Development of a lock-in thermography inspection system for detecting voids in LNG carrier secondary barriers

Abstract

The most widely used liquefied natural gas (LNG) carrier is of the membrane type, consisting of 3 or 4 tanks. The tank is where the LNG is carried, is composed of several membranes to prevent gas leaking or to keep the LNG at cryogenic temperature. Especially, the secondary barrier (called triplex) plays a vital role when the primary barrier fails. It is composed of three layers

a flexible secondary barrier (FSB), a bonding layer, and a resin-based rigid secondary barrier (RSB). Triplex is produced automatically using an automatic bonding machine (ABM) or manually created by a person. During this manufacturing process, voids can be formed in the adhesive layer. It is very important to inspect the triplex and replace it as needed since void may degrade the tightness of the specimen and can lead to gas leakage and explosion. The typical triplex adhesive inspection method is a manual inspection by rubbing the triplex surface with a round metal stick. It is time-consuming and subjective inspection method in that test results vary from person to person. An autonomous inspection system was developed that can automatically inspect triplex, but it has a limitation that only tests standard area with fixed width. Therefore, this paper proposes a portable inspection system that can examine the triplex installed in all areas of the LNG tank.

The inspection system consists of two main modules: (1) inspection module and (2) control and a display module. First, install the inspection equipment in the area where the inspector wants to inspect and start inspection with the control module. An inspection module consisting of two halogen lamps and an infrared camera examine the triplex in real time. After that, the developed inspection area automatic ROI extraction algorithm and void detection and visualization algorithm are used to determine the presence of defects. Finally, the results are shown in GUI-based inspection software from the display module.

A series of lab-scale experiments were conducted to verify the developed inspection system. The proposed inspection system has the following advantages. (1) The developed portable inspection system is capable of testing void defects in all areas installed triplex in LNG tanks. (2) The automatic region of interest (ROI) extraction algorithm can accurately track the ROI regardless of the shape of the inspection area (3) Inspection time is very fast, and the result of the test is intuitive.