Real sea waves are multi-directional, and it's quite different with unidirectional waves. In present, based on the linear theory of wave interaction with an array of circular bottom-mounted vertical cylinders, systematic calculations are made to investigate the effects of the wave directionality on wave loads in real conditions. The time series of multi-directional wave loads can be simulated. The effect of wave directionality on the normal and transverse wave force on an array of tandem cylinders is investigated. It was found that the wave directionality has a significant influence on the transverse force. The biggest transverse force is found to occur on the rear cylinder rather than the front one. This is quite different from the results in unidirectional waves and should be paid much more attention in the design of offshore structures.
Circular cylinder are frequently used as the foundation of offshore structures, for example, include bridges, wind turbine foundations, offshore platforms and floating airports. In the designing of ocean engineering, wave loading is an important factor. For wave interaction with a large-scale cylinder (D/λ>0.15, where D is the diameter of a cylinder and λ is the wave length), wave diffraction is important and should be considered. A superposition eigenfunction expansion method was used by MacCamy and Fuchs (1954) to obtain a linear solution, based on the assumption that the incident wave has a small steepness. For the case of waves acting upon an array of cylinders, the effect of a given cylinder on the incident wave will produce a scattered wave which will in turn be scattered by adjacent cylinders. Thus the computation of the velocity potential must account for the diffraction of the incident wave field by each body and the multiple scattering from other bodies. An exact solution for the diffraction of linear water waves by arrays of bottom-mounted, vertical circular cylinders was first given by Spring and Monkmeyer (1974) using a direct matrix method. It represented an extension of the single cylinder case presented by MacCamy and Fuchs (1954). An accurate algebraic method was developed by Kagemoto and Yue (1986) to calculate the hydrodynamic properties of a system of multiple three-dimensional bodies in water waves. Subsequently, a simplified expression for the velocity potential in the vicinity of a particular cylinder was developed by Linton and Evans (1990) who led to simple formulae for the first-order and mean second-order wave forces on multiple cylinders as well as the free surface profile.