A Variable-Tensioned Transfer Matrix Method for Modal Analysis of Marine Risers and Its Application to Structural Damage Detection

Abstract

Structural damage caused by the vortex-induced vibration (VIV), harsh marine environment, and ultimate loads, etc., often alters marine risers’ natural frequencies and mode shapes. A simple method for calculating variable-tensioned marine risers, especially for the risers with single and multiple damages, is proposed herein to explore the relationships between modal characteristics and structural damage. The approach’ main advantage is that it is suitable for marine risers with various boundary conditions, variable tension, and different damage locations and degrees. Compared with the literature, the natural frequencies and mode shapes are re-examined using the intact marine risers. With the variation of damage degree and location along water depth, the effects of damage on natural frequency and the second derivative of mode curvature (SMC) are studied. To solve frequency and SMC’s insensitivity problem to the node damage, a frequency-square-weighted superposition of the SMC index, i.e., SSMC, is developed. The relationships of SSMC with the single damage and the multiple damages are accurately modeled, and the damaged severities are precisely predicted by fitting a cubic polynomial and a quintic-binary function, respectively. In total, the proposed methods and the numerical results may be useful references to the damage detection of marine risers.