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OCR for page 27
2
ASSESSMENT OF PREVIOUS
EVALUATION METHODS,
PROPOSALS FOR ALTERNATIVE
DESIGNS, AND HISTORICAL
DATABASES
P ast efforts to develop methodologies for evaluating alternative tanker
designs and the proposals that have been submitted for approval to
regulatory authorities for designs that might provide performance equiv-
alent to that of double hulls are summarized in this chapter. In the com-
prehensive report entitled Tanker Spills: Prevention by Design (NRC 1991),
the alternative designs that were available as of the date of that study are
reviewed, and their effectiveness is evaluated. The discussion in this chapter
focuses on more recent design proposals and potential approaches to mea-
suring equivalency. Also discussed are the limitations of historical databases
with regard to spill costs, damage statistics, and collision and grounding
incidents. A brief review of quantitative risk-assessment techniques and
their application to the present study is given in the final section.
PREVIOUS EVALUATION METHODS
ANDTHEIR LIMITATIONS
In 1989 USCG commissioned an NRC study of tanker designs and their
pollution-prevention qualities. The study report (NRC 1991) includes an
assessment of whether other structural and operational tank vessel re-
quirements would offer protection for the marine environment equal to or
greater than that provided by the double-hull design (based on oil outflow
following an accident). The 1991 report describes the evaluation of a
number of design variations known at that time. Several were believed to
27
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
28
have the ability to reduce oil outflow, but either sufficient detail was not
provided or technical features were still evolving, so that further study was
needed to assess the design concept. The NRC report identifies four oil
spill control methods under which all designs can be catalogued:
Barrier: In addition to the outer hull, provides a secondary obstacle
to the loss of cargo in the event of an accident.
Outflow management: Restricts the amount of cargo subject to out-
flow and is either passive (through smaller tanks) or active (by manipu-
lation of hydrostatic balance or cargo transfer).
Increased penetration resistance: Controls the worst case through a
more absorbent hull that transfers the momentum from an impact, re-
sulting in a hull that is crushed rather than ruptured.
Accident response: Minimizes the loss of oil from an accident
through either systems that monitor accident conditions or features that
assist with salvage operations.
A major conclusion of the study was that "the committee did not
identify any design as superior to the double hull for all accident scenarios"
(NRC 1991, xxi). However, the committee recommended that other design
alternatives proposed in the course of future research be considered.
Following passage of the OPA 90 legislation, USCG also com-
missioned a study (Herbert Engineering Corporation 1992) to assess the
environmental performance of alternative designs on the basis of the cal-
culation of three measures of merit:
The likelihood that the design will not spill oil given a collision or
grounding that breaches the outer hull, generally referred to as the "prob-
ability of zero outflow";
The mean or average expected outflow from a collision or grounding;
and
The extreme outflow, which is a measure of the expected outflow in
the most severe collision or grounding.
USCG subsequently submitted a report to Congress on alternatives
to double-hull tank vessel design (USCG 1992), incorporating the conclu-
sions of the NRC and other studies (e.g., Herbert Engineering Corporation
1992). USCG determined that certain alternative designs exhibit superior
mean outflow and extreme outflow characteristics as compared with a
double hull. However, the likelihood of a spill following a collision or
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ASSESSMENT OF PREVIOUS EVALUATION METHODS
29
grounding was found to be higher for all of the alternative designs inves-
tigated. Since double hulls were projected to have fewer spills, the con-
clusion of the study was that none of the alternative designs exhibited
environmental performance equivalent to that of double hulls. The report
notes further that tanker design is only one facet of the "total safety and pol-
lution prevention system" and suggests that other measures affecting safe
vessel operations and management should be pursued. The many short-
comings of existing evaluation methodologies are also cited, and further
work in this area is recommended.
USCG and others have supported work aimed at developing
methods for predicting the effects of a collision or grounding on the hull
structure of a ship. This work will add to current understanding of the
structural performance of tankers during accidents. It will also be needed
for future analyses of accidental oil outflow for any new tanker designs
proposed. It has not, however, led to the development of a comprehensive
methodology for evaluating equivalency to double-hull designs.
The IMO method of comparing alternative designs (IMO 1996)
uses a formula that assigns relative weighting factors to the three param-
eters related to spill size noted above (zero, mean, and extreme outflow).1
The weighting factors used are based on a decision by IMO to select a
formula that would ensure the equivalency of the double hull and the mid-
deck design (discussed below), but they provide no measure of the pos-
sible environmental damage itself (Sirkar et al. 1997). However, the IMO
method is probably the only one that is described fully enough for prac-
tical application to an actual design and in fact has been used for that
purpose (see below).
The committee has reviewed the IMO methodology and con-
sidered the implications of its use, as well as its limitations. In the method-
ology, damage to a ship is described by damage extent and damage
location distributions, which are based on limited historical damage data.
The distributions are the same for each type of design, and there are no
cases in which the outer hull is not damaged. The outflow distribution and
the outflow parameters (zero, mean, and extreme outflow) are determined
by analyzing all possible damage cases corresponding to the damage
distributions. The outflow parameters are then combined into an envi-
ronmental index using weighting factors. The fact that IMO's choice of
weighting factors cannot be related to any real measures of environmental
consequence in itself would appear to eliminate the method from consid-
1Appendix C describes the IMO methodology in detail.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
30
eration. Moreover, the historical distributions of ship structural damage are
not applicable to new designs, particularly to those incorporating structural
innovations. Therefore, the committee believes IMO's work on vessel ac-
cidents, structural damage, and oil outflow probabilities is most valuable,
but cannot be directly adapted to a more rigorous methodology. Both the
structural resistance involved in grounding and collision and a more ap-
propriate measure of relative environmental consequences must be incor-
porated into a new scheme.
As noted, USCG has used an evaluation method in its decisions
concerning double-hull alternatives that incorporates estimates of the above
three oil spill outflow ranges (zero, mean, and extreme). These estimates
are made using the same damage distributions and the same approach
used by IMO. USCG then applies its judgment concerning the primary
importance of the zero outflow factor without resorting to any weighting
factors for these three ranges. USCG places the greatest emphasis on
avoiding all spills because it interprets the Clean Water Act as prohibiting
any such discharges.2 However, the environmental consequence judg-
ment required by this policy cannot be expressed quantitatively in a
rigorous analysis. Currently, then, USCG does not have in place a well-
specified methodology for evaluating the equivalency of alternatives to
double-hull tankers.
A number of researchers have investigated methods of applying
the past work on probabilities of zero, mean, and extreme outflows to
some appropriate surrogate for environmental damage related to spill size.
The most developed and recent work on this approach was published by
Sirkar et al. (1997). The authors attempt to establish an analytical means of
assessing the relative importance of the different measures of merit. The
authors also propose that total cost of a spill as a function of its size could
be used as a surrogate for relative environmental impact from accidental
tanker spills. In conducting their study, the authors found that the available
data on historical spill costs were not sufficient, and they suggest that ad-
ditional data be collected to carry out the proposed analysis. However,
many believe that cost as a function of spill size is not a reliable measure
of environmental impact because several factors other than the size of a
spill (e.g., location, weather conditions) greatly influence its cost. This
issue is discussed below, as well as in the description of the committee's
methodology and its application in Chapters 3 and 4, respectively.
2The Federal Water Pollution Control Act of 1972 states: "The Congress hereby declares that it is the policy of
the United States that there should be no discharge of oil or hazardous substances into or upon the navigable
water of the United States." OPA 90 is an amendment to this act.
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ASSESSMENT OF PREVIOUS EVALUATION METHODS
31
While the present study does not include an evaluation of any of
the specific concepts proposed to USCG or IMO (as presented in the next
section), it is important to note that any future evaluation methodology
adopted must be able to accommodate the review of such proposals.
Some of the important features of an appropriate methodology are dis-
cussed below.
First, it is important that an evaluation methodology be clearly un-
derstood by all parties and consistently applied to all proposals. A specific
set of requirements for proposals should be published, including detailed
instructions as to what design and test data are needed and in what form.
Furthermore, the double-hull standard reference ship used for comparison
must be defined in a clear and unambiguous manner. The application of
the methodology should also be as transparent as possible so that each
person or company submitting a proposal will know all the evaluation cri-
teria to be used.
In addition, the methodology should accommodate the variety of
conceptual approaches that may be expected. From past experience, it
is clear that both passive and active systems in many combinations may
be submitted.3 Also, systems with new and unique materials reflecting
different performance characteristics must be considered, since they pose
unique problems. Finally, it should be noted that some proposed systems
have performance histories while others do not, and the methodology
must provide a way to evaluate and consider the relative merits of both.
PROPOSALS FOR ALTERNATIVE DESIGNS
Even though the U.S. requirements for double-hull tankers reviewed in
Chapter 1 have had the effect of setting an international standard (because
most tankers may trade in U.S. ports at some time), the international com-
munity has investigated a number of alternative designs in recent years.
Design proposals for alternatives to double-hull oil tankers have come
from many sources and continue to be put forward with the anticipation
that, if they have proven merit, they will gain needed support from both
the industry and regulators. Since both USCG (for the United States) and
IMO (representing the international community) have regulations ad-
dressing the design of oil tankers (see Chapter 1), those who propose
3A passive system is defined as one that is integral to the vessel's structure and requires no moving parts or
action by a third party to be effective. A double-hull vessel is an example of a passive system. An active system
is defined as one that requires, in whole or in part, an action by a third party or system to be effective. An
emergency transfer system is an example of an active system.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
32
alternatives usually submit them to either or both of these agencies. The
following discussion therefore draws on reports from both USCG and IMO
to describe the designs that have been proposed.
Alternative Designs Proposed to IMO
In 1992, IMO adopted both the double-hull tanker (submitted by the
United States) and the mid-deck tanker (submitted by the Japanese gov-
ernment) as acceptable design types to replace the world's single-hull
tanker fleet. Adoption of these designs led to the establishment of the
IMO regulation permitting alternative designs that meet the stated re-
quirements for equivalent weighted outflow. The mid-deck design was
reviewed for the 1991 NRC report as well; the conclusion reached was
that the design had potential but needed more study. The only other design
IMO has approved since 1992 is the Coulombi Egg concept, proposed by
the Swedish government, which was determined to meet the IMO equiv-
alency test for design approval in 1997. One other design that received
some international attention is the Polmis concept (proprietary), pro-
posed by a German organization, but it was never formally submitted to
or approved by IMO.
The mid-deck design concept utilizes hydrostatic pressure balance
in lower cargo tanks (below the mid-deck) to prevent or minimize oil loss
upon bottom damage and wide wing tanks to protect against oil loss upon
side damage. It thus has a double side and single bottom, plus a horizontal
deck placed so that internal pressure in tanks below the deck is signifi-
cantly lower than the external sea pressure. The mid-deck design meets
the minimum wing tank width requirements contained in IMO regulations.
According to a report prepared for USCG (Herbert Engineering Corpo-
ration 1992), the mid-deck design has the most favorable extreme outflow
performance and less favorable zero outflow performance following
bottom damage.
The Coulombi Egg design is a special variation on the mid-deck
concept with a mid-deck, cofferdams, and sloping bulkheads in the wing
ballast tanks. It utilizes hydrostatic pressure balance plus overflow into
wing ballast tanks to minimize oil loss upon bottom damage. When orig-
inally submitted to IMO, this design included active systems; since the
submission did not provide an appropriate safety assessment of those
systems, however, they were deleted from the final approved design, and
other modifications were made to the passive system to ensure approval.
The process by which IMO approves an alternative design re-
quires that a proposal be submitted by a government to the international
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ASSESSMENT OF PREVIOUS EVALUATION METHODS
33
organization. To date, only the one proposal noted above (Coulombi
Egg) has been submitted to and approved by IMO. As of this writing, the
committee could find no other active proposal for an alternative design
within the international community.
Alternative Designs Proposed to USCG
Table 2-1 provides a list of design alternatives that were proposed and eval-
uated by various parties prior to USCG's 1992 report to Congress. They in-
clude all of the concepts evaluated in the 1991 NRC report plus those
mentioned above that were submitted to IMO. They also include the
underpressure system, a design still being actively developed by a U.S.
company. This design uses an active vacuum pumping system, which is
then blanketed by an inert gas system in the oil cargo tanks to limit oil
outflow upon bottom damage. The Herbert Engineering (1992) study pre-
pared for USCG before its 1992 report to Congress evaluated the mid-deck,
Coulombi Egg, and Polmis concepts.
Since 1992, a number of additional proposals have been sub-
mitted to USCG for consideration; USCG provided the committee with
information on 14 of these proposals for this study (see Table 2-2). In
addition to these 14, the committee received information on one other
active proposal for a design concept--the central ballast tank design--
TABLE 2-1 Tanker Design Alternatives Proposed to USCG Before 1992
Where Evaluated
NRC IMO Herbert Engineering
Design Concept (1991) (1992) (1992)
Protectively located segregated ballast X
(MARPOL tanker)
Double bottom X
Double sides X
Double hull X X X
Resilient membrane X
Hydrostatic balance X
Intermediate oil-tight deck (mid-deck) X X X
Vacuum systems (underpressure) X X
Smaller tanks X
Penetration-resistant hulls X
Emergency oil transfer systems X
Polmis concept X
Coulombi Egg concept X X
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
34
TABLE 2-2 Tanker Design Alternatives Proposed to USCG from 1992 to 1999
Date Submitted Concept Description Remarks
October 1992 Emergency transfer system (to center tanks)
April 1993 Concrete hull Proprietary
May 1993 External protective hull retrofit For existing tankers
June 1993 Emergency transfer system to containment bag
November 1993 Flexible membrane tank liner
January 1994 Collision-resistant double hull
January 1994 Fiberglass-reinforced plastic tanks Proprietary
March 1994 Onboard oil spill recovery system
November 1994 Takes exception to IMO weighting factors
January 1995 Liquid cargo containment system Proprietary
February 1995 Flexible internal tank liner
July 1998 Self-sealing cylinder for double-hull vessel
March 1999 Arrangement with independent tanks
April 1999 Concrete hull Proprietary
SOURCE: Letter from USCG to TRB, Aug. 12, 1999.
from its developer, Marine Safety Systems, Inc., of Houston, Texas (Marine
Safety Systems, Inc. 1997). This design has not been submitted formally
to USCG for evaluation; however, it has been under development for
several years, and its developer has prepared evaluations of its perfor-
mance using the IMO methodology. The central ballast tank design places
ballast tanks in the center of a tanker and provides an active transfer
system to move oil by gravity from damaged cargo tanks to the central
ballast tank. The design also includes a double bottom to protect against
outflow in the case of bottom damage. As of this writing, only the de-
veloper has prepared a detailed oil outflow performance analysis for the
central ballast tank design.
The 14 concepts submitted to USCG after 1992 reflect a range of
development and evaluation to date. Some include detailed schematics
and test results, while others are merely short letters describing a design
idea. Four of the proposals were submitted as proprietary and thus are
not available for public review. Some of the proposals were submitted for
the purpose of requesting USCG approval of a concept for new tankers,
while others could be considered for both new and existing tankers. One
was intended specifically for existing tankers. Each submittal included a
request that USCG approve the concept under OPA 90.
To date, USCG has not approved any proposed alternative tanker
design concepts as equivalent to the double hull. It has replied to those
submitting proposals that the concepts will be evaluated according to the
three measures of oil outflow noted above using the methodology set
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ASSESSMENT OF PREVIOUS EVALUATION METHODS
35
forth in USCG's 1992 report to Congress. As noted earlier, however, USCG
currently has no fully developed methodology for evaluating alternative
designs.4
LIMITATIONS OF HISTORICAL DATABASES
Cost Data
As noted in Chapter 1, the committee's charge included specific reference
to environmental and spill cleanup costs:
The committee will also develop a generalized spill cost database, which
includes all relevant costs such as clean-up costs and environmental spill
costs, and use this cost database to assist in developing a rationally based
approach for calculation of an environmental index.
The intent of this charge was clearly to apply the historical record of oil
spill consequences, as reflected in reported costs, as a basis for comparing
alternative tanker designs.5 The committee addressed this charge in two
steps: the types of information that would appropriately belong in such a
database were first reviewed, and available existing databases were then
examined to determine whether they met the committee's predefined cri-
teria for inclusion.
The committee considered the types of data that appropriately
belong in a spill consequence database to be applied in a regulatory
setting. From an economic perspective, a regulatory process should in-
volve comparison of available alternatives on the basis of net economic
effects. That is, economists look to monetize all the impacts of a set of reg-
ulatory alternatives and to select the alternative with the greatest net ben-
efits. Impacts can be positive (benefits such as reduced environmental
damage) or negative (costs such as increased investment in equipment
4USCG has allowed use of the IMO methodology for evaluating the equivalency of those double-hull
tankers built prior to the implementation of OPA 90 that are not in full compliance with the act's double-
hull clearance requirements. In such evaluations, USCG has required that both the probability of zero
outflow and the mean outflow be superior to those of the reference tanker having the minimum double-
hull dimensions mandated by OPA 90.
5The term costs as used in the committee's charge encompasses the monetized value of all deleterious human
health, economic, and environmental consequences of oil spills. The committee has chosen instead to use the
term consequences to better reflect the range of monetizable and nonmonetizable effects of oil spills. The term
cost is used more typically in the context of regulatory costbenefit analysis to reflect the costs incurred by reg-
ulated entities (e.g., businesses), government agencies, and the public. Avoided deleterious effects in this context
are generally referred to as benefits. In addition, the economic implications of oil spills are generally referred to
as damages in the context of natural resource damage assessment; that term is used in this report to represent
the physical change in a ship due to a grounding or collision.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
36
needed to meet a requirement). In the present case, a regulatory standard
has already been selected and implemented. At this time, the overall
costbenefit ratio for the double-hull requirement is not up for discussion.
Instead, the committee was asked to consider whether alternative designs
could achieve the same or a more favorable net economic effect relative
to the double hull.
In a simple world, every gallon of product released to the envi-
ronment would result in the same level of economic consequence. There
would be no need to address consequences because alternative tanker de-
signs could be compared solely on the basis of the expected distribution
of oil released. It is the committee's assumption, however, that the conse-
quences of oil spills, on average, are not a constant function of spill size.
For example, an alternative to the double-hull design may reduce the prob-
ability of small spills but increase the probability of a catastrophic release.
Which of these distributions of expected releases is preferred will depend
on an understanding of the consequences of the releases for human health,
the economy, and the environment.
A variety of consequences result from oil spills. However, because
not all of these consequences represent true changes in social welfare (i.e.,
the overall well-being of all members of a society or community), they are
not necessarily relevant for purposes of regulatory review. The following
are examples:
An oil company may pay a fine as a result of a release.
An insurance company may settle out of court with a group of busi-
nesses affected by a spill.
Liability limits may allow a firm to avoid paying some damages, while
additional damages may or may not be collected from a central fund.
In the case of fines, no real economic cost is represented. Instead
fines, while clearly affecting a firm's bottom line, represent transfer pay-
ments between parties and thus not a net change in social welfare.6 In the
case of a settlement amount, one would need to understand the basis of
the settlement to understand whether it represented a true measure of
social welfare loss. For example, a settlement with a private party affected
6A parallel example is automobile speeding tickets. The "price" of these tickets is not based on the economic
benefits expected to result from encouraging lower driving speeds (lives saved, accidents avoided), but on a
variety of political and social factors (e.g., the cost of issuing a ticket, the fine that is perceived to generate
the desired reduction in speeding). Thus, the economic benefit resulting from lower speed limits is not equal
to the revenues generated through additional fines.
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ASSESSMENT OF PREVIOUS EVALUATION METHODS
37
by a spill might reflect the revenues lost by the party, but not the economic
surplus losses suffered by consumers of the good or service provided by
that party. Similarly, the fact that a legally defined liability limit exists does
not change the underlying welfare losses that could result from a spill, only
the financial exposure of the spiller.
Since much of the spill-cost data available in the United States re-
lates to natural-resource damage claims, it is important to understand these
measures of consequence. Specifically, the amount recovered by a natural-
resource trustee may not be equal to the true social welfare loss due to the
releases. For example, trustees can recover the full value of lost revenues
and fees resulting from a release (e.g., a beach closure that results from a
spill might lead to a state park being closed and thus the loss of admission
fees). In addition, trustees are directed to collect damages to "restore, re-
place or acquire the equivalent of" injured natural resources. The courts
have interpreted this clause as allowing trustees to develop a restoration
plan following a spill that may cost more or less than the actual social
welfare loss due to the spill (i.e., there is no strict rule that the costs of
restoration not exceed the economic loss resulting from a release). In the
costbenefit framework typically applied to regulatory review, one is not
interested solely in restoration costs, but also in the economic loss asso-
ciated with the damaged environmental resources prior to restoration. In
some cases, restoration costs may reflect social welfare losses, but in many
cases they will not. Finally, Natural Resource Damage Assessment (NRDA)
settlements and awards do not address the loss of human life or value of
lost product resulting from a release.
Overall, the committee adopted a framework for evaluating the
appropriateness of available data in which economic consequences en-
compass the following:7
The value of lost product,
The cost of spill cleanup and response (private and public),
The social welfare component of third-party damages (fisheries
closed, waterways closed, recreationalists displaced),
The social welfare value of human health impacts, and
The social welfare value of the ecological change that results from
a spill.
7Note that economic, ecological, and human health costs can result from a single spill, and thus consider-
ation of multiple loss categories does not necessarily imply double counting.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
38
The following costs should not be included in such a database:
Fines, penalties, and punitive damage awards.
Unreviewed settlements/court awards, since
Not all cases are pursued (or pursued with equal effort);
It is not always clear what cost categories were included in the
settlement;
Settlements reflect a variety of factors unrelated to the true mag-
nitude of the loss (e.g., litigation risk, political pressures); and
In the case of NRDA, settlements/awards may be based on envi-
ronmental restoration costs, not the absolute level of social welfare
change.
Liability limits.
The committee identified and obtained a number of data sets that
describe the costs associated with oil spills and compared these data against
the criteria described above. These data sets included the following:
Cutter Information Corporation's database (a commercially available
database of spill cleanup and other costs);
USCG's Liability Trust Fund database, which addresses cleanup and
removal costs;
A National Oceanic and Atmospheric Administration (NOAA) data-
base of spill consequences (Helton and Penn 1999); and
Data gathered from protection and indemnity clubs.
In some instances, there is overlap between these data sets.
Overall, however, they reflect varying purposes, time periods, admittance
criteria, and so on; and none meets the criteria described above. In ad-
dition, even if these data sets met all of the committee's criteria, the total
number of salient events they reflect is quite small (fewer than 100 spills).
Given the very large geographic and temporal scale of these databases, as
well as the wide range of spill sizes represented, the resultant information
base is quite sparse.
The committee considered whether it would be possible and ap-
propriate to commission the development of a database that would meet
the criteria defined above, demonstrate a high degree of quality control,8
and include a large number of events. The committee decided that, even
if such a data set could be established at little cost, the difficulties involved
8The need for greater quality control is illustrated by the fact that in some cases a cost estimate reported in
a data set will reflect the costs as of a certain date, when in fact additional costs were incurred after the initial
report.
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ASSESSMENT OF PREVIOUS EVALUATION METHODS
39
in obtaining data from private entities and in estimating environmental
consequences for cases in which no such estimate had been developed
would make the resultant data set of limited use. In addition, because 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 of spill character-
istics (e.g., volume of spill, type of oil) using a modeling approach. The
committee also concluded that the cost of an oil spill does not serve as an
adequate surrogate for environmental consequences. In lieu of costs, there-
fore, the committee chose to use physical consequence measures. More-
over, the committee rejected attempting to measure the biota usage of
affected habitats because of significant variability both seasonally and in
abundance of species. Details on the committee's approach to these issues
are provided in Chapters 3 and 4.
Historical Damage Data
In 1992, IMO gathered data on collision and grounding incidents for the
period 1980 through 1990 in the process of drafting and adopting its in-
terim guidelines for approval of alternative tanker designs under MARPOL
(IMO 1996; see also Chapter 1). These data, provided by the International
Association of Classification Societies (IACS), were used in assessing the
probability of oil outflow using damage statistics for tanker accidents to
provide probability density functions of the location and extent of damage
for a contact accident. The incidents for which data were gathered in-
cluded 62 collision and 68 grounding incidents by single-hull tankers or
combination carriers.
The committee was asked to update and review these data. To
this end, information was requested and received from Lloyds Register,
Det Norske Veritas (DNV), and the American Bureau of Shipping for the
period 1992 to 1998. Some of the data were not used because they were
not in the format needed for analysis and comparison. The remaining data
were limited with regard to the number of contact accidents (18 collision
and 10 grounding). Again, all the data appeared to be for single-hull
tankers. The committee developed histograms of both the original IMO
data and the new data and found that the latter do not significantly change
the original histograms. This finding is not particularly surprising given the
limited number of new contact accidents and the fact that both data sets
were for single-hull tankers. In any case, since the committee's task re-
quired taking crashworthiness into account, historical damage data were
not applicable. Thus the new data were not a factor in the committee's
modeling of collision or grounding events and structural damage.
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
40
Collision and Grounding Data for U.S. Waters
At the committee's request, USCG prepared an analysis of its accident and
pollution data covering all tank vessel collision9 and grounding incidents
in U.S. waters during the past 20 years (1979 to 1999). Each incident is
recorded with the date of its occurrence, vessel name and ID number,
primary and secondary causes of the accident, amount of oil spilled (if
known), and location. A total of 1,900 incidents was recorded during this
time period.
A review of these 1,900 incidents shows that 1,660 could clearly
be identified as either collision or grounding events. Of these 1,660 inci-
dents, 47 percent were collision and 53 percent were grounding incidents.
Most of the incidents recorded did not result in any oil being spilled. Only
55 collision and grounding incidents in this database involved any oil being
spilled; of these, 51 percent were collision and 49 percent grounding. Ac-
cording to these data, then, collision and grounding of tank vessels in U.S.
waters appear to occur with roughly the same frequency.
A review of these same data also shows that the three largest spills
represented about 91 percent of the total amount spilled (23.6 million of
the total 25.8 million gallons). Table 2-3 shows a breakdown of collision
and grounding incidents during the 20-year period based on these data.
Fewer than 50 of these collision and grounding incidents resulted
in a spill of more than 1,000 gallons of oil. The incidents occurred along
all U.S. coasts and within major harbors and waterways, including New
York Harbor, Delaware Bay, Galveston Bay, coastal Pacific Ocean waters,
the Gulf of Mexico, and the Mississippi River. While the data representing
tanker spills are relatively sparse, they provide some indication of the fre-
quency of these types of incidents, where they have occurred, and the
amounts of oil involved. The committee used these data to inform its de-
velopment of accident scenarios and to indicate the possible range of dis-
tribution of accident events.
QUANTITATIVE RISK-ASSESSMENT
TECHNIQUES ANDTHEIR APPLICATION
TO SIMILAR PROBLEMS
For the reasons stated previously, there are serious questions about the re-
liability of data from virtually all existing sources for creating an analytical
9As noted earlier, for the purposes of this discussion, the term collision is defined as including both collisions
(between two moving vessels) and allisions (between a moving vessel and a fixed object).
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ASSESSMENT OF PREVIOUS EVALUATION METHODS
41
TABLE 2-3 Summary of Tank Vessel Collision and Grounding Incidents in U.S. Waters, 19791999
Number of Number of Proportion of Proportion of
Collision Grounding Collision Grounding
Sample Total Number Incidents Incidents Incidents (%) Incidents (%)
All incidents 1,660 780 880 47 53
Polluting incidents 55 28 27 51 49
SOURCE: Letter from USCG to TRB, Aug. 12, 1999.
methodology for use in comparing tanker designs. While the IMO interim
guidelines are useful as a starting point in developing any methodology,
the fact that they were adopted after IMO had already accepted the double-
hull and mid-deck tanker designs as equivalent has led to some question
concerning the analytical soundness of the conclusions thus derived.
Indeed, USCG and others in the United States have raised major questions
about the weighting factors used in the IMO methodology. Moreover,
using this methodology, an alternative design that was only marginally
worse than a double-hull design for the probability of zero outflow but
dramatically better with regard to mean and extreme outflow could rea-
sonably be considered equivalent--something USCG's emphasis on zero
outflow would not permit.
These considerations led the committee to look elsewhere for a
more rigorous and defensible methodology. In the process, the committee
reviewed previous NRC work (e.g., Garrick 1999), as well as work in
progress elsewhere in the marine field. For example, the Technical Uni-
versity of Denmark currently has a contract to develop a risk-assessment
model for vessel traffic in Danish waters. This model will incorporate the
determination of collision and grounding probabilities based on traffic pat-
terns and physical properties of waterways, calculation of damage extent
in cases of collision and grounding, and an outflow calculation based on
hydrostatic and hydrodynamic principles (Pedersen 2000). Similar work
has already been completed for Prince William Sound in Alaska (DNV
et al. 1996). These studies used a risk-based methodology.
For the past 25 years, the engineering community has been using
risk-based methods to understand the inherent safety of complex systems.
Notable in this history was the release of a nuclear reactor safety study (U.S.
Nuclear Regulatory Commission 1975). This report was the first to set forth
the logical questions now common in all formal risk assessments: What can
go wrong? What is the likelihood? and What are the consequences? The
same approach has been used extensively throughout the nuclear industry
(e.g., Pickard, Lowe, and Garrick, Inc., et al. 1981) and in the energy, space,
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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS
42
building, and chemical industries (e.g., Keeney et al. 1978; NRC 1991; Pate-
Cornell and Fischbeck 1994; Rasmussen 1981).
On the basis of these historical applications, the expertise of
committee members, and presentations on the use of risk analysis in pre-
vious work done by NRC, the committee decided to develop a risk-based
methodology that would probabilistically generate realistic accident sce-
narios (for both collision and grounding) and use distributions of envi-
ronmental consequences following a spill to compare the performance
of proposed alternatives against that of a standardized double-hull vessel
for all comparable sizes. The committee also refined or adopted modifi-
cations and improvements now available in the areas of structural damage
assessment for both collision and grounding. The results of those efforts
are detailed in the following chapter.
REFERENCES
ABBREVIATIONS
DNV Det Norske Veritas
IMO International Maritime Organization
NRC National Research Council
SNAME Society of Naval Architects and Marine Engineers
USCG United States Coast Guard
DNV, George Washington University, and Rensselaer Polytechnic Institute/
Le Moines College. 1996. Prince William Sound, Alaska--Risk Assessment
Study. Dec.
Garrick, B. J. 1999. Risk Assessment Methodologies Applicable to Marine Systems.
U.S. Coast Guard, Washington, D.C.
Helton, D., and T. Penn. 1999. Putting Response and Natural Resource Damage
Costs in Perspective. Paper 114. International Oil Spill Conference.
Herbert Engineering Corporation. 1992. Probabilistic Oil Outflow Analysis of
Alternative Tanker Designs. Report CG-D-14-92. National Technical Information
Service, Springfield, Va.
IMO. 1992. IMO Comparative Study on Oil Tanker Design. IMO Paper MEPC
32/7/15. London.
IMO. 1996. Interim Guidelines for the Approval of Alternative Methods of Design
and Construction of Oil Tankers Under Regulation 13F of Annex I of MARPOL
73/78. MARPOL 73/78 1994 and 1995 Amendments. London.
Keeney, R. L., R. B. Kulkarni, and K. Nair. 1978. Assessing the Risk of an LNG Ter-
minal. Technology Review, Vol. 81, No. 1, pp. 6472.
Marine Safety Systems, Inc. 1997. The Central Ballast Tanker. Houston, Tex.
NRC. 1991. Tanker Spills: Prevention by Design. National Academy Press, Wash-
ington, D.C.
Pate-Cornell, M. E., and P. S. Fischbeck. 1994. Risk Management for the Tiles of
the Space Shuttle. Interfaces, Vol. 24, No. 1, pp. 6486.
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ASSESSMENT OF PREVIOUS EVALUATION METHODS
43
Pedersen, P. T. 2000. Risk Assessment Procedures for Fixed Structures in Shipping
Lanes. Technical University of Denmark, Copenhagen.
Pickard, Lowe, and Garrick, Inc., Westinghouse Electric Corporation, and Fauske
& Associates, Inc. 1981. Zion Probabilistic Safety Study. Prepared for Com-
monwealth Edison Company, Chicago, Ill.
Rasmussen, N. C. 1981. An Application of Probabilistic Risk Assessment Tech-
niques to Energy Technologies. Annual Review of Energy, Vol. 6, pp. 123138.
Sirkar, J., P. Ameer, A. Brown, P. Goss, K. Michel, F. Nicastro, and W. Willis. 1997.
A Framework for Assessing the Environmental Performance of Tankers in Ac-
cidental Groundings and Collisions. Presented at SNAME Annual Meeting, Oct.
USCG. 1992. Report to Congress: Alternatives to Double Hull Tank Vessel Design,
Oil Pollution Act of 1990. National Technical Information Service Publication
PB93-128874INZ.
U.S. Nuclear Regulatory Commission. 1975. Reactor Safety Study: An Assessment
of Accident Risk in U.S. Commercial Power Plants. WASH-14--NUREG-75/014.
Washington, D.C., Oct.
Representative terms from entire chapter:
tanker designs
