The PCS is completely interchangeable with the APC and XCB coring tools, thus allowing a pressurized core sample to be taken at any time from the mud line down to indurated formations and/or into basement. The PCS is free-fall deployed and lands in the BHA. The PCS latches into the BHA and is rotated with the BHA during coring operations similar to the XCB. The PCS recovers a nominal 42-mm (1.65-in.)-diameter core sample 86 m (34 in.) long at pressures up to 690 bar (10,000 psi). A schematic of the PCS (Fig. VII-1) illustrates its operation.
The PCS contains six main components, or subassemblies: latch subassembly, actuator subassembly, accumulator subassembly, manifold subassembly, ball-valve subassembly, and detachable sample chamber.
1. Latch subassembly
The PCS latch subassembly is a modified XCB latch. The latch subassembly contains the landing point for the PCS when free-fall deployed, and transmits torque from the BHA to the PCS. The PCS cutting shoe is rotated with the BHA, trimming the core sample to proper size for entry into the sample chamber. The latch subassembly also diverts all flow down the drill string through the inside of the PCS, and holds the actuation ball during deployment and coring operations. When the latch subassembly is engaged by the coring wireline and an upward force is applied, the actuation ball is automatically released, dropping into the actuator subassembly. Finally, the latch subassembly provides a receptacle for attachment of the coring wireline during retrieval of the PCS.
2. Actuator subassembly
The PCS actuator subassembly has a dual latch system which latches the ball valve open during coring operations and closed after actuation. The actuator subassembly catches the actuation ball when released by the latch subassembly, and diverts all flow through the PCS to the actuation piston. After the actuation ball has been released and pressure is applied to the PCS, the actuator subassembly automatically unlatches and strokes through itself, pulling the core tube through the ball valve and into the sample chamber. As the core tube is pulled through the ball valve it mechanically rotates the ball valve closed. During actuation, seals at the top of the core tube are pulled into a seal sub, thus closing the sample chamber at both ends. When the actuator subassembly reaches the end of its stroke, it automatically latches once again, holding the sample chamber closed.
3. Accumulator subassembly
As the name implies, the accumulator subassembly contains an accumulator. As the ball valve closes, a small change in volume occurs. To offset the volume change, the accumulator forces fluid into the sample chamber, thus maintaining bottom-hole pressure. The accumulator also compensates for any fluid loss due to weeping seals as the differential pressure increases inside the sample chamber during retrieval of the PCS. The PCS accumulator subassembly also contains a pressure-relief mechanism. An integral back-pressure valve relieves pressure from the gas-charged side of the accumulator in the event of overpressurization of the sample chamber.
4. Manifold subassembly
The PCS manifold subassembly contains integral valves that enable the sample chamber to be isolated and removed. Two sample ports for collecting gas and/or fluid samples also controlled by integral valves are contained in the manifold subassembly. The sample ports have separate flow paths into the sample chamber; one flow path leads to the inside of the core tube, and the other flow path leads to the annular volume surrounding the core tube. The manifold subassembly also contains a burst disk which vents all pressure from the sample chamber should the internal pressure exceed the designed working pressure of 690 bar (10,000 psi). (This burst disk was deactivated during Leg 146 and will likely be eliminated in future designs.) An optional pressure transducer can also be installed in the manifold subassembly, enabling monitoring of the sample chamber internal pressure.
5. Ball-valve subassembly
The PCS ball-valve subassembly forms the sample chamber lower seal when actuated. The ball valve is mechanically closed as the actuation subassembly pulls the core tube through the ball valve subassembly. The ball-valve subassembly also serves as the connection point for the PCS cutting shoe.
6. Detachable sample chamber
The PCS detachable sample chamber consists of the manifold subassembly, ball-valve subassembly, and pressure case. The sample chamber when detached from the PCS is 92.2 mm (3.75 in.) in diameter and 1.8 m (6 ft) long. When the sample chamber is stroked closed and removed from the core barrel, the two sampling ports and their associated integral valves as well as the integral pressure transducer are easily accessible. A sampling manifold can be attached to either or both sampling ports for sampling fluids and/or gases.