Natural Gas, in order to be liquefied, needs a low temperature and is stored under cryogenic conditions. Typical storage temperature is below-160 ° Centigrade. Apart from the fact that this requires the tanks to be fitted with effective thermal insulation and cooling equipment, the tank shell is made out of a steel alloy containing 9 % Nickel in order to remain ductile at low temperatures; where typical wall thicknesses vary between 10 mm (top rings) and 30 mm (bottom ring). In recent years, a tendency can be observed to switch, for NDT of the welds, from Radiography to Automated Ultrasonic Testing (AUT). Not only does this increase inspection speed and detection capabilities of critical defects; but thanks to the absence of radiation, the AUT activities can also be performed in the vicinity of personnel at work, enabling continuous welding and instant NDT results. Since welding is done with high-alloyed welding consumables, resulting in the weld structure to be anisotropic and coarse grained, ultrasonic techniques using angled longitudinal waves should be used. These techniques include longitudinal beams under 45 ° to 70°, creeping waves and Round Trip Tandem technique (RTT, also referred to as Self Tandem Technique). If performed with conventional dual element Transmitter Receiver Longitudinal (TRL) probes,
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