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doi:10.2204/iodp.proc.327.109.2011

Deployments and recoveries

Downhole deployment and recovery from the drillship

The initial deployment of downhole packages occurs on the drilling vessel during installation of the CORKs (Fig. F2). Individual packages are connected on the rig floor with shackles and electrical isolators. A sinker bar is positioned at the base of the package and connected to a variety of OsmoSampler packages. These packages are connected to a sealing plug, which is connected to 0.95 cm (0.375 inch) diameter Spectra cable. These items (collectively known as the borehole instrument string) are lowered into the borehole from the rig floor using tuggers. Considering that these strings are recovered using cranes from a supporting scientific research vessel and the derrick on the drilling vessel accommodates 30 m high lifts, rope holds are required every ~25 m for maneuvering the string into place on the rig floor. Yale grips are used and removed at the rig floor. Recently, loops of Spectra were hand-spliced into the main Spectra line prior to deployment to simplify the deployment and recovery processes. A video documenting the construction and deployment process of a CORK instrument string with OsmoSamplers is available at www.youtube.com/watch?v=-SNINUg6rz0/.

At this time, only downhole strings from Holes 1253A and 1255A have been recovered with the drillship. An attempt will be made with the JOIDES Resolution in late 2011 during Expedition 336 to recover a third string deployed during Leg 174B (Hole 395A) (Becker, Malone, et al., 1998). Recovery with the drillship is the reverse of the deployment procedure described above: first, a CORK-latching tool attaches the drill string to the CORK wellhead; a wireline tool is then sent down the inside of the drill string and latched to the top plug, releasing any latches to the CORK body; the top plug and string are retrieved using the wireline within the drill pipe; and then 25 m long sections are removed at the rig floor using tuggers.

Submersible deployment of downhole strings

Downhole deployments that do not use the drilling vessel but instead use a scientific submersible are more complicated (Fig. F12; see SINKER in CORK in “Supplementary material”). As before, OsmoSampler packages are shackled together with a sinker bar at the base and Spectra cable and a top seal above, similar to the deployment from the drillship. However, the sinker bar is then placed in a drop weight and held in place with a steel T-handled pin. The drop weight has containers that hold a transponder and a directional transponder (homing device) for navigation. Above the top plug is another line (polypropylene) attached to a series of floats. Polypropylene is used here because it is easy to cut underwater with a knife wielded by a submersible’s manipulator. Once assembled, the entire string with flotation is lowered over the side of the support ship via cranes and then released at the surface. A transponder tracks the location of the string during descent and provides a target for the submersible.

Critical to submersible operations is the weight of the various items on the string. Including the drop weight, the overall weight must be at least 68 kg (150 lb) negative so that the package sinks quickly and does not drift far from the planned drop site. Alternatively, the string can be lowered to 100 m above the seafloor and released with an electrical release coupled to the conductive-temperature-depth (CTD) cable. Once the drop weight is disconnected and the transponder and homing device are recovered by the submersible, the downhole string with floats should weigh ~23–45 kg (50–100 lb) in seawater. The sinker bar should be at least 68 kg (150 lb) in water so that if the string is dropped by the submersible only the sinker bar will land on the seafloor and the string will remain in the water column. A weight of 23–45 kg is easily moved by the submersible and provides ample weight to keep the string from floating away. If the string is buoyant when detached from the drop weight, the submersible must ascend the string and release one or two of the floats. Once the string is in place on the wellhead, the string’s weight should be enough to pull the string into the borehole. If the borehole is producing warm hydrothermal water, additional weight is required to overcome the hydraulic forces pushing up on the string. This situation may require the release of additional flotation. Once the string is in place, the remaining floats are released and recovered by the surface ship. Note that a middle sinker bar ~20 m from the top plug is a good idea if there is a bottom seal. This middle sinker bar will pull in slack Spectra cable and help seal the top plug. The middle sinker bar should be no more than 5 cm (2 inch) in diameter. Keys to a successful submersible operation are having actual weights for all objects on the string, a series of floats that can be released, and simple rigging.

Submersible recovery of downhole strings

Original-style CORKs

To recover original-style CORKs, the latch holding the data logger in place must be released (Figs. F2, F13). A pulling tool specially designed to connect to the CORK is manipulated by a submersible. This tool slides between the CORK body and the data logger. The tool uses the difference in hydrostatic pressure at depth relative to that at the sea surface to drive a piston, releasing the “dogs” (latch) that hold the data logger in place. The tool remains attached to the top of the logger during recovery. With the data logger released from the CORK, the string (data logger, thermistor cable, and OsmoSamplers) is recovered either by floats, rope, wireline system, or remotely operated vehicle (ROV).

Recovery of the Barbados instrument string (Leg 156; Shipley, Ogawa, Blum, et al., 1995) was attempted using a French submersible to unlatch the data logger and float the downhole string to the surface using a bag of kerosene for buoyancy. Only one CORK string was recovered (Hole 948D). The other string (Hole 949C), which contained the OsmoSampler, remains in the borehole waiting to be recovered. Safety issues related to the volume of kerosene resulted in the development of a new recovery method.

The submersible Alvin was used to attach the pulling tool to the data loggers deployed during Leg 168 (Davis, Fisher, Firth, et al., 1997; Wheat et al., 2003a). Once the data logger was released from the latch, the submersible continued to conduct other operations. After the submersible was recovered, the Scripps wireline vehicle was used to latch onto the pulling tool and pull the tool and downhole string out of the borehole (Spiess et al., 1992). However, the Scripps wireline vehicle required a number of technical personnel to operate, and the breaking strength of the fiber optic cable used to hold and manipulate the wireline vehicle placed a restriction on the amount of tension allowed. In attempting to pull out several strings, the maximum allowable tension was reached, and a weak link on the control vehicle was severed.

The weak link severed once during pull-out attempts with the Scripps wireline vehicle, leaving the pulling tool attached to the data logger, which remained in the CORK. The following year a nylon rope was used to retrieve the string. This operation included deployment of the rope with a weight at the bottom and floats on the surface. The Alvin then latched the rope to the pulling tool. Following recovery of the submersible the nylon rope was then recovered using a capstan aboard the R/V Atlantis. Unfortunately, the recovery did not include the OsmoSampler package or sinker bar. The two unrecovered OsmoSampler packages (Holes 1025C and 1026B) were likely lost because of disturbances in the open borehole that resulted in the burial of these packages. As noted above, one must be concerned about placing any sensor or sampler into open boreholes. Using a rope to pull the string out of the hole is still the method of recovery used today, except a dedicated winch system and Plasma rope are used instead of a capstan and nylon rope. This rope-winch system is preferred because the capstan was time consuming, the rope was manually spooled, and nylon stretches significantly in tension so that the deck had to be cleared until the tension was released.

Lastly, the downhole string deployed at South Chamorro Seamount (Hole 1200C) was recovered using the ROV Jason II (Woods Hole Oceanographic Institution) (Salisbury, Shinohara, Richter, et al., 2002; Wheat et al., 2008, 2010a). The Jason II brought the pulling tool to the seafloor, released the latches holding the data logger in place, attached a line from the clump weight on the Medea to the pulling tool, and then ascended. The Jason II was recovered first, followed by the logger, thermistor cable, and OsmoSampler package. Once recovered, a dummy plug was deployed in the CORK to keep it sealed. Years later in 2009 the Japanese ROV Hyper Dolphin (Japan Agency for Marine-Earth Science and Technology) conducted similar operations to recover the dummy plug and deploy new sensors and samplers within the borehole (Fig. F13).

CORK-IIs and L-CORKs

A Plasma rope connected to the top plug of the sample string can be used to recover downhole strings from CORK-IIs and L-CORKs with the aid of a submersible or ROV. Two Plasma ropes exist: one is 3000 m long and the other is 2000 m long. These ropes can access any CORK deployed to date. At the base of the Plasma rope, acting as the anchor, is a milk crate containing a 50 m long Spectra cable, homing device, transponder, float, and ~100 kg of steel plates in two bundles (Fig. F14). The submersible releases one of the steel bundles and positions the milk crate near the CORK. The Spectra cable in the milk crate is attached to the Plasma rope and to a latch that connects to the top plug at the wellhead. Top plugs differ from each other in design, as do the tools required to recover the downhole string. The original-style CORKs require the pulling tool to remove the data logger and downhole string as noted above. Other top plugs require a hook (Hole 1253A), a 3.5 inch Otis GS tool (Holes 1026B, U1301A, and U1301B), or a 2 inch RS pulling tool (Tools International Corporation, part number 40RS1200; Holes U1362A and U1362B). Once the submersible latches into the top plug, the remaining steel plates are released, which allows the float on the milk crate to pull the Spectra cable away from possible tangling hazards (e.g., handles and instruments on the wellhead). After the submersible is recovered, the Plasma rope is retrieved with a dedicated winch. The string is then pulled aboard using a series of crane picks with Yale grips or presewn lifting handles.

Deployment and recovery of seafloor systems

Seafloor sampling and sensor systems that are attached to the wellhead and deployed by the drillship require that they fit within the radius imposed by the drillship’s camera system; a submersible must be used for larger components. Most submersible operations have the samplers and tools on the submersible’s basket; however, some instruments are too large and require separate elevator trips. These sampling and sensor systems are attached to the wellhead via the connectors described above. Similar operations are required to recover instruments from the wellhead.

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