Galloping may take place for non-circular cross sections, such as an ice-coated electric power line or a riser bundle, under current action. This type of instabilities occurs at lower frequency than Vortex Induced Vibrations but with unbounded amplitude, and might be detrimental for riser integrity.
In a recent joint industry project, the CITEPH “Gallopan” project, galloping instabilities were investigated for two types of cylinders: an academic square cylinder and a generic riser tower cross section. Model tests and numerical computations were performed to assess the propensity of both cylinders to gallop.
Experiments on the square cylinder are reported here. Three types of tests were performed in steady flow: loads measurement on fixed cylinder, at various headings; loads measurement on the cylinder with over imposed cross-flow harmonic oscillations; free transverse oscillations. By using analytical calculations, the ability to predict galloping instability occurrence and amplitude, of each of the three above methods, was compared.
Compared to typical results found in literature, these experiments were conducted at a larger scale, and thus with Reynolds number closer to on-site values, i.e. over 105.