Vortex-induced Vibration of a Flexible Free-hanging Circular Cantilever

The behavior of a free-hanging riser of floating offshore structures would be different from a typical at-sea-floor-terminated riser type of oil or gas platforms. For the design purpose, the present study was intended to incorporate some important factors of the riser conditions (i.e. bidirectional vibration, free-end condition, and spanwise variation of response amplitude) for investigating its dynamics characteristics. An experimental investigation on time-dependent motion of a flexible free-hanging circular cantilever subjected to uniform cross-flows has been carried out. The free-end condition cantilever has a 34.4 aspect ratio and a low mass ratio of about 1.24. The cylinder freely oscillates in both inline and transverse to the flow. Reynolds number varied from 10,800 to 37,800. The "jump phenomenon" was found in the inline motion of the cylinder that agrees well with an existing comparable work. At high flow velocities, the 3rd higher harmonic frequencies of the cylinder transverse response became predominant that produce quite different motion characteristics compared to the other existing comparable works with 2-dimensional bottom-end condition. Generally, the results suggested that the flexible free-hanging cantilever generate different vortex wake mode than either, a uniform (a short-rigid flexibly-mounted cylinder) or a linear amplitude variation along the span case (a pivoted cylinder).