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API REPORT 79-28 Document Information:
Title
Review of the State-of-the-Art of Oil Spill Simulation Models
American Petroleum Institute
Publication Date:
Jun 1, 1982
Scope:
1.0 EXECUTIVE SUMMARY
With a huge amount of petroleum being transported by tankers
and pipelines through the marine environment, and with increasing
offshore oil exploration, drilling and production activities, the
potential of an oil spill resulting in damage to our marine environment
and to the critical near shore resources has aroused great concern
amongst government decision-makers, oil industry officials, and the
general public. Assessments have to be made of the potential risk of
damage resulting from exploration and development activity based
upon predictive impact evaluations of the fate of hypothetical or real
oil spills. When an oil spill does occur, planning and execution of
cleanupmeasures to minimize impacts also require the capability to
forecastthe short term and long term behavior of the spilled oil.
A substantial amount of money has been spent by government and the
oil industry over the past decade in an effort to develop the
capability to predict the fate of spilled oil. Numerous models have
been developed and applied, most of which could only predict the
horizontal movement, or advection, of the oil slick. When oil is
discharged into the marine environment it will immediately be affected
by various physical-chemical and biological-chemical processes in
addition to the physical advective processes. Most existing models do
not have the capability to accommodate the effects of such processes
although they are quite important in determining the overall fate of
spilled oil. Over the past few years, a few models have been developed
which combine advective and weathering processes in an attempt to
provide a more comprehensive predictive capability. An inherent problem
in this approach lies in the fact that these weathering processes have
not always been integrated into the advection models in the most
beneficial way; in other words, the later, more inclusive models
essentially wind-up being a melange of ideas that have been strung
together rather than a basic framework within which all process models
are placed.
As is frequently the case in a relatively new technological area,
the burgeoning of ideas in the absence of a well-defined problem
framework has led to a proliferation of techniques designed to meet
some broad objectives. Within this context an assessment of the
state-of-the-art in modeling the fate of oil spills should contribute
significantly to the quality of models developed in the future and the
resultant decisions regarding the potential risk and/or hazard of an
oil spill.
The objectives of this study, chosen with this in mind, are
three-fold:
1. To assess the capabilities of selected oil spill fate models
tosimulate real-world conditions;
2. To provide recommendations for the synthesis of various oil
spill fate process components into a comprehensive "State-of-the-art"
model; and, since this synthesized model will still have shortcomings,
3 . To recommend further improvements that can be made in oil
spill simulation models.
To achieve these objectives, the study was divided into three
phases. In the first phase, available information on existing oil
spill simulation models was gathered and placed in one of the
following process categories: (1) advection, (2) spreading, (3)
evaporation, (4) dissolution, (5) emulsification (water-in-oil), (6)
dispersion (oil-in-water), (7) auto-oxidation, (8) biodegradation, and
(9) sinking/sedimentation. A truly comprehensive oil fate model
was perceived to be composed of submodels for each of these nine
processes. A preliminary review of the information was made with
respect to the techniques used to simulate oil fate processes included
in each model. The submodels for each individual process were grouped
with others found in the literature, but not yet applied (i.e.
"theoretical"algorithms), to allow for a screening and selection of
those with the potential of contributing to the state-of-the-art
modeling technology, based on a set of six criteria as described in
Section 3.0. The main criterion used for the selection was that the
process model or algorithm has a unique treatment which makes it more
realistic, reliable, valid, theoretically sound, or applicable than
others. The selected models were then subjected to a detailed
evaluation in Phase II.
The primary goal of Phase II was to identify the model(s) which
"best" represent(s) the state-of-the-art technologies for a given
process. The factors considered included: theoretical soundness or
realism, verifiability, level of data input, applicability to a range
of conditions and localities, and accuracy of output. Details of
this phase of the evaluation are presented in Section 4.0 of this
report.
Results of the final phase of this study were to be
recommendations for: integrating various component models, based on the
results of the Phase II into a comprehensive oil spill fate model, and
steps to betaken to improve oil spill fate modeling. In the course of
the Phase III work, it was determined that a synthesis of a truly
comprehensive model incorporating all nine oil fate processes could not
be done at this time nor was it necessary for all practical purposes.
There are processes such as auto-oxidation, biodegradation
and sinking/sedimentation for which there are virtually no
models available, and emulsification and dispersion whose mechanisms
are not well understood, although unvalidated models are available for
these two fairly important processes. Unvalidated models for the
dissolution process are also available, but must be regarded as highly
speculative because determination o f the dissolution rate from an oil
slick is imprecise. The only processes which have received a great deal
of attention by researchers and are capable of producing results
with fairly reasonable confidence are advection, spreading, and
evaporation. Models with varying degrees of sophistication and detail
for these three processes are available, and can be assembled to form a
limited comprehensive oil fate model for environmental impact
assessment or oil spill risk analysis purposes. Although the current
state-of-the-art is such that very accurate prediction of oil spill
behavior is yet to be achieved, it is probably fair to state that
reasonable determination of the fate of the spilled oil can be made
using a combination of only three or four processes.
Depending on the purposes for which a model is intended,
the geographical areas to which the model is to be applied, and
the uncertainties of some oil fate processes, one should include
algorithms for only those processes which significantly affect the
spilled oil under those specific conditions and in which there is a
high degree of reliability in the model output. In this way, it is
possible to reduce uncertainty in the model outcome while still
accounting for the most critical factors. This may, however, have an
inherent flaw in that many of the important processes such as
spreading, evaporation, emulsification, and dispersion are highly
interdependent and synergistic, and through interaction largely
determine the behavior and the lifetime of a spill. Although models for
emulsification and dispersion contain considerable empiricism, omission
of either one from an oil fate model will lead to unrealistic results.
For example, emulsification will significantly retard spreading,
evaporation, dissolution, biodegradation and dispersion; if it is
ignored, the effective rates of other related processes will be
unrealistically high, and will result in overly pessimistic estimates
of environmental impacts. The process of dispersion of surface oil into
the water column by breaking waves is also exceedingly important in
that it largely determines the lifetime of the surface slick. Its
exclusion from an oil fate model may, too, lead to overly conservative
and unrealistic impact estimates. Sinking/sedimentation can also
shorten the lifetime of the surface slick, but generally only occurs in
the presence of a significant suspended sediment load (a condition
which becomes more critical in nearshore regions).
It is recommended in this report that additional work be done
on algorithms for emulsification and dispersion since they are so
critical to several of the other processes. As mentioned previously
there is no need, for all practical purposes, for a great amount of
effort to be put into developing a fully comprehensive oil spill fate
model. Such a model should, and likely will, remain a research tool for
the foreseeable future primarily because of the great deal of
uncertainty in the rates and mechanisms of such processes as
biodegradation, auto-oxidation, and dissolution. When these
uncertainties are factored into a more comprehensive model the overall
reliability of the model prediction decreases while the percentage of
additional oil accounted for is probably less than 10%.
Recommended steps for the selection of existing oil spill models,
as well as some factors to be considered in constructing
limited comprehensive oil fate models, are presented in Section 5.0 of
the report. The precise nature of such models will depend upon the
purposes for which they are intended and the geographic areas to which
they are to be applied.
One of the most difficult and frustrating problems encountered in
the course of this project was the general lack of data, collected
from either spills of opportunity or experimental spills, that can be
used to validate or verify models. This lack of data seriously limits
the utility of models, and can be remedied by modest data collection
and analysis programs. The success of such programs lies in
comprehensive planning and execution of extensive data gathering. In
too many cases field data collection efforts of this type are done
strictly according to the wishes of the principal investigator, and
with relative disregard for techniques used by other investigators.
What results is a huge number of data that are virtually unrelatable
from study to study. In order to achieve the desired levels of
comparability it would be useful to construct a crude composite model
from existing algorithms and look at the commonalities of data
requirements and interrelationships between process models with an eye
toward designing appropriate, efficient experimental programs whose
objectives closely match model data requirements. The assumed rate
constants or coefficients employed in the model can then be verified
with such data, or the data can be used to estimate the proper rate
constants or coefficients.
Decision making based on model predictions, is by the very nature
of the uncertainty in the models, highly conservative. The model in use
by the U.S. Department of the Interior falls into this category, and
has significant implications for near-term future energy decision
making. Such conservatism can certainly be either justified or refined
on the basis of the findings of this oil spill fate model evaluation
and initiation of certain recommendations found in Section 7.0.
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