Subsea Free Span Pipeline Damage Detection Based on Wavelet Transform under Environmental Load

During their service life, marine pipelines continually accumulate damage as a result of the action of various environmental forces. Clearly, the development of robust techniques for early damage detection is very important to avoid the possible occurrence of a disastrous structural failure. Most of vibration-based damage detection methods require the modal properties that are obtained from measured signals through the system identification techniques. However, the modal properties, such as natural frequencies and mode shapes are not adequate sensitive indicators of structural damage. In this paper, structural damage was identified based on a new method called Detail Signal Energy Rate Index (DSERI). Damage localization of offshore pipeline was then based on Continuous Wavelet Transform (CWT) and Discrete Wavelet Transform (DWT). For modeling the damage, the stiffness of 1 and 2 elements were reduced in three models and the dynamic signals were measured by finite element analysis. The dynamic signals were analyzed by applying discrete and continuous wavelet transforms. Then, by using Daubechies wavelet, a peak was clearly evident and the exact damage location was identified by the peak with the maximum absolute value in the plots of the detail signal calculated with the DWT and CWT. To eliminate the effects of the boundary conditions on the wavelet-based damage, the authors proposed a new method: DSERI. Component energies were then calculated and used for damage assessment. The results showed that the DSERI were good candidate indices sensitive to structural damage. These component energies could be used for damage assessment including identifying damage occurrence and location of damage. Wavelet coefficient was a good candidate for structural damage identification when damage occured at the middle of pipeline but not sensitive for damaged elements near to supports.