Experimental investigation on vortex-induced force of a Steel Catenary Riser under in-plane vessel motion

A method to identify vortex-induced forces and coefficients from measured strains of a Steel Catenary Riser (SCR) undergoing vessel motion-induced Vortex-induced Vibration (VIV) is proposed. Euler-Bernoulli beam vibration equations with time-varying tension is adopted to describe the out-of-plane VIV responses. Vortex-induced forces are reconstructed via inverse analysis method, and the Forgetting Factor Least Squares (FF-LS) method is employed to identify timevarying vortex-induced force coefficients, including excitation coefficients and added mass coefficients. The method is verified via a finite element analysis procedure in commercial software Orcaflex. The time-varying excitation coefficients and added mass coefficients of an SCR undergoing vessel motion-induced VIV are investigated. Results show that vessel motion-induced VIV is excited at the middle or lower part of the SCR and in the acceleration period of in-plane velocity, where most of the excitation coefficients are positive, while during the deceleration period, the excitation coefficients becomes too small to excite VIVs. Parameter gamma [1] has strong correlation with excitation coefficients. In addition, time-varying tensions contribute significantly to the variations of added mass coefficients under the condition that the ratio of dynamic top tension to pretension exceeds the range of 0.7-1.3. Moreover, chaotic behaviors are observed in vortexinduced force coefficients and are more evident with the increase of vessel motion velocity. This behavior may attribute to the randomness existing in in-plane velocity and its coupling with out-of-plane vibrations.

Automated summary

The method proposed can accurately identify vortex-induced forces and coefficients from measured strains of a Steel Catenary Riser (SCR) undergoing vessel motion-induced Vortex-induced Vibration (VIV). The time-varying excitation coefficients and added mass coefficients of an SCR undergoing VIV were investigated, and significant correlations were found between certain parameters and excitation coefficients. Chaotic behaviors in vortex-induced force coefficients were observed, especially with increasing vessel motion velocity, suggesting randomness in in-plane velocity coupling with out-of-plane vibrations.