Multi-stage centrifugal water injection pumps are used by oil producing organizations for enhanced oil recovery. In one such installation, a 14-stage centrifugal pump driven by a 730 kW electric motor at nearly constant speed of 2975 RPM was used. However, after almost 3 years of operation, it was found that the injectivity of the water disposal well had deteriorated badly due to oil fractions in water which tend to block the underground formation. Due to this problem and because of the speed limitation of the pump, back pressure on the pump discharge had grown up to unacceptable levels and kept on fluctuating. To overcome this problem; installing a variable speed drive (VSD) unit to the existing pump was considered as an option. The hydraulic capacity of the pump was checked and found suitable for higher discharge heads but the dynamic capability of the pump rotor for the intended speed range need to be investigated. The aim of this work is to assess the rotor-dynamic implications of running a fixed speed water injection pump over a variable speed range for new operational requirements. A rotor-dynamic model is developed using finite element method (FEM) for the rotor including impellers, bearings and seals. The model is used to study the existing configuration of the pump and to validate its dynamic behavior under the current operating conditions. The model is next used to simulate the pump rotor-dynamic behavior under various speeds within the proposed operating range. Critical speed maps, stability analysis, unbalance responses and transients of the system are investigated for evaluating the suitability of this pump for the intended operation. The effects of different parameters on the system performance are also examined. The study helps in identifying the operating envelope of the pump.