Wave impact forces on a horizontal cylinder
Creators: name: Prasa, Sundar
Publisher: name: University of British Columbia
Year: 2009

description: Impact forces due to wave slamming on structural elements of offshore platforms have been known to reach very high magnitudes and contribute to accelerated fatigue of members and joints due to the resulting dynamic response. Results of previously reported theoretical analyses vary by as much as 100% with regards to the peak value of the slamming coefficient, and experimental verification of these results has been difficult due to the significant amount of scatter in the data reported by several investigators. The present thesis investigates the slamming force due to non-breaking and breaking wave impact on a fixed horizontal circular cylinder located near the still water level. A numerical model which is based on a combination of slamming, buoyancy, drag, and inertia force components has been developed in order to predict the time history of the vertical force on a fixed horizontal cylinder in waves. The model has also been modified to include the effects of dynamic response and cylinder inclination. In addition, an approach based on an impulse coefficient is proposed for estimating the maximum dynamic response of an elastically supported cylinder. Experiments have been carried out in the wave flume of the Hydraulics Laboratory of the Department of Civil Engineering at the University of British Columbia in order to measure the vertical force on a horizontal test cylinder for a range of regular (non-breaking) wave conditions and cylinder elevations. The data has been analyzed to obtain the corresponding slamming and impulse coefficients, as well as the impulse rise-time and duration. Corrections to the measured coefficients to account for buoyancy, dynamic response and free surface slope are indicated. The coefficients exhibit a considerable degree of scatter, even when the various corrections are taken into account. However, the degree of scatter of the impulse coefficient is notably less than that of the slamming coefficient. The results for the maximum slamming coefficientC0 agree with those of recent studies which observe that C0 may be closer to 2it than the generally accepted value of it. A limited number of tests have also been performed for the case of an inclined cylinder, and the effect of tilt on the maximum slamming force and rise-time is examined. The numerical model for the rigid horizontal cylinder has been used to determine the variation of the maximum non-dimensional vertical force in regular waves as a function of the governing non-dimensional parameters. Statistics of the maximum force obtained from simulations in random waves are compared to corresponding results derived from available analytical expressions, and indicate reasonable agreement in the case of a narrow-band spectrum. The temporal variation of the vertical force predicted by the numerical model is also compared to that of the measured force in regular non-breaking waves. In general, the agreement is quite good for both a horizontal and inclined cylinder. The application of the numerical model to an estimation of a member’s response in a prototype situation is illustrated. It is seen that the approach based on the impulse coefficient is relatively simple, and appears to be effective in estimating maximum responses for conditions under which the method is applicable. Experiments have also been carried out in order to measure the impact forces due to plunging wave action on a horizontal circular cylinder located near the still water level. The vertical and horizontal components of the impact force on the cylinder due to a single plunging breaker have been measured for three elevations of the cylinder, and six locations of wave breaking relative to the horizontal location of the cylinder. A video record of the impact process has been used to estimate the kinematics of the wave and plunging jet prior to impact. The force measurements have been corrected for the dynamic response of the cylinder, and analyzed to obtain slamming coefficients and rise times. It is observed that the cylinder elevation and the wave breaking location relative to the cylinder have a significant effect on the peak impact force. The magnitude of the impact force due to a breaking wave is 4 to 20 times greater than that due to a regular non-breaking wave of similar height and period. In addition to the fluid velocity, the curvature of the water surface has a noticeable effect on the peak impact force.