Wave Actions and Responses for Large-Diameter Monopod Platform Structures

Abstract

The sea-wave loads acting on the fixed offshore structures are estimated by using Airy's linear wave theory and Morison's equation, dissociating the total force into an inertia force component and a drag force component. The contribution of each component of the total force on tubular members can vary significantly based on size specification, from standard pipe members of fixed jacket structures to wide-ranging cylindrical Monopod towers. Inconclusive results can be seen in some published articles in estimating static wave loads using the hydrodynamic module of offshore platforms, indicating that this is still a subject of investigation. A demonstration of an example steel Monopod under Airy's type wave loading is presented. Several finite element offshore structure simulation packages use this simple monopod model for computationally efficient static wave load case simulations. The displacement pattern and the base shear force and bending moment of the Monopod model are calculated. The analytical solution is checked with numerical results of standard commercial FE software packages for verification and comparison purposes. The results show that the wave load calculation module of the finite element-based design programs considered in this study is underestimated, mainly when the contribution of the inertia coefficient to total instantaneous wave force is dominant, like in the monopod case with a large diameter. It can be thought that the differences here are due to the inertia coefficient weighting of the Morrison equation used in wave force calculations.