An experimental investigation of hydrodynamic coefficients for a vertical truncated rectangular cylinder due to regular and random waves

The research into hydrodynamic loading on ocean structures has concentrated mostly on circular cross-section members and relatively limited work has b een carried out on wave loading on other cross-sections such as rectangular sections. These find applications in many offshore structures as columns a nd pontoons in semi-submersibles and tension-leg platforms. The present inv estigation demonstrates the behaviour of rectangular cylinders subject to w ave loading and also supplies the hydrodynamic coefficients for the design of these sections. This paper presents the results of wave forces acting on a surface piercing truncated rectangular cylinder set vertically in a towing tank. The experi ments are carried out in a water depth of 2.2 m with regular and random wav es for low Keulegan-Carpenter number up to 6. The rectangular cylinder is o f 2 m length, 0.2 m breadth and 0.4 m width with a submergence depth of 1.4 5 m from still water level. Based on Morison equation, the relationship bet ween inertia and drag coefficients are evaluated and are presented as a fun ction of KC number for various values of frequency parameter beta, for two aspect ratios of cylinders, equals to 1/2 and 2/1. Drag and inertia coeffic ients obtained through regular wave tests are used for the random wave anal ysis to compute the in-line force spectrum. The results of the experiments show the drag and inertia coefficients are s trongly affected by the variation in the aspect ratios of the cylinder. The drag coefficients decreases and inertia coefficients increases with increa se in Keulegan-Carpenter number up to the range of KC number tested. The ra ndom wave results show a good correlation between measured and computed for ce spectrums. The transverse forces in both regular and random waves are fo und to be small compared to in-line forces. (C) 1999 Elsevier Science Ltd. All rights reserved.