Hydrodynamic Forces on Flexible Offshore Structures

Abstract

The extension of Morison's equation to allow for structural motion in predicting the hydrodynamic force on offshore steel jacket platforms may be based on two different hypotheses: (1) The relative velocity model which replaces the fluid velocity by the relative velocity between the fluid and the structure; and (2) the independent flow fields model which considers the flow to be a superposition of two unrelated flows, one due to the wave‐current action on a rigid cylinder, and the other due to the structural motion in still water. An iterative computational procedure that combines time domain and frequency domain analysis techniques is developed to solve the nonlinear governing equations for both models. Comparison studies are carried out for seastates ranging from the drag dominant through the inertia dominant regimes. Results indicate that the independent flow fields model always predicts a higher structural velocity response, the difference increasing with wave height. There is negligible difference for the inertia dominant range. At intermediate seastates, which are of primary concern for fatigue analysis, the relative velocity model appears to overestimate the damping. Consequently, its use in fatigue life prediction may be questioned. For typical offshore platforms, the applicability of the independent flow fields model diminishes as the seastate approaches extreme values.