A method for evaluating the noise of a platform is presented in which 

 the highest of the ambient noise curves shown in Figs. 5-9 is used as 

 reference norm for natural noise. The number of decibels by which the 

 platform noise exceeds this reference is readily observable from the graphs. 

 The maximum value of this was determined in each of three frequency regions, 

 rather arbitrarily selected to cover the frequency range from 5 to 5000 Hz 

 which encompasses the main noise of the platforms. The three frequency regions 

 are: (1) 0-30 Hz, (2) 30 to 300 Hz, and (3) above 300 Hz. The value of 

 the maximum excess in each of these frequency regions is tabulated for each 

 platform in Table I. Their values in decibels vary from a maximum of 45 

 decibels in the under 30 Hz band for platform A to zero for platform Rincon. 

 The right hand column of Table I gives a noise rating derived by rating the 

 noise excess of each of the three frequency regions separately as follows: 

 an excess of over 40 is designated: N, for noisy; 30 to 40: M for moderate 

 and under 30 Q, for quiet. The three ratings are combined to derive a total 

 rating of Noisy, Moderate, Quiet, or ^ery Quiet. Note that only one platform, 

 the man-made Rincon Island, rates a Q in each frequency region, and so is rated 

 very quiet. This is probably the result of its being supplied by shore power, 

 so it does not need a local generator of prime power, and vibration from 

 machinery on the island does not propagate effectively through the island 

 material and does not radiate efficiently into the surrounding shallow 

 water. The two platforms rated "noisy" have no obvious common relationship. 

 Platform C is a production platform driven by shore-generated power, so it 

 might have been expected to be quiet. Its noisy rating results from relatively 

 high-level spectrum lines in all three frequency regions, especially those 



F-7 



