A trawl gear impact on an underwater pipeline can create a dent that pushes part of the pipe wall inward. External pressure also tends to push the wall inward. Because of that interaction, an impact under external pressure dents the pipeline more severely than the same impact with no external pressure. The interaction has been investigated as part of a wider study of overtrawlability of pipe-in-pipe systems carried out by the National University of Singapore. The problem is important because the need to protect pipelines against trawl gear impact leads to a requirement to trench medium and small-diameter pipelines. Trenching is costly and a frequent source of delays and disputes, and so it is worthwhile to search for ways to eliminate unnecessary trenching. A finite-element model of denting under external pressure for single wall pipe and pipe-in-pipe using hydrostatic fluid element has been established and verified by comparison against published data and current experiment data. Parametric study of different external pressures shows the effect of external pressure on the denting process. The combinations of the internal pressure, external pressure and indentation are considered. The study shows that when the internal pressure of a dented subsea pipe is decreasing, the possibility of buckle propagation for the single wall pipe is higher than it is for the pipe-in-pipe.