Application of wavelet packet transform in subsea pipeline bedding condition assessment

Subsea pipelines provide a vital transportation service to transport natural oil or gas from offshore wells to an onshore location. Free-spanning damage is one of very common problems both in the design phase and during operation of pipelines. The objective of this paper is to identify free span locations and severities using a vibration-based method. Most vibration-based damage detection methods require the modal properties obtained from measured signals through the system identification techniques. However, for a long subsea pipeline without clear boundary, it may not be able to obtain its modal properties reliably. In this study, a signal-based damage detection method using wavelet packet energy is applied to assess pipeline bedding conditions. A damage index called average wavelet packet energy change rate ((WPECR) over bar) based on the covariance of output signals excited by ambient wave force is proposed to identify free-spanning damage. Numerical simulation is firstly used to verify the feasibility of this method. Then ambient vibration tests are carried out on a scaled pipe model to further verify the applicability of this method. Both simulated and experimental studies demonstrate that the (WPECR) over bar is a good index for assessing pipeline bedding conditions. (C) 2012 Elsevier Ltd. All rights reserved.