Saturn researchers at MARIN, the Netherlands’ Maritime Research Institute, are currently in the process of evaluating the use of air bubbles to mitigate the source levels of underwater noise radiated by ships. To test the effectiveness of this measure, air bubbles will be injected in the flow below the ship hull to isolate machinery noise, and in the flow upstream of the propeller disc to dampen the cavity collapse, thereby reducing radiated noise.
In 2021, several actions were performed to prepare the model tests for this purpose:
A 94 m coastal tanker, known as the Streamline tanker, was selected as reference ship for which a scale model will be manufactured.
Different types of porous hoses to inject air bubbles were evaluated using model tests on a similar hull form (see photo below). A configuration for the tests to be performed in 2022 has been proposed. With the model tests, the effect of air bubbles on propeller performance was also assessed as the bubbles were swept into the propeller plane: these first tests showed a decrease in thrust and torque of a few percent but propulsive efficiency was decreased by 1% only.
A metal mid-section of the tanker model was designed. This section will be excited using a shaker and the resulting radiated noise in the basin will be measured with and without bubbles present thereby providing information on the insertion loss.
A system to inject bubbles in the propeller disc was designed and manufactured. First tests were successfully completed.
The tests of these models with the bubble layer below the hull and the injection into the propeller disc are scheduled to be carried out in Q3 of 2022.