Ocean Noise Pollution Reduction, Onboard Underwater Radiated Noise (URN) Estimation and Management for the Existing Maritime Transportation Fleet

Although the most important efforts towards undesired emissions and pollution reduction are focused on decarbonization and greenhouse gas emissions, the ocean noise pollution must not be relegated as of lesser importance. Existing evidence on ocean noise levels since mid-sixties of the past century report an incrementation rate of 3 dB per decade on average, inherently due to the increasing human ocean activity, i.e., maritime transportation. Also, URN impact on marine life is raising increasing concern, including the International Maritime Organisation. The last revision of its Guidelines for the Reduction of Underwater Radiated Noise from Shipping to Address Adverse Impact on Marine Life (IMO Ref. MEPC.1 / Circ.906) is a clear indicator, showing the path to follow.

For existing ships to advance in the implementation of URN reduction measures it is crucial to find a solution that fits well into already operating ships, as for new ships it is possible to include well known URN reduction techniques at the initial stages of the design and construction process.

For the estimation and control of the URN, the proposed method is based on the characterization of the several sources that that have a major contribution to the URN signature of a ship, i.e.,  propulsion system (engine and gears), propeller (both in non-cavitating and cavitating conditions) and other onboard machinery (generators, HVAC, etc.). The characterization was conducted by both onboard and outboard measurements (made in compliance with international standards such as IACS Rec. 176 and Bureau Veritas NR614) and resulted in the corresponding transfer functions for the several sources considered. The results show that an accurate estimation of the URN emitted by the ship can be obtained by measuring vibration levels at the selected onboard sources. The URN estimations can even be produced onboard in real-time using an extension to the Ni-CDS system, a non-intrusive onboard cavitation detection system, that already provides real-time information on the cavitation status of the propeller using accelerometers located nearby inside the hull, being cavitation the most contributing source to URN when occurring. Data analysis techniques merging URN estimations with knowledge on the operational status of the ship at the same time can provide insights and useful information for comprehensive URN management onboard, leading to lesser URN levels and thus, achieving the goals IMO is promoting.