Novel approaches towards mooring integrity in-situ monitoring fatigue crack growth evaluation using acoustic emission parametrical analysis

Abstract

Mooring systems are an integral and complex component of offshore assets. Mooring chains for floating offshore installations, typically designed for a 25 year service life, are subject to fatigue in a seawater environment. Structural components are subjected to harsh conditions and cyclic loading and can present a significant risk to asset integrity and personnel safety. Moreover, the life of old and new structures has to be extended, and the maintenance strategies improved, calling for a more proactive management approach. Structural Health Monitoring (SHM) is a crucial part of asset integrity management and should be enforced in conjunction with traditional inspection programmes. The main objective of SHM is to continuously verify the condition of the mooring system's performance and provide input for the assessment of mooring integrity, complementing periodical in-service inspections. The early detection and monitoring of fatigue crack growth in mooring chains remain a crucial challenge. Current technologies cannot provide information on fatigue cracking, ultimately leading to unexpected structural failure. Thus, its detection and characterisation open the way towards a new, reliable monitoring approach that can be used as an early warning of crack propagation. The present research evaluates the suitability of an SHM technology, Acoustic Emission (AE), to detect fatigue crack growth in mooring chains. Numerical modelling was used to provide an overview of the expected AE wave propagation elementary characteristics. Experimental work was performed to investigate the applicability and the accuracy of AE to monitor and predict fatigue related cracking in mooring systems. The outcome of this investigation results in the definition of a crack sizing method, a data filtering criterion and the establishment of a risk alarm system for the prediction and assessment of fatigue crack growth in mooring chains.