A procedure for estimating mitigation effect of air injection on ship underwater radiated noise

Increasing evidence exists that underwater radiated noise from ships has a negative impact on marine ecosystems. This has led to initiatives aimed at mitigating ambient sound levels generated by shipping. Within the European Union, noise abatement is being addressed by a wide range of stakeholders, with activities focused on developing procedures and thresholds for evaluating noise impact as well as knowledge on the effectiveness of a range of mitigation measures. In the SATURN project, MARIN is studying air injection systems for mitigation of both propeller cavitation noise and machinery noise; the so-called ‘Prairie’ and ‘Masker’ systems, respectively. Model-scale tests have been performed in order to quantify performance in terms of sound level reductions for a range of test conditions. In this paper we elaborate a procedure for applying the processed measurement data to ship-scale underwater radiated noise predictions for assessing mitigation potential during the early design stage. Data reduction of the model test results is explained, leading to a model for estimating the mitigation effect of the two systems. Mitigation metrics for use in noise impact risk assessments are proposed. Limitations of the methodology are also discussed.