Experimental and numerical investigation of a moored floating structure in waves

Floating structures play a major role in a wide range of traditional industrial branches as well as in recently developing sectors such as renewable energies. An accurate pre- diction of the movement of floating structures induced by varying environmental loads is essential for their design. Numerical models nowadays play a key role in the cal- culation of processes in hydraulic and marine engineering, due to their advantage in cost and time efficiency. Nonetheless, the traditional approach of physical model experiments has not lost its importance since the validity of numerical tools has to be ensured. REEF3D is an open-source numerical CFD solver developed at the Depart- ment of Civil and Environmental Engineering at the NTNU. It solves the Reynolds averaged Navier-Stokes equations in two phases using a finite differences method and a level set method to capture the free surface between water and air. Floating struc- tures are represented as a level set function, too. The motion of the function is based on the rigid body dynamics in 6DOF. A directional immersed boundary method is used to implicitly incorporate appropriate boundary conditions at the fluid-structure interface. A set of benchmark data is to be generated at the physical wave flume of the Ludwig-Franzius institute in Hanover, Germany. A simple cuboid wooden barge of constant density is examined. The motion of the barge in various waves, ranging from 2cm to 4cm in wave height and from 0.8s to 2.4s in wave period is tracked. The configuration of the set-up varies from a soft-mooring-system, ensuring free motion of the structure, to a traditional mooring system. The system includes springs of differ- ent material parameters representing the elasticity of the mooring system in a slack and a taut configuration and catenary mooring comprised of a small metal chain. The movement is quantified using an OptiTrack Motion-Tracking System composed of four cameras referencing three infra-red markers positioned on the test object. On basis of these experimental results the influence of the mooring system on the motion of the moored-floating barge is evaluated. The experimental data shall be compared to numerical data. For this purpose, the experimental set-up is reproduced in the numerical wave tank of REEF3D. A focus of the study is the investigation of the different mooring models which are available in REEF3D: an analytical approach not considering hydrodynamic forces and a quasi-static model solving the force equilib- rium on a discretized line taking into account drag forces.