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Question
Hi David,

I refer to API 17F and it says that the SCSSV will always be closed last in cases of emergency (for example, USV, Master valve then the SCSSV) and it will be opened first when we want to open the well (SCSSV, Master Valve and then USV). May I know the reason behind the closing and opening sequence? From my standpoint, i thought it will be more reasonable to close the SCSSV first during emergency to prevent further upsurge of the production fluid from downhole to the mudline. The same goes for when we want to open the well. Please advise. Thanks.

Answer
Andre,
The rationale for the closing and opening sequence is, 1: to verify the integrity of the tree valves before closing any down hole valves. If the SCSSV was closed first the tubing would be losing pressure while the other valves were being closed and a bad surface valve may be missed. If the surface valves are closed first, a leaking valve would be discovered. 2: to maintain pressure on the tubing for the closure of the SCSSV. They operate better with pressure on both sides. Closing the SCSSV first would allow some pressure to bleed off the tubing causing a differential pressure across the valve. 3: the SCSSV may be bad and if it were operated first pressure may escape to the surface.
For opening think the reverse. If the above is followed there will be pressure on both sides of the SCSSV and it will open easily. If, for example, there is 5000 psi below and zero above the valve will not function and one must apply pressure to the tubing to equalize it.

That said; the SCSSV IS an emergency back up and is there for immediate action. If it is a "slow emergency" one can do the right thing for your equipment but the red button is there for a reason. I have had to hit it several times and had the same answer for the managers on shore - I was here, you were there, courage increases with distance.

A similar procedure is used in normal, workaday shut in. Close the choke, then wing or outer valves, then upper masters, lower master valve last, then down hole valves. While closing or opening a valve, some product is moving across it until it is completely closed. This can cause erosion of the gate. The outer valves can be changed by closing the master valves but if the master valves get cut it will take a workover to replace them. This is doubly important with subsea wells with which intervention is difficult or impossible.

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