Estimation of Hydrodynamic Coefficients by CFD for Application to Subsea Protection Covers

Abstract

The background for this thesis is the limiting weather window experienced when deploying subsea structures. By following today's industry standard the allowable sea states for which structure deployment can take place is strict and conservative. One of the reasons is poor estimates of hydrodynamic properties. Better estimates can be obtained by model tests, but they are typically too expensive to be economically feasible. Another option is analyses with computational fluid dynamics. This approach has been investigated in the present thesis, applied to a much used subsea component, namely a subsea protection cover.

In order to understand how the hydrodynamic properties are estimated and quantified, the forces on an object oscillating in still fluid is described. The important parameters affecting the coefficients was discussed, and related to subsea protection covers. The most important parameter was recognized as the Keulegan-Carpenter (KC) number. The interesting KC range with respect to subsea deployment was determined to be $[0,6]$ where the most interesting part is the lower half of the interval.

In order to have data for validation of the numerical simulations, model test was performed for $KC = [0,6]$. The experiments was repeated in three different positions in order to investigate the free surface effect. The experiments yielded good results, except for the smallest KC numbers where errors in the force measurements was noticed.

The numerical simulations performed in the thesis, have been done in OpenFOAM. Since the cover has to move during the simulations, a dynamic mesh model based on a Laplace equation was used. Two different solvers have been set-up for cases with and without a free surface, where the free surface was modeled by potential theory. All the simulations were run with the $k-\omega$ SST turbulence model as the flow was highly turbulent.

Five different cases have been run in total. One validation test case, and four cases simulation the subsea protection cover. The validation test case results was in the upper range compared to other experimental results, but within acceptable limits.

For the cover simulations, one simulated the cover oscillating in an infinite domain, while the others replicated the the experiments.

All the results agreed relatively well with the experimental results for $1.5