Health Monitoring of Submerged Plates Using Ultrasonic Guided Waves

Abstract

Several offshore and marine infrastructural systems involve plate like members in submerged condition and are often subjected to extreme service and environmental conditions leading to deterioration in the form of corrosion loss, fatigue cracking and other mechanical degradations. Submerged condition of such structures often makes them inaccessible for various established non-destructive technologies. Most of these conventional techniques approaches require interruption of normal service and removal of the subject structure from submerged condition. The prohibitive out-of-service inspection costs restrict the frequency of such investigations, thus exposing them to a great risk of unnoticed fatal damages. Hence, there is a need to supplement or replace these methods with an efficient, reliable, in-situ, non-contact and non-destructive monitoring technique for submerged plate assemblies. The present work reports a laboratory study using a pair a mobile non-contact probes arranged in pitch-catch orientation employing immersion coupling utilizing Leaky Lamb waves for assessing and characterizing notch and corrosion damages in submerged plates. The propagation characteristics of the different Lamb wave modes in a submerged plate are studied. The longitudinal wave excited by cylindrical transducer falls obliquely on the submerged plate using surrounding water as natural couplant. The transmitted signal is received by other transducer after it has traversed through the length of the plate. Use of water coupled transducers makes the system non-contact and non-invasive. Interaction of propagating waves with the simulated damages in the form of machined notches has been studied. By comparing the transmitted signals of the healthy plate with that of the notched plate, damage monitoring technique is developed. Specific Lamb wave modes sensitive to near surface and sub-surface damages have been identified. These modes are further exploited for non-contact scanning of the plates to identify and quantify the presence as well as extent of damage. Effective combination of specific Lamb wave modes leads to comprehensive inspection of the submerged plate structures. Exact location of the damage is ascertained by pulse echo monitoring of the plate. Post processing of the ultrasonic data generated from scanning the plate has been pictorially represented in the form of defect maps. v The developed methodology is further successfully applied for monitoring progressive accelerated corrosion in submerged plates. It is observed that corrosion in submerged plates is discernible using ultrasonic guided waves. Through a judicious selection of different Lamb wave modes, not only the corrosion phenomenon can be monitored but also different effects and mechanisms of corrosion can be successfully identified. Combination of the selected guided wave modes could also discern uniform and pitting corrosion in submerged plates. Along with the ultrasonic signals, mass loss, stress-strain behavior and tensile strength of the plates at different stages of corrosion have been monitored. Semi-empirical relationships between the ultrasonic readings and other parameters have been developed. This investigation should be useful in developing a noncontact, non-invasive and non-destructive technique for monitoring progressive corrosion in plates and assessing their deterioration in strength, stiffness and mass loss that would help in the estimation of residual life. The methodology has the potential to develop into a commercially viable real time, non-invasive and in-service corrosion monitoring and evaluation tool for large submerged structures as in marine installations.