Entrepose Contracting

The methane terminal of Dunkerque features three LNG cryogenic storage tanks with a capacity of 190,000 m³. Each tank is comprised of a concrete enclosure and an inner vessel made of welded 9%Ni steel plates. This vessel contains liquid gas at a temperature of 163°C. Inside the concrete “shell”, a 5 mm thick metal skin (the liner) provides leak tightness. « After the construction of the first tank, the air pressure test at 260 mbar revealed a bubble phenomenon on the concrete shell », says Marcio Gomes Rodrigues, Project Control Manager of the Entrepose Projets-Bouygues TP consortium. « Although the test results complied with the cryogenic tank construction standard, the appearance of bubbles led us to suspect a leak tightness defect in the metallic liner in some areas. » 

Sixty sensors installed 

Thus, Entrepose Contracting decided to find the root cause of the bubbles. This turned into quite a challenge, given the dimensions of the tank (90 m in diameter, 50 m in height), the difficulty to implement tests in a confined space (50 cm) between the liner and the vessel and the low test pressure! « The acoustic emission testing method implemented by Cetim seemed to be the only one that could meet our expectations. Cetim provided us with a response within an extremely short deadline », continues Marcio Gomes Rodrigues. The tests were carried out in three days, with difficult access and in hard climatic conditions. Sixty piezoelectric sensors were installed, every 5 m, on the periphery of the liner and made it possible to locate a small size problematic area at the junction between a pipe connection and the liner. At the end of the investigation, the root cause of the bubble phenomenon had been identified.