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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.