Previous   |   Next

doi:10.2204/iodp.proc.301.103.2005

CORK mechanical and hydraulic design

General CORK configuration and operations

CORK observatories work by hydraulically isolating discrete depth intervals, allowing formation pressure, temperature, geochemistry, and microbiology to approach an equilibrium state after the perturbation resulting from drilling has ended (Davis et al., 1992). Intervals of hydrogeologic interest to Expedition 301 studies are all in basaltic basement. The standard approach to creating a basement observatory begins with setting a reentry cone at the seafloor and installing a large-diameter conductor casing through uppermost sediments (Fig. F1). For Expedition 301, this conductor casing had a diameter of 20 inches; Holes 1026B and 1027C were emplaced during Leg 168 and used 16 inch conductor casing. Once the cone and conductor casing are installed, the pipe is tripped, and a bottom-hole assembly is deployed with a system for drilling and/or underreaming a large-diameter hole through the rest of the sediment section and into uppermost basement. A smaller diameter casing is installed and cemented into uppermost basement (16 inch casing during Expedition 301; 10¾ inch casing during Leg 168).

During Leg 168 in Holes 1026B and 1027C, rotary core barrel (RCB) coring commenced after the sediment was cased. During Expedition 301, we drilled into basement with a (noncoring) tricone bit to allow installation of a third casing string (10¾ inch), which was cemented into place at the bottom. Once the casing cement was drilled out in Hole U1301A, no additional drilling was attempted. In contrast, Hole U1301B was subsequently deepened by RCB coring.

Once drilling, casing, and coring operations are completed, CORK casing length and locations for inflation of CORK packers in 10¾ inch casing and/or open hole are selected. Each CORK is deployed on the drill string using a CORK running tool. CORK plugs and instrument strings are deployed though the drill pipe before landing the CORK in the reentry cone. When the instrument string is released and the CORK body is placed in the reentry cone, one or more CORK casing packers are inflated and the CORK running tool is released. The last standard operation is deployment of the submersible/ROV platform in the reentry cone.

CORK mechanical components

The CORK heads deployed during Expedition 301 comprise a 30 inch diameter frame built around a piece of 11¾ inch casing (Figs. F1, F2). Sampling and valve manifolds, sensor packages, data loggers, and samplers are arranged within three 1.5 m high bays, offset by 120°, and separated by steel plate bulkheads and gussets. The gussets provide strength to the CORK head and help to guide the ship's camera system and the submersible platform around the bays during installation and later operations, protecting instrumentation and valves. During Expedition 301, one bay was dedicated to monitoring and logging pressure data, one was dedicated to fluid sampling, and one was dedicated to microbiological sampling and/or auxiliary pressure monitoring or other experiments. Cutouts on the bulkheads and gussets are designed to allow a submersible or ROV to hold on for stability and leverage.

Inside and below the CORK head is the CORK body, which extends downward into the reentry cone. A seal sub (Fig. F3) is located at an appropriate depth so that it can land and seal in 10¾ inch casing near the throat of the reentry cone. Below the seal sub are crossovers and an appropriate length of 4½ inch casing, which extends to depth. The amount of 4½ inch casing that hangs below the CORK body is adjustable at sea, based on hole depth and conditions, available joint lengths, and the dimensions of other CORK components. One or more inflatable CORK casing packers are incorporated into the 4½ inch casing to allow sealing of distinct depth intervals. Additional 4½ inch casing can be attached below the deepest packer protect instruments if the open hole is unstable.

The 10¾ inch seal sub and packers include hydraulic pass-throughs and connectors for monitoring, sampling, and inflation lines (Fig. F3), plumbed to valves and fittings within the CORK head. A ½ inch packer inflation line is attached to the CORK running tool (used to deploy the systems) with a hose and quick-connect so that the drill string can be used to pressurize the inflation line. For systems having more than one packer, packers are connected in series and are inflated at the same time. Packer inflation lines and bodies are allowed to fill with water during initial deployment from the drillship. A check valve in the packer inflation line allows pressure to be trapped internally, and the line can be repressurized later by submersible or ROV if there is reason to suspect incomplete inflation or pressure loss within the packers.

An umbilical tubing bundle was constructed for Expedition 301 comprising four inch lines, four ¼ inch lines, and one ½ inch line, the latter being used for packer inflation (Fig. F4). This umbilical was used for the multilevel CORK in Hole U1301B. This CORK also used a dedicated microbiological sampling tubing constructed from ½ inch Tefzel (a variant of Teflon) hose. The single-level CORK system installed in Hole U1301A used umbilical left over from Leg 196 (six ¼ inch, one inch, and one ½ inch lines), and that deployed in Hole 1026B used umbilical left over from Leg 205 (three ¼ inch and one ½ inch lines). Umbilicals were attached to the outside of CORK casing with zip-ties, stainless steel banding, and duct tape. Centralizers were attached at irregular intervals to hold the 4½ inch casing away from the 10¾ inch casing and borehole wall during deployment and to protect the umbilical and miniscreens.

One challenge during earlier CORK deployments was ensuring that hydraulic lines were filled with water during deployment. Trapped air would lead to excessive compliance within the lines, which would influence pressure responses monitored at the seafloor and could lead to clogging of fluid samplers. In previous deployments that utilized umbilical lines, check valves and cap seals were used to bleed air at the time of deployment. We elected to avoid this complexity by leaving all sampling and monitoring lines open and positioning the open ends and fittings at the highest points on the lines. Valves were closed and some instruments were recovered or replaced during an ROV dive three weeks after Expedition 301.

Top of page   |   Previous   |   Next