater would give 15.8 ppg).

We plan to take 1300 sacks (61 short tons) of cement preblended with of Cello-Flake (cellophane) flakes lost circulation material (LCM). In addition, ~1660 sacks (78 short tons) of Class G neat cement will also be available onboard. This cement will not be combined with LCM because we do not have the recommended mixing capabilities. We also plan to take aboard six 55 gallon drums of sodium silicate liquid "extender." This additive can be used to develop the same volume of cement slurry while using lower quantities of neat cement. The lowest recommended density using the extender would be 11.3 ppg and this would require 68 gallons of extender for every 100 sacks of cement. The preference is to not use the extender unless we are running short on cement.

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