The purposes of the subsea BOP stack used to control pressures in wells are essentially the same as for those with surface BOP stacks. There are, however, several additional complications, which must be taken into consideration. The purpose of this lecture is to describe some of those issues and show some typical examples of equipment, which is required to meet the additional requirements.
The illustration above Fig. 76 is an example of a typical subsea BOP stack. Fig 76
The BOP is designed to allow for disconnecting at the upper part of the BOP under a variety of circumstances while maintaining the well closed in. The parts of the BOP stack that remains in place on the wellhead is called the Blowout Preventer and the parts above which can be disconnected is the Lower Marine Riser Package or LMRP.
Some of the additional special equipment on a subsea BOP stack includes: • Hydraulic connector between the wellhead and the BOP.
• Remote operated ram locking system or integral ram locking systems.
• Hydraulic operated choke line and hydraulic kill line valves for the BOP side outlets. • Riser choke line and riser kill line of fixed pipe.
• Hydraulic connector between the BOP and the LMRP. • An additional annular BOP as a part of the LMRP. • Flex joint or a ball joint.
• Marine drilling riser with attached lines on the outside (kill, choke, booster, hydraulic).
• Riser auto fill valve (Riser fill-up valve). • Telescopic joint.
• Riser tensioning system.
• Hydraulic BOP Control System to function the subsea BOP.
In the following some of the above equipment and systems will be described.
08.01 Model 70 Collet Connector (Cameron Iron Works)
See Fig 77. The Cameron Model 70 Collet Connector forms a tight seal while withstanding the bending stresses and separating forces caused by well pressure, riser tension and vessel motion.
Manual override is possible. The metal- to-metal sealing AX ring gasket is standard.
The connector is functioned by a set of hydraulic cylinders, Fig 78a and 78b. These provide unlocking force that is 80% higher than locking force. The connector locks to the mating hub on the well head or on the BOP via pivoted locking segments/fingers, (called collet fingers). The position of the locking segments is controlled by the position of the surrounding cam-ring. The position of the cam-ring is again controlled by a number of connected hydraulic jacks mounted on the outside diameter of the cam-ring. When the cam-ring is placed in the uppermost position the collet fingers will force the locking segments to take the shape of a funnel, which help to guide the connector into position when landing the BOP on the wellhead.
Fig 78a
Fig 78b Data:
Bore Sizes: 13-5/8 in through 21-1/4 in Rated Working Pressure: 2000 psi through 15000 psi Bending at 10K psi/1000K lb: 1.85 million ftxlb
Preload: 1.4 million ftxlb
Shoulder Angle: 25°/25° (Housing/Connector)
Max Release Angle: 30°
Swallow: 13-3/8 in
Weight: 16600 lb (7530 kg) studded top
Hydraulic Operating Pressure: 1500 psi to 3000 psi
08.02 Model HC Collet Connector (Cameron Iron Works)
The HC Collet Connector (Fig. 79a and 79b) name, is related to High Capacity.
The HC Collet Connector is similar to the popular Model 70 Connector but is designed to provide greater preload forces to withstand higher separating forces.
The features include an annular hydraulic piston in the cylinder, which is an integral part of the housing. This provides substantially higher clamping preload than the Model 70. Secondary unlock is available. The Connector locks to the mating hub via pivoted locking segments/fingers, which form a funnel to guide the connector into position. The metal-to-metal sealing AX ring gasket is standard. The greater clamping force is obtained due to the segment and hub geometry and the large actuating piston area.
Data:
Bore Sizes: 13-5/8 in through 21-1/4 in Rated Working Pressure: 5000 psi through 15000 psi Bending at 15k psi/1000 k lb: 1.85 million ft x lb
Preload: 7 million ft x lb
Shoulder Angle: 25°/25° (Housing/Connector) Max Release Angle: 30°
Swallow: 12-½ in (32 cm)
Weight: 23100 lb studded top
Hydraulic Operating Pressure: 1500 psi to 3000 psi Hydraulic Vol. 18-3/4” 15k RWP: Open 25 gal ,Close 20 gal
Fig. 79a
08.03 Hydraulic operated choke/kill line valves
Cameron MCS Gate Valves (Fig. 80) are compact valves suited for the requirement of subsea choke and kill lines in water depths up to 6000 ft.
Balanced stem prevents fluid displacement and also prevents opening the valve when line pressure is less than sea hydrostatic pressure. Bi-directional sealing allows valves to be spaced closely without liquid lock. Metal-to-metal sealing between gate an seats is utilised.
Rated Working Pressure 10000 psi and 15000 psi
Hydraulic Operating Pressure 1500 psi to 3000 psi
One mean (hydraulic pressure) opens the valve. The valve opens when hydraulic pressure is supplied from the SPM valve in the pod to the top of the actuator. In the actuator the piston is pushing the gate into open position.
Three different means close the valve:
When closing the valve the hydraulic opening pressure is vented from the top of the piston in the actuator. This happens through the open-SPM valve in the pod, which is venting. Simultaneously the hydraulic closing pressure is supplied to displace the piston in the actuator into the valve’s closed position. Further additional forces support the hydraulic closing pressure. One is the spring force. The other is the hydrostatic sea water pressure which is exposed to the end of the tail rod connected to the gate.
In earlier control system layouts, the only two means of closing the valve was the spring force and the hydraulic force generated by the ambient water column working on the tail- rod. This way of operation was called “Failsafe”. This synonym has stuck to the choke and kill valves, although incidents has proven that valves are NOT failsafe, why the valves today are forced closed by mean of hydraulic power supplied by the BOP control system.