BACKGROUND OF THE PRESSURE CORE SAMPLER

Basic Tool and Manifold Descriptions

The PCS (Fig. F1) is a tool designed to retrieve a 1.00-m-long sediment core from depth under pressure (Pettigrew, 1992). The basic principles of a PCS run are as follows. A cutting shoe (Fig. F2) is connected to the bottom of the tool. The tool is deployed, free falling within the drill string to reach the bottom of the hole, where it mounts the bottom-hole assembly. The drill string is turned to cut a core. When the wireline is pulled, a ball drops within the PCS to redirect internal fluid circulation and stroke the core through a lower ball valve that seals the core at pressure. The tool is then retrieved to the rig floor like other cores, such as those from the advanced hydraulic piston corer (APC) or extended core barrel (XCB). The PCS is separated from the drill string and brought to a shipboard laboratory for experiments. Much more expansive descriptions of mechanical and wireline operations of the PCS are presented elsewhere (Pettigrew, 1992).

Once in a laboratory, the adopted protocol for releasing gas from the PCS is as follows (Dickens et al., 2000b). The tool is placed in a mounting sleeve and surrounded with ice to maintain the core at a constant temperature, where gas hydrate will not dissociate at high pressure (Dickens et al., 2000a). A gas manifold system and bubbling chamber (Fig. F3) are attached to a port on the PCS. Incremental volumes of gas are then released through the port and manifold to the bubbling chamber over time until the inside of the PCS is at atmospheric pressure. The PCS is removed from the ice bath and warmed to at least 15C. Additional incremental volumes of gas are collected. Aliquots of gas for various analyses (e.g., hydrocarbon composition by gas chromatography) are taken from individual gas volume increments by releasing gas from the bubbling chamber into a syringe. The PCS is disassembled and the sediment core is extruded. The core is then examined for length and overall condition and is sampled for physical properties, especially porosity.

Previous PCS Operations (Leg 164)

A long and mostly unsuccessful history of pressure coring operations marks scientific deep-sea drilling prior to Leg 164 and drilling operations in 1995 (Pettigrew, 1992; Paull, Matsumoto, Wallace, et al., 1996). Two main problems particularly plagued early operations: (1) tools did not retrieve lengthy cores at pressure, and (2) available manifolds could not easily measure gas volumes. Both problems were mostly overcome during Leg 164 on the Blake Ridge off the southeast margin of the United States. At Sites 995 and 997 during this cruise, 17 cores of between 3 and 100 cm length were recovered by the PCS at pressures between 750 and 4800 psi and were successfully degassed through a manifold to quantify gas volume (Dickens et al., 2000a, 2000b).

Although successful PCS operations during Leg 164 led to long-desired gas concentration profiles across a bottom-simulating reflector (Dickens et al., 1997), they immediately raised several issues relevant to future use in scientific drilling operations. In particular:

  1. Sediment on the Blake Ridge consists of very fine grained and fairly homogenous "nannofossil-bearing clay." Can the PCS be used to collect cores at high pressure on other margins or in other lithologies?
  2. Only one PCS deployment retrieved a full 1.00-m sediment core, and most deployments recovered cores <50 cm long. Can the PCS be modified to collect longer cores?
  3. Many of the cores were unconsolidated after removal from the tool. Can cores be extruded from the tool so they retain their dimensions?
  4. Only cores taken below 150 meters below seafloor (mbsf) were successfully degassed at Sites 995 and 997 because unlithified sediment at shallow depth clogged the manifold. Can the manifold and PCS be reconfigured to collect gas at shallow sediment depth?

New Modifications

Three modifications were made to the PCS prior to Leg 201 in an attempt to improve the length and quality of cores. First, three new cutting shoes for rotary coring were developed (Fig. F2). These shoes are (1) the Christensen auger shoe with carbide cutters, (2) a Rock Bit International (RBI) tapered auger with polycrystalline diamond compact (PDC) cutters, and (3) an RBI standard PDC cutting shoe. Second, to minimize washing of sediment during coring, the cutting shoe was placed ~50 cm ahead of the XCB bit. Third, the 4.32-cm-diameter core barrel was lengthened to 1.00 m to give an effective volume of 1465 cm3 (instead of 1385 cm3 during Leg 164).

Following designs constructed late during Leg 164, a new free-standing, lightweight manifold was constructed for Leg 201 (Fig. F3). However, the basic components and operational principles are the same (see "PCS-M4;" Paull, Matsumoto, Wallace, et al., 1996, p. 25). Several short lengths of high-pressure pipe are connected so that (1) air can be displaced through one valve, (2) gas can enter at high pressure from the PCS through a second valve, (3) pressure can be measured by a gauge or transducer, and (4) gas can be released into a bubbling chamber through a third valve. The bubbling chamber, which consisted of an inverted 1-L graduated cylinder in a plexiglass tube filled with a saturated NaCl solution, was the same as used during Leg 164 (Paull, Matsumoto, Wallace, et al., 1996, p. 25).

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