We were recently engaged to assess the damage reported to the cargo tanks secondary barrier of a series of LNG vessels. The vessels are equipped with a membrane-type cargo containment system composed of prefabricated insulation panels integrating both primary and secondary barriers. These panels ensure the thermal performance and structural integrity required to safely transport liquefied natural gas at –163°C and an absolute pressure of 1,060 mbar.
The primary barrier consists of corrugated 304L stainless steel, engineered with large and small corrugations to accommodate thermal expansion and contraction. The secondary barrier is a laminated composite made of a thin aluminium sheet bonded between two layers of glass cloth and resin. This barrier is embedded within reinforced polyurethane foam insulation panels, which provide both mechanical strength and thermal resistance.
The damage came to light during statutory testing required by the International Gas Carrier (IGC) Code, which mandates verification of the secondary barrier’s integrity at five-year intervals, typically during the Special Survey. Such tests must be approved by the Administration or the vessel’s Classification Society. Over the years, the containment manufacturer has developed several approved testing methods, including:
- SBTT (Secondary Barrier Tightness Test): Identifies loss of tightness but not the exact defect location.
- TAMI (Thermal Assessment of Membrane Integrity): Detects localised cold spots caused by nitrogen leakage from the primary to the secondary space, using thermal imaging.
- AE (Acoustic Emission Test): Applies a vacuum of -500 mbg to the secondary space while the primary remains at atmospheric pressure; ultrasonic emissions indicate potential leakage points.
In the cases we investigated, testing confirmed that although the primary barrier remained intact, the secondary barrier exhibited ruptures due to perforation and cracking at multiple locations. Since LNG containment systems must provide two independent barriers, any breach in the secondary membrane—designed as the critical fail-safe layer—requires immediate rectification. The ruptures were found within the laminated composite material of the secondary barrier.
Repairs, performed under the supervision of the containment system manufacturer and Classification Society, involved cutting sections of the corrugated stainless-steel primary barrier from inside the tank to access the damaged areas. The affected insulation panels containing the secondary membrane were removed, inserted and the primary barrier sections were then reinstalled and welded. These operations require specialised equipment and highly qualified personnel, and can only be undertaken at facilities pre-approved by the containment manufacturer.
LNG transport is an advanced, high-precision field with zero tolerance for failure. Maintaining all components of the containment system in optimal condition is essential to ensuring safety and regulatory compliance.
