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London, United Kingdom

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Witlox H.W.M.,DNV Software | Stene J.,DNV Software | Harper M.,DNV Software | Nilsen S.H.,Statoil
Energy Procedia | Year: 2011

Projects in carbon capture and storage technologies for energy production involve the transport of vapour, liquid and supercritical CO 2 and CO 2/hydrocarbon gas mixtures via pipelines and process systems with subsequent injection into wells, e.g. offshore under the seabed. In addition several chemical companies often store and transport large quantities of CO 2 and this may also represent a hazard. There is a need to model potential loss of containment scenarios for risk assessment and design purposes for such installations. It is observed that several models used in quantitative risk analyses and hazard assessment studies are not able to take into account modelling of the thermodynamics of CO 2 in case of accidental releases from dense or supercritical conditions. Statoil together with DNV therefore initiated a project to further improve the Phast code for modelling of CO 2 releases. The work and methodology derived in this project have mainly been developed by Det Norske Veritas (DNV), but with significant co-operation and input by Statoil. The consequence modelling package Phast examines the progress of a potential incident from the initial release to the far-field dispersion including the modelling of rainout and subsequent vaporisation. The original Phast 6.54 models allow the released chemical to occur only in the vapour and liquid phases. The new Phast 6.6 models were extended to also allow for the occurrence of fluid to solid transition in case of CO 2 releases. This applies both for the post-expansion state in the discharge model, as well as for the thermodynamic calculations by the dispersion model. Here it is assumed that no solid deposition occurs on the ground. The current paper documents work regarding modelling by Phast 6.6 of discharge and atmospheric dispersion of carbon dioxide, including a detailed sensitivity analysis for a wide range of scenarios (base cases) including high-pressure cold releases (liquid storage) and high-pressure supercritical releases (vapour storage) from vessels, short pipes or long pipes. The objectives of this work were to examine the effect of input parameters on key output data, to ensure robustness of the models, and to identify further model improvements where deemed to be necessary. © 2011 Published by Elsevier Ltd.


Bettis R.J.,HSL Fire and Process Safety | Jagger S.F.,HSL Fire and Process Safety | Witlox H.W.M.,DNV Software | Harper M.,DNV Software
Journal of Loss Prevention in the Process Industries | Year: 2013

This paper describes the results of the first experimental stage of Phase IV of a Joint Industry Project (JIP) on liquid jets and two-phase droplet dispersion. The objective of this stage of the JIP was to generate experimental rainout data for non-flashing water and xylene experiments. See the overview companion paper I for a wider overview of the problem, model implementation and associated model validation.A range of orifice sizes (2.5 and 5 mm) and stagnation pressures (4-16 barg) were applied. Measurements included flow rate, initial droplet size, plume concentrations/temperatures for a range of downstream locations, and distributed rainout. Instead of the Phase Doppler Anemometry method used for droplet size measurements earlier in the JIP, a photographic technique was applied in an attempt to include measurement of the larger (non-spherical) droplets. This enabled a more accurate evaluation of the initial droplet size distribution and a much clearer understanding of the droplet morphology. The results showed that the droplet behaviour in the jet is more complex than had been anticipated with the mass distribution dominated by a very small number of large non-spherical droplets. Consequently a large number of spray images were required to evaluate an accurate size distribution. Distributed rainout was measured by weighing the amount of rainout in trays positioned along the jet direction. The rainout results showed a good degree of repeatability and internal consistency. They indicated that an increasing proportion of the released material did not rainout for increasing pressure. Rainout distance also increased with increasing pressure. Evaporation of the liquid was confirmed by temperature measurements, which showed the effect of evaporative cooling. Xylene concentration measurements (up to 1%) were carried out using a direct reading photoionization detector calibrated for xylene (measuring vapour only). For a limited dataset, the accuracy of these measurements was estimated by means of comparison against an alternative more time-consuming concentration method (xylene absorption onto a charcoal filter; measuring both vapour and liquid). The concentration measurements displayed several consistent qualitative features. For example, at a given downstream distance, the peak concentration increases with increasing pressure and nozzle diameter and the vertical height at which the peak is achieved increases. The cross-stream profiles displayed a consistent tendency to increased concentration at the edge of the jet, and the reason for this has not been established. Finally recommendations are provided for potential future work. © 2012 Elsevier Ltd.


Kay P.J.,University of Cardiff | Bowen P.J.,University of Cardiff | Witlox H.W.M.,DNV Software
Journal of Loss Prevention in the Process Industries | Year: 2010

This paper describes the results of the first stage of Phase III of a Joint Industry Project (JIP) on liquid jets and two-phase droplet dispersion. This stage included scaled experiments for water, gasoline, and cyclohexane for a range of superheats and nozzles with different aspect ratios. Additional experiments for butane and propane were conducted as a validation exercise and are discussed in the companion paper. Moreover this paper provides recommendations for atomisation correlations in the regimes of mechanical break-up, transition to flashing, and fully flashing. The objectives of this scaled experimental programme are to: (i) provide confidence in the previously proposed modelling methodology (Phase II) across a broad range of initial conditions (ii) update the models' correlations to generalise further its applicability (iii) recommend further model improvements. Development of new correlations for Sauter Mean Droplet diameter (SMD) and droplet size distribution is based on a best fit of the current scaled experimental data. The new data endorses the previous tri-functional Phase II approach including regimes for mechanical break-up, transition to flashing, and fully flashing, which is hence updated in the new Phase III SMD model. Considerable effort is devoted to capturing the full droplet size range under low-superheat conditions. Also, new enhancements in PDA technology were adopted to enable better quality data under high-superheat conditions. The priority recommendation for further model improvement is better characterisation of the poor quality releases under low-superheat conditions, where this work indicates that extremely broad droplet size distributions are likely. A companion paper (Part I) presents a more general overview of the dispersion problem, implementation of the correlations and subsequent performance against both the current scaled experiments and additional large-scale butane experiments. © 2010 Elsevier Ltd.


Witlox H.W.M.,DNV Software | Harper M.,DNV Software | Oke A.,DNV Software | Bowen P.J.,University of Cardiff | Kay P.,University of Cardiff
Journal of Loss Prevention in the Process Industries | Year: 2010

Many accidents involve two-phase releases of hazardous chemicals into the atmosphere. This paper describes the results of a third phase of a Joint Industry Project (JIP) on liquid jets and two-phase droplet dispersion. The aim of the project is to increase the understanding of the behaviour of sub-cooled non-flashing and superheated flashing liquid jets, and to improve the prediction of initial droplet size, droplet dispersion and rainout.Phase III of the JIP first included scaled experiments for materials with a range of volatilities (water, cyclohexane, butane, propane and gasoline). These experiments were carried out by Cardiff University including measurements of flow rate and initial droplet size across the full relevant range of superheats. See the companion paper II for further details of these experiments and the derivation of new refined correlations for droplet size distribution and Sauter Mean Diameter. Furthermore large-scale butane experiments were carried out by INERIS (France) to ensure that for more realistic scenarios the derived droplet size correlations are accurate.Model validation and model improvements were carried out by DNV Software, including validation of release rate and initial droplet size against the above scaled and large-scale experiments. New correlations for droplet size distribution and Sauter Mean Diameter (SMD) were implemented into the Phast discharge model. These were compared against a range of other droplet size and rainout correlations published in the literature, in conjunction with validation against an extensive set of experiments. It was shown that the new droplet size correlation agrees better against experimental data than the existing Phast correlation. To further improve the rainout prediction, the Phast dispersion model (UDM) was also extended to allow simultaneous modelling of a range of droplet sizes and distributed rainout (rather than rainout at one point). © 2010 Elsevier Ltd.


Gant S.E.,UK Health and Safety Laboratory | Kelsey A.,UK Health and Safety Laboratory | McNally K.,UK Health and Safety Laboratory | Witlox H.W.M.,DNV Software | Bilio M.,UK Health and Safety Executive
Journal of Loss Prevention in the Process Industries | Year: 2013

A methodology is presented for global sensitivity analysis of consequence models used in process safety applications. It involves running a consequence model around a hundred times and using the results to construct a statistical emulator, which is essentially a sophisticated curve fit to the data. The emulator is then used to undertake the sensitivity analysis and identify which input parameters (e.g. operating temperature and pressure, wind speed) have a significant effect on the chosen output (e.g. vapour cloud size). Performing the sensitivity analysis using the emulator rather than the consequence model itself leads to significant savings in computing time.To demonstrate the methodology, a global sensitivity analysis is performed on the Phast consequence model for discharge and dispersion. The scenarios studied consist of above-ground, horizontal, steady-state discharges of dense-phase carbon dioxide (CO2), with orifices ranging in diameter from 1/2 to 2 inch and the liquid CO2 stagnation conditions maintained at between 100 and 150 bar. These scenarios are relevant in scale to leaks from large diameter above-ground pipes or vessels.Seven model input parameters are varied: the vessel temperature and pressure, orifice size, wind speed, humidity, ground surface roughness and height of the release. The input parameters that have a dominant effect on the dispersion distance of the CO2 cloud are identified, both in terms of their direct effect on the dispersion distance and their indirect effect, through interactions with other varying input parameters.The analysis, including the Phast simulations, runs on a standard office laptop computer in less than 30 min. Tests are performed to confirm that a hundred Phast runs are sufficient to produce an emulator with an acceptable degree of accuracy. Increasing the number of Phast runs is shown to have no effect on the conclusions of the sensitivity analysis.The study demonstrates that Bayesian analysis of model sensitivity can be conducted rapidly and easily on consequence models such as Phast. There is the potential for this to become a routine part of consequence modelling. © 2013.


Witlox H.W.M.,DNV Software | Harper M.,DNV Software
Journal of Loss Prevention in the Process Industries | Year: 2013

Many accidents involve two-phase releases of hazardous chemicals into the atmosphere. This paper describes the results of the fourth phase of a Joint Industry Project (JIP) on liquid jets and two-phase droplet dispersion. The objective of Phase IV of the JIP was to generate experimental rainout data for non-flashing experiments, and to develop recommendations for the best methodology to predict rainout [total rainout mass and its spatial distribution ('distributed' rainout)]. Phase IV of the JIP first included rainout experiments by the UK Health and Safety Laboratory (HSL) for sub-cooled releases of water and xylene with a range of orifice sizes and stagnation pressures. See the companion paper II for further details. Secondly model validation was carried out by DNV Software for these experiments using different correlations for the initial droplet size (Sauter Mean Diameter, SMD), i.e. the CCPS SMD correlation and the Phase III JIP SMD correlation. The validation includes flow rates, droplet size, distributed rainout and cloud temperature drop. Subsequently validation was considered for a wider range of experiments from the literature (sub-cooled and superheated releases) for both SMD and total rainout. Adopted rainout methods comprised both methods including explicit modelling of the droplets (using an extended version of Phast dispersion model UDM), as well as more simple methods based on rainout correlations without droplet modelling. Recommendations are made for the most accurate droplet size and rainout modelling. A modified CCPS UDM droplet size correlation has been shown to agree best against experimental rainout data. © 2012 Elsevier Ltd.


Xu Y.,DNV Software | Worthington D.,DNV Software
1st CCPS Asia-Pacific Conference on Process Safety 2013, APCPS 2013 | Year: 2013

The complex and densely packed nature of offshore facilities presents a series of challenges in terms of the potential for fire hazards to develop as well as the ability to model them. In the former, the key issue is the increased potential for escalation, i.e. hazards originating in one area could pose a risk on equipment and people in other areas, and the difficulties to evacuate workers. With the later, the challenge is being able to understand the extent to which the geometry impacts on the development of the hazard and account for this in the modeling. Modfire, a new empirical fire model, has been developed specifically for fires in offshore modules for offshore QRA. It extends the application of simple fire models developed for pool fires and jet fires for onshore QRA to the offshore environment. Using discharge results as the input, Modfire predicts flame shapes with consideration of the interaction between a fire and surrounding boundaries, without the need of huge computing power. The predicted fire characteristics are dependent on release conditions, boundary geometries and ventilation. This paper presents the modeling approach of Modfire and validation work undertaken against CFD predictions. © Copyright (2013) By AIChE. All rights reserved.


Witlox H.W.M.,DNV Software | Harper M.,DNV Software
Process Safety Progress | Year: 2014

Many commonly used atmospheric dispersion models cannot accurately simulate time-varying releases of hazardous chemicals in the atmosphere. A new version of the Phast Unified Dispersion Model (UDM) has been developed to more accurately simulate time-varying effects resulting from a decreasing discharge rate in a vessel or pipe and/or resulting from a time-varying pool (following rainout or from direct spill). The new model includes effects of along-wind diffusion which reduces concentrations in the far-field. This is particularly relevant for toxic releases. The new formulation presumes a number of "observers" to be released at successive times from the point of discharge or the upwind edge of the pool. The UDM first carries out pseudo steady-state calculations for each observer, where the release data correspond to the time at which the observer is released, and where the observer picks up vapor while traveling over the evaporating pool. Subsequently, effects of along-wind diffusion are included by means of Gaussian integration of observer concentrations over the downwind distance. The above new UDM formulation has been verified against the time-varying HGSYSTEM model HEGADAS-T for the specific case of dispersion directly from a pool. Furthermore, it has been tested for more general cases including elevated releases with rainout. © 2013 American Institute of Chemical Engineers.


Witlox H.W.M.,DNV Software | Harper M.,DNV Software | Oke A.,DNV Software | Stene J.,DNV Software
Process Safety and Environmental Protection | Year: 2014

This paper discusses the validation of discharge and subsequent atmospheric dispersion for both unpressurised and pressurised carbon dioxide releases using the consequence modelling package Phast. The paper first summarises the validation of the Phast dispersion model (UDM) for unpressurised releases. This includes heavy gas dispersion from either a ground-level line source (McQuaid wind-tunnel experiments) or an area source (Kit-Fox field experiments). For the McQuaid experiments minor modifications of the UDM were made to support line sources. For the Kit Fox experiments steady-state and 20-s finite-duration releases were simulated for both neutral and stable conditions. Most accurate predictions of the concentrations for finite duration releases were obtained using the UDM Finite Duration Correction method. Using experiments funded by BP and Shell and made available via DNV's CO2PIPETRANS JIP, the paper secondly summarises the validation of the Phast discharge and dispersion models for pressurised CO2 releases. This modelling accounted for the possible presence of the solid CO2 phase following expansion to atmospheric pressure. These experiments included both high-pressure steady-state and time-varying cold releases (liquid storage) and high-pressure time-varying supercritical hot releases. Both the flow rate and the concentrations were found to be predicted accurately. The above validation was carried out with no fitting whatsoever of the Phast extended discharge and dispersion models. © 2013 The Institution of Chemical Engineers.


Borges V.,DNV Software | Hickey C.,DNV Software
Safety, Reliability and Risk Analysis: Beyond the Horizon - Proceedings of the European Safety and Reliability Conference, ESREL 2013 | Year: 2014

RAM analysis is a well-established methodology used to predict the performance of systems taking into account the Reliability, Maintainability and Availability. An even more challenging application of this methodology relates to oil and gas developments where a large number of parameters are interrelated and the interdependency of the systems must be taken into account. In order to achieve a comprehensive picture of the production efficiency, not only the reliability and maintainability of the system must be taken into account but also the manner the system is operated. This paper presents an example of how the operability term is crucial to oil and gas development, showing a base case and sensitivity analysis. © 2014 Taylor & Francis Group, London.

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