Impact Loads on Circular Cylinders

In the past, testing of water impact problems such as breaking waves impacting on risers, lowering of subsea structures through the splash zone and free fall lifeboat performance has been carried out either by model tests or simplified methods such as empirical formulas and potential flow calculations. The simplified methods generally give reasonable results for simple geometries such as wedges or two-dimensional cylinders. When more complex structures are to be analyzed model tests are required, increasing the cost and time spent significantly. In addition, scale effects may also alter the results.Recent advances in computer technology and computational fluid dynamics (CFD) have made it possible to simulate many of the problems that earlier only could be studied using model tests. This allows for more tests being performed at a lower cost compared to model tests and with higher accuracy than simplified methods. In addition, the CFD simulations are normally performed in full scale, thus avoiding scale effects.This thesis explores the possibility of simulating water entry problems using the CD-adapco CFD-software STAR-CCM+ with focus on circular cylinders. The reason for choosing this shape is that it is a simple geometry while still being a realistic problem, since this is the most widely used shape for structural members in the offshore industry. Also, for numerical methods the initial phase of impact of a cylinder is numerically difficult to solve, because of the almost flat surface on the cylinder bottom. This means that a method solving water impact of cylinders correctly is believed to also being able to simulate water entry of other geometries. Two-dimensional simulations have been performed for constant velocity and free-falling cylinders. Good agreement is found between the CFD results and empirical lines by Campbell and Weynberg (1980). The results for free-falling cylinders show fairly good agreement with experimental data by Greenhow and Lin (1983), nonlinear boundary element method results by Sun (2007), and CFD simulations by Zhu (2006).Three-dimensional simulations have been performed to analyze the water impact of a cylinder with 8 degree impact angle assuming constant velocity. The results are compared to experiments performed by Campbell and Weynberg (1980) and strip theory calculations. Good agreement is found between the methods