The computer prediction of wave loads on large vertical cylinders of arbitrary section extending from the seabed or deep water and piercing the free surface is summarised. One important reference geometry conforming to this situation corresponds to a pair of neighbouring circular cylinders. Wave force experiments for this case are described and a comparison of the results with the computer predictions is made. The force on one cylinder may be significantly increased by the presence of the neighbour.


Offshore structures of large horizontal dimension have now found extensive application and reliable predictions of the wave induced forces on these structure have become of enormous economic importance. At present such predictions are usually based on wave diffraction computer programs (Refs. 1,2) in which a body of arbitrary geometry is represented as a distribution of a finite number of point wave sources over its submerged surface. These programs are extremely costly and it is thus of some importance to investigate cheaper methods of prediction for various special geometries. The geometry considered here corresponds to one or more large vertical cylinders of arbitrary section extending from the seabed or deep water and piercing the free surface. The cylinders are represented by a distribution of vertical line wave sources over their immersed surfaces and it is thus possible to achieve considerable savings, both in terms of programming complexity as well as in computer time and storage requirements. An equivalent approach has been used previously by Hwang and Tuck (5) in the context of harbor resonance and by Ijima, Chou and Yumura (6) in the context of wave diffraction around off shore breakwaters. The theory and numerical approach have been described in some detail in a separate paper by the author (8), and the method has been summarised and put into perspective in relation to the general topic of wave forces on offshore structures in a recent book by Sarpkaya and Isaacson (9). Applications of the method lie in the efficient calculation of wave forces on neighbouring circular storage tanks and caissons, isolated or neighbouring tanks, caissons and berths of square or rectangular section, and various other offshore structures conforming to the geometry described above. The case of two neighbouring circular cylinders is considered here as one particular reference geometry of this situation. This case has also been treated by Lebreton and Cormault (7) using a point wave force representation, and by Spring and Monkmeyer (10) by a separate approach. Experiments involving the measurement of forces on one cylinder in the presence of another are described, and a comparison with the computer predictions shows good agreement. The loads on one cylinder may be considerably increased by the presence of a neighbour.


Wehausen and Laitone (12) have outlined the theoretical approach to obtain the wave forces on bodies of arbitrary shape. The investigation here concerns the forces exerted by a regular wave train on one or more vertical cylinders of arbitrary section as depicted in Fig. 1. The fluid is assumed incompressible and inviscid and the motion irrotational; and the wave height is assumed to be sufficiently small for linear wave theory to apply.

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