Laboratory experiments were conducted in a three dimensional wave basin with a multi-directional wave maker to measure the diffracted waves due to a cylinder array and the wave forces acting on cylinders in a directional sea condition. An array of two or three cylinders was installed in the wave basin. The wave height between the cylinders is significantly affected by the cylinder's spacing and it increases as the cylinders get closer. The interaction between the diffracted waves from the cylinders, however, becomes less appreciable when the spacing between cylinders is larger than twice of the cylinder diameter. The wave forces in the principal direction for a two-cylinder array are almost the same as those acting on an isolated cylinder when the spacing is twice of the diameter. However, they tend to increase with decreasing distance between the cylinders and clear differences are confirmed in a comparison with the isolated cylinder case. It has also been found that the spreading of directions of the incoming waves reduces the wave force in the principal direction for an isolated cylinder. In case of a cylinder array, however, it has been observed that the spreading parameter has a less effect on the wave force. This indicates that the design based on unidirectional waves cannot always give a conservative estimation.
The directionality of incoming waves becomes important as the water depth of a construction site of offshore structures is increasing, because the effect of wave refraction becomes less than the shallow water region. Thus, it should be considered properly in the design of them. In case of an isolated circular cylinder, the superposition of the linear solutions obtained by MacCamy and Fuchs (1954) is applicable when the incident wave has a small steepness. Ikegaya et al. (1994) measured the pressures at certain locations on an isolated cylinder surface and effectiveness of this technique was examined.