An Empirical Procedure for Fatigue Damage Estimation in Instrumented Risers

Vortex-induced vibration (VIV) can lead to significant fatigue damage accumulation in deepwater marine risers. In order to assess the effects of VIV and to ensure riser integrity, field monitoring campaigns are often conducted wherein riser response is recorded by a few data sensors distributed along the length of the riser. It is possible to empirically estimate the fatigue damage at “key” critical fatigue-sensitive locations, where sensors may not be available as part of the spatially distributed discrete measurements. In this study, two empirical techniques — Proper Orthogonal Decomposition (POD) and Weighted Waveform Analysis (WWA) — are sequentially applied to the data; together, they offer a novel empirical procedure for fatigue damage estimation in an instrumented riser. The procedures are briefly described as follows: first, POD is used to extract the most energetic spatial modes of the riser response from the measurements. Often, only a few dominant POD modes preserve most of the riser motion kinetic energy; other modes are less important. Identified POD mode shapes are discrete as they are defined only at the available sensor locations. Accordingly, a second step in the proposed procedure uses WWA to express each dominant POD mode as a series of riser natural modes that are continuous spatial functions defined over the entire riser length. Based on the above empirically identified modal information, the riser response over the entire length is reconstructed using backward procedures — i.e., compose identified natural modes into the POD modes and, then, assemble all these dominant POD modal response components into the derived riser response. The POD procedure empirically extracts the energetic dynamic response characteristics without any assumptions and effectively cleans the data of noisy or less important features, which makes it possible for WWA to identify dominant riser natural modes — all this is possible using the limited number of available measurements from sensor locations. Application of the entire procedure is demonstrated using experimental data from the Norwegian Deepwater Programme (NDP) model riser.