Liquefied Natural Gas fires: The damage on structures

In most regions of the world natural gas is the fastest growing energy resource. This trend has been persistent since the new millennium and is expected to continue, driven by the low greenhouse gas emission regulations that drive the energy industry to meet international climate change agreements.

Traditionally, the supply chain for natural gas involves transport via pipelines over huge distances; something the Oil & Gas industry is accustomed to thanks to the many years of experience in design and safety know how. However, with the depletion of large natural gas reservoirs in accessible locations and increasing demand, it is clear that significant quantities of natural gas need to be obtained from new reserves which might not be as easy to get to.

The Oil & Gas industry has shifted its attention towards large, isolated gas reservoirs that are too remote for pipeline construction. One solution to the transport challenge is to cool the natural gas to approximately –162 oC (–259 F). The cooling produces Liquefied Natural Gas (LNG), which is easy to transport by cargo ship or truck in this more compact form.

One thing to be aware of is that accidents can sometimes happen and, if LNG escapes its transportation container it can form a flammable gas cloud and it can also expose the structures to very low cryogenic temperatures. These low temperatures are very damaging and affect the passive fire protection coatings on the structure. Subsequently, this might prevent their normal function during fires.

It is therefore imperative that this hazardous environment is understood and that correct methodologies are employed to prevent catastrophic failures.

Structures and protection materials in hazardous environments

The materials used in the LNG industry can be exposed to various harsh conditions. These conditions may include thermal stresses due to the temperature differences between the LNG and the ambient environment. There could also be internal and external mechanical loads and accidental events such as fire and explosion, which adds to the range of situations that materials need to be able to tackle. Scenarios such as a leak from a pressurised LNG tank may lead to such an exposure.

Contact of a cryogenic liquid such as LNG with metallic and non-metallic objects will cause rapid cooling and consequent shrinkage. Structural steels are particularly vulnerable because they are subject to a specific transition at low temperatures in which their material properties change.

Passive fire protection materials are applied to steel structures in the Oil & Gas industry. These enable resistance to fire by providing insulation which increases the time for the steel to degrade and lose mechanical strength, which can subsequently lead to collapse. The passive fire protection material is composed of numerous polymer chains and, as such, it assumes the characteristics of glass at sufficiently low temperatures – including hardness and brittleness. This relates to the ease with which cracks can form and propagate.

The properties associated with cooling to cryogenic temperatures will affect the performance of the materials and will not provide the expected resistance to a fire. Therefore, a meticulous approach must be undertaken during the design of such structures to ensure that the scientific principles are understood and proper material selection is made to limit or avoid catastrophic failure.

Conclusion

Using LNG offers economic benefits, flexibility, and security of supply advantages over gas pipelines and other technological alternatives. Many countries in Europe and Asia are embracing LNG for these reasons and the Oil & Gas industry is adapting accordingly.

However, new hazardous environments are being created where materials can be exposed to unfamiliar and technically challenging scenarios. With the growth of the LNG industry, materials can be subject to severe temperature transitions which will result in changes to their mechanical properties. As the demand from the Oil & Gas industry for natural gas increases, the need to understand the performance of the materials which make its extraction and transport possible also increases. In response to this, new methodologies to assess the provision of sufficient fire resistance need to be continually applied and reviewed.

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