[Federal Register Volume 75, Number 168 (Tuesday, August 31, 2010)]
[Notices]
[Pages 53371-53374]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2010-21588]
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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
[email protected].
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