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3
DESCRIPTION OF THE
PROPOSED METHODOLOGY
A s noted in Chapter 1, the committee developed an overall method-
ology for use by USCG in evaluating the relative environmental
performance of alternative tanker designs following a collision or grounding
accident. This methodology is intended for use in conjunction with other
factors to determine whether USCG should approve an alternative design
to a double-hull tanker. The key components of the methodology, the data
needed for its implementation, and the nature of the results to be ex-
pected are described in this chapter. The status of the methodology's
development and the efforts required to enable its routine application
are also reviewed.
OVERVIEW
If a tanker runs aground or collides with another vessel,1 the severity of
the damage to the ship caused by the impact and the amount of oil spilled
depend on the design of the tanker, its loading condition, mitigation ef-
forts by its crew, the location of the impact, and the type of accident. Once
the oil is in the water, the environmental, economic, and financial conse-
quences of the spill will depend on the volume of oil spilled, the type of
oil, the location of the spill, resources at risk, seasonality, and the weather
conditions at the time of and after the accident, as well as any recovery
and cleanup efforts. A rigorous evaluation of the environmental perfor-
mance of a tanker should take all these variables into account. Moreover,
all possible accident scenarios and their outcomes should be considered.
In developing its methodology, the committee considered all the above
factors, but to accomplish its mandate had to adopt some simplifica-
1Collisions with fixed objects, such as piers and bridges, are not considered within the committee's illustration
of the methodology. However, a more complete future evaluation should take this type of incident into
account.
44
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DESCRIPTION OF THE PROPOSED METHODOLOGY
45
tions. The simplifications were necessary because of the limitations of
existing technology for modeling physical phenomena and the redun-
dancy of factors that did not introduce new information into the analysis.
Throughout the study, the committee took care to ensure that the simpli-
fications were not introducing bias into the methodology.
The methodology has three main components or steps:
1. Structural damage and oil outflow calculation,
2. Consequence assessment, and
3. Design comparison.
The results of the first two steps feed into the design comparison. The di-
vision of each step into tasks is shown in Figure 3-1. Each task involves
both theoretical and methodological challenges, which are discussed later.
An overview of each step is provided in the remainder of this section;
each step is then reviewed in detail in the sections that follow.
Structural Damage and Oil Outflow Calculation
The first step in evaluating the environmental performance of a tanker
design is calculation of the structural damage and oil outflow in pos-
1. Structural Damage and Oil Outflow Calculation
· Collect data on collision and grounding accidents
· Develop distributions of accident factors
· Generate accident scenarios
· Calculate structural damage
· Determine oil outflow
3. Design Comparison
· Calculate differences in consequence metrics
· Analyze design differences
2. Consequence Assessment
· Model oil fate and transport
· Simulate environmental conditions
· Select impact measures
· Develop consequence metric (ratio based)
FIGURE 3-1 Components of the methodology.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
46
sible accident scenarios. To this end, the accident scenarios, or collision
and grounding events, must be selected. The parameters (accident fac-
tors) that define the collision and grounding events include the speed
of the vessel, its loading condition, and the type of obstacle or colliding
vessel. The accident factors used by the committee were selected to rep-
resent conditions in U.S. waters, specifically in areas with a high density
of tanker traffic. The factors are defined with distributions that represent
the range of their possible values, as well as the frequencies at which
these values occur and their correlation structure. By sampling from
these distributions, a large number of collision and grounding events
were generated that model possible accidents and their relative fre-
quencies in the selected areas of tanker traffic. For example, the com-
mittee chose a distribution of vessel speeds that are likely to occur in
the areas where tankers operate. This distribution of speeds was then
used to construct a distribution of accidents that are more or less likely
to occur at those speeds.
Once the collision and grounding events have been defined,
ship damage models are used to determine the structural damage for
each accident and each design. The inputs into the models include the
accident factors and a description of the vessel (definition of the hull
form, compartments, and hull structure).2 The output from the damage
models is the damage extent and location, in other words, the size of the
hole and its location on the hull of the ship. Once the size and location
of the hole on the bottom or side of the ship are known, the resulting oil
outflow can be determined. The calculation is carried out for each accident
scenario.
Consequence Assessment
The second component of the evaluation process is the assessment of
environmental consequences from an accidental spill. If the environ-
mental impact of a spill were to increase linearly with the volume spilled,
the oil outflow could be used to measure the performance of alternative
designs. However, since the impact of the spill is dependent on many
factors other than the volume (e.g., product type, environmental condi-
tions, location), one cannot assume that the relationship between the
spill volume and the consequences is linear (see Chapter 2). Therefore,
2The committee developed this methodology for evaluating new tankers. If several simplifying assumptions
about aging and fatigue can be made, however, it may be possible to study retrofit options for existing ships
as well.
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DESCRIPTION OF THE PROPOSED METHODOLOGY
47
to perform an unbiased evaluation of alternative designs, it is necessary
to assess the consequences of the oil outflow by taking into account all
relevant variables that affect the outcome.
The committee concluded that past efforts to use the reported cost
of a spill as a surrogate for its overall consequences did not provide a ra-
tional basis for comparison of designs. Therefore, the committee decided
to carry out environmental impact modeling using SIMAP, a modifica-
tion of a stochastic version of the National Resources Damage Assessment
Model for Coastal and Marine Environments (NRDAM/CME).3 SIMAP
models oil fate and transport and allows for random sampling of weather
conditions on the basis of historical weather data. It provides a number
of consequence measures, such as the area of the sheen, the toxicity in
the water column, and the length and area of oiled shoreline. Spill cleanup
costs were included in the analysis at first but were later excluded because
of the uncertainty in the cost models. The economic impacts due to cargo
loss and third-party damages are excluded from the methodology: the
impact of cargo loss is considered negligible compared with the environ-
mental impact, while third-party costs, which can be significant, are as-
sumed to be linear with the environmental consequence measures. These
assumptions are discussed in more detail later in the report.
Because a distribution of weather events is included, the envi-
ronmental impact modeling produces distributions of values for each
consequence measure and for each simulated spill. In addition, the con-
sequences are measured at a number of threshold levels, which represent
the intensity or the quantity of the physical measure (e.g., the concentration
of toxic components in the water column or the thickness of the slick on
the water). The committee decided to limit the evaluation of the conse-
quences to the physical measures instead of extending it to the impact on
biological resources. The intent was to keep the analysis as rigorous as pos-
sible without necessitating difficult decisions on what threshold levels
would damage biological resources and how different types of biological
resources are valued. This decision is also addressed in more detail in
later sections of this chapter. These distributions of impacts are then com-
pared with each other to determine a single overall consequence metric
(or function) that will capture the relative damage caused by spills of dif-
ferent sizes.
3NRDAM/CME and the SIMAP modification were developed by Applied Science Associates (ASA) for the De-
partment of the Interior.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
48
Design Comparison
In the third and final component of the methodology, oil outflow values
from the first step are transformed using the consequence metric from
the second step. For each accident scenario, the performance of the alter-
native design is compared with that of the reference double-hull tanker,
and a difference is calculated. These distributions of differences capture
the difference in the environmental impact of the two designs. Equivalency
in performance can be determined by evaluating these distributions either
qualitatively or quantitatively.
STRUCTURAL DAMAGE AND
OIL OUTFLOW CALCULATION
The determination of oil outflow after collision or grounding is divided
into the following tasks:
1. Definition of the collision or grounding event and its probability
of occurrence,
2. Determination of the structural damage that results from the given
collision or grounding event,
3. Determination of the outflow that results after the structure is
damaged, and
4. Evaluation of active systems that modify outflow when applicable.
Each of these tasks is discussed in detail in the following subsections.
Definition of the Collision or Grounding Event and
Its Probability of Occurrence
The methodology includes the assumption that the probability of alter-
native designs encountering a potential collision or grounding scenario
is the same. Features of a design that reduce the risk of encountering dan-
gerous situations, such as navigational aids, can be incorporated into any
alternative design. However, the probability of occurrence (or frequency)
of an event relative to the frequency of all other events is included in the
methodology through the description of the accident factors that define
the collision and grounding events. In addition, whether the ship runs
aground in a given scenario depends on its draft, and the effect of pos-
sibly different drafts of alternative designs is taken into account in the
methodology. Accident factors, which define the condition of the tanker
before the accident and the type of colliding vessel, are described in more
detail in Chapter 4.
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DESCRIPTION OF THE PROPOSED METHODOLOGY
49
Accident factors, which are defined by a distribution, must be se-
lected so that they represent conditions in the studied area. For example,
vessel speed should have a distribution that includes all possible speeds
along with their likelihood. Transit and maneuvering speeds will have a
high likelihood in the distribution. Speeds outside of the typical range
must also be included, but their likelihood will be small. An example of
the distribution for speed is shown in Figure 3-2. Once the initial distri-
butions are known, the factors are sampled from the distributions to gen-
erate accident scenarios. Collision and grounding incidents that are more
likely will have a higher frequency of occurrence.
Determination of Structural Damage
During a collision between two ships or a grounding (when a ship strikes
a fixed object on the bottom), contact forces occur between the two
bodies. The magnitude of the contact force depends on a number of
characteristics of the striking and struck bodies. The effects of these char-
acteristics are to decelerate the motion of the moving ship, perhaps suf-
ficiently to bring it to a stop, and to damage structural members in the
region of contact. In an extreme case, the damage may result in internal
flooding, leading to loss of buoyancy and stability. If sufficiently severe,
the damage may cause the ship to capsize or founder, resulting in its total
loss. Even if the ship remains afloat, interior tank spaces may be pene-
trated, allowing spillage of any liquids they contain.
0.08
0.07
Events 0.06
0.05
Grounding 0.04
of 0.03
0.02
Proportion0.01
0.00
0 2 4 6 8 10 12 14 16 18 20
Speed of Ship (knots)
FIGURE 3-2 Distribution of grounding vessel speeds.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
50
Analytical Methods for Assessing Damage During Collision or Grounding
The magnitude and extent of any structural damage depend on the lo-
cation and magnitude of the forces of contact, as well as the strength
properties of the structural members in that vicinity. The collision or
grounding event occurs over a time interval ranging from a few seconds
to a few minutes, during which time the forces of contact undergo vari-
ations in direction and magnitude. The behavior of these contact forces
depends on the initial speed and mass, as well as other properties of the
ship, and on the behavior of the affected structural members. The forces
will change in an irregular manner with time as the ship decelerates and
as different structural members undergo deformation and rupture. An
analysis of ship behavior in a collision or grounding has the twin goals of
predicting the forces of contact and the behavior of the ship structure
under the action of those forces. Since these effects, force and structural
response, are interrelated, it is necessary to perform simultaneous analyses
of the two. This is in contrast to the usual structural design problem in
which the forces acting on the structure are predicted first, and the struc-
tural members are then designed to withstand those forces.
The basic problem to be solved in the collision or grounding may
be stated in terms of energy relations as follows. Before the collision or
grounding, the ship is moving forward at velocity V, and its kinetic energy
(KE ) is given by the following expression:
KE = 1 2
2(m + m)V
where m is the actual mass of the ship and m is the added mass of an
equivalent quantity of water, which accelerates with the ship.
During the collision or grounding, KE is transformed or dissi-
pated through several mechanisms. A part of the KE is transformed into
potential energy of the ship corresponding to changes in draft, heel, and
trim. KE is dissipated through friction, acting principally between the con-
tacting surfaces of the two ships or the ship and the object upon which
grounding occurs. KE is also transformed into strain energy of plastic de-
formation, fracture, and tearing of structural members. Finally, hydro-
dynamic effects, including water friction and radiating waves, dissipate
some of the KE, but this is a minor term and may usually be neglected.
Plastic deformation and fracture of structural members involve
highly nonlinear aspects of material behavior; therefore, the forces of
contact between ship and ship or ship and ground are strongly nonlinear
functions of the relative motions of the two. As a result, the problem of
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DESCRIPTION OF THE PROPOSED METHODOLOGY
51
finding the time history of vessel motion (and damage progression) after
collision or grounding occurs must usually be solved as a step-by-step
integration in time of the equations of vessel motion.
An example is provided by a common model of grounding in
which a rock pinnacle is modeled as a circular vertical cone with a
rounded vertex. As the bottom of the ship contacts the cone tip, vertical
and horizontal forces act on the ship at the point of contact. These forces
cause rigid body motions of the ship in its six degrees of freedom, but
principally in pitch, heave, roll, and surge. The forces also cause defor-
mation, possibly including yield and fracture, of the ship's structural
members in the vicinity of the contact point. The problem is then solved
in a series of time steps. At each step, using the velocity at the end of the
previous step, an incremental motion is predicted, structural deforma-
tions are determined that are consistent with the relative motion between
the ship and struck object, and the force associated with those deforma-
tions is determined. The integral of this force over the increment of
motion equals the work done by the contact force in this step. This work
must, in turn, equal the incremental reduction of KE. From the remaining
KE, the velocities at the next step are determined. The entire process ends
when the remaining KE equals zero and the ship has come to rest.
Obviously, the analysis of a process as complex as this requires
certain simplifying assumptions to keep the magnitude of the analysis
within practical bounds. These simplifications include assumptions con-
cerning the external kinematics of the ship and the behavior of the ma-
terial undergoing deformation and fracture. Methods that have been
developed and used for this purpose include the following:
Statistical analysis of previous casualty data,
Detailed nonlinear finite-element methods, and
Macroscopic finite-element or superelement methods.
In general, the statistical methods inherently involve the behavior
of structures typical of the ships forming the database and may include sim-
plifying assumptions regarding the structural arrangements. They cannot
be applied with any confidence to comparative studies in which the subject
ship has an innovative structural arrangement. Since such comparison of
alternative arrangements with a known arrangement, the double hull, is
a key element of the present work, the statistical approximation methods
cannot be applied in all evaluations.
Finite-element methods are widely accepted in the analysis of
complex structures such as those used in ships. The most advanced im-
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
52
plementations are capable of treating nonlinear material and structure be-
havior, including yield, fracture, and buckling. A detailed nonlinear finite-
element analysis requires great effort to build the structural model itself
and considerable computational effort to arrive at a solution. The repet-
itive analyses needed to investigate large numbers of casualty scenarios
render these methods impractical for purposes of the present study.
Superelement methods are essentially a variation on finite-element
methods. Whereas the latter methods utilize large numbers of small simple
elements to represent the structure, superelement methods use relatively
few large, sophisticated elements, each incorporating the material and be-
havioral properties of a relatively large portion of the structure. An example
might be the entire stiffened plate panel contained between two web
frames. The necessary computational effort is greatly reduced with such
methods, yet if appropriately defined elements are used, the results are
of sufficient accuracy for the present application.
A limited number of computer codes based on superelement
methods have been developed. The program DAMAGE, developed by
the Joint Massachusetts Institute of Technology (MIT)Industry Project
on Tanker Safety, which falls into this category, was used to analyze
grounding damage during the application of the committee's method-
ology. DAMAGE is available as software and has the widest range of ap-
plicability of published simplified methods (Tikka and Chen 2000). Other
published work includes that of Wang et al. (1997), which is applicable to
raking damage only, and that of Pedersen (2000) on grounding on soft soil.
The collision study was carried out using a simplified collision
analysis tool, SIMCOL, which was developed by Brown and his students
at Virginia Tech. Other simplified approaches for analyzing collision in-
clude (a) the collision module in the program DAMAGE; (b) the ALPS/
SCOL simplified finite-element code, developed at Pusan National Uni-
versity, Korea; and (c) the simplified approach developed at the Tech-
nical University of Denmark. (More details on these methods can be
found in Alternative Tanker Designs Collision Analysis on the accompa-
nying CD.) SIMCOL was selected because of its availability and applica-
bility to analyzing a large number of collision scenarios. The validation
and limitations of DAMAGE and SIMCOL are discussed later in this
chapter and in Chapter 4.
Limitations of Modeling the Physical Phenomena
Several factors limit the ability to achieve an accurate model of collision
or grounding scenarios:
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DESCRIPTION OF THE PROPOSED METHODOLOGY
53
Random errors: Virtually every aspect of any attempt to model the
behavior of a ship and its structure in a collision or grounding event is
subject to random uncertainties, including
a. Randomness of the conditions in the environment (e.g., waves,
current),
b. Ship conditions at the time of collision (e.g., speed, draft, heading),
and
c. Structural conditions (e.g., corrosion, construction tolerances,
material imperfections).
Systematic errors: These errors are due to shortcomings of analytical
methods, imperfect knowledge of the relevant phenomena, and the need
to reduce the computations to a manageable level.
The random errors of Types a and b can be addressed by the
choice of conditions under which simulations are run. This is accomplished
by conducting simulations for a very large number of cases covering all
expected conditions of loading and the environment. Type c includes
effects that are routinely treated when a reliability analysis is performed
on a structure. These effects, such as corrosion, unrepaired dents, and
cracks, are a consequence of deterioration of the structure over the life of
the ship. Type c also includes construction tolerances and material imper-
fections, which are due to the ship not being built precisely as designed:
the thickness of steel plate as received from the mill is not exactly as spec-
ified by the designer because of rolling imperfections; welded members
are not in the exact locations and alignments shown on the plans; and ma-
terial properties may not be precisely as assumed or specified as a result
of chemistry, heat treatment, and other aspects of the steel-making process.
Many of these items are covered by specifications on dimensions plus al-
lowable tolerances in manufacture, and these tolerances can be incorpo-
rated in the analysis procedure.
The systematic errors are of more concern since they are charac-
teristic of the analysis methodology. In a complex analysis such as that
using DAMAGE or SIMCOL, especially when applied repeatedly to a large
number of cases, simplifying assumptions must be made to keep the com-
putational effort within reasonable bounds.
In DAMAGE, simplified analytical expressions are developed for
the behavior of the different structural elements under various conditions.
For example, for a panel of bottom plating, expressions are derived for
plate indentation without fracture in the initial stages of contact with an
obstacle, for plate splitting as the ship continues to move over the obstacle
with increased penetration, and for tearing and wrinkling of the plate
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
54
ahead of the obstacle. Similarly, expressions are derived for the behavior
of stiffeners and webs attached to the plating.
Two types of assumptions are involved in modeling the response--
those concerning the material itself and those concerning the structural
deformations. Material behavior is defined by a stressstrain curve, which
describes the stressstrain relationship up to the fracture strain. The kine-
matic assumptions concern the geometry of deformation. An example is
the behavior of a plate being deformed by contact with the vertex of the
cone, which models the obstruction. The model assumes that the deflected
shape consists of "flaps" with plastic hinges at their edges, whereas the
real deformation pattern involves curved edges.
The structural model in DAMAGE includes only the cargo block.
The effect of the bow and the stern on the structural behavior in the
damage region is neglected. The model is built with conventional struc-
tural members, and the materials used are limited to those that can be
described with the stressstrain curve and the assumed failure modes. In-
novative structural designs using new materials would require extensions
to the current program.
DAMAGE is limited to modeling powered grounding on a single
pinnacle. The obstruction, modeled as a pinnacle, is defined by its apex
angle and the tip radius. Other types of obstructions or groundings (e.g.,
grounding on a reef or soft soil, drift grounding) currently cannot be
modeled in DAMAGE.
In general, the validity of the combined assumptions can be tested
only by experiment. Validation of DAMAGE has been limited to a few test
cases, but it has been found to predict the overall damage extent well. It
predicts average forces, but it does not capture peak forces as the ship
advances relative to the obstruction. (For details see Alternative Tanker
Designs Grounding Analysis on the accompanying CD.)
The SIMCOL collision model uses modified procedures derived
from the statistical work of Minorsky (1959) to determine energy ab-
sorption by the horizontal members; reaction forces on the vertical
members are determined by applying simplified analytical models that take
into account the mechanics of the structural behavior. Total forces are a
superposition of the forces acting on vertical and horizontal members.
Only conventional structural members can currently be analyzed by
SIMCOL, and assumptions on material are similar to those in DAMAGE.
SIMCOL can be used to analyze collisions between two vessels,
but not collisions with a solid object, such as a bridge or a pier. The striking
ship bow is modeled in SIMCOL as wedge-shaped and rigid. Only the
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DESCRIPTION OF THE PROPOSED METHODOLOGY
59
risks that necessitate a higher degree of evaluation of the performance
of these systems.
CONSEQUENCE ASSESSMENT
While the first component of the methodology (damage and outflow cal-
culation) is based on techniques that have been developed previously,
the consequence assessment component is a new approach. As discussed
in Chapter 2, the committee determined that data on historical spills were
not sufficient to allow for the development of a response function de-
scribing the relationship of spill volume, oil type, and other factors to spill
consequence. Instead, the committee chose to apply existing models of
oil spill transport and fate, as well as consequence, to generate such a re-
lationship. The committee's approach to this effort, which involved the
following three tasks, is described in this section.
1. Selection of an existing oil fate and transport model and its appli-
cation to generate estimates of the expected physical consequences of
a broad range of hypothetical spills (e.g., meters of shoreline oiled, area
of slick).
2. Consideration of whether response functions could be developed
that describe the relationship of spill size to
Value of lost product,
Response cost,
Environmental consequence, and
Economic and social consequences.
3. Using the results of Tasks 1 and 2, establishment of equivalency
ratios that describe the expected consequence of a spill in terms of the
consequence of a standard-sized spill (referred to as the reference spill).
These tasks are discussed further below.
It is important to note that this approach to spill consequence
modeling differs significantly from earlier efforts (e.g., Astrup et al. 1995;
Michel and Moore 1995; Michel et al. 1996; Sirkar et al. 1997). These other
efforts involved using reported costs of historical oil spills as surrogates
for environmental damage, an approach rejected by the committee. Sirkar
et al. also recognized the limitations of available cost data and instead con-
sidered a range of hypothetical cost curves. They called for the compilation
of additional data on spill costs to support the generation of experience-
based cost relationships. However, the committee does not believe that an
accurate and consistent set of cost data can be compiled for purposes of
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
60
consequence assessment, and thus pursued the alternative approach de-
scribed below.
Modeling of Oil Spill Transport and Fate
Three primary decisions were required to model the expected physical
consequences of a broad range of hypothetical spills. First, a tool to
model the fate of oil in the environment had to be selected. Such tools
allow for an understanding of the likely trajectory of oil in the envi-
ronment under any number of hypothetical conditions. Second, the pa-
rameters of this model needed to be defined, including the case study
locations for which the model would be run, spill sizes, oil type, and
weather. Finally, the metrics that would be used to describe the physical
consequences of each modeled release event had to be identified.
Several models are available that simulate the transport and fate
of oil in the marine environment. Three were considered by the com-
mittee: (a) NRDAM/CME, developed by the U.S. Department of the In-
terior for purposes of natural resource damage assessment; (b) the Outer
Continental Shelf model, developed by the Department of the Interior's
Minerals Management Service for use in assessing the environmental im-
plications of decisions concerning off-shore oil leases; and (c) USCG's
TAP model, developed for oil spill response planning.
Of these three models, NRDAM/CME is the only one that (a) ad-
dresses a range of sites throughout the United States (the TAP model is
available only for San Francisco Bay) and (b) allows for thresholds of
concern to be varied. In addition, as noted earlier, ASA has developed a
stochastic version of NRDAM/CME, SIMAP, that makes it possible to ef-
fectively generate a large number of modeled scenarios with a reasonable
level of effort. It is this model that the committee applied to the present
analysis.
The SIMAP model is described in detail by French et al. (1996,
1999) and French and McCay (2001). The model has undergone ex-
tensive peer review, been applied widely for both damage assessment
and spill response planning purposes, and been accepted for use in
natural resource damage assessment by both the U.S. Department of the
Interior and NOAA.
The physical consequences of a spill event will depend largely
on the location of the event, the volume and type of oil spilled, and the
weather at the time of and following the spill. For example, the location
of a spill will determine shoreline oiling potential; the characteristics of
the product will determine the extent to which it partitions to the water
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DESCRIPTION OF THE PROPOSED METHODOLOGY
61
column and sediments; and the weather following a spill will determine
the direction and speed at which the slick will travel, the distance it will
travel, and the extent to which the oil will be dispersed in the envi-
ronment.4 The committee selected ranges for each of these factors in
modeling spill consequence across various spill volumes, as discussed in
Chapter 4.
The physical consequences of a spill can be described by a range
of metrics. Commonly used measures include extent of shoreline oiling,
area of slick, oil constituent concentrations in the water column, residual
oil constituent concentrations in sediments, and various measures of
the "dose" of oil borne by different environmental media [e.g., hours
of exceedance of a threshold polycyclic aromatic hydrocarbon (PAH)
concentration in the water column]. The committee used four physical
consequence metrics (the area of the slick, the length and area of oiled
shoreline, and the toxicity in the water column) to generate the equiv-
alency ratios, as described in Chapter 4.
Response Functions of Spill Size to Consequence
The ultimate implications of an oil spill are defined in terms of the envi-
ronmental, economic, financial, and social consequences of the event.
For example:
The owner(s) and insurer(s) of the ship's cargo suffer a financial loss.
The responsible party and other entities incur costs associated with
the response to the spill and its cleanup.
The public may suffer loss of use of an oiled resource (e.g., a beach
closure).
The public may suffer a loss associated with the reduced quality of
an environmental resource (e.g., the loss of nursery habitat for an endan-
gered fish species).
Private parties may suffer losses (e.g., a commercial fishery may be
closed for some period of time).
An important resource for a local community (e.g., an artesianal
fishery) may be lost or diminished in value.
The committee gathered information on the relationship of these factors
to spill size. In particular, the committee considered the extent to which
4Weather here is used to refer to wind, currents, tides, wave energy, and similar factors.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
62
these spill outcomes are more or less a function of the physical conse-
quence measures discussed above.
One option available was to attempt to model each of these mea-
sures of consequence individually. For example, it is possible to estimate
the likely number of visitor days that would be diminished in value as a
result of a hypothetical spill event given information on the location,
extent, and duration of beach oiling; the time of year of the spill event;
and beach use levels. Similarly, given the volume of oil spilled, the spill
location, and the weather conditions at the time of a spill, it is feasible to
generate an estimate of expected cleanup costs for a hypothetical spill
event. The committee examined a range of measures of spill consequence,
as discussed below.
Environmental Impact Measures
The committee considered the use of several environmental impact mea-
sures. In particular, SIMAP provides two categories of environmental con-
sequence metrics. The first involves measures of impacts to wildlife,
water column organisms, and so on. The second involves measures of
impact across shoreline type (e.g., rocky versus wetland). These mea-
sures may be better than the physical impact measures discussed above
at reflecting the likely true consequence of a spill event. For example,
equivalent areas of surface water swept by a slick at two locations may
affect dramatically different numbers of biota.
Despite the potential advantages of considering environmental
consequences,5 the committee chose not to do so, for several reasons.
First, there is a great deal of uncertainty in the available measures of
species abundance and other factors used to describe environmental re-
sources present at each modeled site, and these measures will vary with
weather, time of year, and other factors. That is, the environmental con-
sequences predicted by SIMAP may significantly over- or understate the
actual consequences that would occur from a given spill. Therefore, the
results generated by SIMAP for these measures of damage are generally
less well accepted by the professional community than are physical
impact measures.
Second, in selecting case study sites for use in modeling spill con-
sequence, the committee's goal was not to obtain precise measures of loss
5Environmental consequences refer to the impact of an oil spill on the affected habitat, whereas physical
consequences refer to physical measures of oil concentration in the water or on the shore. In the committee's
methodology, physical consequences are used to measure environmental consequences, and the term envi-
ronmental impact is used in the report to describe this measure.
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63
at these specific locations, but to model consequence under a range of pos-
sible physical conditions. Including the environmental consequence mea-
sures in the model would result in an increased risk of assigning too much
weight to the specific conditions found at these sites.
Finally, there is reason to believe that environmental consequence
will, on average, be linearly related to physical consequence. That is, the
physical consequence measures discussed above probably serve as good
indicators of likely environmental consequence when considered across
all potential spill locations and conditions.
Value of Lost Product
One category of economic loss associated with oil spills is the value of
the lost product. Early in the analysis, the committee determined that,
while larger quantities of product may have lower value per unit (and
thus the value of lost product from a large spill may not be as great as
that from a small spill, per unit volume), the overall effect of omitting this
factor from the analysis would be quite small. Thus, this measure was not
included in the analysis.
Response Costs
The committee expended a great deal of effort generating a relationship
that would describe oil spill response and cleanup cost as a function of
spill size. The committee considered historical spill cost data and ex-
isting models of spill cleanup and response costs, as well as primary es-
timates of spill response and cleanup costs generated for each of the
hypothetical spill locations. The committee asked a recognized expert
in the field, Dr. Dagmar Etkin, to prepare a set of reports summarizing
available information on spill response and cleanup costs and to provide
information that would make it possible to estimate response costs for
each of the modeled spills. (See Shoreline Cleanup Cost Modeling and
Mechanical Containment and Recovery Cost Models on the accompa-
nying CD.) After reviewing these reports, the committee concluded that
(a) the historical record of spill response and cleanup costs is not suffi-
cient to allow for the estimation of a cost consequence function; (b) esti-
mates generated using existing models of spill response and cleanup cost
demonstrate a great deal of uncertainty, especially for spills at the low and
high ends of the modeled spill size range; and (c) in many cases, the best
predictive models are simply linear transformations of the physical con-
sequence measures already being considered by the committee.
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All efforts to estimate spill response costs for modeled spill events
rely to some extent on historical cost data. Since the number of spills for
which such data are available is small (especially for larger spills), either
inconsistencies in the reported cost estimates or case-specific circum-
stances could introduce significant bias into the modeling effort. In ad-
dition, the committee determined that the degree of uncertainty inherent
in any modeled cost estimate will be greatest for the smallest and largest
hypothetical spill events, a factor that could have led to additional bias
in the final consequence function. Since the best predictive models are
linear transformations of physical consequence measures, the committee
decided that these measures are sufficient proxies for response and
cleanup costs.
As noted below, the committee recommends that additional
consideration be given to the development of a robust spill cleanup and
response cost function, and that the consequence model developed here
be tested for sensitivity to inclusion of alternative measures of consequence.
Third-Party Damages
The committee considered the development of detailed estimates of third-
party damage for each of the hypothetical spill events. The focus of this
discussion was on such consequences as beach closures, recreational
fishing closures, interruption of other recreational activities, commercial
fishing/shellfishing closures, and interruption of marine transportation.
As for the biological consequence metrics discussed above, the
committee determined that (a) the level of precision that could be
achieved in establishing these consequence measures would be low and
could vary significantly across case study sites (e.g., determining the im-
pacts of a beach closure would require information on numbers of vis-
itors to the beach at the time of the spill, which would generally not be
known with any certainty); (b) there is good reason to believe that these
measures of consequence are, in general, linearly related to physical
consequence, already captured in the modeling efforts as described
above; and (c) the case study sites were not chosen as specific locations,
but as indicative of the kinds of locations at which oil might be spilled
(that is, shifting the case study site a few miles could result in a different
pattern of expected third-party losses). Thus, as for specific measures of
environmental consequence, consideration of site-specific third-party
losses might have resulted in assigning disproportionate weight to a par-
ticular modeled location.
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DESCRIPTION OF THE PROPOSED METHODOLOGY
65
Establishment of Equivalency Ratios
As discussed above, the committee ultimately selected a modeling ap-
proach that (a) uses SIMAP to estimate the physical consequences of a
range of hypothetical oil spills at a set of case study sites and (b) assumes
that a set of physical consequence measures are reasonable proxies
for the financial, economic, social, and environmental consequences of
oil spills.
Since the committee's approach does not rely on a single metric
to report consequence (such as dollar damages), it was necessary to
combine the physical consequence measures used into a single mea-
sure that could be employed to compare the relative consequences of
spills of different sizes. The approach selected involves the use of "equiv-
alency ratios," which define the expected consequence of a spill of a
given size relative to the expected consequence of a standard reference
spill. That is,
physical consequence of modeled spill
Equivalency ratio = (e.g., meters of shoreline oiled)
physical consequence of reference spill
(e.g., meters of shoreline oiled)
If consequence is constant across spill size per gallon spilled, the
equivalency ratio should equal the ratio of the size of the modeled spill
to the size of the reference spill. That is, if the equivalency ratio is greater
than the ratio of spill size, the consequence per gallon of oil spilled is
greater for the modeled spill than for the reference spill. If the equiva-
lency ratio is less than the ratio of spill size, the consequence per gallon
of oil spilled is less for the modeled spill.
The specific analytic steps the committee followed in generating
these ratios and the ultimate consequence function are described in
Chapter 4.
DESIGN COMPARISON
The design comparison involves combining the results of the other two
steps in the methodology (outflow and consequence analyses). Since
both the alternative design and the reference double-hull vessel have
been run through the same set of accident scenarios, their performance
for each scenario can be directly compared. However, since the conse-
quence analysis showed that a direct oil outflow comparison would not
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66
be adequate, the outflow values need to be transformed using an overall
summary consequence metric based on the equivalency ratio. By taking
this step for each accident scenario, not only can the better design be de-
termined, but also the relative impact on the environment can be as-
sessed. If both designs perform poorly for a particularly severe accident
scenario, both may spill considerable, though different, amounts of oil
into the environment. This difference in environmental impact may not
be significant; massive damage has already occurred. However, the same
difference could be very important if one design did not leak and the
other did for a much less severe accident scenario. When repeated across
thousands of realistic scenarios, this pairwise analysis done at the scenario
level using the overall consequence metric allows for a comprehensive
evaluation of the designs. The resulting distribution of differences describes
the relative environmental performance of the alternative design and allows
for a determination of equivalency.
LIMITATIONS OF THE METHODOLOGY
While the committee's methodology is sound, it has some limitations in
its current stage of development. The four most significant limitations are
discussed below, along with suggestions for further development work
to address them.
Vessel Structure
Analysis of a vessel of nonsteel or nonconventional construction would
require the use of material performance characteristics dissimilar to those
used in the methodology and would necessitate modifications to the for-
mulae used in the analysis. To this end, the programs used in the analysis
of outflow, DAMAGE and SIMCOL, would have to be modified. Making
these modifications would require specific determination of both the ma-
terial properties and the performance of those materials.
Site-Specific Factors
The committee's approach involves modeling a large number of hypo-
thetical releases from tanker accidents at given case study sites. To the
extent that these case study sites are not representative of typical spill lo-
cations, this approach will produce biased results. The sensitivity of the
final consequence function to the chosen set of sites could, however, be
tested by considering a larger number of case study sites.
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67
Physical Consequence Measures
The committee's approach involves estimating the extent to which physical
consequence predicts total consequence. As noted above, the committee
assumed that various physical consequence metrics can be used as proxies
for the expected environmental and third-party costs of a spill, as well as
the expected response and cleanup costs. Several limitations may be intro-
duced by this assumption. The most significant of these is that third-party
costs and response and cleanup costs are likely to be disproportionately
larger for small spills. That is, the physical consequence measures used by
the committee are likely to understate third-party and response and clean-
up costs for small spills. Further investigation of this limitation could lead
to more confidence in the final outcomes.
Modeled Grid Size Used in SIMAP
SIMAP estimates the physical consequences of spills using a geographic
grid size that varies by location. In the case of small spills, the amount of
oil in the water may not be sufficient to register the physical consequence
measures. Thus while small spills do cause environmental harm, the
version of SIMAP used by the committee assigns some modeled spills a
physical consequence of zero. The result is understatement of the con-
sequences of some small spills. The SIMAP modeling approach does
adjust for this factor to a certain extent by assuming that the physical con-
sequence of a spill can never be less than the smallest measurable unit.
However, it would be useful to do additional modeling with smaller grid
sizes to understand the potential bias introduced by this factor and pos-
sibly modify the model for future uses.
SUMMARY OF THE METHODOLOGY
In summary, the methodology proposed by the committee involves two
initial steps--one to calculate oil outflow following a tanker accident and
the other to assess the consequence or impact of an oil spill. These two
steps are then combined in a third and final step--the comparison of
tanker designs.
The oil outflow calculation involves the computation of a series
of accident scenarios causing specific damage to a ship that, in turn, re-
sults in a specific quantity of oil spilled. Each design to be evaluated will
be subjected to the same series of accident scenarios, but because they
are different designs, different amounts of oil will be spilled.
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68
The consequence assessment involves assigning a rational metric
for the environmental consequence of a series of oil spill events of varying
quantities, locations, and conditions, and then constructing and using a
multiplier to transform the outflow differences for two designs determined
in the first step into consequence differences. The committee carried out
this step, and the resulting consequence curve can be applied to different
design comparisons. The consequence curve needs further refinement, as
discussed above, but once the refinement has been completed, this step
does not need to be repeated for each design comparison.
The third and final step, design comparison, involves comparing
the results of the measurement of differences in consequence for the design
being evaluated with the reference vessel. Under some conditions, one
design may have lower consequences and thus be a better performer,
but under other conditions the opposite may be true. The methodology
provides the data and analyses needed to make this comparison for a
large number of individual conditions and to conclude whether one
design is equivalent, inferior, or superior to another.
As noted above, the committee believes its methodology is sound,
systematic, and well specified, and represents a significant improvement
over methods proposed and used in the past. Chapter 4 describes the ap-
plication of the committee's methodology in more detail and provides ex-
amples of the necessary computations, descriptions of the analyses, and
illustrations of the resulting graphics that would be used to compare actual
designs.
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ABBREVIATION
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Representative terms from entire chapter:
physical consequence
