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EXECUTIVE SUMMARY
T o reduce the risk of oil spills from accidents, the United States re-
quires that all tankers calling on U.S. ports in the future be fitted
with a double hull. The world tanker fleet is changing to double hulls in
accordance with both U.S. law and a similar provision in an international
agreement. Even as this change has been occurring, however, some or-
ganizations have proposed alternative designs as equivalent or superior
to a double hull in preventing the outflow of oil in an accident. U.S. law
allows for the evaluation and approval of alternative designs that can be
determined to have performance equivalent to or better than the double
hull in protecting the environment. To date, however, the United States
Coast Guard (USCG) has not made that determination for any alternative
design. The International Maritime Organization (IMO) has approved two
designs as equivalent to the double hull; because most tankers might trade
with the United States at some time, however, the U.S. double-hull regu-
lation has thus far set the standard for the world.
Proponents of alternative tanker designs have approached USCG
and the U.S. Congress with proposals they believe will offer performance
equivalent to or better than the double hull and have asked whether
these designs would be accepted under U.S. law. Moreover, some have
proposed that regulations be based on performance criteria for designs
instead of prescriptive criteria. They believe that if a method of evalu-
ating equivalence to double hulls can be developed, a better design can
be invented, whereas technological innovation will be discouraged if
only a single prescriptive design, such as the double hull, remains a fixed
requirement. Some proponents also believe that U.S. regulations should
be more consistent with international law, especially in an area that has
so close a connection to international trade.
1
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
2
STUDY PURPOSE
In the above context, Congress requested that the present study be un-
dertaken by a committee under the auspices of the Marine Board of the
National Research Council's (NRC) Transportation Research Board to
determine whether a methodology could be established for measuring
the equivalency of alternatives to double-hull designs with regard to en-
vironmental performance. Congress made this request through the USCG
Authorization Act of 1998 but specified that the investigation be con-
ducted independently of past USCG policy on double-hull equivalency.
The committee was charged to develop a rationally based approach for
assessing the environmental performance of alternative tanker designs
relative to the double-hull standard. The proposed methodology was then
to be applied to double-hull tankers and alternative designs to demon-
strate that it could be used for an assessment. The committee was asked
to ensure that the proposed methodology would be applicable to con-
ditions prevailing in U.S. waters. The committee's charge also included
refining and adjusting existing tanker damage extent functions used for
measuring the crashworthiness of tank vessel structures. In addition, the
committee was to develop a generalized spill cost database and use this
database in formulating a rationally based approach for the calculation
of an environmental index.
BACKGROUND
Status of Design Proposals
A number of organizations have submitted proposed alternatives to double-
hull designs to either USCG (for the United States) or IMO (representing
the international community), both of which have developed regulations
addressing minimum tanker design standards. Under the IMO guidelines,
the double hull and two other designs--the mid-deck design and a special
variation on that concept proposed by the Swedish government and ap-
proved in 1997--are permitted. In addition to the alternatives submitted to
IMO, several other concepts have been developed in the United States.
Status of Existing Methodologies
While considerable effort has gone into schemes for measuring the en-
vironmental performance of tankers following accidents, none of these
approaches carries the analysis through to a conclusion based on relative
damage to the environment from an oil spill. The IMO method of com-
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EXECUTIVE SUMMARY
3
paring designs is based on a formula that assigns relative weighting factors
to three outflow parameters related to spill size (zero, mean, and extreme
outflow). Use of these weighting factors was based on IMO's decision to
select a formula that would ensure the equivalency of the double-hull
and mid-deck designs. However, these factors provide no measure of the
possible environmental damage itself. Moreover, the IMO methodology
uses historical distributions of ship structural damage that are not ap-
plicable to new designs, in particular to designs based on innovative struc-
tural concepts.
In its recent decisions on whether to accept double-hull alterna-
tives, USCG applied a method that employs estimates of the same three
outflow parameters used by IMO (zero, mean, and extreme). Judgment
is then applied regarding the importance of the zero outflow factor.
USCG puts the greatest emphasis on avoiding all spills because it inter-
prets the Clean Water Act as prohibiting any such discharges. However,
the environmental consequence judgment involved in this method has
not been expressed quantitatively.
Other researchers have investigated methods of applying past
work on the probabilities of zero, mean, and extreme outflows to some
surrogate for environmental consequences, such as cost (see below).
However, none have assembled the cost data needed to yield conclusive
results.
Limitations of Cost Data for Measuring
Environmental Damage
As noted, the committee's charge included development of a generalized
spill cost database that could assist in formulating a rationally based ap-
proach for calculation of an environmental index. The goal behind this
charge was to apply the historical record of oil spill costs as a basis for
comparing alternative tanker designs.
The committee identified and obtained many data sets that de-
scribe the costs associated with oil spills. However, the existing cost data
for past oil spills have been gathered irregularly and are difficult or im-
possible to obtain. Moreover, because of extreme variability in the cost
data associated with environmental damage assessment, as well as in
third-party cost data, past data are neither reliable nor comparable.
The committee also considered whether it would be possible and
appropriate to develop its own database that would reflect a high degree
of quality control and include a large number of events. For example, in
some cases a cost estimate reported in a data set will reflect the costs as
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
4
of a certain date, when additional costs were incurred after the initial
report. The committee concluded that the difficulties involved in obtaining
data and in estimating environmental consequences would make the re-
sultant data set of limited use. In addition, considering the fact that very
few large-scale spills actually occur, it would be difficult to generalize
from these limited events to generate a consequence model. As a result,
the committee decided to develop a response function relating spill con-
sequences to spill characteristics (e.g., volume of spill, location of spill,
type of oil) using a modeling approach. Combining the expertise of com-
mittee members with a review of risk analysis in other fields, the committee
decided to develop a risk-based methodology that uses environmental
consequences following a spill as a means of comparing the performance
of proposed alternatives against that of a standardized double-hull vessel
for each of the comparable sizes.
DEVELOPMENT OF A NEW METHODOLOGY
Given the status of previous efforts to establish a methodology for com-
paring the environmental performance of alternative tanker designs, the
committee concluded that the development of a new approach was war-
ranted. The methodology developed by the committee is divided into
three main components: (a) structural damage and oil outflow calcu-
lation, (b) consequence assessment, and (c) design comparison.
For the first component, the committee selected scenarios (col-
lision and grounding events) that represented conditions in U.S. waters,
specifically in those areas with a high density of tanker traffic. Once the
collision and grounding events had been identified, the committee used
damage models to determine the structural damage and resulting outflow
in each accident.
The second component of the methodology involves the as-
sessment of consequences from an accidental spill. The committee used
an environmental impact model that predicts oil fate and transport and
allows for random sampling of weather conditions on the basis of historical
weather data. This model provides a number of physical consequence
measures, such as the area of the sheen, the toxicity in the water column,
and the length and area of oiled shoreline. The committee decided to limit
the assessment of consequences to these physical measures instead of
extending it to impacts on biological resources. Doing so would keep the
analysis as systematic and well specified as possible without necessitating
difficult decisions as to what threshold levels would damage biological
resources and how those resources are valued.
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EXECUTIVE SUMMARY
5
In addition to the physical consequence measures, the committee
considered several nonenvironmental measures of consequence, in-
cluding spill cleanup and response costs, the value of lost product, and
third-party damages. These measures were eliminated from the final
analysis for several reasons. The value of lost product, measured on a per
gallon basis, is not expected to vary significantly with spill size. More im-
portant, the collective best judgment of the committee was that the
physical consequence metrics used in the analysis are reasonable proxy
measures for likely third-party losses and cleanup and response costs.
The final component of the methodology involves the com-
parison of two designs. By subjecting each design to the same set of ac-
cident scenarios, one can directly compare the resulting performance of
the designs for each scenario. Since the relative impact on the envi-
ronment can be assessed for each design, the better-performing design
is evident for each accident scenario.
APPLICATION OFTHE NEW METHODOLOGY
The committee prepared two examples to demonstrate the application of
the methodology and perform an initial test of its validity. Both examples
involved comparing the performance of a double-hull design with that of
a single-hull design. One example used vessels with a 150,000-deadweight
ton (DWT) capacity and the other vessels with a 40,000-DWT capacity
design. None of the alternative designs proposed to USCG was available
to the committee in sufficient detail to be used in the examples.
The committee selected four case study locations in U.S. waters:
Big Stone Anchorage (Delaware Bay), Galveston lightering area (Gulf of
Mexico), Carquinez Strait Bridge (San Francisco Bay), and Farallon Islands
(offshore San Francisco). These locations have large volumes of tanker
traffic, and adequate oil spill modeling data are available for each. These
four locations also demonstrate sufficient variation in site characteristics and
conditions to provide an adequate test of the components of the method-
ology. The committee ran a total of 80,000 accident scenarios: 10,000 col-
lision and 10,000 grounding events for each of two designs (single-hull and
double-hull) of the two different sizes (150,000 and 40,000 DWT).
To determine environmental consequences for these events, the
committee conducted a separate analysis and generated a set of conse-
quence functions for the necessary range of oil outflows, also considering
such factors as weather, oil types, and geography for the selected locations.
With four sites, 200 weather events, two oil types (crude and product), and
seven spill volumes, a total of 11,200 spills was simulated in the models
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
6
used. The seven spill volumes were chosen to represent the full range of
possibilities.
The consequence functions selected by the committee represent
ratios of the environmental consequence for a spill of a certain size to
that for a 500,000-gallon reference spill. The consequence function is not
linear. That is, for spills smaller than the reference spill size, the conse-
quence for each gallon spilled is relatively greater than for the reference
size, and that difference continues to increase as the spill size decreases.
The opposite is true for larger spills. For example, for a spill of 100 times
the reference size (a spill of 50 million gallons) the environmental conse-
quence function would be only about 8 times greater. To test the validity
of this result, the committee also conducted several sensitivity analyses--
for different oil types, different case study sites, and different consequence
metrics. These analyses provide upper and lower bounds for the con-
sequence function graph and can be used to make the final design com-
parisons more accurate and complete.
Each of the 80,000 outflows was converted to a consequence
measure, relative to the 500,000-gallon spill equivalent. By calculating and
analyzing the differences in this measure across the scenarios, one can de-
termine the relative performance of two designs. However, this result is not
meant to be predictive of environmental consequences for any specific spill.
Since the committee did not attempt to determine the likelihood
of any of the scenarios actually occurring, the analysis cannot be used to
determine the real savings that might accrue through use of a particular
design in a given time period. The latter is a very different and more dif-
ficult problem that would require a detailed risk analysis of a specific port
area with defined operations and traffic patterns. This is the type of
analysis that would be required, for example, if one wanted to determine
whether the additional costs of an alternative design were justified.
CONCLUSIONS AND RECOMMENDATIONS
The committee developed a rationally based approach for assessing the
performance of alternative tanker designs on the basis of their relative
ability to prevent environmental damage from oil spills following col-
lision and grounding accidents. This methodology can be used as a tool
by regulatory authorities in determining whether to approve an alter-
native to the double-hull tanker design. First, however, a few other things
need to be accomplished: (a) peer review of the methodology; (b) testing
of the methodology; and (c) a comprehensive review of the methodology
by stakeholders, including the tanker industry and environmental groups,
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EXECUTIVE SUMMARY
7
as well as regulatory, oversight, and review organizations. The method-
ology is a significant improvement over existing methods; however, it
needs further refinement to enhance its accuracy and reliability. On the
basis of its work, the committee makes the following recommendations.
Overall Methodology
Recommendation 1: USCG should use the proposed method-
ology for evaluating alternative tanker designs and at the same
time undertake a program to refine the methodology to address
the issues discussed in this report.
Recommendation 2: USCG should institute a standard pro-
cedure for evaluating specific designs submitted as equivalent
to a double-hull design. This procedure should include the
methodology proposed by the committee for assessing equiva-
lency on the basis of environmental consequences from oil
spills following collision and grounding accidents. Other ap-
propriate factors, such as those associated with the safety and
operation of the vessel, will have to be evaluated in conjunction
with the use of this methodology.
Recommendation 3: To continue and validate the work of the
committee, USCG should apply the committee's methodology to
compare other alternative designs with the double hull. The
committee suggests that one alternative assessed be the mid-
deck design, which is available in a detailed form and has al-
ready been evaluated by IMO.
Double-Hull Reference Ships
Although the committee's charge referred to comparing alternative
designs with the double-hull standard, the committee did not select
a standard double-hull design. To test its methodology, the committee
selected one available double-hull design without regard to whether it
represented an accepted standard. Since each design may have qualities
and characteristics that differ from a minimum standard in a significant
way, selection of one standard by which all alternatives would be mea-
sured in the future would represent a policy decision. In using the
methodology, however, a critical first step is to define such standard
double-hull reference ships in a number of size ranges, thus enabling all
proposed new designs to be measured on the same basis.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
8
Recommendation 4: USCG should define in sufficient detail and
make available the standard reference ships needed for the
methodology. This concept is similar in nature to the reference
ships currently used by IMO. In developing the standard ref-
erence ships, USCG should refer to the discussion of design of
double-hull tank vessels in the 1998 NRC report entitled Double-
Hull Tanker Legislation: An Assessment of the Oil Pollution Act
of 1990.
Need for Vessel Design Details
The committee's approach to developing this methodology entailed
rigorous computational methods that included analyzing the crash-
worthiness of ship structures, calculating oil outflows in specific accident
scenarios, and modeling spills and their complex behaviors while re-
ducing the results to numerical values. The methodology is necessarily
complex and requires substantial detail in all input values, including
complete design details for any vessel to be evaluated. Given these com-
plexities, it would be unreasonable to expect that the methodology could
be used to evaluate a concept in the absence of a complete ship design.
The committee concludes that if an alternative design is to be evaluated
by USCG, sufficient design and analysis detail must be available.
Recommendation 5: Anyone proposing an alternative design
should be required to submit to USCG not only a complete de-
scription, including design plans, but also an analysis of the
design and its performance within the framework of the
models used in the proposed methodology, including such as-
pects as outflow under different accident scenarios. Sufficient
information should also be provided to allow USCG to perform
an independent review of the proposed design. In addition,
USCG should prepare specific instructions for those who wish
to submit alternative designs, including a list of required design
plans, structural and mechanical details, and relevant calcula-
tions. The format and organization of a submission should also
be specified.
Consideration of Active Systems
Several designs for oil tankers, including the double hull, protect against
oil spills by creating an arrangement of the ship's structure that will prevent
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EXECUTIVE SUMMARY
9
or mitigate oil outflow. These "passive" approaches may create a void
space between cargo oil tanks and the sea, or locate tanks where they are
less likely to leak or be punctured, or use hydrostatic pressure balance to
prevent leaks after a puncture. In contrast to passive systems, some alter-
native designs incorporate "active" systems with the use of valves, sensors,
piping, pumps, or other mechanical devices that would be activated after
an accident to mitigate oil outflow. Active systems present additional factors
to be considered when evaluating alternatives. Their unique characteristics
pose multiple types of risks that need to be considered in conjunction with
relevant operational protocols. These complexities add an overlay to the
proposed methodology that the committee could not test within this study
because of a lack of sufficient detail on any active system.
Recommendation 6: Any submittal to USCG of an alternative
design that includes an active system should contain a quanti-
tative life-cycle risk analysis, along with supporting information,
so that independent verification can be accomplished by either
USCG or others. In addition, USCG should develop the capability
to review and evaluate all of the risk assessment factors that
might be presented in such a submittal.
Components of the Methodology
In its outflow analysis, the committee concluded that use of historical
data, and therefore the IMO methodology and other methods based on
such data, is not appropriate for evaluating new tanker designs. Accord-
ingly, the methodology proposed in this report uses direct computational
tools instead of historical data to determine the crashworthiness of either
double-hull or alternative designs. The structural damage databases cur-
rently available, including the one updated by the committee, include
only single-hull tankers and combination carriers. Collecting new data
would not provide a usable database for the purposes of this method-
ology since data on innovative designs simply do not exist.
In addition, in developing the methodology, the committee con-
cluded that existing computational tools for determining damage extent
and outflow are not fully validated, and methods are based on simplifying
assumptions whose effects on the results are not yet entirely understood.
The committee believes that the computational tools used for this study
provide a better comparative method than the current approach based
on the use of historical damage data, although further work is needed to
validate and improve these tools.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
10
Recommendation 7: USCG should undertake a program to
collect collision and grounding data in sufficient detail for use
in validating both collision and grounding analyses. The USCG
accident investigation report should routinely include data of the
detail and extent necessary for this purpose. The data should be
stored in a format that is easily accessible and conveniently
usable by researchers. USCG should encourage others, through
IMO, to collect detailed accident investigation data in a uniform
manner. In addition, USCG should initiate a program for the
continued development of grounding and collision analyses.
The following areas need the most development:
Addition of other than powered grounding on a single
pinnacle,
Addition of collision with solid objects,
Addition of a deformable bow in the collision model, and
Further development of the collision model at the struc-
tural member level.
As more data become available, USCG should maintain a con-
tinuing program of testing and validation of the collision and
grounding analysis tools.
As discussed earlier, the consequence analysis performed by the
committee indicated that the relationship between spill size and envi-
ronmental consequence is not linear. In other words, the impact of spills
increases with volume, but the marginal impact of each gallon spilled de-
creases. Thus the evaluation of an alternative design based on outflow
alone would not be valid and could yield a misleading result. This con-
clusion led the committee to select an approach that could relate mea-
sures of environmental damage to each oil spill scenario. Moreover, as
explained above, the committee chose to use physical consequences, in-
stead of historical spill costs, as the most consistently measurable and
comparable method of evaluating environmental consequences.
Recommendation 8: The committee recommends that USCG
take the committee's findings on evaluating environmental
consequences of spills into account in its regulatory initiatives
relative to environmental impacts of oil spills, including cost
benefit analyses.
In its design comparison, the committee concluded that a complete
distribution of the differences in environmental impact (impact differences)
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EXECUTIVE SUMMARY
11
is necessary for such comparisons because it provides information on the
regions where one design performs better than another. The use of simple
descriptive statistics, such as the mean, is not sufficient and can be mis-
leading. Furthermore, it is important to compare the impact differences
between alternative designs event by event, instead of comparing the
cumulative impacts of designs for all events. The impact difference is a
function of the difference in the consequences of the outflows from each
design, and since this relationship is not linear, it is not a function of the
outflow difference. For example, the impact difference for an event in
which one design spills 200,000 gallons and the other spills no oil can be
larger than that for an event in which one design spills 60 million gallons
and the other 70 million gallons, even though the actual outflow difference
is much larger in the latter case. The methodology proposed by the com-
mittee yields a distribution of impact differences for each event, which in
turn provides information on the magnitude of the impact differences as
well as their frequency.
Recommendation 9: The committee recommends that USCG
propose to IMO that it replace its current guidelines with a ra-
tional methodology for evaluating alternative tanker designs
based on the principles presented in this report.
The committee understands that to implement all of its recom-
mendations will require substantial time and effort on the part of USCG
but has neither estimated the cost involved nor determined whether
USCG has the necessary resources available. Therefore, the committee
cannot propose an appropriate schedule for the recommended tasks, nor
can it set priorities for this work relative to USCG's other responsibilities.
The committee does, however, believe that the work presented to illus-
trate the proposed methodology provides a foundation that can be used
by USCG in its implementation efforts.
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Representative terms from entire chapter:
double hull
