The spectral analysis method is presented in the paper for predicting random wave force on large square section caissons. A comparison of the coefficients of group effect with the results of equivalent circular caissons and of square caissons in regular waves has been made. It is shown that the effect of the neighboring caissons in random waves decreases and tends quickly to an isolated caisson case with the increase of caisson spacing. In general, when the ratio of caisson spacing to the side of caisson is greater than 4, the group effect may be neglected.
The problem of the interaction of large square caissons has a number of possible field applications in offshore and ocean engineering. Considerable research has been carried out to estimate the regular wave force on large square caissons resting on seabed and piercing the free surface (Isaacson, 1978, 1979, Mogridge, 1976a, 1976b, Wang,1990). With the development of offshore and ocean engineering, random wave force on large caissons become important since waves in the sea are really random. The spectral analysis method is presented in the paper for predicting interaction of large square caissons in random wave. It uses Bretschneider spectrum as incident wave spectrum. For each component wave of the spectrum, it can be considered as a regular wave train. Thus the wave action on the caissons for each component wave can be determined "by using the source representation method for regular waves. The random wave action on the caissons can be obtained by superposing all the regular wave action of each component wave together. The expected value of the maximum wave force, the wave force for different exceedance probability, and the coefficient of group effect for each caisson can be obtained from the random wave force process by the method proposed in the paper.