Stuck Pig on a Waxy Crude Line
At 14:20, a 12-inch, 48-mile crude oil line is launching a brush pig at 900 bbl/h. MAOP is 1,200 psig; the variable-speed pump discharge limit is 950 psig. Within 18 minutes, launcher pressure rises from 620 to 780 psig, receiver pressure falls 40 psig, and flow drops to 610 bbl/h. The pig transmitter last reported near milepost 22 two hours ago and has not updated since. The line carries waxy crude, and ambient temperature is 28°F and falling. The controller has a pig-tracking contractor on call but no confirmed sighting. A second pump is available but currently offline. Operations wants to raise pump speed to 'push the pig through.' The adjacent 12-inch line is at 640 psig and can accept a transfer, but only after valve lineup and approval. SCADA shows no sudden pressure wave, and the launcher trap pressure is steady.
1. What immediate actions should the controller take, and what must be avoided?
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Treat the trends as a possible stuck pig, not a pump problem. Hold pump speed, verify launcher and receiver pressures against independent gauges, confirm flow by tank gauge or meter, and check the pig transmitter status. Notify the supervisor and pig-tracking contractor. Do not raise pump speed to push through; that can overpressure the section or damage the pig or line, and it does not address a possible wax blockage. Do not exceed 950 psig pump discharge or 1,200 psig MAOP. Keep the launcher trap pressure stable and confirm valve lineup. If pressure continues to climb, reduce rate or shut down and isolate the segment. The adjacent line transfer is not an immediate fix; it requires approval and lineup and may not relieve the stuck-pig condition.
2. How would you distinguish a stuck pig from a wax blockage or a transmitter failure?
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No single data point proves it. A transmitter failure would not normally cause a simultaneous flow drop and receiver pressure fall; cross-check with independent pressure gauges and tank-volume change. A wax blockage tends to build gradually and may show a rising differential over hours, while a stuck pig often produces a sharper upstream rise and downstream decay as the pig stops. The last pig signal at milepost 22 is useful but not conclusive. Compare the current differential with the same rate on the previous pig run. If the pig transmitter is stale and pressures continue diverging, assume stuck until proven otherwise. A pressure wave or flow reversal suggests something else. The conservative response is the same: stop adding energy, locate, and verify.
3. What conditions would you require before attempting to push the pig through with higher pressure?
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First, know where the pig is and that the line can safely take the pressure. Verify the pig is not at a valve, tie-in, or low-point trap, and confirm the section MAOP, test pressure, and relief settings. Get engineering approval for any pressure above the normal operating limit but never above MAOP. Have the tracker on site and a stop-pressure agreed in advance. Confirm receiver and upstream facilities can handle the pressure and that valves are correctly lined up. Notify operations and emergency response. Set a maximum differential and a stop criterion, such as no movement after a specified pressure increment. If the pig does not move at the approved limit, stop, reduce pressure, and plan excavation or another recovery method. Pushing harder is not a substitute for locating.