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SPM VALVES, REGULATORS AND REDUNDANCY SYSTEM

An Example SPM Valvennn

An Example SPM Valve

The valve is a poppet type in which a sliding piston seals at the top and bottom of its travel on nylon seats.

In the normally closed position a spring attached to the top of the piston shaft keeps the piston on the bottom seat and prevents the power fluid from passing through the valve to the exit port. Power fluid pressure, which is permanently present, also assists in keeping the valve closed by acting on a small piston area on the spindle. In this position fluid from the valveu2019s associated operating piston is vented through the sliding piston at ambient conditions.

When pilot pressure is applied to the valve the sliding piston moves up and seals against the upper seat which blocks the vent ports and allows regulated power fluid to flow through the bottom section of the valve to function the BOP. Note that the pilot fluid therefore operates in a closed system whilst the hydraulic power or control fluid is an u2018openu2019 circuit with all used fluid being vented to the sea.

Regulatorsnnn

Each subsea control pod contains two regulators u2013 one to regulate pressure for the manifold and one to regulate the pressure for operating the annular preventers. Some control systems incorporate a third regulator so that the operating pressure of each annular preventer can be individually manipulated.

Typical regulators are 1 u00bdu201d hydraulically operated, stainless steel, regulating and reducing valves. As shown above the output line of each regulator is tapped and the pressure routed back to a surface gauge through the umbilical. This readback pressure is used to confirm that the subsea regulator is supplying the power fluid at the pressure set by the pilot surface regulator.

Redundancy nnn

Principle of Control System Redundancy

The two subsea control pods are functionally identical. When a pilot control valve (rams close for example) is operated on the hydraulic control manifold a pilot signal is sent down both umbilical’s so that the associated SPM valve in each pod u2018firesu2019.

If the pod selector valve is set on yellow then power fluid is sent only to this pod and it is only through the SPM valve in this pod that the fluid will reach the ram operating piston. The pod selection has no effect on the pilot system.

Once the yellow pod SPM valve u2018firesu2019, the power fluid passes through it to a shuttle valve, the shuttle piston of which moves across and seals against the blue pod inlet. The fluid then passes through the shuttle valve to move the ram to the close position. Fluid from the opposite side of the operating piston is forced out through the u2018ram openu2019 shuttle valve and vented through the u2018ram openu2019 SPM valve and into the sea. The schematic above illustrates the principle

Note that if the blue pod was now selected to open the rams, then the power fluid would flow to the ram through the u2018openu2019 SPM on the blue pod but the fluid from the u2018closeu2019 side of the piston would be vented through the yellow pod SPM since the u2018closeu2019 shuttle piston would still be sealing the blue pod inlet port.

The shuttle valves should be located as near as possible to their relevant ports on the BOP stack since the system is redundant only down as far as the shuttle valves. The image below shows a NL Shaffer shuttle valve.

NL Shaffer Shuttle Valve