Subsea Operations
Subsea Operations tests subsea completion and intervention well control: tree valves, control systems, emergency shutdown, and barrier verification. It assumes you know how surface commands affect subsea barriers.
Core rules
- A subsea tree carries production master and wing valves, annulus master and wing valves, and often a crossover or chemical injection valve. Know each valve's fail-safe position.
- The SCSSV is a downhole barrier; tree valves are surface-controlled subsea barriers. Both must be verified, not assumed.
- ESD (emergency shutdown) shuts in the well and closes tree valves; PSD (process shutdown) protects the process and may not close all tree valves. Do not treat them as the same.
- The THRT runs and tests the tubing hanger; it is not a permanent barrier after retrieval.
- Emergency disconnect sequences must be known: what closes, what remains, and how pressure is monitored.
Worked example
An ESD is initiated during a subsea completion. The correct response is to confirm the ESD signal, verify that the tree valves have moved to their fail-safe closed position, monitor tubing and annulus pressures, and confirm the SCSSV status. Do not reopen the tree or override the ESD until the cause is cleared and the supervisor authorizes it. If hydraulic supply is lost, assume valves have gone to fail-safe closed, but verify by pressure response rather than by indicator alone.
Traps
- Confusing ESD with PSD.
- Assuming a valve is closed because the indicator says so.
- Forgetting that a tree valve may fail open or closed depending on design.
- Treating the THRT as a permanent barrier.
- Not knowing which barriers remain after emergency disconnect.