Introduction to consequence modelling

Transcription

1 QUESTIONS AND ANSWERS Introduction to consequence modelling PRESENTERS: Kehinde Shaba (DNV GL - Software), Mark Hunter (DNV GL Oil & Gas, Risk Advisory) 1. I've found that some chemicals cannot be individually tracked in Phast. Why? As far as we are aware the material to track feature is designed to allow users to track concentrations of a material (if modelling a pure material), or any components in a mixture (if modelling a mixture) and there are no restrictions. If you are having trouble working with this feature and selecting a material, please send an to our support desk and we will be happy to help. The address is software.support@dnvgl.com. 2. We use Phast and other tools. What are some simpler modelling tools you can recommend? There are a range of approaches to consequence analysis which includes calculators on certain websites as well as spreadsheet solutions. A cursory web source will identify several options. 3. We are using Phast 7.11, dispersion / explosion modelling with consideration of congested area etc. is possible? Yes, this is possible, though the 3D explosions / extended explosion modelling optional license add-on is required in order to be able to use these features. 4. Is there a definition of what a congested volume is? The TNO yellow book (CPR14E Methods for the Calculation of Physical Effects) gives a good overview of congestion and how it s modelled for explosions. This paper presentation gives a general overview with examples: _Siting_Hazard_Distance_Predictions_Using_the_TNO_Multi- Energy_Model/links/568a890308aebccc4e1a0452.pdf?origin=publication_list We ran a webinar last year that explored explosion modelling in a great amount of detail. The webinar was titled Explosion modelling in onshore facilities - Considerations for congested and confined regions. Please click here to access the webinar material and related content. The webinar also includes a list of refences that will be useful with regards to definitions, methodologies etc. 5. Is Phast the right tool to perform gas dispersion modelling from a flare tip during a cold vent scenario? Yes, Phast can certainly be used in this scenario and will give reliable results.

2 6. Compared to other models, for example Trace, what are the advantages and disadvantages of Phast? Phast has many features that make it unique when compared to other consequence analysis models. For example, Phast includes: - A proprietary dispersion model called the UDM that is suited to handle all type of releases vapour, liquid and two phase as well cloud behaviour (dense, buoyant, active to passive transition etc.) - Direct linking to a QRA tool, Safeti - Extensions for modelling multicomponent mixtures - A 3D congested explosion modelling extension that enable detail explosions modelling full 3D (i.e. 3D cloud interaction with regions of congestion) - The model is approved for LNG dispersion modelling in the US by the regulator (PHMSA) amongst others. Phast is also widely seen as an industry standard tool for consequence modelling and is used all around the world by various stakeholder groups from regulators, to operators, to consultants, to academia. etc. Please see the Phast product page online for more details. 7. My interest is particularly around the physical property models that are used in Phast. Given that many mixtures in the industry are far from ideal (both in liquid and vapour phases) have you conducted any study around the models that are available in Phast and whether choosing different models might impact the results of the consequence modelling study? Predicting properties of materials over a range of physical conditions is a complex and challenging exercise. This is especially the case when materials are not ideal. Phast includes a property system calculator for both pure and multicomponent property calculations (including equilibrium calculations). It makes use of the DIPPR database for pure component thermosphysical data and for additional flammable/toxic properties. Various equations of state can be utilised More details of the property calculations can be found in the theory document provided with the program Property System Theory (Available from the help section under Technical documentation ). This documents also includes details of the verification and validation of the model (i.e. comparison versus experimental data or other physical property models). In the specific case of multi-component mixtures, Phast includes a dedicated extension that allows for more rigorous modelling of mixtures. More details of this add on can be found on the product page here. 8. You mentioned it was best to handle products that are mixtures by tracking the individual components. Can you explain how that is done? As discussed in the response to question 9 above, Phast includes a dedicated extension that allows for more rigorous modelling of mixtures covering - discharge, flashing, rainout, pool evaporation etc. This ensures that the right material composition is considered as part of the dispersion modelling. For e.g. in a mixture of heavy and light hydrocarbons, following a release, the vapour phase will be rich in the more volatile components and the liquid phase in the less volatile ones. If modelling of mixtures is important to you, we would recommend you use this add on. 2

3 On a separate note, it is possible to track individual components of a mixture in a dispersing cloud. This can be done using the material tracking feature discussed in Question 1 above. 9. Can we have one example showing multi-component CM scenario using Phast? Unfortunately, the short duration of the webinar did not allow for an example to be shown. However, we are happy to provide a demonstration to any interested parties. Please contact Software.communications@dnvgl.com if this is of interest to you. 10. For QRA studies we specify a point location for leak and keeping in mind the dynamics of dispersion/consequences Phast models. This can vary the LSIR outcome. What would be the Best Industry practice for choosing the location of leak so that it gives the most practical scenario of LSIR. If the analysis is considering general overall risk on a plant, then the usual practice would be to put the release point either, a) roughly in the centre of the section of equipment it is representing a release from; b) at the location of the largest inventory within that section, typically a large vessel. If the aim is to assess hazardous distances, then it may be more appropriate to put the release point at the point in the section closest to the edge of the equipment/ plant so as to capture the largest possible distance away from the release equipment that a release might travel. 11. When I use open source software for conducting CM like ALOHA, is it still reliable? The proprietors/developers of other tools are the best placed persons to comment on their reliability. It is a well-established fact that issues with accuracy can arise with simplified tools as well as the fact that they tend to be designed to give conservative results (which can result in safety mitigation costs that are unnecessary and excessive). 12. Can Phast consider inference from nearby buildings/structures when modelling toxic clouds? In 3D? To a limited degree yes. Dispersion in open areas will be different to say a built-up area (with large buildings etc.). This is because of changes in atmospheric turbulence which affect dilution, missing etc. Phast includes a surface roughness parameter setting that allows users to select the environment in which the cloud is dispersing. This has an impact of the predicted hazard ranges. Note that specific modelling of the effects on toxic gas dispersion of 3D objects or ground undulations in the path of the cloud cannot be modelled in Phast. 13. Can Phast handle mixtures, either 2 hazardous chemicals or a chemical in water? Yes, mixtures of two hazardous chemicals can be modelled successfully in Phast. 14. Often we characterize risk as the potential severity of injury. How does one correlate impact values from Phast as kw/m 2, kpa/impulse, ppm toxic/time exposure at a given coordinate to injury versus fatality potential? Such correlations are often explicitly stated and e.g. 100% fatality is assumed on exposure to a flash fire hazard. As such no further correlation is needed as the flash fire hazard range in Phast determines the 100% fatality level. The same applies for fire and explosion hazards (i.e. certain hazard levels are understood to result in certain harm levels). Toxics hazards tend to be handled differently largely because impacts are based on a combination of two parameters concentration and exposure duration. The injury potential is calculated using a probit 3

4 equation. (Note that threshold concentration based approaches are still in use but are generally see to be less accurate (and possibly more conservative) than probit based approaches). In the case of toxic dispersion calculations, Phast will output distance versus probability results (footprint, downwind etc.) based on the material probit. In Quantitative Risk Assessment (QRA), where the goal is to calculate the risk of death associated with a hazard, this calculation is carried directly in Safeti (a QRA software from DNV GL) and the impact value is mapped to a fatality severity using a methodology selected by the user, e.g. for thermal radiation, selecting specific radiation levels which correlate to set facility probabilities. 15. Do you have a plan to develop the consequence model under upwind condition? Yes, we do. It is one of the features we intend to develop soon, and one that will significantly extend the capabilities of the dispersion model (the Unified Dispersion Model, UDM) in Phast. At present, the nest major update to the UDM involves including along wind diffusion and gravity spreading to allow for better modelling of short duration and time varying releases (with rainout). This will be made available in Phast/Safeti 8.0 that is scheduled for release in late Read more about the changes here. After this major update, we will explore further changes to the UDM which will include considerations of upwind dispersion modelling. 16. I have always found it difficult inserting a layout (in auctocad) in my PHAST modelling. Please can you throw more light on this We are sorry to hear that you are having trouble with this functionality. The process was significantly simplified in Phast 7.x and users simply have to connect to a CAD file (on Tool ribbons, you will find CAD under the Input group. See image below). Alternatively, users can go to the map tab, right click at the root of tree and select External connection. This will open several options of which CAD is one. (See image below). 4

5 If you are still having problems after exploring the above, please contact our support team at and we will be happy to take this further. 17. Relation between Consequence Model results and actions to be implemented (example SIS or other,..)? The results of the consequence modelling will identify whether there is an issue and the extent. A follow up exercise will be to determine what actions can be taken to mitigate the problem (e.g. increase separation distance etc.). 18. What does LSIR take into account? LSIR is short for Location Specific Individual Risk. It is a cumulative measure of the risk of death from a range of hazards at a particular location. Consequently, the LSIR value calculated in Safeti will take reflect any hazardous scenarios that are predicted to impact the specific location and will cover fires, explosions and toxic gases as appropriate. 5

6 19. Is it real to have a consequences radio of 50 meters of 32 kw/m 2 coming from a Jet Fire of 50 m coming from a hole size of 15 mm? Is the operational pressure maintained after the release? Is it real to have a consequence radio of 50 m of a Jet Fire coming from a hole size of 15 mm? We are unable to comment on specific results, but if the scenarios have been set up correctly, then Phast s validated models can be relied on to give trusted, validated results. The exact results possible will depend on the input parameters used for the case. Jet fires are momentum driven events and external factors such as weather are relatively minor drivers. The key drivers are the hole size and system pressure. In most cases, following a release, the pressure in the system will decay and the jet fire hazard range will be expected to diminish as well. A full time varying release profile can be modelled in Phast. For the jet fires calculations, the user is required to specify a rate to be used to model the corresponding jet fire hazard range. As a default, the initial rate is used (and this will give the most conservative output). Users can vary this default to give jet fire hazard ranges at other times e.g. after 1 min, 5 min, 10 min etc. 20. When will Phast be able to handle topography i.e. Norwegian fjords? See response to question 12 above. 21. How is a boilover scenario, such as described in LASTFIRE, modelled in SAFETI/Phast? If confined to a bund, then the aftereffects of a boilover could be modelled by a standalone pool fire, restricted to the size of the bund. The immediate boilover itself, with liquid potentially spraying from the top of the tank, cannot be modelled. 22. What is the practice in assumption of modelling for toxic H 2S release hole? This will depend on the specific case. Typical inputs needed will be: - Release hole sizes - Pressure, temperature - Mass/volume in the section - Probit values to be used (for probit analysis)/threshold concentration (for concentration based assessment) - Local weather conditions 23. Is capping of dispersion contours a standard practice? Yes because dispersion calculations are usually done to a concentration of interest e.g. lower flammable limit. Without such a threshold defined, it will be impossible to determine the extents of the cloud footprint. The concentration for dispersion modelling is capped at a lower bound (which is user modifiable) to give an end point for calculations. The contours themselves will be plotted depending upon the levels selected by the user. 24. What is the basis in selection HK, TNO and UK HSE in probit? The different regulatory authorities have developed their probits/ criteria based on experience, experimental data, past incidents and an understanding of public risk tolerability. These factors will 6

7 vary between jurisdictions, though there is broad high-level agreement between them on the overall governing principals. This is where social consent is a key factor. Assumptions should be agreed by all stakeholders involved. 25. How the Phast sense congested area? In Phast 3D Explosions (i.e. Phast with the 3D Explosions add-on), users can draw the location of their congested regions on the map. The congested region is defined on a full 3D basis (with the height input by the user as well. Also, note that congested areas can be elevated e.g. piperacks (i.e. they do not have to start at ground level). As is standard, the location of scenarios are defined on the map. For a consequence modelling run, the coordinates of the dynamic cloud are tracked by Phast and compared to the location of the congested region. This allows Phast to know when a cloud has come into contact with a congested area. Please note that congested area modelling with cloud/congested area interaction requires the 3D explosion/extended explosion modelling optional license add-on. 26. What is the difference between toxic and flammable averaging time and on what basis? Please see response to Question 42 below. 27. Consequence effect depends on only release rate or on inventory and release rate? This will depend upon the type of scenario and the type of hazard being considered. For example, for a jet fire, the initial hazardous area for people will depend only on release rate. For an unbunded pool fire, the size of inventory will determine the ultimate size of the pool. (Note that these are two very discrete examples, and in general, there are a number of factors to consider in identifying the key inputs.) 28. How Phast takes into account protection layers like detection, protection, control, mitigation etc.? This largely depends on the protection layers that need to be considered. In some scenarios in Phast (e.g. Long Pipeline), it is possible to define various valves types (e.g. shutdown valve) on a pipeline and model how these will impact on the duration of a release. This can be useful in determining the need for, and location of valves along a pipeline route. 29. Will there be a webinar in the future about how to use Phast? Plans for future webinar topics are under consideration and all suggestions are welcome. Please send suggestions to Software.communications@dnvgl.com. Please note detailed courses on how to use Phast are available from DNV GL. Please see this webpage for details: Are these results (from CM) used as input in SIL assessments? Consequence modelling results could certainly be used as inputs to a SIL assessment. The specifics will depend upon the system being analysed and the type of study being carried out. 31. What is the database that is used for the frequencies in these assessments? DNV GL recommends the use of the UK HSE s Hydrocarbon Release Database (HCRD). For detail, see for example these publications 7

8 Does Phast have the ability to model both the toxic and flammable hazard as one model or are these two separate models Yes, both toxic and flammable effects from one release can both be modelled. Phast incorporates very intelligent decision calculation logic that means results are generated where they are credible. For example, every material in the database is classed as flammable, toxic or both. A flammable material will only result in flammable calculations; toxic materials will yield toxic results. Material classed as both will give both sets of results. Also, the phase plays a role. For example, flammable vapor releases will give rise to jet fires whilst liquid releases will yield pool fires. 33. Typically your model will have potentially multiple release points - can Phast do this from a plot plan or do you model multiple releases and then plot onto the plot plan? Multiple release points (scenarios) can be defined directly on a plot plan. Once defined, it is also possible to move the location around - for e.g. to view the same results but at another location. 34. Will CM in 10 years all be CFD models? What's your opinion? Conceptually, the major benefit of phenomelogical models (i.e. tools like Phast) versus CFD are: - the ability to get results in a very short amount of time - the ability to run large amounts of cases in an efficient way (cost and time). - Nominal expertise required (versus CFD which is often a specialist undertaking) It is expected that these key differentiators versus CFD based tools and will remain so for some time to come. It is difficult to say what will happen in 10 years, but nevertheless, there is a clear trend to improving visualization and moving to 3D approaches. For e.g. tools like Phast now have the capability to model objects in 3D and take into account the shielding effect of walls/buildings from fire hazards. Read more about this here and see image below. This 3D modelling capability will be extended to dispersion and explosion modelling in the near future. Whilst not a CFD based approach, it enables users to see their results in 3D and do more detailed calculations. 8

9 35. If engaging a CM specialist, how many years' experience should I look for in a junior and senior? This is largely a matter of opinion and there are no hard and fast rules. One approach might be that a junior is from 0 years experience, senior 5+ years. 36. When we are modelling physical effects using Phast, interactions between different substances (that can react with each other) are/can be considered? Currently, reaction modelling in Phast is only supported for Ammonia (reaction with water) and Hydrogen Fluoride (reaction with the atmosphere). 37. There are any tools that deals specifically with environmental effects? There are a variety of tools available focussed on environmental effects, it would depend upon which specifically are of interest. 38. Phast can be used for academic purposes? Is there any special licence for universities, researches? Yes. We do have license solely for academic and research purposes. Please contact DNV GL for license queries: Software.communications@dnvgl.com 39. Can you point us to authoritative guidance on scenario selection. The identification of releases is OK, what I mean is how to cut down on the potentially vast number of cases with wind conditions, etc. Please see the Useful references slide in the presentation pack. 40. Mixture plays different from single material. What do you recommend to include maximum how many materials in mixture? Typically, 5 or 6 components, representing the most dominant components in a mixture, is a sensible upper limit on the number of components to consider. 41. BLEVE needs to be modelled as standalone only? Yes, there is a standalone BLEVE model in Phast. 9

10 42. We have a case with a toxic in a short pipe section where there is a valve on one side and a hose connection on the other. The hose fails at the connection so the leak is catastrophic but very brief, i.e. like a "puff". Phast uses time averaging but this release rate is much shorter. How do we model that? Scenarios like this are at the boundary between short duration and instantaneous releases. Initially, we would suggest trying both approaches and comparing the results although it is thought this would probably be handled best using the instantaneous release model in Phast. Averaging time and release duration are two related but distinct concepts. A further concept is exposure duration. Release duration is as the name suggests the duration of a release from an orifice e.g. if a release rate is calculated to be 5kg/s and the mass involved in 100 kgs, the release duration will be 20s (assuming a constant rate). Averaging time relates to the concentrations measured at locations downwind of a release as a passive cloud transverses that location. Due to variations in cloud concentration, a sensor located at some point downwind of a release will measure a range of concentrations as the cloud moves over it. This necessitates the need to evaluate an average concentration over a time period. See image below: Reference: ion%20models.htm Very short averaging times are the norm for flammable materials (typically high concentrations). In Phast the default averaging time is 18.75s. and this is taken to apply for flammable materials. Practically speaking, an averaging time of 18.75s is essentially the same as using the initial peak concentration. Whereas longer averaging times are needed for toxic calculations to capture the fact that impacts tend to be over a longer duration. Exposure duration can be thought of as the duration for which the receptor (i.e. a person or sensor) is exposed to the cloud. For toxic releases, the ideal case would be where the averaging time equals the exposure duration so that a good estimate of the cloud concentration at that location can be achieved. 10

11 Phast includes an option that allows users to set the averaging time as equal to exposure time and we would recommend this to be used for all toxic calculations. 43. For toxic and flammable material, to use flammable modelling or toxic? See response to Question 32 above. 44. Is Phast capable of modelling internal explosion within equipment and the consequent projectile distance Phast can model an explosion due to the build-up of a gas cloud within a congested equipment area. The effects modelled would be the blast overpressure level though, not necessarily the projectile travel distance. 45. How do we decide about the best scenario that can represent the best actual scenario that can happen for the facility under study? Any criteria? This would be a fundamental basis of consequence modelling and too large a subject to be answered here. It will typically be up to user knowledge and/ or experience. Please see the Useful references slide in the presentation pack for some sources which go into this subject in detail. 46. I've heard about explosion overpressure. Is it different from the explosion pressure 100mbar shown in the slide? No difference, the 100 mbar plotted is the explosion overpressure. 47. I have a doubt regarding the benefits obtained by using Phast. As per webinar, Consequence result can be used for flare stack height. Also, I have heard that Phast can be used to locate gas/fire detector location. Phast can be used for these purposes, but user skill will be required in setting up applicable jet fire details for a flare design, or gas dispersion scenario modelling for gas/fire detector location. In the latter case, the basic dispersion results will need to be post-processed to determine what the optimum detector placement will be. This is a service DNV GL offers, please contact us if this is of interest or further detail is required. 11