Forces on bilge keels in regular and irregular oscillating flow

This paper presents a validated methodology to calculate the oscillatory loads on bilge keels of ships operating at zero forward speed in irregular sea states. To calculate these loads, the local relative fluid velocities acting normal to the bilge keel are combined with a KC dependent drag coefficient. The local relative velocity to the bilge keel is obtained from 3D potential flow calculation which implies that the hull geometry and bilge keel location are incorporated and that all wave velocities (from radiation, diffraction and the incident wave) are accounted for. The KC dependent drag coefficient of the bilge keel geometry is calculated by 2D CFD simulations in harmonic flow oscillations utilizing a rectangular fluid domain. This method is verified to experimental data from the literature. With the present approach it is possible to quantify the ultimate load on the bilge keel in design extreme conditions and to obtain the long term load distribution necessary for fatigue analysis. It respects the vessel heading and sea state parameters. Model tests for several FPSO vessels have been used to validate and calibrate the methodology. The calculation method is currently further evaluated and applied for structural design analysis on bilge keels by SBM Offshore.

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