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.


FPSO vessels are designed to operate continuously without interruption for about 20 to 25 years. The turret mooring allows the vessel to freely weathervane and so to show the least resistance to the combined forces of wind, waves, and current. This minimizes the exposure to beam seas. But for various reasons the preferred FPSO mooring might be a spread moored system. In that case, the probability of beam seas is defined by the selected vessel compass heading in combination with the distribution of wave environments at site location.

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