Computations and measurements of the global drag force on a Tension-Leg Platform

Abstract

We present experimental and computational results for the global drag force on a model Tension-Leg Platform (TLP) in uniform current for various values of the Reynolds number in the subcritical regime (Re = 104 - 1.06×105). The objective of the experiments, which were performed in a towing tank on a representative TLP configuration, was to provide data suitable for validating numerical model predictions. The purpose of the simulations was to determine the extent to which Large-Eddy Simulations (LES), when used as a truly predictive tool, can be relied upon to provide reliable estimates of the total drag force. Thus the simulations were performed only once with no further computations performed in order to bring about closer agreement with the measurements. The dependence of the computed results on the numerical grid was checked using the Grid-Convergence Index (GCI) method applied to results from benchmark flows. These results also served to assess the dependence on the value of the Smagorinsky constant in the model for the sub-grid scale terms. It was found that, at the highest value of Reynolds number considered, the variation between the measured and predicted values of the global drag coefficient was under 10% - a result which is in line with the limitations inherent in both.