
[Federal Register: August 31, 2010 (Volume 75, Number 168)]
[Notices]               
[Page 53371-53374]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr31au10-131]                         

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DEPARTMENT OF TRANSPORTATION

Pipeline and Hazardous Materials Safety Administration

[Docket No. PHMSA-2010-0226]

 
Liquefied Natural Gas Facilities: Obtaining Approval of 
Alternative Vapor-Gas Dispersion Models

AGENCY: Pipeline and Hazardous Materials Safety Administration, (PHMSA) 
DOT.

ACTION: Notice; issuance of advisory bulletin.

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SUMMARY: This advisory bulletin provides guidance on the requirements 
for obtaining approval of alternative vapor-gas dispersion models under 
Subpart B of 49 CFR part 193.

FOR FURTHER INFORMATION CONTACT: Charles Helm at 405-954-7219 or 
charles.helm@dot.gov.

SUPPLEMENTARY INFORMATION: 

I. Background

    The Pipeline and Hazardous Materials Safety Administration (PHMSA) 
issues federal safety standards for siting liquefied natural gas (LNG) 
facilities. Those standards require that an operator or governmental 
authority control the activities around an LNG facility to protect the 
public from the adverse effects of thermal radiation and flammable 
vapor-gas dispersion. Certain mathematical models and other parameters 
must be used to calculate the dimensions of these so-called ``exclusion 
zones.''
    In the case of vapor-gas dispersion, two different models may be 
used where appropriate: (1) The DEGADIS Dense Gas Dispersion Model 
(DEGADIS), an integral model that simulates the downwind dispersion of 
dense gases in the atmosphere, and (2) FEM3A, a dispersion model that 
accounts for additional cloud dilution which may be caused by the 
complex flow patterns induced by tank and dike structures.
    The use of alternative vapor-gas dispersion models is also 
permitted, if those models take into account the same physical factors 
as the approved models, are validated by experimental test data, and 
receive the Administrator's approval. Conservatism, field testing, 
post-testing data evaluation, and correlative analysis are critical to 
satisfying these conditions.
    In addition, PHMSA's federal safety standards incorporate by 
reference the National Fire Protection Association (NFPA) NFPA 59A: 
Standard for the Production, Storage, and Handling of Liquefied Natural 
Gas. That consensus

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industry standard is issued by the Technical Committee on Liquefied 
Natural Gas of the NFPA.
    Several years ago, the NFPA 59A Technical Committee tasked the Fire 
Protection Research Foundation (FPRF), a nonprofit entity that performs 
research for the NFPA, with developing a tool for evaluating the 
suitability of LNG vapor-gas dispersion models. The FPRF subsequently 
contracted with the Health & Safety Laboratory; the research agency of 
the United Kingdom Health & Safety Executive, to examine the modeling 
of dispersion of LNG spills on land and develop guidelines to assess 
those models.
    An expert panel, including representatives from Sandia National 
Laboratories, PHMSA, the Federal Energy Regulatory Commission (FERC), 
NFPA, the United States Coast Guard, and other stakeholders, assembled 
to provide guidance and comment on the development of those guidelines. 
That effort led to the creation of the Model Evaluation Protocol (MEP) 
as described in M.J. Iving et al., Evaluating Vapor Dispersion Models 
for Safety Analysis of LNG Facilities Research Project: Technical 
Report (Apr. 2007) (available at http://www.nfpa.org) (Original FPRF 
Report), and supplemented in S. Coldrick et al., Validation Database 
for Evaluating Vapor Dispersion Models for Safety Analysis of LNG 
Facilities: Guide to the LNG Model Validation Database, Version 11.0 
(May 2010) (available at http://www.nfpa.org) (Supplemental FPRF 
Report):

    The MEP is based on three distinct phases: scientific 
assessment, model verification and model validation. The scientific 
assessment is carried out by obtaining detailed information on a 
model from its current developer using a specifically designed 
questionnaire and with the aid of other papers, reports and user 
guides. The scientific assessment examines the various aspects of a 
model including its physical, mathematical and numerical basis, as 
well as user oriented aspects. * * * The outcome of this scientific 
assessment is recorded in a MER, along with the outcomes of the 
verification and validation stages * * *.
    [In] [t]he verification stage of the protocol[,] * * * evidence 
* * * is sought from the model developer and this is then assessed 
and reported in the MER. The validation stage of the MEP involves 
applying the model against a database of experimental test cases 
including both wind tunnel experiments and large-scale field trials. 
The aim of the validation stage is * * * to quantify the performance 
of a model by comparison of its predictions with measurements.

    Funded by a grant from PHMSA, the National Association of State 
Fire Marshals (NASFM) then convened a panel of its own experts, and 
that panel performed an independent review of the MEP and produced a 
separate technical report, National Association of State Fire Marshals, 
Review of the LNG Vapor Dispersion Model Evaluation Protocol (Jan. 
2009) (NASFM MEP Report); see also National Association of State Fire 
Marshals, Review of the LNG Source Term Models for Hazard Analysis: A 
Review of the State-of-the-Art and an Approach to Model Assessment 
(Jun. 2009) (NASFM Source Term Report).
    After carefully considering the information provided in the 
Original FRPF Report, Supplemental FPRF Report, and NASFM MEP Report, 
PHMSA is issuing further guidance on the standard for obtaining 
approval of alternative vapor-gas dispersion models, particularly the 
requirement for validation by experimental test data. That guidance is 
based on the MEP's three-stage process for evaluating such models, but 
includes modifications to address the concerns of other stakeholders, 
including NASFM and FERC.

II. Advisory Bulletin (ADB-10-07)

    To: Owners and Operators of LNG Facilities.
    Subject: Liquefied Natural Gas Facilities: Obtaining Approval of 
Alternative Vapor-Gas Dispersion Models.
    Advisory: In seeking the Administrator's approval of an alternative 
vapor-gas dispersion model, a petitioner may demonstrate that its model 
has been validated by experimental test data by using the three-stage 
process described in the MEP. A petitioner may also submit a MER as 
evidence of its completion of the MEP.
    The model developer or an independent body may complete the MER, 
which should contain certain information about the proposed model, 
including general information (Section 1), information for scientific 
assessment (Section 2), information for user-oriented assessment 
(Section 3), information on verification (Section 4), information on 
validation (Section 5), and other administrative details (Section 6). 
The validation portion of the MER should include the validation 
database described in the Original FPRF Report and Supplemental FPRF 
Report, with appropriate consideration of the additional guidance 
provided below.
    This guidance relates to some of the concerns raised in the NASFM 
MEP Report and by other interested parties, including FERC, and is 
organized to correspond with the affected sections of the MER. These 
suggested practices may require modification in individual cases, and 
the proponent of an alternative model may establish its suitability by 
any other appropriate means, subject to the Administrator's approval.
    1. Section 2.1.1.2 Source Geometry Handled by the Dispersion Model 
should describe and clearly state the limitations of the model related 
to its ability to handle different source terms, including:
    a. Ability to handle the dispersion of vapors from a transient 
(i.e., flowing) and irregular liquid pool geometries, including 
vaporization from geometries with high aspect ratios (i.e., long 
trenches) in the cross-wind and parallel-wind direction.
    b. Ability to handle the dispersion of vapors from a vaporizing 
regular liquid pool geometry (circular, squared) source term.
    c. Ability to handle the simultaneous dispersion of vapors from a 
combination (i.e., multiple sources) of the phenomena above.
    d. Use of any sub-models to simulate the phenomena above.
    2. Section 2.2.2.1 Wind Field should describe and clearly state the 
limitations of the model related to its ability to model low wind 
speeds (i.e., less than 2m/s) and its ability to model fluctuating wind 
speeds.
    3. Section 2.2.2.3 Stratification should describe and clearly state 
the limitations of the model related to its ability to model 
atmospheric stabilities (e.g., F stability). The description should 
indicate if temperature and/or turbulence profiles may be invoked at 
the upwind boundary or if forcing functions may be invoked.
    4. Section 2.2.3.1 Terrain Types Available and Section 2.3.12 
Complex Effects: Terrain should describe and clearly state the 
limitations of the model related to its ability to model sloping 
terrain, including any special methods to model (e.g., gravity vector 
adjustment, sub-model for adjusting Cartesian grids, etc). Unique 
modeling characteristics that may alter the terrain should be described 
(e.g., Cartesian Grid, Porosity-Distributed Resistance methodology, 
etc).
    5. Section 2.2.4.1 Obstacle Types Available and Section 2.3.13 
Complex Effects: Obstacles should describe and clearly state the 
limitation of the model related to its ability to model complex 
geometries, including the limitations based on the grid or mesh options 
available (reference can be made to Section 2.4.3.1 Computational 
Mesh). Unique modeling characteristics that may alter the obstructions 
should be described (e.g., Cartesian Grid, Porosity-

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Distributed Resistance methodology, etc).
    6. Section 2.3.1.5 Turbulence Modeling should describe and clearly 
state the limitation of the model related to its ability to model 
turbulence, including the turbulence sub-models available (e.g., 
Algebraic, Favre- or Reynolds-Averaged Navier Stokes, Reynolds Stress 
Transport, Spalart-Allmaras One-Equation, K-Epsilon Two Equation, K-
Omega Shear Stress Transport, Large Eddy Simulation, Detached Eddy 
Simulation, etc).
    7. Section 2.3.1.7 Boundary Conditions should describe and clearly 
state the limitation of the model related to its ability to model 
certain boundary conditions, including the boundary condition 
specifications available (e.g., wall functions, full-slip, no-slip, 
partial-slip, inlet/outlet boundaries, injection boundary, periodic 
boundary, mirror/symmetry boundary, etc).
    8. Section 2.3.11 Complex Effects: Aerosols should describe and 
clearly state the limitations of the model related to its ability to 
model different source terms, including:
    a. Ability to handle the dispersion of vapors from a flashing 
source term.
    b. Ability to handle the dispersion of vaporized aerosol formed 
from mechanical fragmentation or other means of a high pressure 
release.
    c. Ability to handle the dispersion of vaporization from aerosol 
that has settled out (i.e. rainout).
    9. Section 2.4.3.1 Computational Mesh should clearly state all 
features of the computational mesh (e.g., Automatic, Manual, 
Structured, Unstructured, Cartesian, Curvilinear, Body-fitted, H-Type, 
C-Type, O-Type, Triangle/Tetrahedral, Quadrilateral/Hexahedral, 
Adaptive, Multi-Block, etc).
    10. Section 2.4.3.2 Discretization Methods should describe and 
clearly state the limitation of the model related to its numerical 
solution methodologies, including a description of the temporal 
discretization methodologies available (e.g., Implicit, Explicit, 
Multi-Stage Schemes, Order of Runge-Kutta, MUSCL, QUICK, Courant-
Friedrchs-Lewy limitations, etc) and description of the spatial 
discretization methodologies available (e.g., Central Schemes, Upwind 
Schemes, etc).
    11. Section 2.6 Sources of Model Uncertainty should describe and 
clearly state all known uncertainties described in previous sections 
and any uncertainties due to any other physical parameters and 
assumptions inherently built into the model.
    12. Section 2.6.4 Sensitivity to Input should include a parametric 
analysis. Alternatively, a sensitivity analysis of the validation study 
may be referenced, as described below in Section 6.2 Evaluation Against 
MEP Quantitative Assessment Criteria.
    13. Section 2.7 Limits of Applicability should summarize the 
limitations of the model described in previous sections and any other 
limitations inherently built into the model.
    14. Section 6.2 Evaluation Against MEP Quantitative Assessment 
Criteria should provide the following as part of the submitted 
validation phase:
    a. An uncertainty analysis that accounts for model uncertainty due 
to uncertainty in the assumption of input parameters specified by the 
user.\1\ The model uncertainty analyses should address the following:
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    \1\ Model uncertainty due to the uncertainty of the physical 
parameters and assumptions inherently built into the model is not 
required to be quantified, although these limitations should clearly 
be stated in the scientific assessment.

    i. Analysis of source term(s). Certain models have built-in 
source models that are able to calculate the flashing, mechanical 
fragmentation and subsequent aerosol formation and rainout, 
resultant liquid trajectory, flow and vaporization. It is 
recommended that the built-in models be used, where appropriate and 
applicable, as those are the most likely to be used during hazard 
analyses. For models without built-in source models, it is 
recommended that appropriate source term model(s) \2\ be used that 
provides an accurate depiction of the experiment that can be 
inputted into the dispersion model as it should generally produce 
better fidelity. Alternatively, simplified source term inputs may be 
used with justification provided for the selection of pool 
diameter(s), vaporization rate(s), and other specified sources along 
with a sensitivity analysis of the vaporization rate and resultant 
pool diameter(s). A source term based on an instantaneously formed 
pool with a vaporization rate and pool size equal to the discharge 
rate (mass balance) based on empirically selected vaporization rates 
of 0.085kg/m\2\/sec and 0.167kg/m\2\/sec should be included in the 
sensitivity analysis.
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    \2\ Source term models may be supplemented with an evaluation in 
accordance with Model Assessment Protocol (MAP) published by the 
FPRF in Ivings, et al., LNG Source Term Models for Hazard Analysis: 
A Review of the State-of-the-Art and an Approach to Model Assessment 
(Mar. 2009) (available at http://www.nfpa.org) or equivalent Health 
and Safety Executive report, LNG Source Term Models for Hazard 
Analysis: A Review of the State-of-the-Art and an Approach to Model 
Assessment, RR789, 2010 (available at http://www.hse.gov.uk/
research/rrhtm/rr789.htm).
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    ii. Analysis of boundary conditions, including wall conditions, 
slip conditions, surface roughness, thermal properties, and any 
other parameters specified for the boundaries that may otherwise 
have a significant influence on the model results. The analysis 
should demonstrate the impact of the boundary conditions on the 
analysis. This may be accomplished by demonstrating that the 
boundary conditions do not have a significant influence on the 
analysis (i.e., boundaries are sufficiently far away not to 
influence the flow field of the vapor cloud) and/or through a 
sensitivity analysis of the boundary conditions. For boundary 
conditions associated with the ground, a sensitivity analysis, 
including any bounds (e.g., a no-slip v. free-slip) of the boundary 
conditions should be evaluated.
    iii. Analysis of wind profile. Certain models are only able to 
provide steady-state wind profiles and/or direction. Other models 
are able to input/calculate transient, fluctuating, or periodic 
(e.g., sinusoidal) wind profiles and directions. It is recommended 
that the most accurate depiction of the wind field be used, as it 
should provide better fidelity. The wind field throughout the domain 
should be fully established before the source term initializes. 
Surface roughness sensitivity analysis should be included based on 
user guide documentation or other recommended and generally accepted 
good engineering practices that represent surface roughness for the 
area.
    iv. Analysis of sub-models. Certain models contain multiple sub-
models (e.g., turbulence models) that may be selected by the user. 
It is recommended that the most appropriate and applicable sub-
models be used, as it should provide better fidelity. Technical 
justification for the selected sub-models should be provided. If 
multiple sub-models may be appropriate and applicable, sensitivity 
analysis should be used for a range of sub-models. Any specification 
in associated coefficients may also be subject to sensitivity 
analysis, where warranted.
    v. Analysis of temporal discretization/averaging. Certain models 
may specify different time-averages. Time averages should reflect 
the time averaged data of the experimental measurements or less. 
Where time averages cannot be specified to reflect the time-averaged 
data of the experimental measurements, sensitivity analyses or 
corrections should be provided.
    vi. Analysis of spatial discretization/averaging and grid 
resolution. An analysis should evaluate the effect of any spatial 
averaging by the model. For Computational Fluid Dynamics (CFD) 
models, a grid sensitivity analysis should be provided that 
demonstrates grid independence or convergence to a grid independent 
result (e.g., Richardson extrapolation). If overly cost-prohibitive, 
it may be acceptable to selectively refine grids in the areas of 
principal interest only based on user guide documentation or other 
recommended and generally accepted good engineering practices.
    vii. Analysis of geometrical representation for sloped and 
obstructed cases. Certain models may not be able to model sloped and 
obstructed flow fields. Others may be limited in the representation 
of slopes (e.g., change in gravity vector), or in the representation 
of complex shapes or curvatures by simpler geometries (e.g., to fit 
a Cartesian grid). The effect of these simplifications should be 
discussed or evaluated.

    b. An uncertainty analysis that accounts for model uncertainty due 
to

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uncertainty in the output used for evaluation. The analyses should 
address the following:

    i. Analysis of spatial output. Certain models may be limited in 
the output of the cross wind concentration profile (e.g., Gaussian 
concentration profiles in the cross-wind direction). 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. The centerline 
concentration of the model may not necessarily be representative of 
the maximum concentration measurement location. Any interpolations 
and extrapolations used to determine concentrations should be 
documented, evaluated and discussed. If a model cannot represent the 
actual location of the sensor relative to the centerline, the effect 
of these simplifications should be discussed or evaluated.
    ii. Analysis of temporal output. Certain models may be limited 
in the temporal resolution that can be outputted. Any interpolations 
and extrapolations used to determine concentrations should be 
documented, evaluated and discussed. If desired, transient data of 
the model and experimental data may be provided to supplement the 
maximum arc wise values to allow for more detailed comparisons with 
the experimental data, including the evaluation of discrepancies due 
to spurious experimental or model results.

    c. An uncertainty analysis that accounts for experimental 
uncertainty due to uncertainty in the sensor measurement of gas 
concentration,\3\ where known. Other sources of uncertainty may also be 
included.
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    \3\ Experimental uncertainty due to the sampling time, time 
averaging, spatial/volumetric averaging, cloud meander, and other 
errors associated with the experiment are not required to be 
quantified, but the analysis may benefit from them being evaluated 
or discussed.
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    d. Graphical depictions of the predicted and measured gas 
concentration values for each experiment with indication of the 
experimental and model uncertainty determined from the analyses 
described above. Vertical error bars should be used to represent the 
uncertainty.
    e. Calculation of the specific performance measures (SPMs) below in 
addition to those specified in the MEP:
[GRAPHIC] [TIFF OMITTED] TN31AU10.000

    f. Calculation of SPMs specified in the MEP for each experiment and 
data point in addition to the average of all experiments.
    g. A tabulation of all simulations, including all specified input 
parameters, calculated outputs.
    h. A tabulation of all calculated SPMs.\4\
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    \4\ If the model predictions are outside the experimental 
uncertainty interval or MEP SPMs, this does not necessarily mean 
that the model is unacceptable, but may alternatively impact the 
safety factor associated with the model usage.
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    i. All relevant input and output files used.

    Issued in Washington, DC, on August 24, 2010.
Jeffrey D. Wiese,
Associate Administrator for Pipeline Safety.
[FR Doc. 2010-21588 Filed 8-30-10; 8:45 am]
BILLING CODE 4910-60-P

