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Expedition 336 CORKs: mechanical and hydraulic features overview

The overall design, shipboard assembly, and installation of the Expedition 336 CORKs generally followed the plans outlined in detail elsewhere (Fisher et al., 2011, 2005) with some notable exceptions as outlined below. An overall schematic of a CORK observatory as shown in figure F2 in Fisher et al. (2011), applies well to the North Pond CORK observatories. For detailed step-by-step CORK installation details, see the operations summary presented in the Expedition 336 Preliminary Report (Expedition 336 Scientists, 2012a).

Expedition 336 CORKs were initiated by jetting in conductor casing with a steel reentry cone into the uppermost sediments. These components are described in more detail elsewhere (Graber et al., 2002). The conductor casing is installed to hold the reentry cone in position, thereby preventing the cone from sinking in unconsolidated sediments until a longer casing string can be cemented at depth. The conductor casing deployed in Holes U1382A and U1383C has a 16 inch outside diameter (OD). Hole U1383B has a conductor casing with a 20 inch OD. Once the cone and conductor casing were jetted in to the seafloor to support the reentry cone and deeper casing and keep them from sinking in unconsolidated sediments, a hole was drilled (without coring) through the underlying sediment and into the top few meters of basement. For the 20 inch OD conductor casing, an 18½ inch tricone bit was used; 16 inch casing spanning the thickness of the sediment column was then installed (i.e., the top was latched into the reentry cone casing hangers and the bottom was cemented below the sediment/​basement interface). Basement was then drilled with a 14¾ inch tricone bit to allow installation of a second (in the case of 16 inch casing being the largest) or third (in the case of 20 inch casing being the largest) steel casing string (10¾ inch OD) within the upper basement. Holes were deepened by RCB coring once the 10¾ inch casing string was installed and cemented into place.

The remaining 4½ inch OD casing and wellhead (i.e., the section of the CORK that extends above the reentry cone above the seafloor) portions of the CORK were then assembled together before installation with the rest of the CORK system at the seafloor. The wellhead portion of the CORK was constructed from concentric 4½ and 10¾ inch steel casing sections, with parts of the larger casing omitted or cut away between horizontally oriented 30 inch OD bulkheads that house the sampling bays (Fig. F3). Seafloor sampling and valve manifolds, sensor packages, data loggers, and samplers are arranged within three bays, offset by 120°, and separated by vertical gussets. The gussets are intended to provide strength to the wellhead and help to guide the ship’s camera system and the submersible platform around the bays during CORK installation and later operations, protecting instrumentation and valves. One bay is dedicated to monitoring pressure data, the second to fluid sampling, and the third for a flowmeter and microbiological sampling or auxiliary pressure monitoring or other experiments (Fig. F3). Cutouts on the bulkheads and gussets are designed to allow a submersible or ROV to hold on for stability and leverage, and signs attached to the gussets indicate the directions that valves should be turned during operations. The Expedition 336 CORK wellhead gussets were strengthened by welding 1 inch thick × 3 inch wide steel plates along their length; this was done to improve resistance to strain induced by pressure applied to the CORK during installation and unlatching the running tool.

The interior CORK casing strings, which spanned the length from the seafloor to the bottom of each CORK observatory, were composed of various combinations of resin-coated steel casing and drill collars (perforated and unperforated), crossover stubs, fiberglass casing (perforated and unperforated), packer elements, and external umbilicals terminating at miniscreens. The composition of each CORK string was dependent on the depth of the borehole and the placement of the monitoring sections within horizons of interest (Fig. F3), as discussed in the following sections.

CORK sealing and isolation elements

CORK systems are intended to hydraulically isolate intervals of interest within the formation at depth from the overlying ocean. This requires the use of multiple seal components (see figs. F2, F4 in Fisher et al., 2011). The 10¾ inch OD casing systems deployed in Holes U1382A and U1383C included three independent mechanisms for sealing the formation inside 16 inch casing:

  1. O-rings in a ring-shaped tapered seat, welded to the bottom of the 10¾ inch casing hanger, provided a seal between the 10¾ and 16 inch casing hangers. Casings were designed so that the 10¾ inch casing would be sealed against the 16 inch casing immediately upon installation, using rubber O-rings and metal sealing surfaces in the casing hangers held in place by gravity and a mechanical latch (see fig. F4A in Fisher et al., 2011).
  2. Swellable packer elements were bonded to the outside of a single section of 10¾ inch casing in each hole (see fig. F4B in Fisher et al., 2011). These elements use a Freecap FSC-11 elastomer (developed by TAM International, that expands in seawater. They have an initial external diameter of 14¾ inches. Full expansion of the swellable packer elements to the inside diameter (ID) of the 16 inch casing (15 inch ID) requires several weeks to months and, hence, does not provide a seal immediately following casing installation but should provide a reliable casing seal over subsequent months to years.
  3. Both the 16 and 10¾ inch casing strings in both Holes U1382A and U1383C were cemented. Cellophane cut in 1 inch square pieces was used as a lost circulation material and added to the cement to clog pores and fractures adjacent to the borehole. Cement was deployed around the shoe of the 10¾ inch casing without a cement retainer by backfilling the hole after the casing was landed and latched mechanically into place. Although this cement may not have formed a complete hydrologic seal between the 10¾ inch casing and the formation, the cement should have helped to separate the main borehole area from the dead (annular) space outside the 10¾ inch casing, and this should improve the quality of future geochemical and microbiological samples. Postinstallation monitoring of pressure within the annular gap of the cased interval should allow for quantitative assessment of the quality of the cement seal at depth.

The main CORK seal (see fig. F4C in Fisher et al., 2011) is located at the base of the CORK wellhead, where it seals against the 10¾ inch casing hanger in the throat of the reentry cone. Holes were drilled and tapped through the CORK landing seal ring for packer inflation, pressure monitoring, and fluid sampling lines. Below the landing seal ring are crossover subs and 4½ inch casing that extends to depth below the seafloor. Hydraulic packers were used to isolate observatory intervals. The hydraulic packer was inflated using pumped seawater after the CORK had been lowered into position. The final CORK seal component employs the weight of the top plug, instrument string, and a sinker bar to hold an O-ring seal against a tapered area in the top of the wellhead and additional landing seals within the borehole that define hydraulic zones of interest (see figs. F2, F4F in Fisher et al., 2011).

CORK sampling components

Expedition 336 CORKs included multiple perforated and screened components to permit pressure monitoring and fluid sampling while protecting downhole instrumentation from basement collapse. Perforated resin-coated steel drill collars (6¾ inch OD) were deployed above a bullnose and below the deepest hydraulically inflatable packer in each hole, with lines of 2 inch holes separated by 9 inches running vertically up four sides of the collars (Figs. F3, F4; also fig. F5 in Fisher et al., 2011). The collars provide weight in order to pull and guide the lower end of the CORK into the hole during deployment of the CORK string, keeping the string in tension should it “hang up” on a ledge during deployment. At shallower sampling horizons, either perforated resin-coated steel 4½ inch casing or perforated fiberglass casing was used to span the region of interest (Figs. F3, F4).

Formation pressure is monitored and borehole fluids are sampled at the CORK wellheads via the umbilicals terminating at wire-wrap miniscreens installed at depth (Fig. F5). Stainless steel screens are used for pressure monitoring and standard geochemical sampling, whereas microbiological sampling is conducted through titanium screens. Three forms of tubing umbilicals were used during Expedition 336: (1) plastic-jacketed “flatpack” containing three ¼ inch OD stainless steel tubes for pressure monitoring, along with a single ½ inch OD hydraulic packer inflation line; (2) plastic-jacketed flatpack containing three ⅛ and three ¼ inch OD stainless steel tubes, mainly for geochemical sampling; and (3) plastic and woven metal–jacketed umbilical constructed around a ½ inch OD polytetrafluoroethylene (PTFE) tube specifically for microbiological sampling (Fig. F6). Umbilical tubes for each CORK were deployed with the tubes being passed through the inflatable and swellable packers, as required (Fig. F7). Final connections were made at the top of the CORK casing, connecting sampling and monitoring lines to tubes that were preinstalled to pass through the seafloor CORK seal.

Each CORK installed in Holes U1382A and U1383C included a lateral 4½ inch casing section that extended at a ~15° angle up from below the lower bulkhead (i.e., “lateral CORK” [L-CORK]; Fig. F8). The angled lateral casing penetrated the lower bulkhead with an offset of 9¼ inches from the CORK centerline, terminating at the top with a large-diameter (4 inch) ball valve. The ball valve was modified to include welded valve handle stops, and holes were drilled through the valve body to avoid trapping air in dead space around the ball, which could lead to development of a large differential pressure and damage during deployment. A custom ring clamp on top of the ball was deployed with a dust cover to prevent fouling of the valve.

CORK component surface treatments

To minimize microbiological and geochemical perturbation of the borehole environment by corrosion of “standard” CORK components, specialized components and coatings were used during Expedition 336 (see Orcutt et al., 2012). Specifically, steel components deployed within the borehole sampling environments were coated with various resins and epoxies to minimize rusting (Fig. F4), and fiberglass casing (Fig. F9) was used in some locations as a replacement for steel casing. Table T1 lists the coatings that were used on different components of the downhole assembly, and Orcutt et al. (2012) and Orcutt et al. (2010) provide detailed descriptions of the properties of these materials. During Expedition 336, uncoated steel and scratches on steel were touched up manually during assembly (Fig. F4). Other specialized components like greases (thread lubricant, or “dope” as it is commonly referred to in drilling) (Table T2) and sealants, were also used (see Orcutt et al., 2012 for more details). In part, running fiberglass casing for the first time in the drilling program’s history drove the selection of new lubricants and sealants.