Water hammer is a known pressure pulse or surge that may occur by the instant shut-in of a valve in a flow line. Sudden momentum change may create a pressure cyclic pulse that could cause damage to valves, bending parts in tubing, and/or joints. Usually this effect has been well managed in surface facility design; however, it tends to be overlooked in subsurface well design. Additional possible impact by water hammer in subsurface wells could be on the sandface completions. The severe water hammer could cause failure of formation integrity, resulting in sand production. It may also damage the wellbore and downhole completions. Especially for deep sea water injection and/or production operations, water hammer effect needs to be thoroughly investigated and properly managed because it could be more severe due to longer flow line and higher flow rate. The purpose of this study is to have a comprehensive investigation on water hammer effect for an actual water injection well in Chevron's deep water project with different design parameters and operating parameters. The design parameters include a) height of vertical riser; b) tubing diameter; c) injectivity index (skin or completion type); d) sandface wellbore length; and e) well deviation. The operational parameters include a) injection rate; b) closing time; and c) injection water temperature. Multiphase transient fluid flow model OLGA is used for the water hammer simulation. Results of the water hammer parameter study for optimum well design and operating strategy are reported here. It is shown that the impact of water hammer can be significantly mitigated or eliminated at well design stage or by adjusting the operating parameter(s).