Underwater acoustic emission monitoring – Experimental investigations and acoustic signature recognition of synthetic mooring ropes

Abstract

Mooring ropes are essential components of offshore installations, and synthetic ropes are increasingly preferred because of their favourable cost to weight ratios. In-service condition of these materials is traditionally monitored through costly visual inspection, which adds to the operating costs of these structures. Acoustic Emissions (AE) are widely used for condition-monitoring in air, and show great potential underwater. This paper investigates the AE signatures of synthetic mooring ropes subjected to sinusoidal tension-tension loading in a controlled environment, using a large-scale dynamic tensile test rig. With a linear array of 3 broadband (20Hz to 50kHz) hydrophones, four main signatures are identified: low-to high frequency, low-amplitude signals (50Hz to 10kHz), low-amplitude broadband signals (10kHz to 20kHz), high amplitude signals (10Hz to 48kHz) and medium-amplitude signals (500Hz to 48kHz). These AE types are related to different stages of rope behaviour, from bedding-in to degradation and failure. The main findings are that the failure location and breaking load can be identified through the detection of AE. The occurrence of high amplitude AE bursts in relation to the applied tensile load allows the detection of an imminent failure, i.e. prior to the failure event. These initial results indicate that AE analyses can enable the integrity of synthetic mooring ropes to be monitored.