Appendices 5-13 - Nautilus Cares - Nautilus Minerals

2 Methods
2.1! Source modelling
Cavitation in water occurs when the pressure in a particular location drops below the
saturated water vapour pressure. Bubbles of water vapour then form - the water
effectively boils, but due to a lowering of pressure, rather than the increase in water
vapour pressure with temperature. When these bubbles move into a region of higher
pressure they implode violently, producing a sharp, impulsive sound.
For a propeller, there is a low pressure region on the forward face of each blade which can
(and usually does) result in the continuous formation of cavitation bubbles, which
subsequently move into a region of higher pressure and implode. The combined effect of
the implosion of many cavitation bubbles is high intensity, broadband noise, usually
modulated at the propeller blade rate (the shaft rotation rate multiplied by the number of
blades).
Because of its importance for passive sonar detection of ships and submarines, cavitation
noise has been extensively studied (see eg. Ross, 1987), however this has been in the
context of ships travelling at speed, rather than ships holding station on DP. The source
model used in this report is therefore based on measurements made by one of the authors
of underwater sound levels produced by a rig tender (Pacific Ariki) on DP (McCauley
1998). The characteristics of the Pacific Ariki are given in Table 1, and the measured,
third octave source spectrum is shown in Figure 2. The Pacific Ariki is a much smaller
vessel than the proposed mining vessel (see Table 1). Levels have therefore been
extrapolated to those to be expected for a larger vessel by assuming that a constant
proportion of the mechanical power is converted to acoustic power. This relationship has
been found to hold reasonably well for surface vessels operating at their normal cruising
speed (Ross 1987). The source level corrections given in Table 1 are therefore given by:
Correction 10
( P % vessel
) 10 log
&
# (dB)
' PAriki
$
where P vessel is the total installed thruster power on the vessel, and P Ariki is the total
installed thruster power on Pacific Ariki. The resulting source spectrum is shown in
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