This study investigates the causes to exposure of the 16A Bach Ho-Long Hai (BH-LH) gas pipeline in a coastal area, Ba Ria-Vung Tau province, Vietnam. A Delft3D numerical model was developed to simulate hydrodynamics, wave propagation, and sediment transport under four simulations scenarios. Historical data on bathymetry, waves, currents, and sediment properties were employed, and the model was calibrated and validated using field measurements. Simulations were conducted for Southwest and Northeast monsoon seasons, before and after construction of the Nam Con Son 2 (NCS2) pipeline to see whether or not construction of this new pipeline has contributed to the exposure of the existing BH-LH pipline. The simulation results revealed that the seabed erosion and pipeline exposure risks of the latter have existed even before construction of the former, particularly during the Northeast monsoon (NEM) season. However, the additional construction of NCS2 pipeline have significantly increased flow velocities and erosion potential near the BH-LH pipeline during the NEM conditions. Maximum erosion depths increased from about 1.0 m to more than 1.5 m after construction of the NCS2 pipeline. The results of the Delft3D-based hydro-dynamic modeling in this research could highlight the influence of the sediment transport process on the pipeline exposure and the need to consider properly various adverse factors as well as to have a continuous monitoring and adaptive management in subsea pipeline construction.