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BLOCK AND OPEN FUNCTION AND CONTROL FLUID CIRCUIT

Block Functionnnn

Operating Sequence – Block Function

The block function is used to vent a pilot control valve. By doing this individually on each valve a leak in the control system or the preventers can be located and isolated. By centring and venting all the valves when the accumulator unit is first being pressurised unintentional and inadvertent operation of the various other positions and functions can be eliminated

When the u2018blocku2019 button is pressed, both the solenoid valves are actuated in such a way as to apply pressure to both sides of the air operator. This causes the pilot control valve to be centred which then allows both the pilot u2018openu2019 and u2018closeu2019 lines to be vented. The springs in both the SPM valves then push the spindles down so that they seal against the bottom seats and block the flow of any power fluid through the valves. At the same time this also vents both sides of the BOP ram operating cylinders.

Open Functionnnn

Operating Sequence – Open Function

This sequence is the parallel opposite of the CLOSE function. As shown above, when the u2018openu2019 button is pressed, the solenoid valves on the hydraulic control manifold are actuated and allow air pressure to move the operator on the pilot control valve to the u2018openu2019 position. The solenoid valve on the left in the diagram vents the u2018closeu2019 side of the operating piston.

The pilot fluid can then flow down to the subsea control pod where it lifts the spindle in the RAMS OPEN SPM valve thus blocking the vent and allowing power fluid to flow through the valve. From the pod the power fluid travels through the u2018openu2019 shuttle valve to the u2018openu2019 sides of the BOP ram operating cylinders. Simultaneous reciprocal actions in the RAMS CLOSE SPM valve allows the fluid from the u2018closeu2019 side of the operating cylinders to be vented.

Control Fluid Circuitnnn

Subsea Control System – Hydraulic Schematic

In addition to the control fluid circuits used to operate stack functions such as ram or annular preventers, the control system must also perform other functions such as control of subsea regulators, provide readback pressures, latch/unlatch the subsea control pods and charge the subsea accumulators.

The above Subsea Control System Hydraulic Schematic shows a typical control fluid circuit. The hydraulic fluid is mixed, pressurised and stored in accumulator bottles by the hydraulic power unit. A pilot operated accumulator isolator valve is provided to allow the pumps to charge the subsea accumulators. When control fluid is used, it passes through a totalising flowmeter in the hydraulic control manifold and then through the pod selector valve which directs it to the chosen subsea pod.

After passing through the jumper hose and the subsea hose bundle to the control pod, the fluid supplies the hydraulically operated subsea regulators. These reduce the fluids pressure to that required to operate the particular BOP function desired. The fluid is also routed to a SPM valve in the pod which is controlled by the accumulator isolator valve on the hydraulic control manifold. In the open position this SPM valve allows the control fluid to charge the stack mounted accumulator bottles. This operation is described in more detail in Section Shuttle valves allow the bottles to be charged from either pod.