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API REPORT 79-28 Document Information:
Title
Review of the State-of-the-Art of Oil Spill Simulation Models Phase II
American Petroleum Institute
Publication Date:
Aug 1, 1981
Scope:
1.0 INTRODUCTION
This report summarizes the results of the second of a three-phase study
project Raytheon Ocean Systems Company (ROSC) is under taking for the
American Petroleum Institute (API). The objective of the project is to
review, evaluate and then provide a comprehensive view of the
state-of-the-art, both theoretical and applicable, for modeling the
fate of the spilled oil in the marine environment. The result of the
project will be recommendations for the construction of an oil fate
model framework with state-of-the-art components which is suitable for
environmental impact assessment or oil spill risk analysis, exclusive of
socioeconomic considerations.
Oils, especially crudes, are a mixture of many different petroleum
components that, upon release into the marine environment, will undergo
various physical, chemical, and biological processes before reaching
their ultimate destination. In general, these processes have been more
specifically categorized into the following processes: advection,
spreading, evaporation, dissolution, emulsification (water-in-oil
emulsion formation), dispersion (oil-in-water emulsion formation),
auto-oxidation, biodegradation, and sinking/sedimentation. As
illustrated in Figure 1.0-1, these processes are perceived as the
factors which affect the ultimate fate of the spilled oil in the marine
environment and they constitute the components for a comprehensive oil
spill fates model. Advection affects the movement of oil slicks due to
external forces. Spreading, also affects oil slick movements, but is a
phenomenon resulting from a balance of external forces such as gravity
and inertia, and internal forces such as viscosity and surface
tension. Evaporation and dissolution remove fractions of the oil. Of
all the physical-chemical forces acting on oil, evaporation is by far
the most significant in reducing oil volume. Emulsification
and dispersion are two processes which result in formation
of water-in-oil and oil-in-water emulsions, causing a change in
the physical-chemical properties of oil (e. g., volume, viscosity,
etc.) and making cleanup operations more difficult. Oil dispersed into
the water column will then be subject to subsurface
advection, biodegradation and/or sinking/sedimentation and be exposed
to an environment different from that at the surface. Auto-oxidation,
or more specifically, photo-oxidation, can cause alteration
and transformation of many petroleum hydrocarbons through
interaction with energy from natural sunlight. The resulting products
may have considerably different properties from their parents, and may
even be toxic, which would have major environmental
consequences. Biodegradation also alters and/or transforms many
petroleum hydrocarbons through the action of microbial populations
and/or the ingestion or retention by macro-organisms. Given time
and appropriate environmental conditions, biodegradation will
eventually remove oil from the environment by converting the oil into
such final products as carbon dioxide and water.
The sinking/sedimentation process may eventually remove oil
fractions into the bottom sediment where they may remain locked in
the sediment, they may be resuspended and reintroduced into the
water column, or they may undergo further biological or
physical-chemical alteration in-situ. It is also possible that during
these dimentation process the oil does not reach the bottom but
becomes neutrally buoyant at some level and remains in suspension for
a period of time.
The ultimate aim in modeling the fate of spilled oil is to develop a
comprehensive model incorporating sub-models, or algorithms, for each
of these processes. Given the uncertainties in many of these processes,
however, it may not be feasible or desirable to develop and use such a
model since the uncertainties in each of the algorithms would be
magnified in the final prediction. It may be more useful to model only
the most significant processes leaving the less important ones out of
the calculation and there by reducing uncertainty in the outcome. To
date, most of the oil spill models simulate only one process -
advection. A few models incorporate advection and spreading and a few
further incorporate a number of weathering processes such as
evaporation and dissolution to provide a more comprehensive oil spill
fates modeling. Different techniques with varying degrees of
sophistication have been applied in the existing oil spill models to
simulate each of these processes. In order to facilitate the assembly
of a comprehensive oil fate model with state-of-the-art components,
there has to be an evaluation of individual process models. This allows
for the selection of whole models or components of models which
best represent the state-of-the-art technologies.
Thus, information on some thirty-five oil spill models (Table 1.0-1)
was gathered and reviewed during Phase I of this study. There view
included the identification in each oil spill model, the processes
included, and the model (or modeling technology) used for each
individual process (process model). A preliminary evaluation of the
spectrum of process models employed by different oil spill models for a
given process was then performed to identify process models which
represent potential state-of-the-art technologies and which merit
further evaluation in Phase II. The main criterion used for such
identification is that the selected model has a unique treatment which
makes the model more realistic, reliable, valid, theoretically sound,
or applicable than others. These selected component process models are
summarized in Table 1.0-2, which is taken from the Phase I addendum
report.
The goal of the Phase II evaluation is to attempt to identify the
model(s) which "best" represent(s) the state-of-the-art technology, for
a given process so that a comprehensive oil spill fates model with
state-of-the-art components can be assembled. The determination or the
assessment of which model(s) "best"represent(s) the state-of-the-art
technology for a particular process can be made by evaluating various
models against the following general criteria:
• Theoretical soundness or realism
• Verifiability
• Level of data input (amount, sophistication, availability)
• Application (applicability, range, ease, cost)
• Accuracy of output
In order for a model to be acceptable for application, it must be based
on theoretically sound and/or realistic assumptions. In considering the
soundness of the assumptions one must not over look the practical aspect
of the problem, namely, such that the model could become overly complex
to the extent that its outputs are practically unverifiable or that its
requirements for input data are too large or too unrealistic to be
practical. Of course there liability of a model is also related to its
verifiability. The applicability of a given model is the next factor to
be considered in the model evaluation. A model with applicability to a
wide range of geographic and physical conditions is certainly a
desirable one, but such a feature should not be overburdened by
excessive cost or at the expense of ease of operation. Thus, in
considering applicability one must also consider the associated cost
and ease of application. Finally, the model should not only produce
realistic results, but also accurate results. Accuracy is the ultimate
goal toward which every modeler strives. Unfortunately, this
last criterion is usually not attainable for most of the oil
spill models. There is a general lack of laboratory or field
data collected during actual or experimental spills that is useful
for the assessment of model accuracy. There is even a lack of
sufficient data to validate a model. Therefore, the evaluation of the
various selected models is generally limited to assessing: the realism
of the inherent or underlying assumptions made in deriving
model formulations; model verifiability and input requirements; the
extent to which the model may be applicable, including ease of
application and the associated computational effort.
This report is organized into four sections. Following
this introductory section, Section 2 provides details on the
evaluation of the various models, on a process-by-process basis.
Section 3 discusses the synthesis of various component processes into
a comprehensive oil fates model. Finally, Section 4 briefly
summarizes the results of this evaluation phase.
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