The long term motivation for this work has been to find hydrodynamic coefficients needed to predict vortex induced vibrations (VIV) for free spanning pipelines. For such cases one must expect that the proximity of the seafloor will influence hydrodynamic forces, which means that coefficients found from experiments where wall effects have been eliminated are not necessarily valid. The paper presents results from a three dimensional large eddy simulation of flow over a forced oscillating cylinder close to a plane wall. Hydrodynamic forces are computed at a frequency of 0.425 Hz and Reynolds number of 24 000, and the motion amplitude is half diameter of the cylinder. The minimum gap distance between the cylinder and the wall has been varied from 0.2 to 2.5 D, where D is the diameter of the cylinder. The results show that the hydrodynamic forces on the cylinder are significantly influenced by the gap ratio. The excitation force shows a slight increase when the average gap is decreased from 2.5 to 0.5 D, but further reduction changes the trend so that the coefficient becomes negative for a minimum gap of 0.25 D. Hence, the dynamic lift force will change from being an excitation force to a damping force at this distance for the given flow and motion parameters. A sharp drop in drag force is observed at gap ratio less than 0.3 as the cylinder is still out of the boundary layer on the plane wall. The average lift force is also seen to be influenced by the gap. At a large distance from the wall, the average lift is obviously zero, but an average force that will push the cylinder away from the wall is found for gaps smaller than 0.3 D. Results are also reported for the cross-flow added mass as a function of varying gap. In addition to these results, spectra and time histories for the hydrodynamic forces are presented, and also coefficients for higher order force components. Vortex shedding is observed at the minimum gap ratio of 0.2. Some references to experimental work are made, but verification of these results cannot be obtained without new experiments.

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