Estimation of Hydrodynamic Force on Rough Circular Cylinders in Random Waves and Currents (RESEARCH NOTE)

Most of the Codes of Practice (API, BSI, DnV, NPD) uses Morison's equation to estimate hydrodynamic loads on fixed and moving offshore structures. The significant difference in the prediction of the loads mainly arises from the assumption of the values of hydrodynamic coefficients. In this paper by analysing a full scale set of data in large KC's numbers collected from Delta Wave Flume in the Netherlands the effects of random waves (JONSWAP spectrum) and uniform current over the artificially rough cylinder have been investigated. The prediction of water particle kinematics has been made from the surface elevation measurements using Stokes fifth order wave theory. By using the Weighted Least Squares technique the hydrodynamic coefficients have been estimated. A comparison between measured force and predicted force shows that although the accuracy of prediction over the whole data is about 90%, the errors on the local peaks are significant (24%) the case which is of interest in the ultimate limit state involving a single extreme wave.