Mooring systems with submerged buoys: influence of buoy geometry and modelling fidelity

Abstract

Mooring systems often make use of submerged buoys (SBs) in order to make the moorings compliant. In this paper we present the dynamic effects of changing the buoy geometry or the buoy model fidelity on the mooring system response. Three cylindrical SBs with increasing slenderness (height/diameter) are studied for a mooring leg with two polyester ropes and a SB. The results show a large impact of SB geometry on the mooring dynamics. A larger height/diameter ratio (with preserved mass and buoyancy) is shown to be beneficial as it gives both smaller tension force magnitudes and, more importantly, avoids slack-snap occurrence in the upper cable. We further present a comparison between four numerical methods for SB dynamics: (i) a high-fidelity model using computational fluid dynamics (CFD); (ii) the Morison equation with slender body drag force approximation using numerical quadrature; (iii) the Morison equation with an independent evaluation of the fluid drag due to translation and rotation; and (iv) a translating Morison model which simulates a vertical cylinder in three degrees of freedom with no rotation. All methods are used together with a high-order finite element mooring dynamics solver. The results show that the translating method is inadequate to model this mooring configuration. The remaining three methods agree moderately well, but the Morison formulations give larger motions and higher tensions compared to the CFD results. We show that the quadrature drag model is better suited to model the drag moment on SBs than the independent model, and that the improvement increases with increasing slenderness of the buoy. The uncertainty, sensitivity and importance of the hydrodynamic coefficients of the buoy are discussed and examined by a regression analysis from the CFD data.