A methodology for CFD simulations of floating structures in waves coupled with mooring dynamics

Prediction of the responses in wave fields of moored floating structures, such as floating offshore wind turbines (FOWT), wave energy converters (WEC), aquaculture farms (AF), or floating breakwaters (FB), requires high-fidelity yet efficient coupled analysis between Computational Fluid Dynamics (CFD) and mooring dynamic models. Among the coupled models available to users for the open-source CFD software OpenFOAM and mooring libraries like MoorDyn, we can find foamMooring and FloatStepper. These address issues such as added mass instability, and offer different computational methods for free-surface capturing and rigid body motion. If we also consider the various dynamic mesh techniques, users have multiple options for the general configuration of a simulation. Bearing this in mind, the present study proposes an efficient and accurate methodology for simulations with OpenFOAM coupled with the lumped-mass mooring model Moordyn for a simple floating structure, with the intention of applying it to more complex structures, or even performing survivability studies. Pursuing this goal, different dynamic mesh techniques, free-surface capturing methods and the available algorithms for rigid body motions including an added-mass correction algorithm, are investigated systematically and validated against experimental data. In the course of this work, new features have been added to the lastest version of Moordyn and also coupled with the non-iterative algorithm FloatStepper. The surface elevation, floating body motions and mooring line tensions were considered to compare the different settings. The results showed that all the tested options were accurate, but the overset technique was the most efficient in terms of computational time.