Anticipating and controlling transient response is a critical design activity for ensuring both safety and integrity of the operational subsea system. Predicting transient effect, commonly known as surge pressure, is of high importance for offshore industry. It involves detailed computer modelling that attempt to simulate the complex interactions between flowline and fluid, aiming at efficient flow assurance and consequently flowline and riser systems integrity. Bijupirá and Salema water injection systems, located in the Campos Basin, offshore Brazil, have been operating since 2003. The operational teams have raised concerns, whether the system is adequately designed to protect the subsea system against possible surge pressures during the event of sudden closure of a valve. Researches, referred to transient effects, clarify that is necessary to evaluate the system performance under current and desired operating conditions. The main goal of this paper is to predict the surge pressure in flowline and riser of the water injection system due to valve closure. A supplemental simulation has been performed in order to evaluate conditions that would keep the water injection surge pressure below Maximum Allowable Operating Pressure (MAOP) of the subsea equipments. According to the simulations results, the maximum surge pressure occurs throughout the manifold and jumper region, and the worst case occurs when all valves are simultaneously closed in Bijupirá and Salema production fields. The maximum surge pressures verified in simulations may reach values greater than the operating desired pressure, which may cause damage to the water injection system integrity. In order to avoid surge pressures greater than MAOP, which corresponds to 255 bar, the simulations indicated that when a pressure of 230 bar occurred on the turret, injection flow rate should not exceed 12000 bbl/day. Therefore, conventional techniques to solve complex problems in this area need to be improved, and computational simulations may contribute to establish the operating control system and guarantee its integrity during operating life.