Leaking is one of the major problems that often to occur in a pipeline system, whether it is for carrying a multi-phase or a single-phase liquid. The study case, an oil and gas company has suffered leaking problems on all three waterflood pipeline systems in one of their offshore sites. To overcome these problems, they are planning to execute one of their projects, Pipeline Repair and Replacement Project (PRRP). This certain project needs several preliminary studies to be executed, and one of the studies is about the water hammer phenomenon. This research proposes a study on the effect of water hammer phenomenon on two kinds of pipes, which are rigid pipe and flexible pipe. In total, four pipes are used, API 5L X52 for the rigid pipe and three varieties of Reinforced Thermoplastic Pipe (RTP) for the flexible pipe. The methodology uses a dynamic multiphase flow simulator, OLGA v.2017.2.0, to obtain the transient pressure profile of the water hammer phenomenon. The main variables of this research are the Young's modulus of elasticity and operating parameter. The Young's modulus of elasticity is varied based on the material composition of each pipe. The operating parameter varied is the volumetric flow rate with each value of 10.000, 15.000, and 20.000 STB/d. The result shows high pressure surge and long transient duration for API 5L X52 Pipe. For RTP, pressure surge and transient duration varies based on the reinforcement ratio. Higher pressure surge occurs for RTP with 11,84% reinforcement ratio than API 5L X52 because of the dimension differences, but still faster transient duration. For the other two ratio varieties, which is 2,96% and 5,92%, significantly lower pressure surge and faster transient duration. For each volumetric flow rate varieties, significant effect on all pipes and the result corresponds to the basic theory. For safety requirements, all kinds of pipes are still safe to be applied because there's no pressure surge that exceeds the Pressure Safety High (PSH) set point.