11:15 - 11:45
Thursday, 19 September 2019
T2.8 Lessons Learned – Investigating failures of LNG containment systems
Natural gas is one of the fastest growing energy resources. Liquefied natural gas (LNG) offers a very effective way to commercialize and exploit remote natural gas reservoirs and distribute natural gas to regions that are difficult to reach by pipeline. The last few decades saw rapid development of LNG facilities and their associated hardware and systems including storage systems. Hence, there is currently a growing population of ageing LNG storage systems. Characterizing their condition, mitigating failures, and assessing the remaining useful life of these storage systems is crucial for LNG asset managers. These analyses require complex multidisciplinary evaluations involving civil engineers, metallurgists, and material and thermal scientists.
This paper will provide a description of some key failure modes in LNG containment systems with particular focus on two systems: 1) pre-stressed concrete tanks and 2) membrane systems.
The first part of the discussion will cover the main components of the containment systems such as 1) the tank exterior shell, consisting of concrete and shotcrete, pre-stressing wires, and rebars; 2) tank primary containment (liners); and 3) insulating layers that consist of a variety of insulating materials including perlite, glass foams, polymer foams, plywood layers, and combinations of these. These components should be selected, designed and constructed to perform well over a long life span under normal and upset conditions.
In pre-stressed concrete tanks, the shotcrete and concrete outer shell are exposed to atmospheric CO2 causing carbonation and to other environmental conditions such as rain, snow, and cycles of freeze-thaw cycles. Such conditions may eventually lead to delamination and concrete spalling and may contribute to the corrosion of the pre-stressing wires and rebar. Visual inspections, acoustic testing, concrete sampling and corrosion testing, such as half-cell potential, can assist in evaluating the severity of these conditions.
Poor tank design combined with the high temperature differentials experienced by these systems can cause damage to the liner if it carries excessive loads. Typically, video inspections performed on the inside of a tank can provide data to, at least preliminarily, evaluate the conditions of the liner. Operators can choose to replace damaged areas of the liner if necessary and economically viable. The performance of the insulating foams and the plywood layers can be significantly impaired by moisture intrusion or poor moisture control, which can eventually lead to the development of condensations and thermal bridges within the insulation. In some cases, moisture probes and temperature readings may be able to assist in characterizing these conditions. Hence, it is very critical to provide a system that effectively blocks moisture from entering the insulation layers and directs water away from the containment. Thermal bridging can lead to ice-lensing, which subsequently can cause significant localized pressures in various tank components and cause structural failures.
The technical discussion will be accompanied by a review of selected historical failures of containment systems, real-world case studies and forensic investigations. The authors are experts in LNG systems, thermal sciences, and metallurgy and corrosion, and civil engineering and have years of experience in forensic investigations.