LNG Model Evaluation Protocol & Validation Database Update

Transcription

1 2017 NFPA Conference & Expo LNG Model Evaluation Protocol & Validation Database Update Presented by: James Stewart U.K. Health & Safety Executive and Daniel Gorham Fire Protection Research Foundation

2 Overview Session Learning Objectives LNG Facilities and NFPA 59A The LNG Model Evaluation Protocol (MEP) - What is the LNG MEP and what is its purpose? The LNG Model Validation Database Updating the LNG MEP and Database - Why have the LNG MEP and Database been updated? - What changes have been made? - How does this affect the process of gaining approval for alternate vapor dispersion models for use in LNG siting applications? Reviewing Updated Versions of Approved Models Key Messages

3 Session Learning Objectives The aims of this NFPA Conference & Expo 2017 Education Session are to: Provide information on how U.S. energy needs have lead to increased attention for LNG facilities and NFPA 59A Explain the purpose of the LNG Model Evaluation Protocol (MEP) and the LNG Model Validation Database Describe the updates and changes that have recently been made to the LNG MEP and Database

4 Existing LNG Import/Export Terminals Source: Source: Source:

5 Proposed LNG Export Terminals Source: Federal Energy Regulation Commission (FERC)

6 History of NFPA 59A AGA begins work on LNG 1960 Standard prepared by Sectional Committee on Utility Gas tentatively adopted 1966 Committee on LNG established to develop standard with scope broader than utility gas plant applications Draft submitted to NFPA as basis for new standard 1967 First official edition of NFPA 59A adopted 1971 First edition of NFPA 59A developed under broadened scope

7 Purpose To provide minimum fire protection, safety, and related requirements for the location, design, construction, security, operation, and maintenance of LNG plants. Source:

8 Scope and Application Applies to: (1) Facilities that liquefy natural gas (2) Facilities that store, vaporize, transfer, and handle LNG (3) The training of all personnel involved with LNG (4) The design, location, construction, maintenance, and operation of all LNG facilities Does not apply to: (1) Frozen ground containers (2) Portable storage containers stored or used in buildings (3) All LNG vehicular applications, including fueling of LNG vehicles (NFPA 52 and 30A)

9 LNG Facilities and NFPA 59A LNG Facility Siting Requirements NFPA 59A requires that any LNG container or transfer system must have a dispersion exclusion zone The size of such zones must be determined using an approved vapor dispersion model, one of: DEGADIS 2.1 FEM3A PHAST v6.6/6.7 FLACS v9.1 r2 49 CFR 193 Subpart B, states that alternative models may be used, subject to the approval of the Administrator In the case of LNG facilities the administrator is the Pipelines and Hazardous Materials Safety Administration (PHMSA)

10 LNG Facilities and NFPA 59A LNG Facility Siting Requirements PHMSA state that alternative models may be used in LNG siting applications provided that the models take into account the same physical factors as the approved models, are validated by experimental test data, and receive the Administrator s approval The LNG Model Evaluation Protocol should be used as a means of evaluating alternative models The LNG MEP, and the accompanying LNG Model Validation Database, have recently been updated to take account of additional requirements imposed by PHMSA

11 LNG Model Evaluation Protocol What is the LNG Model Evaluation Protocol (MEP)? The LNG MEP is a document outlining the process that should be used to assess the suitability of vapor dispersion models for evaluating hazard distances in LNG siting applications What is the purpose of the LNG MEP? The purpose of the LNG MEP is to provide a comprehensive evaluation methodology for determining the suitability of models to accurately simulate the dispersion of vapors emanating from accidental spills of LNG

12 LNG Model Evaluation Protocol What does the LNG MEP involve? The LNG MEP comprises three distinct phases: Scientific Assessment Model Verification Model Validation Each stage must be completed during an evaluation of a model against the LNG MEP The outputs are then recorded in a Model Evaluation Report This process must be followed when seeking approval to use an alternate vapor dispersion model for LNG siting applications

13 LNG Model Evaluation Protocol LNG MEP: Scientific Assessment Involves critically reviewing the physical, mathematical and numerical basis of a model by someone independent of the model development Key details of the model should be available for scientific assessment Model produces output suitable for assessment against MEP SPM Model should be based on accepted/published science Model accepts a credible source term Model accounts for the effects of wind speed Model accounts for the effects of surface roughness on dispersion Model accounts for the effects of atmospheric stability on dispersion Model accounts for passive dispersion Model accounts of gravity-driven spreading Model accounts for the effects of buoyancy on dilution Numerical methods are based on accepted/published practice practice Model is suitable for evaluation against MEP Model contains necessary physics with appropriate mathematical implementation Model is based on suitable numerical methods

14 LNG Model Evaluation Protocol LNG MEP: Model Verification Model verification is the process of checking that a computer implementation of the model accurately represents its mathematical basis In the LNG MEP, verification is treated as a passive element of the scientific assessment, rather than an exercise in its own right Evidence of model verification should be provided to demonstrate that the model being evaluated has been suitably verified PHMSA makes the ultimate decision about model suitability and limitations

15 LNG Model Evaluation Protocol LNG MEP: Model Validation Model validation is the most significant stage of the LNG MEP Scientific assessment and model verification give an indication of model suitability, whereas validation assesses how well a model actually performs for scenarios of practical interest Model validation for the LNG MEP involves comparing model predictions to experimental data for a range of dense gas dispersion scenarios The experiments included in the model validation database were selected to test the key physical processes involved in the dispersion of LNG

16 LNG Model Validation Database LNG Model Validation Database The Database is an Excel spreadsheet used during model validation for the LNG MEP It contains data from 33 experiments taken from 8 different series of tests Model predictions of gas concentrations at specified locations are entered into the Database for each experiment Max. arc-wise concentrations and Statistical Performance Measures (SPM) are then automatically calculated by the spreadsheet and then used for quantitative assessment of the model

17 LNG Model Validation Database LNG Model Validation Database Details of each experiment are given in separate Excel worksheets These details comprise: - Basic test information - Details of the substance released - Details of the conditions of release - Atmospheric conditions The Database also lists sensor locations and averaging times/windows for each experiment

18 LNG Model Validation Database LNG MEP: Model Validation The Database produces graphs of predicted vs. measured maximum arc-wise gas concentration for each experiment Two of the worksheets give tabulated values of SPM, colorcoded to show whether or not the suggested quantitative performance measures are met, e.g. for Maplin Sands trial 27

19 LNG Model Validation Database LNG MEP: Model Validation The final worksheet of the Database contains scatter plots of predicted vs. measured concentrations to give a visual representation of model performance Five such plots are produced to show model performance for: - All Trials - Field Trials - Wind Tunnel Trials - Unobstructed Trials - Obstructed Trials

20 LNG Validation Database Guide Guide to the LNG Model Validation Database Version 12 The Database is described in: Stewart et al. (2016), Guide to the Model Validation Database Version 12

21 LNG Validation Database Guide Guide to the LNG Model Validation Database Version 12 The Database guide describes the Model Validation Database and how it should be used as part of the LNG MEP It gives details of each series of experiments and describes the experimental uncertainties The methods used in the Excel spreadsheet to automatically calculate the SPM are also outlined

22 Updating the LNG MEP & Database Aims of the update: To add new features to the Database and amend the MEP to incorporate the additional requirements given in the PHMSA Advisory Bulletin PHMSA To incorporate previously-omitted experimental data and to correct errors within the Database To automate the calculation of SPMs and maximum arc-wise gas concentrations in the Database to reduce user effort during model validation To provide more extensive documentation and guidance on using the Database and MEP

23 Updating the LNG MEP & Database Deliverables from the LNG MEP & Database Update The 2nd Edition of the LNG Model Evaluation Protocol M. J. Ivings et al. (2016), Evaluating Vapor Dispersion Models for Safety Analysis of LNG Facilities, 2nd edition The LNG Model Validation Database version 12 An updated guide to the LNG Model Validation Database J. R. Stewart et al. (2016), Guide to the LNG Model Validation Database Version 12 The two reports are available from the NFPA research reports webpage 1 and the Database is available as an Excel spreadsheet on request 1

24 Updating the LNG MEP & Database Requirements of the PHMSA Advisory Bulletin PHMSA Maximum arc-wise gas concentrations - Method for determining these is now prescribed Experimental and modeling uncertainty - Discussion of experimental uncertainty incorporated into the Database Guide Additional SPM: - Concentration Safety Factor (CSF) - Concentration Safety Factor to LFL (CSF LFL ) - Distance Safety Factor to LFL (DSF LFL )

25 Updating the LNG MEP & Database Maximum Arc-wise Gas Concentrations Comparison of measured and predicted maximum arc-wise gas concentrations forms a key part of the LNG MEP model validation exercise Ensuring all models are compared in a like-for-like manner is fundamental for consistent model evaluation PHMSA has prescribed the method which should be used when applying for approval to use an alternate vapor dispersion model for use in LNG siting applications

26 Updating the LNG MEP & Database Maximum Arc-wise Gas Concentrations The maximum arc wise concentration should be based on the location of the experimental sensor data that produced the maximum arc wise concentration relative to the cloud centerline (PHMSA )

27 Updating the LNG MEP & Database Maximum Arc-wise Gas Concentrations: What s the difference? Difference between the PHMSA-prescribed method and the common 1 alternative is subtle but potentially significant Predicted max. arc-wise concentration Cloud centerline Predicted max. arc-wise concentration Cloud centerline Sensors Gas source Common Method PHMSA Method 1 The method often used is that of Hanna et al. (1993) where the max. arc-wise concentration at a given downwind distance is taken as the max. at any position along the arc at the height of the lowest experimental sensor

28 Updating the LNG MEP & Database Maximum Arc-wise Gas Concentrations: What are the benefits of the PHMSA approach? The method is conservative, it will typically result in larger exclusion zones for LNG sites It accounts for cases where the plume bypasses the gas sensors It encourages the development of plume meandering models

29 Updating the LNG MEP & Database Experimental and Modeling Uncertainty Following guidance given in the PHMSA Advisory Bulletin PHMSA , sensitivity analyses must be undertaken during the model validation stage of the LNG MEP The updated Database Guide gives an overview of the experimental and modeling uncertainties for each test series included in the Database These uncertainties are based on the review of the DEGADIS model by FERC and on experimental analysis reports This information should be used to guide sensitivity analysis studies conducted as part of the LNG MEP process

30 Updating the LNG MEP & Database Additional Statistical Performance Measures (SPM) The Advisory Bulletin PHMSA requires calculation of three additional SPM from the model validation data: - Concentration Safety Factor (CSF): CSF = c p c m - Concentration Safety Factor to LFL (CSF LFL ): CSF LFL = c p LFL - Distance Safety Factor to LFL (DSF LFL ): DSF LFL = X p,lfl X m,lfl These parameters are automatically calculated within the Database to ensure that a consistent approach is used to evaluate all models

31 Updating the LNG MEP & Database Additional Experimental Data Time-varying concentration data has been processed to produce point-wise concentrations for the Maplin Sands (1980) trials Shell

32 Updating the LNG MEP & Database Additional Experimental Data Point-wise concentration data has been added for the Thorney Island (1982) experiments

33 Updating the LNG MEP & Database Additional Experimental Data e.g. Thorney Island 45 Point-wise concentrations at multiple sensor heights Highlighting of sensor locations where negligible concentrations were measured Negligible (below 0.1 % v/v) concentrations excluded from SPM calculations

34 Updating the LNG MEP & Database Corrections to the Database Adjustment of sensor height for one of the downwind measurement locations for CHRC Trials B and C Height used in earlier version of Database coincided with location of the dike CHRC A, Havens and Spicer (2006)

35 Updating the LNG MEP & Database Corrections to the Database Corrected ambient and release conditions to be consistent with original experimental reports Adjusted surface roughness height to coincide with inflow profiles from experiment Corrected geometry sizes and positions for obstructed scenarios

36 Updating the LNG MEP & Database Further Database Changes Visual representation of concentration sensor locations Transformed sensor locations for use with models in which the wind is aligned with the grid

37 Concentration (%) Concentration (%) Concentration (%) Concentration (%) Updating the LNG MEP & Database Further Database Changes Cloud widths are calculated in cases where the gas cloud is not bifurcated in the cross-wind direction Plots (see right) are created for the Database user to assess cloud shape The user then indicates whether or not the cloud is bifurcated Long Time-averaged Predicted Arc-wise Concentration Profiles (at 1 m elevation) 57 m Arc Measured Predicted Cross-wind distance (m) 400 m Arc Measured Predicted Cross-wind distance (m) 140 m Arc Measured Predicted Cross-wind distance (m) Note that only concentration values at the lowest sensor height are shown here and are included in any cloud width calculations m Arc Measured Predicted Cross-wind distance (m)

38 Updated Versions of Models Reviewing Updated Versions of Approved Models A new process has been proposed for the expedited review of updated versions of models already approved by PHMSA The approval of a model for use in LNG siting applications is only valid for the exact version of the model evaluated However, models are frequently updated to fix errors or to include additional features or model improvements PHMSA intend to provide the option of an expedited review for updated versions of approved models so as to avoid unduly impeding the use of new code developments

39 Updated Versions of Models Reviewing Updated Versions of Approved Models The new process is based on the comparison of the updated model to the previously-approved version The extent to which the updated model is assessed is based on whether the changes are deemed to be major or minor Here, a minor change is meant to address four issues: - Bug fixes in the software - Changes to the Graphical User Interface (GUI) - Addition of compatible hardware support - Modification of sub-models not relevant to LNG dispersion A major change makes advances to the scientific basis of the model for modelling LNG dispersion

40 Updated Versions of Models Reviewing Impact of Model Updates: Minor Changes Assessing an updated model that has undergone only minor changes requires the model to be run against a subset of the LNG validation database experiments This subset includes the following 6 tests: Burro 8 Thorney Island 47 Coyote 5 CHRC B Falcon 1 BA Hamburg DAT223 Point-wise concentration predictions for these tests are then compared to the results from the approved version of the model The two versions of the model should use identical setups for each of the tests

41 Updated Versions of Models Reviewing Impact of Model Updates: Minor Changes The next step is to determine if any changes in model predictions are significant In this context a change in predicted concentration is considered to be significant if there is: A 1% relative difference in predictions from the two versions of the model where the absolute predicted concentration is > 10% v/v Or A 0.1% v/v absolute difference in predictions from the two versions of the model where the absolute predicted concentration is 10% v/v

42 Updated Versions of Models Reviewing Impact of Model Updates: Minor Changes If there are no significant differences between the model predictions for the validation subset then the following items should be submitted to PHMSA as a record of the review: - A short report to describe the changes to the model since the previous version was approved - Point-wise concentration predictions for the subset of validation test cases - Model input files for the subset of validation test cases If there are significant differences as a result of model changes, the minor changes must be treated as major changes and the model should be reviewed accordingly

43 Updated Versions of Models Reviewing Impact of Model Updates: Major Changes For models that have been updated to include a major change, e.g. to a LNG dispersion related sub-model, the process for evaluating the updated version of the model is more rigorous In this scenario the model must be evaluated against the three stage LNG MEP process, i.e. Scientific Assessment to assess the impact of the model updates Model Verification to ensure that the model is verified to an equal or higher standard than the previous version of the model Model Validation against the entire LNG model validation database with significant differences in model predictions highlighted and explained

44 Updated Versions of Models Reviewing Impact of Model Updates: Major Changes On completion of an evaluation of an updated model with major changes from the approved version, the following should be submitted to PHMSA: - A change-log report to describe changes to the model since it was previously approved by PHMSA. This report should include results of the scientific assessment, verification and validation of the updated model in accordance with the LNG MEP - Model input files for the LNG MEP model validation exercise - Access to the updated and currently-approved versions of the model

45 Updated Versions of Models Major or Minor update? Run subset of validation cases Minor Major Are any differences significant? Yes Evaluation against full LNG MEP No Submit following to PHMSA: 1. Short report 2. Point-wise concentration predictions 3. Model input files Submit following to PHMSA: 1. Change-log report 2. Model input files 3. Access to new and currently approved versions of model

46 Key Messages Summary Updated versions of the LNG Model Evaluation Protocol (MEP), Model Validation Database and Database Guide are available from the NFPA When seeking approval to use alternative vapor dispersion models for LNG siting applications from PHMSA, the latest version of the MEP and Database must be used A streamlined process to gain approval of an updated version of a model currently approved for determining LNG facility dispersion exclusion zones has been defined PHMSA are currently working towards using this new approach

47 References PHMSA Advisory Bulletin, PHMSA /filename/ADB-10-07%20LNG%20Facilities.pdf DEGADIS 2.1 evaluation for LNG MEP PHAST v6.6 and v6.7 evaluation for LNG MEP FLACS 9.1r2 evaluation for LNG MEP Hanna et al. (1993) Havens J. and Spicer T. (2006) Vapor dispersion and thermal hazard modelling, Final topical report to Gas Technology Institute under sub-contract K , October 2006.

48 HSE's Health and Safety Laboratory (HSL) is a multi-disciplinary laboratory whose work delivers high quality science to meet the needs of industry and government in the UK and overseas. Our commercial customers can commission services and research using our state-of-the-art scientific laboratory in Buxton, U.K. as well as analytical expertise from other parts of HSE s science base. This presentation and the work it describes were undertaken by the Health and Safety Laboratory under contract to Oak Ridge National Laboratory. Its contents, including any opinions and/or conclusion expressed or recommendations made, do not necessarily reflect policy or views of the Health and Safety Executive.

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