Risers are known to generate high amplitude tonal sound (singing) due to inner corrugations. In the prediction of the singing characteristics both the source strength and the damping must be known. A complicating factor is that in real risers, the gap width varies throughout the riser due to stretching and compression during manufacturing, transport, installation and operations. Therefore, the effect of the gap width on the damping and source strength is needed.

For a set of geometries, the scattering matrix was measured via a passive damping method. This is done for two sets of geometries at two diameters (30 and 49mm) in which the gap width was varied (all cavities the same width or the width varied along the length). In this paper, the results of the 49mm will be presented. The measurements were done at different velocities (up to 60 m/s) and at 3 different pressures (1, 4, 6 bara). In this way th ebackground zero flow damping was measured as well as evaluating the influence of the gap and the Reynolds number on the source strength. Based on the experiments, a proportionality of the source with teh Reynolds number was found as Source ~ Re^0.25. With respect ro the gap width a scaling of gap^1.55 is found.