Fires may follow from any hydrocarbon release given exposure to the ignition sources present at the facilities in question, and can result in fire loads beyond the design limits of the facilities. Fire analysis is therefore in many cases a significant part of a QRA.
The main objective of the fire analysis is to provide a sound basis for setting design fire loads such that the risk for personnel, assets or environment is within acceptable limits. Further, fire analysis is used for evaluating impairment of escape ways (smoke/radiation), and evaluating how the actual layout affects the consequences of fires. The aim is to provide useful advice on escape/layout design or to describe the fire risk for an existing plant.
Fires may be modelled using analytical/empirical models describing the shape radiation, smoke, effect of wind, and so on. Such models are used in Lilleaker’s risk analysis tool ASAP, as described under QRA.
Sometimes significantly more accurate modelling of fires is required, and in such cases we apply the CFD tool KFX (Kameleon FireEx). Here, fires can be modelled in 3D, taking into account all relevant aspects affecting the fire, and results for heat radiation, temperature, smoke etc. can be measured and plotted as required.
In some cases, a full probabilistic fire analysis may be required. An overview of the main stages in probabilistic fire analysis is as follows, using the CFD tool KFX and the risk analysis tool ASAP:
Fire frequencies and durations are calculated in ASAP which provides transient calculations of the leaks, gas dispersion, detection and ignition. The CFD tool KFX is applied for incident heat/temperature calculations on targets of interest. Combining the results of these two programs gives the fire exceedance frequency on each target.
Example of a fire simulation plot, illustrating heat and smoke:
Explosions may follow from any hydrocarbon releases given exposure to the ignition sources present at the facilities in question, and can result in explosion loads beyond the design limits of the facilities. Probabilistic explosion analysis is therefore in many cases a significant part of a QRA.
The main objective for the probabilistic explosion analysis is to provide sufficient decision support for the process of defining design explosion loads (where the objective is to make sure that the design of the plant/installation can withstand a certain level of explosion loads, such that the risk for personnel, assets or environment is within acceptable limits).
An overview of the main stages in probabilistic explosion analysis is as follows, using the CFD tool FLACS and the risk analysis tool ASAP:
Frequency of ignited gas clouds is found using the risk analysis tool ASAP, where leak frequency and leak duration is combined with a time-dependent model for dispersion, detection and ignition. The CFD tool FLACS may be applied for supporting ASAP with a) modelling more accurate ventilation calculation and b) more accurate gas dispersion calculation, based on a 3D model of the facilities. Explosion simulations are performed with the FLACS 3D model, to the degree of accuracy necessary for the purpose.
The resulting explosion loads are combined with the frequency of the corresponding ignited gas cloud sizes for producing so-called exceedance curves for the defined explosion barriers and equipment which need to withstand the explosion loads.
Structural Response Analysis
Structural response analysis may be applied in order to investigate further the consequences of fires and explosions.
A KFX (Kameleon FireEx) model is established by importing the Clients 3-D model. Based upon the risk analysis credible fire scenarios are selected and simulated by KFX. Heat load on the structure is exported to FATHS and imported to the non-linear structural analysis tool USFOS. The transient behaviour of the structural steel is simulated, deformations are shown and time to collapse is assessed. Based upon this the need for PFP on structural elements can be determined.
Example of a structural fire response calculation result showing utilization and structural deformation of a cantilevered offshore derrick:
A FLACS model is established by importing the Clients 3-D model. Based upon the risk analysis credible explosion scenarios are selected and simulated by FLACS. Explosion load on the structure is exported to suitable non-linear finite element codes (such as Usfos, LS-DYNA or Ansys) for analysing the dynamic response of the structures, often measured as degree of utilisation, strain and deformation.
Example of structural explosion response calculation result showing utilization and structural deformation of an offshore deck structure:
Gas Dispersion Analysis
Gas dispersion analysis, either from jet leaks, liquid pools or underwater leaks emerging at surface, may be performed as part of the probabilistic explosion analysis. However, it may also be relevant for investigating exposure of air intakes or other specific ignition sources.
We perform gas dispersion analysis either using FLUENT, FLACS or KFX, depending on the purpose or availability of 3D model.
Prior to simulating the gas dispersion, which is quite time consuming, one must agree on the leak cases to be investigated, i.e. the leak sources, fluids, degree of congestion/obstruction as well as the wind conditions.
Example of 2D plot showing the extent and concentration of a gas dispersed in a hazardous area:
Verification of Gas Detector Layouts
The gas detector layout is designed by the technical safety discipline using experience and following standard guidelines, which for the most cases will result in an adequate layout. Gas dispersion analysis may be used for verification purposes of a gas detector layout. The verification activity may be performed during engineering/construction phase (in order to avoid deficiencies emerging during operation) or in some cases, as a result of inadequacies suspected/proven during operation.
The verification activity may uncover parts of areas with inadequate gas detector coverage, and different detection limits (high / low alarm or action, as % of LEL) and different choices of voting may be evaluated.
The CFD tool FLACS is used in these analyses, where a 3D model of the installation is made, including both point and line gas detector in their exact positions. Once modelled, the gas detectors may be turned on or off, and detector limits or voting may be altered in order to evaluate the effect of these detectors or parameters. A representative choice of time-dependent gas dispersion simulations is run, and features like time to detection and probability of detection is found for each simulation. The percentage of gas clouds detected before they become dangerous (e.g. giving explosion pressures above design loads) is evaluated.
The method may be applied to different type of gas detector, e.g. hydrocarbon gas, H2S or other toxic gases.
Example of 3D plot showing the extent of a toxic gas dispersed in a hazardous area, for determining location of gas detector around the equipment:
PFP optimisation for piping and equipment
Credible fire scenarios are established with ASAP, KFX (Kameleon FireEx) or extracted from standards. Input from process discipline such as P&IDs, depressurisation capacities and inventories are required. Vessfire or similar tools is applied to calculate heat rise, loss of strength, pressure rise due to boiling and time to rupture for piping and equipment. The equipment and piping that should be protected with PFP are identified. Alternative measures such as improvement of depressurisation capacity, change of piping class, etc. is evaluated.
Example calculation result showing the strength of a fire exposed pipe vs. time with pressure,temperature and utilization:
Transformer Explosion Analysis
Oil filled transformers are present in many different applications both onshore and offshore. They represent explosion risks as follows: An electrical failure (e.g. short circuit) in the transformer may produce a strong arc which causes pyrolysis of the insulating oil and causes a breach of the transformer. The gaseous pyrolysis products and oil mist leaks out, and upon exposure of ignition sources may give a secondary explosion affecting the surroundings.
In transformer explosion analysis, we evaluate the potential of gas leaking from the transformer (depending on short circuit current, disconnection time, arc energy). The gas dispersion resulting from the primary transformer explosion is modelled with the CFD tool FLACS, taking into account the ventilation conditions of the area where the transformer is located. The secondary explosion, i.e. ignition of the gas from the decomposed oil, is also simulated with FLACS, and the resulting explosion pressures may be estimated.
Based on the results, mitigating measures can be proposed for lowering transformer explosion risk.
When combined with an assessment of transformer explosion historical frequencies, the personnel, asset or environment risk from transformer explosions may be estimated.
Energy exceedance curve:
Lilleaker perform studies for gas dispersion from cold vent/ unignited flares, as well as for radiation from ignited flare.
The purpose for cold vent studies is to investigate whether detectable and/or flammable gas may expose ignition sources at the facilities (hazardous areas, non-hazardous areas, helicopter deck) or nearby locations (neighbouring facilities).
The purpose for flare radiation studies is to investigate whether radiation on areas is within the acceptable limits, either for personnel (e.g. according to API 521/ ISO23251) barriers (e.g. a firewall) or equipment (e.g. as specified by vendor).
Studies of cold vent/unignited flare may be performed either using FLUENT, FLACS or KFX, depending on the purpose or availability of 3D model, whereas radiation from ignited flare is performed using KFX, see case displayed below.
Example of contours of radiation onto a FPSO topside from an ignited flare:
Helicopter deck analysis is performed to evaluate the air flow conditions above helicopter deck at e.g. offshore installations, vessels or buildings.
The layout of the helicopter deck and its surroundings impact the flow field and create turbulence above the helideck, and may cause problems for helicopter operations if turbulence exceeds certain limits (e.g. given by the standard CAP 437 issued by the Civil Aviation Authority).
We use the CFD tool FLUENT, where a 3D model of the helideck and surroundings is made for simulating the wind conditions at helicopter deck, applying a turbulence model which is applicable for the purpose. Turbulence levels are plotted and evaluated against the limits for helicopter operation.
Outlets of exhaust or other hot gases may cause temperature gradients above helicopter deck if the outlets are in the vicinity. Temperature gradients cause problems for helicopter operation if the gradient exceeds certain limits (e.g. given by the Norwegian standard NORSOK C-004). Further, unignited flaring may ignite if exposing the helicopter flight path (requirement to max gas concentration is given in e.g. NORSOK S-001).
Similar as for turbulence issues we use the CFD tool FLUENT for simulating the hot gas / flammable gas exposure of helicopter deck / path. Temperature gradients are plotted and evaluated against the limits for helicopter operation.
Example of turbulence plot for a helicopter deck on an offshore installation:
Example of hot gas plume for a helicopter deck on an offshore installation:
Natural Ventilation Analysis
The purpose of natural ventilation analysis is to assure that ventilation rates in naturally ventilated hazardous areas fulfil applicable requirements for operation.
The CFD tool FLACS is used in these analyses, where a 3D model of the installation is made and ventilations simulations are run for relevant wind conditions. The simulation results, given as air changes per hour for a given area, are combined with probabilities of the wind conditions in order to evaluate fulfilment of requirements (e.g. minimum 12 air changes per hour for 95% of the time in a hazardous area). Further, it can be important to consider so-called “stagnant zones” in hazardous areas, where ventilation is very low and gas accumulation can therefore become significant even if very small leaks should occur.
Example of calculated air changes per hour for a hazardous area, given one wind speed for 12 different wind directions:
CFD simulation is a very effective and intuitive tool for proposing possible improvements for increasing natural ventilation.
Example of plot of velocities in an area:
Mechanical Ventilation Analysis
The purpose of mechanical ventilation analysis is to assure that ventilation rates in mechanically ventilated areas fulfil applicable requirements for operation.
The CFD tools FLACS or FLUENT may be used in these analyses, where a 3D model of the installation is made and ventilations simulations are run for relevant ventilation rates. The simulation results, given as air changes per hour for a given enclosed area, are evaluated against requirements (e.g. minimum 12 air changes per hour). Further, it can be important to consider so-called “stagnant zones” in mechanically ventilated areas, where ventilation is very low.
Wind Chill Analysis
The purpose of Wind Chill Index (WCI) analysis is to evaluate the Wind Chill Index (WCI) for outdoor working areas, which must fulfil applicable requirements for operation.
The CFD tool FLACS is used in these analyses, where a 3D model of the installation is made and ventilations simulations are run for relevant wind conditions. The simulation results are combined with probabilities of the wind conditions in order to evaluate fulfilment of requirements (e.g. not to exceed WCI = 1000 W/m2 in more than 2% of the time). WCI analysis is input to outdoor operation analysis.
Pecentage of wind chill above 1000 W/m2 for a horizontal plane of an offshore platform (NORSOK requirement is 2%)
Outdoor Operations Analysis
Outdoor operation analysis is performed to evaluate whether outdoor operations can be performed within working environment criteria for Wind Chill at workplaces in naturally ventilated working areas. The analysis is an extension of the WCI analysis, wherefrom the results are evaluated further regarding how to meet the criteria for workplaces:
Evaluate possibility of e.g.
- Layout changes
- Weather protection
- Acceptable limitations for operation at workplaces (coldest months, wind conditions)