Numerical Investigation of Effects of Bow Flare Angle on Greenwater Overtopping a Fixed Offshore Vessel

The effect of bow flare angle on greenwater overtopping is numerically investigated using CFD with a VOF surface capturing scheme. A two dimensional fixed rectangular box with a flare angle is adopted to represent a Floating, Production, Storage and Offloading Unit (FPSO) with flare. Deep water conditions are assumed. The incident waves are focused wave groups based on the New Wave formulation, which represents the most probable free surface elevation around a large crest and includes much of the spectral properties of the underlying random sea state. Bow flare angles of 10, 30 and 50 degrees are analysed, which cover the typical range of a FPSO unit. For the box model, the length L, draft D and freeboard f are modelled as L/D = 15 and f/D = 0.25, which are typical dimensions for a FPSO. A range of wave steepness and relative length are considered, which lead to a variety of overtopping conditions including the hammer fist (HF), plunging plus dam break (PDB) and dam break (DB) greenwater events observed for vessels with no bow flare angle. The numerical simulations are performed using the open source CFD toolbox OpenFOAM of version 2.4.0 together with a fully nonlinear wave generation and absorption utility waves2Foam. The effect of the bow flare angle is quantified by analyzing changes in freeboard exceedance, overtopping volume and horizontal momentum flux of water on deck flow for different bow flare angles. The results indicate that, for the special case in which the vessel is fixed, the bow flare angle can significantly reduce each of these quantities. It is also shown that trends in the overtopping volume and momentum flux can be explained reasonably well using the modified dam break model proposed by Zhang et al. [14].