In jack-up structure and foundation designs, the accurate prediction of drag and wave forces is important but difficult. For the assessment of hydrodynamic forces SNAME T&R Bulletin 5-5A provide a useful empirical approach. This approach was developed largely based on wind tunnel measurement data in 90s of last century. However, in some cases reported, drag forces calculated by the SNAME recommended practice differ appreciably from those obtained from wind tunnel tests. To investigate this puzzle, it is worth having a comprehensive comparative study among SANME practices, CFD (Computational Fluid Dynamic) and wind tunnel measurement. In this paper, a typical jack up leg configuration is selected. The hydrodynamic forces on its leg due to solely current, solely waves and combined wave and current are calculated using the aforementioned SNAME, CFD and wind tunnel test approaches. Through the comparison of the calculated drag and wave forces from different approaches, this paper tries to confirm the conservativeness adhered in the SNAME approach compared to the CFD and model test results.
Historically, hydrodynamic loads on jack-up legs have been estimated based solely on wind tunnel measurements. While recognizing that wind tunnel tests can only account for steady current situations and not wave oscillatory flow situations, wave tank tests should have been conducted to assess their relative magnitude. Unfortunately, such studies do not seem to have been conducted thoroughly thus far, nor do they seem to have been very well reported in published literature for reasonably large KC (Keulegan-Carpenter) number. Perhaps this situation is due to high costs and the model scale restriction. With the advent of CFD (Computation Fluid Dynamics) simulation techniques, it has become possible, at a relatively modest cost, to revisit this issue with a view to understand the magnitude of wave loads on jack-up legs due to large wave oscillatory flow.
