Infrared Techniques for Practical Defect Identification in Bonded Joints in Liquefied Natural Gas Carriers

Abstract

A robust and reliable method for the identification of defects in adhesively bonded joints used in the membrane containment system used to store liquefied natural gas during transport on board ships is required. The adhesively bonded interface of the membrane system may contain low volume defects known as kissing defects, which are extremely difficult to detect using portable inspection techniques. A novel methodology for detecting such defects is described in the paper. To demonstrate the approach simulated kissing defects were produced in a controlled fashion in a representative secondary membrane bond using silicon grease contamination. The defects were detected using an active thermographic approach known as pulsed phase thermography (PPT) which detects subsurface defects by monitoring the effect they have on the propagation of heat through a component. Due to the low volume of kissing defects, they generally have minimal effect on the heat propagation, so the detection was made possible by application of a small load generated by application of a vacuum. The vacuum can be set-up using a reusable chamber and a standard vacuum pump and therefore is portable and can be applied on-site and in a shipyard during construction.