Numerical Modeling of Water and Mooring Line Interaction for a Floating Barge at Lake Kivu using Immersed Boundary Method

Abstract

Mooringsystemsaremechanismsusedtostation-keepfloatingplatformsusingmoor ing lines and anchors. The stability performance of a mooring system is estimated by the moored structure’s motion, mooring line tension and anchor’s holding ca pacity. Modeling of the dynamic effects of mooring system is a challenging task because of the complexities encountered if inertial, torsional, elastic, bending and frictional effects of mooring lines are to be considered. Numerical models that have been developed to analyze the stability of mooring systems include lumped mass, finite element, finite volume and finite difference methods. However, some mooring systems are still failing due to inaccurate estimation of hydrodynamic forces acting on the mooring lines, poor mooring line tension, and anchor’s holding capacity con trol. The failure of mooring system has also been attributed to the failure to capture flow induced vibration due to vortex shedding and wake evolution in the vicinity of mooring lines after fluid flow on it during the design. This research focused on numerical modeling of water and mooring line interaction using immersed boundary method coupled with finite volume. The drag forces acting on the mooring lines was estimatedandtheeffectofflowfrequencyondragforceswasinvestigated. Therecir culation length and frequency of shedding were quantified when fluid flowed past the mooring line at different Reynolds number. The immersed boundary method acts as a suitable link between the fluid and solid meshes. The effect of damping on drag forces was appropriately captured through the immersed boundary method. Thus the accurate resolution of drag forces improved the overall stability of the floating xvi barge at Lake Kivu. Considering the condition of Lake Kivu waters of 6m/s as wind speed, 0.05m as amplitude, 0.4Hz as flow frequency of oscillation and a mooring line of 0.05m as diameter, the estimated mean average drag force acting on a mooring line was found to be 6.3N per unit length. By increasing flow frequency from 0.4, 0.8, 1, 10, 60 and 100Hz, it was found that the drag force fluctuation amplitude increased by 67% for flow frequency less than 10. Nevertheless, the mean average drag force was not affected. The drag force increased by almost 30% and by 1.7% for fluctuation amplitude of the drag force but drag coefficient decreased by 0.18% with Reynolds number increase. This study also found that at low Reynolds number (less than 40) the flow pattern behind the mooring line remains symmetric without shedding. Fluid flow at Reynolds number of 100 and 185 presented a shedding of vortex with a frequency equal to the lift force fluctuation frequency. The maximum recirculation length measured was 2.4m behind the mooring line at a Reynolds number of 40. The maximum Strouhal frequency obtained is 0.2 at a Reynolds number of 185.