Integrated Power Systems for Offshore Vessels. Control, trends and challenges

The offshore vessels require a high precision speed control and dynamic positioning. Adverse weather conditions and/or rough waves give rise to high disturbances that must be rejected by the control system to keep the vessel position in the area or station where the vessel operation is being executed. Other requirements related to the safety, reliability and robustness must be obviously ensured. Since more than a decade the power system of this kind of vessels is based on a diesel-electric power plant where the propellers and thruster units are controlled by electrical drives. In this power system the required electrical energy/power is generated through diesel gensets, and distributed to the propulsion drives (and to the rest of vessel utility loads) through a power grid. In this work the most commonly used integrated power system architecture is first introduced thus identifying the control functions and their interdependencies. Some trends and new power system topologies to improve the energy efficiency are then also presented and described. The emergence of new solutions based on DC grids allows the diesel gensets to operate at variable speed and makes the integration of the energy storage systems or even some renewable energy systems easier. However, they present some technical and design challenges in order to ensure a stable and robust solution by design. One of them is related to the stability analysis of the DC grid when multiple nonlinear constant power loads (negative impedance) are connected and thus interacting on the same DC bus