Interference Effects Between Large Cylinders In Waves
Authors: de St. Q. Isaacson Michael 
Published: December 2007

Computer predictions of wave load on large vertical cylinders of arbitrarysection extending from the seabed or deep water and piercing the free surfaceare summarized. Wave-force experiments for the case of a pair of neighboringcircular cylinders are described, and results are compared with computerpredictions. The force on one cylinder may be increased significantly by thepresence of its neighbor.

Introduction

Offshore structures with large horizontal dimensions are being usedextensively, and reliable predictions of wave-induced forces on thesestructures have become very important economically. Presently, such predictionsusually are based on wave diffraction computer predictions usually are based onwave diffraction computer programs in which a body of arbitrary geometry isprograms in which a body of arbitrary geometry is represented as a distributionof a finite number of point wave sources over the structure's submergedsurface. These programs are extremely versatile but fairly costly, and wheneverpossible may be complemented by alternative prediction methods that may becheaper or more accurate, but that are restricted to special bodyconfigurations. The geometry considered here corresponds to one or more largevertical cylinders of arbitrary section, extending from the seabed or deepwater and piercing the free surface. The cylinders are represented by adistribution of two-dimensional wave sources over their horizontal contours.Thus, it is possible to achieve considerable savings, both in terms ofprogramming complexity and in computer time and storage requirements. Anequivalent approach has been used previously by Hwang and Tuck in the contextof harbor resonance, and a related Green's function method by Ijima et al. inthe context of wave diffraction around offshore breakwaters. The theory andnumerical approach were described in some detail in another paper. The methodwas summarized and put into perspective in relation to the general topic ofwave forces on offshore structures recently by Sarpkaya and Isaacson. Themethod can be applied to calculate efficiently the wave forces on neighboringcircular storage tanks and caissons, isolated or neighboring tanks, caissonsand berths of square or rectangular sections, and various other offshorestructures conforming to the geometry described above. The case of twoneighboring circular cylinders is considered here as one particular referencegeometry of this situation. This case also has been treated by Lebreton andCormault using a point wave-source representation, and by Spring and Monkmeyerand Chakrabarti by a separate approach applicable to multiple circularcylinders. Experiments involving the measurement of forces on one cylinder inthe presence of another are described. A comparison with computer predictionsshows good agreement. Loads on one predictions shows good agreement. Loads onone cylinder may be increased considerably by the presence of anothercylinder.

Theory

Wehausen and Laitone outlined the theoretical approach to obtain the waveforces on bodies of arbitrary shape. This investigation concerns the forcesexerted by a regular wave train on one or more vertical cylinders of arbitrary, section (Fig. 1). The fluid is assumed incompressible and inviscid, the motionis assumed irrotational, and the wave height is assumed small enough for linearwave theory to apply.

JPT

P. 505