Fire safety is an intrinsic part of the design process for developing hydrocarbon facilities. With the range of codes and standards used in the Oil & Gas industry, the required safety levels play a key part in the construction and operation phases.
The core purpose of fire safety is to protect people, production processes and flow, as well as to protect equipment against fire damage by complying with the statutory requirements. These requirements include a quantification of risks and the achievement of an acceptable level of safety. It is therefore crucial that protection is not compromised in any way, and that it provides a specified period to ensure a safe evacuation and controlled facility shut down in the event of a fire.
The fire safety of structures in the onshore and offshore hydrocarbon industry requires specific requirements for each sub-section, such as protected load-bearing or separating elements. The starting point for fire safety evaluation will always focus on the evidence of fire testing within the elements of construction in accordance with a range of relevant standards, such as EN 1363 for hydrocarbon fires and ISO 22899 for jet fires. However, these individual fire resistant elements come into their own in harsh and hazardous offshore and onshore environments. Therefore, the assessment of the entire structure requires stability when exposed to these environmental factors, and it is particularly important that the individual fire safety products still function and fulfil their original fire protection tasks after lifetime environmental exposure within the context of the whole structure. With appropriate and rigorous assessment and relevant mandatory/ voluntary product certification, a product can normally be considered to be trusted to deal with a particular application.
In recent years the Oil & Gas industry has increased its attention towards natural gas and the large, isolated reservoirs that require transport via ships. To allow the transportation to be carried out the natural gas is cooled to cryogenic temperatures (-160oC, -260oF) and liquefied to produce Liquefied Natural Gas (LNG). The new challenge here is that the performance of a fire safety product is not just determined by the product performance in the fire testing and approval processes. The reason for the physical behaviour is that as the temperature decreases, the ability of commonly used steel (low carbon) to exhibit plasticity is reduced. Therefore, in accidental cases, the steel will crack instead of plastically deform, which is the commonly expected behaviour in ambient and high temperatures. Considerably less energy is dissipated during cracking, in this case known as cleavage, as compared with the plastic deformation. As a result, the cracks can grow very fast and can lead to collapse. This phenomenon is what makes the protection of low carbon steel to low temperatures a requirement, and the material used to achieve this is commonly referred to as Cold Spill Protection (CSP).
On the other side of the exposure, are the protection materials used for hydrocarbon fires. They are known as Passive Fire Protection (PFP) and are composed of numerous polymer chains which obtain the characteristics of glass at sufficiently low temperatures, including its hardness and brittleness. In the event of a leakage of the cryogenic LNG, the fire protection can be damaged as cracks can easily form and propagate. The resultant fractures will weaken the fire protection system before the ignition of the natural gas has occurred and subsequently reduce the fire resistance performance. With this in mind, methodologies such as ISO 20088 need to be used to simulate thermal stresses and the deterioration damage that can occur in a real leakage scenario to obtain the cryogenic performance of the product. Ideally, the two systems, CSP and PFP, are combined or applied together to provide protection to cryogenic leak followed by fire.
The performance of the protection system is also largely determined by the quality of the installation process. It is vital that the installation strictly complies with the manufacturer’s instructions and has been tested and approved. For example, in LNG facilities the main structure or pipes can be subject to significant thermal expansion or contraction during operation or accidental leakage, which can lead to significant thermal stress and failure if the installation of the protection system deviates from the designed specification.
Given the complexity of the installation of most fire safety products, installation by competent contractors is essential, particularly in order to maintain the product’s integrity and warranty. It is recommended that a specialist contractor/applicator under a recognised third party certification scheme is employed to install fire protection systems.
Regular inspection is also crucial to identify deterioration or damage so that remedial work can be carried out promptly to maintain a continued level of protection. It is especially critical in cryogenic applications as the leakage might not lead to a fire, but cracks in the coating can form, which will compromise the structural integrity of the coating for future damage against fire and explosions.
The extent of any damage can normally be identified and assessed through routine risk assessment. This requires a robust risk management system which covers: identification of the fire safety element; determination of the criticality of each element; assessment of each anomaly noted; risk assessment; and allocation of urgency rating for remedial works. Anomalies in the fire safety system identified by inspections should be repaired based on the risk assessment specific to the location and evaluation of the damage.
In the hydrocarbon industry, accidents such as fire and explosion, and deterioration damage in the case of LNG leakage, can lead to structural integrity failures. Therefore, legislation and guidance in many parts of the world impose increasingly strict requirements on life safety products such as fire protection. The level of fire safety associated with each facility is normally achieved as a result of various fire safety standards or codes. In general, prescriptive standards or codes adequately serve the needs of regulators or approval authorities in the vast majority of traditional Oil & Gas constructions. However, a test method which captures the combined cryogenic and fire exposure which is likely to occur in the current trend of developments in the hydrocarbon industry is yet to be developed. Without such a rigorous testing methodology the physical behaviour of the exposure scenario – cryogenic leakage followed by a fire – is underrepresented by standalone tests such as the jet fire and separate cryogenic exposure tests.
The fire/cryogenic testing standards do not explicitly address the concerns on safety of life or properties over the full lifetime of a structure. Over time a less conventional “engineering” approach has increasingly been adopted in modern designs, bringing a challenge to the more fundamental fire safety designs.
The need to develop a combined cryogenic/fire testing method becomes even more apparent when we look at state-of-the-art developments. One such research effort is the use of computer models to obtain a quantification of the structural integrity during combined fire/cryogenic scenarios in the Oil & Gas industry. Describing the physical processes that occur during the exposure, and under the relevant demanding environments, is challenging. The successful implementation has the added benefit of being able to evaluate the structural integrity of steel structures by implementing a material damage model, and to predict failure or the residual strength that a structure can retain and resist collapse. Furthermore, by modelling, a full structure can be investigated where the cost of performing a full scale test is prohibitive.
The complication is further increased when we consider the fact that LNG is stored under atmospheric pressure conditions and transported via pressurized pipe networks. A leak from a pressurized vessel, or even the base of a large tank, may initially lead to the liquid jetting followed by a pool formation. Worsening the case, two different fire types can form from this release i.e. pool and jet fire, and each has its own characteristics and effects on the protection or structure. Furthermore, classical analysis methods in engineering are based upon assumptions of continuum behaviour, which means that the material properties can be averaged on a large scale and thus allow hand calculations to be carried out, or widely used engineering software to be implemented. However, this becomes inadequate when low carbon ferritic steel structure (widely used in the Oil & Gas industry) is cooled to cryogenic temperatures. In these cases, we have to consider the impact of small cracks and defects inside the material whose properties can no longer be averaged as they start to behave as non-uniformities in the structure. To analyse them, and predict their growth and reduction in the strength of the structure, is critical to ensure that collapse does not occur during cold temperature exposure and the fire resistance is not compromised.
The extent of damage can be significantly controlled by the application of cold spill and fire protection materials, and the computer analysis tools can aid in the design stage. However, it forms part of the weakest link model i.e. the manufacturing of such protection, its installation, and inspection should be carried out accordingly to ensure that the product will perform as expected from the experimental stage. A failure of one system will trigger a systematic error in the ability of the protection to operate as expected, thus highlighting the importance of the certification process and the adoption of a risk management system.
In order to ensure that a hydrocarbon fire safety system remains relevant to the needs of the Oil & Gas industry, the characteristics of integral fire safety need careful consideration. Central to good fire protection in engineering or safety specification is the use of genuine and appropriate product testing, assessment and certification. Robust testing within the overall production of industrial structures will ensure they remain fit for purpose, their fire safety elements are installed correctly and that over their lifetime they are managed and maintained well.
For more information, go to www.exova.com/sectors/fire/
Nikola Stoyanov
