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

Increasing evidence exists that underwater radiated noise (URN) from ships has a negative impact on marine ecosystems. This has led to numerous initiatives aimed at monitoring and 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 and machinery noise; the so-called "Prairie" and "Masker" systems. 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 URN 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 and power requirement of the two systems. We subsequently apply the model to a number of test cases, covering both measured and calculated source levels. Limitations of the approach will also be discussed.