Oceans of noise - Whale and Dolphin Conservation Society

produced as the bubble collapses and this process, “cavitation”, is responsible for the loud “hiss”
often associated with ships. Causes of propeller noise are:
• Cavitation
• Propeller-induced vibration
3.4.1.3 Hydrodynamic noise
This type of noise is distinguished from propeller noise in that it does not originate at the
propeller but is caused by the flow of water past a physical structure such as the hull of a vessel or
the legs or risers of platforms. Causes of hydrodynamic noise are:
• Vortex-induced vibration
• Resonant excitation of cavities, plates, and appendages
• Turbulent flow within pipes
3.4.1.4 Impulsive noise
Impulsive sounds are those created by the rapid expansion and collapse of an air bubble (seismic
air gun) or from the instantaneous application of pressure to a solid structure (pile driver).
Impulsive sounds are typically short-lived and characterised by rapid rise times. Causes of
impulsive noise are:
• Explosions, for example during explosive wellhead decommissioning (decommissioning)
• Airguns used during seismic surveys (exploration)
• Pile drivers (installation)
3.4.2 Rig and platform noise
Rigs and platforms come in various forms, shapes and sizes and are found across a wide range of
depths from coastal to oceanic waters. They fall into three general categories (1) man made
islands / caissons, (2) fixed platforms and (3) drill ships / semi-submersibles (Figure 3.2). Their
design, construction and local oceanographic conditions will affect both the path of the sound into
the water column and how much sound is transmitted (Figures 3.3 and 3.4).
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