Water hammer can result in the rupture of pipes, and significant damage to pipe supports is inevitable during the operation of an offshore plant. In this study, the dynamic behaviors of the water hammer caused by closing valves and starting pumps for the seawater treatment system were evaluated by using the 1D numerical simulation model based on the method of characteristics. Before conducting an analysis of a complex piping network, the 1D numerical simulation tools were validated by a comparison between the numerical results and the results from both static and transient experiments that have been conducted in other studies. For the case study, the effects of valve flow characteristics and valve closing time on surge pressure were investigated, and the equal percentage butterfly valve was recommended in order to reduce the surge pressure with a shorter valve closure time and lower weight compared to other valve types.