This paper, authored by Torgeir Kirkhorn Vada of DNV Software, describes the role of a linear frequency domain analysis for offshore structures, and some of the important aspects that must be addressed when doing such an analysis. The two main challenges in this analysis, in addition to creating a good quality panel model, are:
How to include the viscous effects
How to handle the mooring system
For offshore structures both FLS (fatigue limit state) and ULS (ultimate limit state) analyses are normally carried out.
The main purpose of the hydrodynamic analysis will in many cases be to compute the loads to be applied in a following structural analysis. These loads are typically:
Inertia load due to the acceleration of the vessel
Pressure load on the outer hull and on compartment walls if there is an internal fluid
Line loads on beam elements and loads from risers and mooring lines
In addition, global loads are often computed. Also the motions may be of significant interest on their own, in particular for computing relative motion/air gap.
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