Analysis of Underwater Radiated Noise Based On Vessel Hull Vibrations.


This article discusses the issue of ocean pollution, specifically the impact of increased ambient noise on marine life, caused by anthropogenic (human-induced) sources such as maritime shipping and seismic exploration by the oil/gas industry. It explains that low frequency noise can negatively affect marine mammal species, particularly big whales, whose auditory range overlaps with the frequency range of ship-generated noise.

The article also provides an overview of the science of underwater acoustics and discusses the major sources of onboard vibrations, including machinery vibrations and hull girder vibrations. It further explains the transmission of vibrational acoustics, both airborne and structure-borne, and the different types of waves that can occur in a structure, including longitudinal, shear, and bending waves.

Key highlights
  • Pollution in the ocean is not just about harmful substances, but also about increased ambient noise.
  • Human-induced acoustic sources are closely related to maritime shipping and seismic exploration by the oil/gas industry.
  • The science of underwater acoustics began in 1490 with Leonardo da Vinci’s observation that ships can be heard from a great distance by placing a tube in the water.
  • Onboard vibrations are produced by machinery components like engines and motors and can be categorized into torsional, axial or longitudinal, and lateral or transverse vibrations.
  • Vibrational acoustics and transmission can be air-borne or structure-borne, and wave propagation in structures is much more complicated than in air, involving longitudinal, shear, and bending waves.
Key Challenges
  • Limited regulatory framework and enforcement to control noise pollution from shipping and other human activities.
  • Difficulty in monitoring and measuring the extent of underwater noise pollution due to the vastness and complexity of the marine environment.
  • Technological limitations in reducing noise pollution from existing vessels and equipment.
  • High costs of implementing noise reduction measures and transitioning to quieter technologies in the shipping and oil/gas industries.
Major Opportunities
  •  Once we are able to calculate the vibratory forces on the hull due to different machinery, we can use them directly to calculate the noise radiated.
  •  We have enough literature which provides us with noise estimates of the
    machines onboard. Using those estimates and by using transmission path
    analysis we can estimate URN.
  • While applying Mindlin model to hull plates, dynamic pressure variation i.e. linearly
    proportional to depth outside and based on positioning of machinery components
    like diesel engine and motors inside has to be taken into account.
  • Vibration exciting functions from various machinery components can be
    superimposed vectorially in the Mindlin model for most accurate results.

“Low frequency noise i.e., less than 1,000 Hz could affect several marine mammal species, specifically the big whales, whose auditory range overlaps & Ocean ambient noise results from both natural and anthropogenic(human-induced) acoustic sources. The latter are closely related to maritime shipping and seismic exploration by the oil/gas industry.”

Akshay Kumar Tomar & Arnab Das