Design and Control of a Water Ballast Management System for an Offshore Floating Structure.

The design of a water ballast control system for an offshore platform located in the Venice lagoon is being considered. The primary aim of the system is to maintain balance in roll and pitch, even in the presence of external disturbances, such as waves, wind or static imbalances on the platform. The structure's balancing action is generated by water tanks that distribute masses of ballast water, generating a rotational moment which returns the structure to the desired position. Managing and allocating the water masses is the ultimate goal of the control system. First, a hydrostatic balance study of the structure is conducted to determine the required ballast water amount, optimal tank size and position for meeting hydrostatic balance requirements. The platform's low-frequency motion and mooring system allow its dynamic model to be approximated from a non-linear 6-degree-of-freedom model to an LTI model that is decoupled in heave, roll, and pitch. This linear model is transformed into the equivalent state space model and used in a feedback control system loop to design a robust controller via H infinity minimization with LMI optimization approach. The controller receives the error between current and desired angles and outputs the required balancing moment to restore equilibrium. The controller's transmitted information is translated into an optimization problem, which is then solved to determine the ideal water ballast configuration. To achieve the optimal water ballast arrangement within the tanks, the control action is performed through a system of pumps and valves. The system automatically opens or closes the valves to fill or empty the tanks accordingly. Special attention is given to the analysis of this actuation system due to its delay in moving water masses, which impacts control system performance by slowing down its dynamics. A Simulink model has been developed to simulate and test various control functions. The goal is to select the optimal operating frequency for the floating platform while considering the limitations of the actuators and the possible environmental disturbances and static imbalances. To sum up, the implementation of a water ballast control system through robust control techniques can be achieved using a simplified dynamic model. One can simulate its performance to obtain the best possible result before testing it in a real environment.