Deep subsea gas pipelines are in place or being built in places like the Baltic, the Mediterranean, the Black Sea, and Southeast Asia. At full depth, the height of the water column can produce pressure that exceeds the gas pressure inside the pipeline, particularly toward the delivery end; in those regions, a loss of containment will result in water ingress into the pipeline instead of gas egress.
We discuss the detection of water ingress in such pipelines. We present the results of multiphase simulation of ingress at several locations and sizes in two flow regimes. Time scale of the ingress depends strongly on location, which is equivalent to the difference between gas pressure in the pipeline and ambient external pressure from the water column. Larger sizes have greater tendency toward cyclic behavior in the simulator. The expectation that the accumulation of water in the pipeline would displace gas, producing a downstream flow surge, is shown to be naive. Instead, accumulating water decreases the gas flow cross-section, throttling down gas throughput and reducing delivery flow. At equilibrium, water ingress will act as a regulator, causing pressure at the point of ingress to be equal to ambient external pressure.
Output of the multiphase simulation is converted to instrument measurements, then simulated in a conventional leak detection RTTM. Ingress is detected as a conventional leak prior to any water reaching the downstream delivery. Location information is partially inverted compared to conventional leaks, but properly corrected can be used to distinguish gas egress from water ingress.