On the Huse-Muren model for wake interaction with vertical offshore cylinders

Abstract

There are many offshore applications that incorporate vertical cylindrical-shaped structural elements such as marine risers, TLP tendons and near shore pier designs. A better understanding of complex wake flows on the drag dominated force predictions is very much of interest for offshore design problems. This study investigates the Huse-Muren analytical wake model, whose mathematical formulation was founded on the earlier definitive studies of Prandtl and Schlichting. Developing a deeper understanding of their key findings that were based upon central assumptions and key approximations is central to this research. Analysis of their model leads to the development of dimensionless expressions for the drag coefficient correction ratio. Both the DNV-RP recommended drag force coefficients and some of Sarpkaya's U-tube measurements are used to investigate whether their analytical model could be used to bound the experimental data. The Keulegan-Carpenter number, A0/D ratios, Sarpkaya's Beta parameter and Reynold's number are used to interpret the range of model applicability. For the current cross flow case, variables identified in Huse and Muren's formulation lead to a dimensionless expression involving the period of in-line oscillation and the velocity of the cross-current resulting in a parameter similar to the Keulegan-Carpenter number. The ratio of this parameter to the Keulegan-Carpenter number allows one to address the drag coefficient correction ratio where a cylinder is simultaneously subjected to in-line oscillatory flow and a steady cross current coming from the transverse direction. A numerical simulation is performed to investigate the behavior of the correction ratio in this flow condition, and the results are presented and discussed.