<p>Dynamic positioning systems (DPS) on marine vessels exhibit actuator redundancy, with more actuators than degrees of freedom. A control allocation unit is employed to address this redundancy. Practical systems often feature time-varying elements in the effectiveness matrix due to factors such as changing operating conditions, nonlinearity, and disturbances. Additionally, not all thrusters require engagement at each step to counteract disturbances and maintain position. Control efforts can be generated by selecting some thrusters based on their instant effectiveness, while others can remain on standby. Therefore, introducing a control allocation method that calculates the effectiveness matrix online and selects the most efficient thrusters could be effective. This paper introduces a fault-tolerant control allocation strategy for DPS with a varying effectiveness matrix. Specifically, the investigation focuses on a case study featuring eight azimuth thrusters used on a drilling rig. At each time step, the effective matrix is calculated online, followed by the selection of the four most effective thrusters based on the actuator effectiveness index, with the four serving as backups in case of a fault. The proposed strategy has been validated through simulation results, demonstrating advantages such as robustness against changes in the effectiveness matrix and reduced energy usage by the thrusters.</p>