Terrain slugging is extremely likely to occur sometime during the life of a deep subsea pipeline/riser system, But, can the terrain slugs be mitigated In this study, we performed transient multiphase flow simulations to explore the feasibility of sea floor separation to mitigate terrain slugging and to optimize fluid transport from wellhead to downstream processing. We considered the effects of separator location and separation efficiency on production system behavior. We found that there is an optimal amount of gas underflow into the liquid line and there is an optimum location for subsea separator. Our study also covered how the operational transient processes, such as startup and shutdown, would affect the performance of the pipeline/separator system. It is necessary to have a good understanding of the performance of subsea separation system before a reliable one can be designed. The results of this study are aimed toward improving the design of sea floor processing for ultra-deepwater production. [SO195-0738(00)00403-9].