Identification of Ringing Events for a Slender Tubular Marine Structure

Abstract

Marine structures can experience loads coming from steep, non-linear waves. A transient response of the structure can be amplified in some circumstances due to phenomenon known as “ringing”. Exact conditions of the ringing response excitation are not well known and various definitions of what constitutes a ringing event appear in bibliography. This article aims at identification of ringing events for a slender marine structure subjected to the second order nonlinear irregular waves. Loads on the structure are calculated from Morison equation with extensions known as Rainey’s slender body theory. Consistent second order formulation for the flow kinematics above mean water level is employed. The excitation load is non-linear due to wave non-linearity, Morison equation’s drag term and non-linear inertia terms which can be found in Rainey’s formulation. The structure’s dynamic response is obtained by Finite Element calculations in time domain. Overall randomly-excited vibrations are analysed by statistical and spectral methods, and application of Continuous Wavelet Transform (CWT) is presented. The simulations show a number of sudden, high amplitude resonant events which can be classified as ringing.