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5
CONCLUSIONS
AND RECOMMENDATIONS
he committee has carried out its charge to develop a rationally
based methodology for assessing alternative tanker designs on the
basis of their relative ability to prevent environmental damage from oil
T
spills following collision and grounding accidents. The committee has
tested this methodology by applying it to two existing tanker designs
and presented the results of those applications. Finally, the committee
has researched and produced related analyses that support, explain, and
demonstrate the methodology, including its limitations. The committee's
conclusions concerning these efforts are presented in this chapter. Also
presented are the committee's recommendations for further refinement
of the methodology and other actions to enhance its future use, as well
as for its application by USCG in making decisions about the acceptability
of alternatives to the double-hull design for operation in U.S. waters.
OVERALL METHODOLOGY
The methodology developed by the committee for comparing alternative
tanker designs is described in Chapter 3; its application for comparing
two designs is illustrated in Chapter 4. The committee concludes that the
methodology can be used as a tool by a regulatory authority for consid-
ering alternative designs and represents a significant improvement over
existing methods. The methodology could also be used by both the in-
dustry and regulatory authorities to rate the relative environmental per-
formance of two designs following collision and grounding accidents. At
the same time, the methodology needs further refinement to enhance its
accuracy and reliability. Such efforts should encompass peer review of
the methodology; testing of the methodology; and a comprehensive
review by stakeholders, including the tanker industry, environmental
groups, and regulators. The methodology is not dependent on the
113
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
114
software used by the committee. If other software is found to provide
more accurate results for structural damage and outflow or the conse-
quences of spills, it can be used instead.
The discussion in Chapters 3 and 4 reviews those refinements the
committee believes to be most critical, as well as factors that will limit the
implementation of the methodology until further refinement is accom-
plished. In developing the methodology, for example, the committee used
a limited set of model specifications concerning spill size and location, as
described in Chapter 4. The committee believes that the construction of
additional model scenarios would yield more information about the rel-
ative effects of spill size and location and thus improve the accuracy of the
modeling technique. Furthermore, certain applications of the methodology
may require enhancements to accommodate various design features. For
example, the tool used to calculate structural damage must be modified if
a subject vessel under review is not of conventional steel construction.
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 ad-
dress 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 appro-
priate factors, such as those associated with the safety and oper-
ation 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
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CONCLUSIONS AND RECOMMENDATIONS
115
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 char-
acteristics that differ from a minimum standard in a significant way,
selection of one standard by which all alternatives would be measured in
the future would represent a policy decision. In using the methodology,
however, a critical first step is to define such standard double-hull ref-
erence ships in a number of size ranges, thus enabling all proposed new
designs to be measured on the same basis.
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 FORVESSEL 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 reducing
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 complexities, 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 con-
cludes 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,
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
116
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
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 char-
acteristics pose multiple types of risks that need to be considered in con-
junction 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.
In particular, a quantitative life-cycle risk analysis conforming to require-
ments specified by USCG would be needed as part of the approval process
for an active system. The committee believes that the techniques for con-
ducting such an analysis exist; however, it could not apply these tech-
niques within the limited resources available for this study.
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 OFTHE METHODOLOGY
The methodology developed by the committee has three components or
steps: (a) analysis of oil outflow following a collision or grounding ac-
cident, (b) analysis of the consequences of the oil outflow, and (c) com-
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CONCLUSIONS AND RECOMMENDATIONS
117
parison of the design relative to the environmental performance of the
double-hull standard.
Outflow Analysis
The committee concluded that the use of historical data, and therefore the
IMO methodology and other methods based on such data, is not appro-
priate for evaluating new tanker designs. Accordingly, the methodology
proposed by the committee uses direct computational tools instead of
historical data to determine the crashworthiness of either double-hull or
alternative designs. The structural damage databases currently 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 methodology since data on in-
novative 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 their applicability is limited to eval-
uating structural arrangements that use conventional members and
materials--plates, webs, girders, and steel. Grounding types are limited
to powered groundings on a single pinnacle. Methods are based on sim-
plifying assumptions whose effects on the results are not yet entirely un-
derstood. 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.
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 conve-
niently 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,
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
118
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.
Consequence Analysis
The consequence analysis performed by the committee indicated that the
relationship between spill size and environmental consequence is not
linear. The committee's application of the methodology demonstrated that
the consequence of each additional gallon spilled is greater for small than
for large spills. In other words, the impact of spills increases with volume,
but the marginal impact of each additional gallon spilled decreases. Thus
the evaluation of an alternative design based on outflow alone would not
be valid and could yield a misleading result. This conclusion led the com-
mittee to select an approach that could relate measures of environmental
damage to each oil spill scenario.
The existing data on cost for past oil spills have been gathered ir-
regularly and are difficult or impossible to obtain. Because of extreme vari-
ability in the cost data associated with environmental damage assessment,
as well as in third-party cost data (which together constitute the prepon-
derance of costs in most spills), past data are neither reliable nor compa-
rable. Therefore the committee chose to use physical consequence instead
of historical spill costs as the most consistently measurable and comparable
method of evaluating environmental consequences. The committee does
not believe that further efforts to collect and analyze historical data on spill
costs would lead to any improvements in the development or application
of its methodology or other similar efforts.
Recommendation 8: The committee recommends that USCG
take the committee's findings on evaluating environmental con-
sequences of spills into account in its regulatory initiatives rel-
ative to environmental impacts of oil spills, including cost
benefit analyses.
Design Comparison
The committee concludes that a complete distribution of the differences in
environmental impact (impact differences) is necessary for comparison of
designs because it provides information on the regions where one design
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CONCLUSIONS AND RECOMMENDATIONS
119
performs better than another. The use of simple descriptive statistics, such
as the mean, is not sufficient and can be misleading. Furthermore, it is im-
portant to compare the impact differences event by event instead of com-
paring the cumulative impacts of designs for all events. As noted above,
the impact difference is not a linear 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,
even though the outflow difference is larger in the latter case. The method-
ology proposed by the committee yields a distribution of impact differ-
ences 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.
Representative terms from entire chapter:
alternative designs
