Session: 08-06-01 non-presentations

Paper Number: 125615

125615 - Comparison of Shallow-Water Side-by-Side Motion Prediction by Application of Cfd, Model Tests and Diffraction Analysis 

Motion assessment of complex floating side-by-side configurations is typically performed in model tests or in diffraction analysis. However, both methods have their disadvantages. In model tests  the presence of basin walls and blockage effects may have an influence on the motions response while in diffraction analysis viscous effects, nonlinear effects from shallow water waves and free-surface sloshing in the gap between the vessels are  taken into account in a simplified manner. With the growth of computational power, Computational Fluid Dynamics (CFD) is becoming more accessible for motion analysis as well. This method has the potential to remove or provide insight in the influence of the disadvantages that the other methods have, provided that the CFD results are shown to give reliable predictions.

The objective of this paper is to investigate and compare the motion response for a side-by-side FLNG-FSU configuration from the three approaches mentioned above, i.e. model tests, diffraction analysis and CFD simulations. For the diffraction analysis, MARINs code DIFFRAC was employed and for the CFD simulations MARINs CFD code ReFRESCO was uitilized. The side-by-side FLNG-FSU configuration is moored in shallow water (h/gT²<0.01) and encounters steep waves (H/ gT²≈0.004). The CFD simulations were performed on model scale for regular waves, with and without the presence of basin walls, in captive conditions and moored floating conditions. The mooring was modeled  by a quasi-static catenary line model in MARINs time-domain code aNySIM, which is coupled to the CFD code.

The CFD results with basin walls present show an agreement within 20% in motion response compared to the model tests. The CFD simulations without basin walls present gave a roll RAO that is twice as low compared to the CFD results with basin walls present. This indicates that due to the presence of the basin walls, the roll motion RAO prediction in the model tests may be too high. Waves are radiated and diffracted outward from the side-by-side configuration which are then reflected back by the basin side walls and affecting the motion response. The diffraction analysis was expected to poorly predict the motion response due to the steep nonlinear waves and the lack of viscous effects in the modelling. However, the results agree fairly well with the CFD simulations without basin walls present.

Presenting Author: Maarten van der Leij MARIN

Presenting Author Biography: Maarten van der Leij is Project Manager at MARIN (Maritime Research Institute Netherlands). He graduated from Martime Technology in the track of Hydrodynamics from Delft University of Technology in 2019 and joined MARIN shortly after. In the role of Project Manager, he mostly researches the wave-frequent and low-frequent motion behaviour of offshore constructions by means of frequency domain, time domain and/or CFD analysis and by means of physical model tests.

Authors:

Maarten van der Leij MARIN
Arjen Koop MARIN
Frederick Jaouen MARIN