By Thomas Lloyd, Frans Hendrik Lafeber and Johan Bosschers (MARIN)
Last year, in Issue 2 of SATURN’s Research Newsletter, we reported on the scale model tests performed at MARIN to study the performance of two air injection systems for reducing ship underwater radiated noise. These are the Masker system, aimed at mitigating machinery noise by isolating the vibrating hull plating from the surrounding water, and the Prairie-like system, whereby bubbles are injected into the propeller cavitation with the aim of dampening the cavity collapse (a dominant sound generation mechanism). The laboratory measurements were made in MARIN’s Depressurised Wave Basin facility, with the scale models – incorporating in-house developed air injection systems – towed through the water by a carriage and sound measured using hydrophones mounted in the basin.
Figure 1: Photograph of scale model used for Masker system tests mounted under the towing carriage in MARIN’s Depressurised Wave Basin.
We previously presented preliminary results for the changes in sound level spectra for each system for a selected test condition. For the Prairie-like system, we call this the ‘source level attenuation’, where source level refers to the magnitude of the sound generated by a particular source, or combination of sources. This name was chosen since the Prairie-like system reduces (or changes) the source level strength by dampening the implosion of the cavitation. However, the equivalent term for the Masker system is `insertion loss’, since the change in measured sound level results from the air bubble layer being ‘inserted’ between the vibrating hull and the water.
Since the last newsletter we have analysed all of our measurements and compared the performance of the Masker and Prairie-like systems across different test conditions. The results for the Masker system were also compared to data available in the open literature for a so-called `Big Bubble Curtain’, used to reduce underwater radiated noise from pile-driving activities. Both measurements showed similar spectral shape, the insertion loss being strongly dependent on frequency. Following this, two data fits were made for further use in application cases; a `conservative’ fit representing the average fit of the measured spectrum and an `optimistic’ fit, which more closely corresponds with the Big Bubble Curtain data (not shown). Only the optimistic data fit is used in the following results. For the Prairie-like system only the measured source level attenuation is presented, although we have plans to develop a simplified model for this quantity.
Figure 2: Air injection system performance derived from scale model tests: `conservative’ and `optimistic’ data fits for Masker system insertion loss and Prairie-like system source level attenuation.
To further exploit our measurement results, we applied the predicted mitigation performance data for both systems to a ship-scale test case for which radiated noise level (RNL) spectra are available for a range of ship speeds. The RNL is distinct from the source level since it accounts for the interference pattern generated when a source is placed close to a free water surface – as is the case for a ship. By making assumptions about the contribution of machinery noise to the total RNL, we are able to apply the mitigation effect of the Masker and Prairie-like systems to the two main source mechanisms separately and combine the results to obtain an estimate of the total mitigated RNL. Example results from this exercise are shown in the following figures, for a low ship speed (without propeller cavitation) and a high ship speed (in which case propeller cavitation noise dominates the RNL).
Figure 3: Unmitigated and mitigated radiated noise level spectra for ship-scale application case at 8 knots (no propeller cavitation). Mitigation effect from Masker system only. Measurement data taken from Arveson and Vendittis (2000).
At 8 knots, the mitigation effect results solely from the Masker system and is centred at around 1 kHz, with up to 20 dB reduction in RNL. However, for the 16 knots condition, the Prairie-like system is mostly responsible for the changes in RNL, of up to 10 dB. The largest reductions are focused close to 50 Hz, which corresponds to the centre frequency of the spectral hump caused by tip vortex cavitation.
Figure 4: Unmitigated and mitigated radiated noise level spectra for ship-scale application case at 16 knots (fully developed propeller cavitation). Mitigation effect predominantly due to Prairie-like system. Measurement data taken from Arveson and Vendittis (2000).
Ongoing work is focused on formalising the procedure for applying the measurement results to application cases, in order to provide stakeholders with a simplified method for predicting the mitigation effect of air injection systems. As part of this work, we aim to include hearing sensitivity data from marine animals such that the mitigation effect can be weighted for a specific (group of) species.
More information on the analysis of the model scale tests and application of the data to ship-scale cases can be found in the following publications:
Klinkenberg, Y., Bloemhof, F., Kamphof, H. J. & van Rijsbergen, M., 2023. ‘Measuring air bubble layer characteristics around the hull of a ship model’. Proceedings of the 7th International Conference on Advanced Model Measurement Technology for the Maritime Industry. 24th-26th October, Istanbul, Turkey.
Lloyd, T., Lafeber, F. H., Bosschers, J., Kaydihan, L. & Boerrigter, B., 2023. ‘Scale model measurements of ship machinery noise mitigation by air injection’. Proceedings of 7th International Conference on Advanced Model Measurement Technology for the Maritime Industry. 24th-26th October, Istanbul, Turkey.
Lloyd, T., Lafeber, F. H., Bosschers, J., 2024. ‘Ship URN mitigation by air injection: model-scale experiments and application to full-scale measurement data’. Proceedings of 8th International Symposium on Marine Propulsors, 17th-20th March, Berlin, Germany.
Lloyd, T., Foeth, E.-J., Lafeber, F. H., Bosschers, J., Klinkenberg, Y., Birvalski, M., Kaydihan, L. & Lidtke, A., 2024. ‘SATURN Deliverable 4.3. Impact of mitigation measures on source level of a single-screw vessel’. Technical report, Wageningen, The Netherlands.
Lloyd, T., Lafeber, F. H., Bosschers, J., 2024. ‘A procedure for estimating the mitigation effect of air injection on ship underwater radiated noise’. Inter-noise, 25th-29th August, Nantes, France (Forthcoming).
References
Arveson, P. T. & Vendittis, D. J., 2000. ‘Radiated noise characteristics of a modern cargo ship’. The Journal of the Acoustical Society of America, 107(1), pp. 118–129.
Bellmann, M. A., 2014. ‘Overview of existing noise mitigation systems for reducing pile-driving noise’. Proceedings of inter-noise 2014. 16th-19th November, Melbourne, Australia.