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Expedition 336 CORKs: detailed configuration

This CORK geometry description provides the reader with a fairly detailed picture of the overall CORK configuration. Many variables have to be considered when configuring a CORK installation and virtually every CORK installation to date has been different (for recent summaries see Fisher et al., 2011; Wheat et al., 2011). CORKs are configured to meet the specific science needs for a given borehole to be instrumented. Expedition 336 CORKs had some unique features, which are outlined in more detail below.

CORK wellhead

The CORK wellhead (Fig. F8) geometry is defined by several factors. The maximum wellhead OD is limited by the requirement to pass the wellhead through the vibration-isolated television (VIT) camera system guide sleeve over the wellhead for reentering a borehole (Figs. F10, F11). The VIT frame guide sleeve clamps around the drill string, which acts as a guide during deployment of the VIT for reentry. The VIT must pass over the wellhead, all the way to the bottom of the stinger, to be able to see the reentry cone. The minimum ID of the VIT guide sleeve is 32 inches. Thus, the maximum OD of the wellhead has been limited to 30 inches to allow the VIT guide sleeve to easily pass over.

The wellhead inner mandrel OD is kept small to allow as much room as possible for attaching instruments to the wellhead (Figs. F12, F13). Note that all instruments attached to the wellhead must reside within a 28 inch OD cylinder, centered on the inner mandrel, to allow the VIT frame to pass over.

The wellhead ID is intended to be “full open” (i.e., having the same or larger ID to the drill string). However, typically, a landing shoulder is incorporated into the CORK wellhead for landing a top plug, which reduces the minimum wellhead ID to ~3⅝ inches.

The distance from the top of the instrument bays to the top of the wellhead is somewhat fixed in an effort to keep it as short as possible while providing an adequate guide for the VIT frame guide sleeve and a stable connection point for the running tool. The distance from the bottom of the instrument bays to the bottom of the wellhead is fixed, based on landing the wellhead on top of the casing hanger inside the reentry cone throat, and positioning the instrument bays just above the reentry cone rim for easy access. Thus, the only length variable is the height of the instrument bays. The cross section of the wellhead instrument bays is an area bounded by a 30 inch diameter circle circumscribed around a 4½ inch diameter circle and divided into three equal sections by vertical panels positioned 120° apart. The CORK bodies deployed during Expedition 336 have 96 inch long bays.

CORK stinger

The term “CORK stinger” refers to the part of the CORK that is suspended below the wellhead (Fig. F14). The stinger is composed of various casings (some perforated and some not), packers, various crossover subs, landing seats, sampling screens, umbilicals, and so on.

CORK stinger casing selection and considerations

Two primary considerations have to be addressed when choosing the casing for making up a CORK stinger. The first consideration is geometry of the borehole and the stinger elements to be included (packers, umbilicals, screens, etc.). The second consideration is strength of the materials.

In terms of geometry, the maximum OD of the casing is dictated by the hole diameter that the stinger will be deployed in. Typically, open holes for CORK stingers are cored, or drilled, with a 9⅞ inch OD bit. Thus, the maximum stinger component OD that can be deployed is 8¼ inches. The minimum casing ID is generally chosen to match that of the drill sting, 4 inches, to be completely open to the drill string.

The second important geometry consideration for choosing a casing size is the inclusion of elements on the outside of the casing, which will restrict the outer diameter size. The most common elements to consider are packers (Figs. F7, F9), miniscreens (Figs. F5, F15), centralizers (Figs. F5, F14), and umbilical sampling lines (Figs. F5, F6). For example, if packers are to be installed in the stinger to seal hydrological horizons, they will limit the outer diameter because of the thickness of the packer mandrels and sealing material. CORK packers, whether inflatable or swellable, utilize a dual mandrel design that allows pass-throughs to be installed. Pass-throughs are stainless steel or titanium tubes that pass under the packer sealing element and are sealed within the packer body. This provides for a continuous sampling line to pass through the packer without defeating the overall packer borehole seal. The maximum number of pass-throughs is achieved with an inner mandrel that is 4½ inch OD × 4 inch ID inside a 7 inch OD outer mandrel. Thus, even if larger diameter casing is used in the stinger, they must be crossed over to mate with the 4½ inch OD packer inner mandrel. Geometry considerations are also important for the selection of umbilicals and miniscreens to be attached to the outside on the casing, which may further restrict the diameter of the tubing. For example, the umbilicals used during Expedition 336 range in size from ⅛ to ½ inch OD (Table T3).

Finally, if a casing with an ID >4 inches is used to make up the stinger, landing seats with IDs small enough to land the internal instrument string components on must be installed (Fig. F14). If the internal instrument string is to be deployed or recovered through the drill string, then the maximum OD of the internal instrument string components is limited by the drill string ID. Thus, the landing seats installed in the stinger must have a small ID.

The second primary consideration to be addressed when choosing casing for making up a CORK stinger is strength, compressive strength more so than tensile strength. When the stinger is lowered into the borehole, the possibility exists for the stinger to hang up and stop moving downhole. When this happens, the only indication the driller has is a slight decrease in the total load supported by the ship. If the drill string continues to be lowered after the stinger stops moving, the stinger casing will be placed in compression and can quickly become overstressed to the point of failure. Tables T4 and T5 present comparisons of the strength and stretch characteristics of various casing suitable for use in CORK stingers. As shown, the thick wall 5½ inch OD drill pipe has the highest area moment of inertia and thus is the stiffer of the standard casing shown. However, all of the casings have relatively low values for critical column loading, the load at which a particular casing is subject to begin buckling. Note that the critical column loads shown are conservative because they are based on the weight per unit length of the casing, a distributed load case, whereas the equation used to calculate the values applies to a point load at the top of the casing. Also note that the critical column load is not a function of the yield or tensile strength of the casing. The only mechanical property of the casing that enters into the critical column loading is the modulus of elasticity, which is virtually equal in all standard casing steels. However, how much a particular casing will buckle before it fails is a function of its yield and tensile strength.

To help prevent buckling of the stinger, a “plumb bob” is attached to the bottom of the stinger (Fig. F14). The plumb bob is simply a section of very thick walled, heavy casing. Drill collars with 8¼ and 6¾ inch OD have been used with good results. The plumb bob serves three purposes. First, it serves as a guide for the stinger as it is lowered into the borehole. A guide shoe, or bullnose, is attached to the bottom of the plumb bob to guide the bottom of the stinger through ledges or tight spots in the borehole. Second, the weight of the plumb bob helps keep the stinger in tension, thus preventing it from buckling. Third, the larger diameter and long length (tens of meters) keeps the stinger better aligned within the drilled hole.

CORK stinger sampling and experimental umbilicals and screens

CORK umbilicals range from electrical wiring (or cables), to individual tubes (or hoses), to single elements containing multiple tubes (Fig. F14). The umbilicals are attached in three places: (1) to the wellhead at the top, (2) to the outside of the stinger along its length, and (3) to the downhole sampling ports. The umbilicals provide a means for sampling multiple zones within the borehole for pressure measurements, collecting geochemistry or microbiological samples, and also for transmitting electrical signals from downhole sensors or hydraulic signals to seafloor sensors. The materials used in fabricating the umbilicals range from stainless steel and/or titanium tubing of various ODs, to copper wires, to Teflon lined hoses (Tefzel), and more. However, no copper wire connectors were incorporated within Expedition 336 CORKs. Pass-throughs in the packers provide for a continuous umbilical path up the stinger. The umbilicals are connected at the wellhead to various valves and sampling ports. Umbilical size is limited by its effective diameter, which is the overall diameter created when the umbilical is attached to the stinger. The effective diameter cannot be any larger than the largest stinger component and should be slightly less for protection while being lowered into the borehole. Typical umbilical maximum OD is ~1 inch.

CORK screens are used to prevent the downhole sampling ports from clogging because of particulate infiltration. Historically, two screen configurations have been used with CORKs: full annular screens and miniscreens. Full annular screens are used when it is critical that an annulus has a large contact area with the formation. Full annular screens require severing the umbilical at the screen and then making up the ends to the screen pass-throughs during deployment. In contrast, only miniscreens were installed during Expedition 336 (Fig. F15). Miniscreens are easily installed and can be positioned anywhere along the stinger, except for over packers, the plumb bob, and pipe joints. Miniscreens are typically ~1 inch in diameter and ~1 m long and are fabricated from stainless steel or titanium.