ABSTRACT

An experimental investigation of the Morison force coefficients and the effect of structural damping on the forcing of dynamically responding vertical cylinders has been conducted in order to evaluate the suitability of existing mathematical models. It was observed that useful values of the Morison force coefficients CM and CD could be obtained from measured cylinder restraint forces by both a constant-valued least squares method and a nonlinear systems analysis approach with little practical differences. Force coefficients C''M and C''D were obtained experimentally from pluck tests and the values of C''M were found to agree closely with classical predictions and those from radiation damping considerations, whereas C''D appeared to be greatly dependent upon the viscosity-frequency parameter b. It was found that the selection of CD is critical to the prediction of hydrodynamic damping, and that for cylinders operating in the inertia dominant range (KC < 5) the drag dependent hydrodynamic damping mechanism is significant despite the relatively minor role drag force would have in the actual hydrodynamic forcing process itself.

INTRODUCTION

An offshore structure is often found to be primarily excited in its fundamental mode of vibration by the prevalent hydrodynamic loading. Consequently it is observed to respond principally in the alongwave direction so that a simplified modelling of its in-line response x0(t) at the mean water level (MWL) can be expressed, where ω0 is the structure's natural circular frequency, ζs is the critical damping ratio for the structure, m0 is the effective mass, ma is the added mass; and F''0(t) is a version of the equivalent hydrodynamic forcing at the MWL. This paper will discuss a damping model for such a structure, which includes radiation, hydrodynamic and structural damping and shows the relationship between the hydrodynamic and structural contributions. The model was tested with an experimental programme that included pluck tests in air and still water.

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