Subsea tree safety control system
An embodiment of a method for limiting the probability of failure on demand of a subsea test tree (“SSTT”) includes the steps of providing a safety shut-in system for actuating a safety valve of the SSTT, the safety shut-in system including a surface control station positioned above a water surface connected via an umbilical to a subsea control system positioned below the water surface to actuate the safety valve; and diagnostically testing the safety shut-in system without actuating the safety valve.
1. A method for limiting the probability of failure on demand of a subsea test tree (“SSTT”), comprising:
utilizing a safety shut-in system comprising a surface control station positioned above a water surface and connected via an umbilical to a subsea control system positioned below the water surface and operationally connected to the SSTT to actuate a safety valve of the SSTT in response to a signal received from the surface control station;
transmitting an electric current to the subsea control system that is insufficient to actuate the safety valve;
calculating the implied impedance to the electric current; and
determining if a fault mode of the SSTT has occurred.
2. The method of claim 1 , wherein the subsea control system does not include a microprocessor.
3. The method of claim 1 , comprising actuating the safety valve via DC actuation.
4. The method of claim 1 , comprising maintaining the safety valve in an as-is position for a selected time delay upon electric failure of the safety shut-in system.
5. The method of claim 4 , comprising actuating the safety valve to a safe state upon passage of the selected time delay.
6. The method of claim 4 , wherein the subsea control system does not include a microprocessor.
7. The method of claim 4 , comprising actuating the safety valve via DC actuation.
8. The method of claim 3 , wherein the subsea control system does not include a microprocessor.
9. The method of claim 1 , comprising demultiplexing the electric current below the water surface.
10. The method of claim 9 , wherein the subsea control system comprises a diode steering circuit for the demultiplexing the electric current.
11. The method of claim 4 , comprising demultiplexing the electric current below the water surface.
12. The method of claim 11 , wherein the subsea control system comprises a diode steering circuit for the demultiplexing the electric current.
13. A method for limiting the probability of failure on demand of a subsea test tree (“SSTT”), comprising:
utilizing a safety shut-in system comprising a surface control station positioned above a water surface and a subsea control system positioned below the water surface and operationally connected to the SSTT to actuate a safety valve of the SSTT in response to a signal from the surface control system;
diagnostically testing the safety shut-in system without actuating the safety valve; and
maintaining the safety valve in an as-is position for a selected time delay upon electric failure of the safety shut-in system.
14. The method of claim 13 , comprising actuating the safety valve to a safe state upon passage of the selected time delay.
15. The method of claim 13 , wherein the subsea control system does not include a microprocessor.
16. The method of claim 13 , comprising actuating the safety valve via DC actuation.
17. The method of claim 16 , wherein the subsea control system does not include a microprocessor.
18. The method of claim 13 , comprising demultiplexing the electric current below the water surface.
19. The method of claim 18 , wherein the subsea control system comprises a diode steering circuit for the demultiplexing the electric current.
20. The method of claim 18 , comprising actuating the safety valve to a safe state upon passage of the selected time delay.