Retrofit-induced changes in the radiated noise and monopole source levels of container ships

The container shipping line Maersk undertook a Radical Retrofit to improve the energy efficiency of twelve sister container ships. Noise reduction, identified as a potential added benefit of the retrofitting effort, was investigated in this study. A passive acoustic recording dataset from the Santa Barbara Channel off Southern California was used to compile over 100 opportunistic vessel transits of the twelve G-Class container ships, pre- and post-retrofit. Post-retrofit, the G-Class vessels' capacity was increased from similar to 9,000 twenty-foot equivalent units (TEUs) to similar to 11,000 TEUs, which required a draft increase of the vessel by 1.5 m on average. The increased vessel draft resulted in higher radiated noise levels (<2 dB) in the mid- and high-frequency bands. Accounting for the Lloyd's mirror (dipole source) effect, the monopole source levels of the post-retrofit ships were found to be significantly lower (>5 dB) than the pre-retrofit ships in the low-frequency band and the reduction was greatest at low speed. Although multiple design changes occurred during retrofitting, the reduction in the low-frequency band most likely results from a reduction in cavitation due to changes in propeller and bow design.

Automated summary

The container shipping line Maersk underwent a Radical Retrofit to improve energy efficiency of twelve sister container ships. This study investigated the potential noise reduction benefits of the retrofitting effort. Analysis of a passive acoustic recording dataset from the Santa Barbara Channel revealed that the post-retrofit G-Class ships had significantly lower monopole source levels in the low-frequency band, likely resulting from a reduction in cavitation due to changes in propeller and bow design.