Acoustic emission technique to monitor crack growth in a mooring chain

Mooring chains are designed for an operational life of 25 years. However, owner/operator data show that a Floating Production Unit (FPU) experiences a mooring line failure every 9 years on average, and periodic inspections are necessary to monitor their integrity. An established condition monitoring technique based on Acoustic Emission (AE) has the potential to be deployed to mooring chains. AE is based on the principle that elastic energy waves are spontaneously released by a material undergoing plastic deformation and crack growth. The purpose of this paper is to present a feasibility study of AE testing to monitor initiation and propagation of stress corrosion cracks in a mooring chain link, in order to establish whether crack initiation and crack extension phases can be detected and discriminated in the course of a static tensile test. The tensile test was carried out in artificial seawater (3% NaCl solution) using a single grade R5 link of diameter 160 mm for 3.7 months where a crack was initiated from a notch. AE features were plotted against time and these plots were extensively discussed with the view of contributing to the closure of a knowledge gap in the experimental literature.