Evaluation of Hydrodynamic Loads on Offshore Truss Structure Using Large Eddy Simulation Method

The hydrodynamic loads on structures built in the coastal and offshore regions are conventionally evaluated by semi-empirical equations such as Morison Equation based on the assumed current speed, its direction and the wave information. Simulation of the motion of water around the structure can give more realistic estimates of the dynamic loads and their distributions including the changes in time if the unsteadiness due to waves and turbulence are important.

A Large-eddy Simulation (LES) has been conducted of the flow around typical oil-production jacket platform in a sea of relatively shallow water depth. The LES solves for a three-dimensional flow with moving free surface using appropriate sub-grid turbulence and wall models. The structure consists of various slender tubular members in truss configuration. It is assumed rigid and the pressure distribution obtained by the LES is integrated over individual members and over the entire structure to evaluate the loads. The flow approaching the jacket platform is assumed known but is possible to be obtained from the larger-scale simulation reflecting the details of the bathymetry of the surrounding sea bed and the metocean conditions. The sea bed is assumed a rough surface with known roughness height.

The simulation results are examined in detail. They indicate that the loads on upstream members are larger but more steady than those on the downstream members due to flow separation and vortex shedding off individual members. Comparisons with the Morison formula are made and will be shown in the full paper.