Vortex Induced Vibrations Of Slender Marine Structures: Inverse Analysis

Abstract

With the development of the offshore engineering and the increasing of water depth, VIV (vortex induced vibrations) becomes a big challenge of the design of slender marine structures. In engineering field, we use some empirical VIV prediction codes like VIVAVA and Shear 7 to predict VIV. A key issue for using the codes is to establish a data base for hydrodynamic coefficients. Such coefficients have so far been found from experiments. An alternative way to get the force coefficients is to measure the response in flexible beams in current, and get the local force using some mathematical calculation method, which is called inverse analysis. In this project, we analyzed 44 NDP riser model tests. First we apply inverse analysis method to estimate the excitation force coefficients for one test and make comparison with the previous results from the rigid pipe model test. And we can find out the inverse analysis method is quite an efficient way to calculate the force coefficients. Second we compare the results with the existing models in VIVAVA for excitation and damping coefficients. We can see that the model in VIVAVA can give more reasonable results of the damping coefficients compared with inverse analysis when the non-dimensional frequency is outside the excitation range.Third we calculate the fatigue damage from the varying frequency components in order to find the contribution from the primary cross flow frequency and higher order frequency. We find that the fatigue generated by higher order frequency components is as important as that from the primary cross flow frequency component and cannot be neglected. We can introduce a parameter which can be used to find out the total fatigue damage from the primary frequency fatigue damage.