Diffraction of irregular waves, focused wave groups, and random seas by an
array of vertical bottom-mounted circular cylinders is investigated using t
heoretical, computational and experimental methods. This is an extension of
our study of such an array in regular waves, reported in Part 1. Linear fo
cused wave group theory is reviewed as a method for predicting the probable
shape of extreme events from random wave spectra. Measurements are present
ed of the free surface elevation distribution in the vicinity of a multi-co
lumn structure in an offshore basin when subjected to irregular waves havin
g peak frequencies and significant wave heights in the range 0.449 < k(p)a
< 0.555 and 0.114 < H-s < 0.124 respectively, where a is the cylinder radiu
s. Analytical linear diffraction theory is extended for application to focu
sed wave groups and random seas. Experimental irregular wave data are analy
sed for comparison with this theory. Linear diffraction theory for random s
eas is shown to give an excellent prediction of incident wave spectral diff
raction, while linear diffraction theory for focused wave groups works well
for linearized extreme events. Due to the phase shifting of incident wave
spectral components, diffraction is shown to generate focused wave groups i
n the vicinity of the cylinder array.