On semisubmersible production platforms often a number of Riser Guide Tubes (RGT) are used to protect the risers through the wave splash zone from the floater pontoon and up to a balcony at deck level. Then the question comes up as to whether Vortex Induced Vibrations (VIV) of the RGT’s may be a problem. The present paper reports on part of the effort to investigate that matter. Here a computer programme was developed to analyse the situation and since model tests were made to clarify the behaviour of the RGT’s, also data were available against which the computational predictions could be compared. Unfortunately verification of the computational model turned out to be problematic. Most important it turned out that water entry forces dominated the force picture, and since flow in the region between the platform legs was quite complicated it was hard to know whether vibrations were due to VIV or water entry. VIV response is strongly dependent on the time histories of the relative particle velocity, which were not measured in the experiment, and could not be accurately estimated due to pronounced flow nonlinearities, refraction effects and platform motions. Also the experimental database on which the computer code relied for the force coefficients appeared to be too summaric at low VIV amplitudes for our purpose. This is understandable, since for VIV in steady currents only large motions have been of importance.

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