Reliability as a metric for structural health monitoring systems performance: application on fixed jacket offshore structures

This research work addresses the need for reliability performance metrics for Structural Health Monitoring (SMH) systems on a fixed offshore jacket platform. The most common metric used to quantify SHM performance is the probability of detection (POD), it only quantifies the probability of detecting damage of a specific size. While POD is of high importance, it does not fully quantify the total performance of SHM systems. SHM systems can produce outcomes on the detection of damage, its predicted location of damage on the monitored structure and severity of damage detected at the predicted location. These outcomes are of interest for maintenance of the monitored structure. It is paramount to quantify the quality of information attained from all three SHM outcomes. This work proposes the use of Reliability as a metric to quantify outcomes from SHM systems. Furthermore, it also proposes the use of conditional reliability to quantity the interdependence of the predicted damage severity, given damage was localized, and damage was detected. The dependency of the Monitored structured reliability on the reliability of the SHM system was also proposed a metric of overall SHM performance. Probability performance metrics were also proposed in this work to extend the widely used POD, to other outcomes of SHM systems. The Probability of accurate localization and probability of accurate assessment were presented. Additionally, conditional probabilities were also proposed to address the interdependencies between SHM outcomes. Methods used in this work to estimate reliability include the First order reliability method (FORM), and Monte Carlo simulation. Finite element models of Fixed offshore platform were created, the modal response under specific loading conditions, and simulated damage events were used as inputs for Vibration based SHM methods. Hit/Miss techniques were implemented to calculate the probability-based parameters for each SHM outcome. A framework was developed to capture the uncertainties associated with the SHM process, with a limit state function generated for each SHM outcome. Fatigue reliability was implemented using Paris Law to estimate the reliability of structure members in the Fixed offshore platform. Bayes method was implemented to calculate conditional reliabilities. The outcomes from the proposed approach presented in this thesis, present a pathway to quantify SHM system performance beyond damage assessment. iii Presented in this work is a comparison between two SHM systems, where their performance outcomes are quantified for damage detection, localization, and severity assessment. The contribution of the overall SHM systems reliability on the predicted monitored structure members predicted fatigue reliability was presented. Furthermore, a comparison between different vibration based SHM systems at all levels of SHM performance was also presented. In addition, the fusion of multiple SHM systems and their potential contribution to the improvement in predictions of the monitored structure’s reliability was also described. This methodology can stand as a generic approach that can be applied to non-vibration based SHM systems or structures that are not fixed offshore platforms.