Scale Model Measurements of Ship Machinery Noise Mitigation by Air Injection

Underwater radiated noise (URN) from shipping is a main contributor to the rising noise levels in the oceans. There is growing evidence that this has a negative impact on marine life. There are mitigation methods available, but many are still in the development phase. One such method is presented here: an air injection system for reducing URN that is generated by ship machinery. For these measurements, a dedicated ship model incorporating an air injection system and a simplified ship structure was developed based on a tanker hull geometry. A scaled metal midship section was designed using finite element method simulations, aiming to obtain a representative structure with sufficient modal density at frequencies of interest. This mid-section was excited using a shaker provided with white noise signals while the ship model was towed over hydrophones to measure the noise radiating from the metal midship section. Performance of the air injection system is then quantified using the difference between the sound levels measured with the air injection system turned off and on – termed the insertion loss. The dependence of the insertion loss on shaker input signal, model speed and air flow rate are investigated.