Passive Fire Protection: Reducing Risk, Saving Lives and Assets

Fires and explosions and the consequences to lives and assets are a particular concern in high-risk process and oil and gas industries.

Designing key elements to withstand the consequences of gas explosions and hydrocarbon fires, as well as reducing the risk of escalation, are important aspects in process plant design. In many cases, required mitigation measures can also have a significant impact on project and future maintenance costs.

It’s impossible to completely eliminate the risk of catastrophic events. Therefore, it’s important to effectively protect personnel from those events and reduce escalation risk to keep the event contained to a single fire area and prevent its spread to other areas or the whole installation.

Excessive conservatism in consequence modelling doesn’t always result in safer designs, as the positive effect of highly effective mitigation measures might be masked by the simplification and conservatism.

During the last 30 years the use of advanced CFD for explosion simulation – especially for offshore installations – has become an industry standard.

Embracing similar advanced tools for fire-related consequence modelling has been much slower. With the advent of new software tools, ever-faster desktop computers and improved user interfaces, more advanced fire simulation models are becoming more accessible to a wider range of fire safety engineers.

Thus, CFD simulations are becoming a viable option for many aspects of fire safety engineering, including escape route studies and PFP optimisation.

Fires on offshore facilities

Several different types of hydrocarbon fires can be distinguished on offshore facilities. Pool fires occur as a result of ignition of liquid fuel spill and can be either static pool or a flowing liquid.

If the liquid is released at high pressure the resulting fire is known as a jet spray fire. Following a gas leak an immediate ignition would result in a gaseous jet fire.

If ignition is delayed, the released gas is dispersed and mixed with air, resulting in a flash fire or possible explosion.

The most common fires that PFP protects against are pool and jet fires, which can engulf the system – both structure or equipment – in a flame or expose it to high, radioative heat fluxes for prolonged periods of time.

Passive fire protection, unlike active fire protection, does not need any external activation means or input from personnel. The common PFP material used varies from mineral based, such as rock wool, to organic, resin-based – ie, intumescent coatings – and composites.

The use of PFP in the oil and gas industry, particularly on offshore facilities, has many advantages. Applying PFP to structure and process equipment allows time for safe evacuation by personnel and for firefighters to tackle the fire.

This is particularly crucial on offshore facilities, where escape and evacuation is more critical than on onshore facilities. Prevention is done by maintaining containment of additional process equipment and isolating the fire to a single fire area.

Key structural elements must also be protected from fires to prevent loss of structural integrity, which could further exacerbate spread due to falling heavy objects or even structural collapse of the whole installation.

By preventing escalation the PFP implementation helps protect people and assets. However, PFP increases the risk of corrosion and leak frequencies from process equipment.

PFP also increases equipment dimensions, causing a rise in explosion design loads. The inspection and maintenance of PFP requires more personnel in process areas and creates a larger potential for ignition sources and greater number of exposed people.

Excessive use of PFP on structure and equipment can make fire and hot gases much hotter. IT also  increases the total weight of installation and the facility’s installation and maintenance costs. Optimal, risk-based, proportionate use of PFP is therefore required to provide necessary protection.

PFP implementation

The definition of the fluctuating heat load expected on a system – structure or equipment – can be useful in assessing the need for PFP. The reasoning is that there is no need to apply high heat load PFP in areas where these loads will not occur.

While a number of standardised methodologies for determining risk-based explosion dimensioning loads exist (e.g. Norsok Z-013), there are currently no standardised, detailed methodologies for defining changing heat loads in a similar way.

A simplified approach can be followed to define heat loads on a system. The approach starts with a screening of the areas of concern to identify critical flammable fuel inventories/SCEs. On an offshore facility, these are normally the inventories containing hydrocarbons.

Assessment of the type of fire expected is done by investigating the representative mixture in the inventory and conditions of containment. In process areas, the operating pressure in most inventories is usually relatively high and therefore jet fires are the most common.

A first screening of inventory volumes is conducted to determine whether a fire occurring near an SCE is large enough or present for a critical period of time. For instance, a leak from a hydrocarbon segment given the segment characteristics – volume, pressure, temperature, representative mixture – can be evaluated to determine whether a leak rate above a cut-off value (0.1 kg/s for example) could happen for at least five minutes.

The effect of isolation and depressurising on the segment is taken into account by the evaluation. In addition, the segment’s leak frequencies can be included to evaluate the frequency of leak and potential fire occurrence from different segments.

After the screening stage, areas with potential critical fires that could expose the SCE’s to hazardous situations are subject to further consequence analysis to define the expected heat loads on the system.

Specific system targets in the area are identified such as firewalls, separator vessels and shutdown valves. Fire scenarios are set-up to expose the selected targets to representative heat loads.

The fire scenarios are simulated with a suitable CFD consequence modelling software which has been validated for high momentum gas jet fires such as FLACS-Fire. CFD simulations tools are used rather than simplified models and empirical correlations in order to account for the various scenario variables such as fire obstruction by geometry, leak characteristics and weather conditions.

Other more advanced programmes that simulate the material response to heat loads can be used for this purpose (more advanced Non-Linear Finite Elements Analysis).  For vessels and pipes, it is assumed that the running medium would absorb part of the heat loads applied on the target and thus dissipate more of the received heat radiation.

The extent of this reduction will depend on the flow rate inside the pipe. Therefore, time-dependant heat transfer models are to be used for this purpose. Heat generation due to fire fluxes will compete with heat dissipation due to flowing medium until the fire decays.

Steel temperature is monitored during the heat generation and dissipation process to detect whether the steel temperature would reach critical values. In any of the described methods, where the heat transfer calculations indicate a failure in a given time range which isn’t compatible with the safety function and performance requirement, the PFP is implied or increased and the calculations are repeated until the system is observed to withstand the expected heat loads.

The optimisation of PFP application would significantly reduce both installation and maintenance costs.

Balancing act

The importance of adequate PFP implementation on offshore facilities is undisputable. Prevention of escalation for at least sufficient time to allow safe evacuation is a minimum requirement with respect to safety.

The extensive use of CFD allows the inclusion of various affecting parameters such as fire interaction with geometry, release characteristics and weather conditions. The calculated incident heat loads are then used to assess whether the system would withstand the heat without protection or if implementation and where the increase of PFP is required according to a specific performance requirement.

This optimisation of implementation of PFP provides an important balance between the benefits and drawbacks.

The costs of installation and future maintenance of PFP in areas where it does not contribute to safety can be reduced and limited significantly, while system protection can be achieved while limiting the disadvantages.

Free Download: The Video Surveillance Report 2023

Discover the latest developments in the rapidly-evolving video surveillance sector by downloading the 2023 Video Surveillance Report. Over 500 responses to our survey, which come from integrators to consultants and heads of security, inform our analysis of the latest trends including AI, the state of the video surveillance market, uptake of the cloud, and the wider economic and geopolitical events impacting the sector!

Download for FREE to discover top industry insight around the latest innovations in video surveillance systems.

Download The Video Surveillance Report