Environmental Performance of Tanker Designs in Collision and Grounding Method for Comparision -- Special Report 259 (2001) / Chapter Skim
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The world tanker fleet is changing to double hulls in accordance with both U.S. law and a similar provision in an international agreement.
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Congress made this request through the USCG Authorization Act of 1998 but specified that the investigation be conducted independently of past USCG policy on double-hull equivalency. The committee was charged to develop a rationally based approach for assessing the environmental performance of alternative tanker designs relative to the double-hull standard.
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Limitations of Cost Data for Measuring Environmental Damage As noted, the committee's charge included development of a generalized spill cost database that could assist in formulating a rationally based approach for calculation of an environmental index. The goal behind this charge was to apply the historical record of oil spill costs as a basis for comparing alternative tanker designs.
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The committee used an environmental impact model that predicts oil fate and transport and allows for random sampling of weather conditions on the basis of historical weather data. This model provides a number of physical consequence measures, such as the area of the sheen, the toxicity in the water column, and the length and area of oiled shoreline.
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. To determine environmental consequences for these events, the committee conducted a separate analysis and generated a set of consequence functions for the necessary range of oil outflows, also considering such factors as weather, oil types, and geography for the selected locations.
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To test the validity of this result, the committee also conducted several sensitivity analyses-for different oil types, different case study sites, and different consequence metrics. These analyses provide upper and lower bounds for the consequence function graph and can be used to make the final design comparisons more accurate and complete.
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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.
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In developing the standard ref erence ships, USCG should refer to the discussion of design of double-hull tank vessels in the 1998 NRC report entitled Double Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Need for Vessel Design Details The committee's approach to developing this methodology entailed rigorous computational methods that included analyzing the crashworthiness of ship structures, calculating oil outflows in specific accident scenarios, and modeling spills and their complex behaviors while reducing the results to numerical values.
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Components of the Methodology In its outflow analysis, the committee concluded that use of historical data, and therefore the IMO methodology and other methods based on such data, is not appropriate for evaluating new tanker designs. Accordingly, the methodology proposed in this report uses direct computational tools instead of historical data to determine the crashworthiness of either double-hull or alternative designs.
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This conclusion led the committee to select an approach that could relate measures of environmental damage to each oil spill scenario. Moreover, as explained above, the committee chose to use physical consequences, instead of historical spill costs, as the most consistently measurable and comparable method of evaluating environmental consequences.
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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 recommendations 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.
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Since the passage of OPA 90, several ship designers and other proponents of alternative tanker designs have approached the U.S. Congress with proposed designs they believe offer performance that is equivalent to or better than the double hull.
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. Specifically, the committee was charged to accomplish the following: Develop a rationally based approach for assessing the environmental performance of alternative tanker designs relative to the doublehull standard.
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Double bottoms, double sides, double hulls, and a myriad of other structural arrangements have been proposed and used, and studies have provided evidence to show their effectiveness. By the 1990s, however, a consensus had emerged among regulators -- especially in the United States -- that a double-hull standard would offer the best protection against oil spills following collision and grounding accidents.
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When NRC (1991) issued a report comparing alternative designs with double hulls, it noted that there were no accepted criteria for measuring equivalency but concluded nonetheless that no alternative proposed to date was superior to the double hull for all accident scenarios.
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Flag Tanker Fleet The U.S. flag tanker fleet is not engaged in transporting oil imported to the United States, but rather consists of a small number of vessels either carrying refined products from one U.S.
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Tanker Accidents and Oil Spills The most recent data on oil spills in U.S. waters show a reduction in both the number of spills and the amount of oil spilled during the past decade at the same time that oil shipments and tanker traffic have been increasing.
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Table 1-3 gives the largest tanker accidents worldwide during the past 25 years, ranked by amount of oil spilled. The Exxon Valdez spill was by no means the largest (it is 26th on this list)
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While no attempt is made in this report to identify which causes have been most significant, a few comments are useful as context. Even though the world tanker fleet is gradually changing to double-hull construction, this change cannot as yet have contributed in a large way to a reduction in oil spills because of the difference in time frames.
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Location Atlantic Empress 1979 84 Caribbean Castillo de Bellver 1983 79 Atlantic Amoco Cadiz 1978 68 France -- Atlantic Odyssey 1988 43 Canada -- Atlantic Haven 1991 42 Mediterranean Irenes Serenade 1980 37 Mediterranean Hawaiian Patriot 1977 31 Hawaii -- Pacific Independentza 1979 29 Bosporus Strait Urquiola 1976 28 Spain Braer 1993 25 Shetland Islands Jacob Maersk 1975 24 Portugal Aegean Sea 1992 22 Spain Sea Empress 1996 21 United Kingdom Nova 1985 21 Persian Gulf Khark 5 1989 20 Morocco -- Atlantic Epic Coloctronis 1975 18 Caribbean Katina P 1992 16 Mozambique Assimi 1983 16 Gulf of Oman ABT Summer 1991 15 Angola -- Atlantic Andros Patria 1978 15 Bay of Biscay British Ambassador 1975 14 Pacific Pericles GC 1983 14 Persian Gulf Tadotsu 1978 13 Strait of Malacca Juan A Lavalleja 1980 11 Algeria Thanassis A 1994 11 South China Sea Exxon Valdez 1989 11 Alaska -- Prince William Sound Burmah Agate 1979 11 Texas -- Gulf of Mexico Athenian Adventure 1988 11 Canada -- Atlantic Borag 1975 10 East China Sea St. Peter 1975 10 Columbia -- Pacific Using its existing data, USCG compiled for the committee an analysis of 1,660 tank vessel collision8 and grounding incidents in U.S.
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Congress promulgated OPA 90. The intent of this law was to minimize oil pollution through improved tanker design, enhanced operational safety, and other actions designed to improve oil spill cleanup capabilities.
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; 3. Allowable designs: OPA 90 only allows double hull con struction whereas MARPOL 13F also allows the mid-deck design, and provides for acceptance of other possible alternatives."
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SCOPE AND APPROACH The focus of this report is on the committee's development and application of a computational methodology for comparing the environmental performance of alternative tanker designs. A description of how the methodology was developed is given, an initial example of its application is provided, its strengths and limitations are reviewed, and ways in which it can be further developed and implemented are discussed.
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The committee also investigated historical data on oil spill costs and the potential for using these data to calculate an environmental index. Finding that historical data do not provide a basis for evaluating new design concepts, the committee developed its own models of tanker accidents, structural damage, oil outflow, and resulting environmental consequences.
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ORGANIZATION OFTHIS REPORT Previous methods used to evaluate alternative tanker designs; alternatives proposed to date; and the various historical databases on spill costs, damage statistics, and collision and grounding are reviewed in Chapter 2. The methodology developed by the committee is detailed in Chapter 3.
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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)
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Following passage of the OPA 90 legislation, USCG also commissioned a study (Herbert Engineering Corporation 1992) to assess the environmental performance of alternative designs on the basis of the calculation 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 "probability 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.
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The IMO method of comparing alternative designs (IMO 1996) uses a formula that assigns relative weighting factors to the three parameters related to spill size noted above (zero, mean, and extreme outflow)
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Therefore, the committee believes IMO's work on vessel accidents, 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 appropriate measure of relative environmental consequences must be incorporated into a new scheme.
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ports at some time) , the international community has investigated a number of alternative designs in recent years.
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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.
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. 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)
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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.
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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 criteria for inclusion. The committee considered the types of data that appropriately belong in a spill consequence database to be applied in a regulatory setting.
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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.
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In some cases, restoration costs may reflect social welfare losses, but in many cases they will not. Finally, Natural Resource Damage Assessment (NRDA)
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; 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.
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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, therefore, the committee chose to use physical consequence measures.
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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 incidents, 47 percent were collision and 53 percent were grounding incidents.
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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 doublehull 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.
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1992. Probabilistic Oil Outflow Analysis of Alternative Tanker Designs.
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1992. Report to Congress: Alternatives to Double Hull Tank Vessel Design, Oil Pollution Act of 1990.
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OVERVIEW If a tanker runs aground or collides with another vessel,1 the severity of the damage to the ship caused by the impact and the amount of oil spilled depend on the design of the tanker, its loading condition, mitigation efforts by its crew, the location of the impact, and the type of accident. Once the oil is in the water, the environmental, economic, and financial consequences of the spill will depend on the volume of oil spilled, the type of oil, the location of the spill, resources at risk, seasonality, and the weather conditions at the time of and after the accident, as well as any recovery and cleanup efforts.
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Structural Damage and Oil Outflow Calculation · Collect data on collision and grounding accidents · Develop distributions of accident factors · Generate accident scenarios · Calculate structural damage · Determine oil outflow 3. Design Comparison · Calculate differences in consequence metrics · Analyze design differences 2.
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The parameters (accident factors) that define the collision and grounding events include the speed of the vessel, its loading condition, and the type of obstacle or colliding vessel.
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It provides a number of consequence measures, such as the area of the sheen, the toxicity in the water column, and the length and area of oiled shoreline. Spill cleanup costs were included in the analysis at first but were later excluded because of the uncertainty in the cost models.
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In addition, whether the ship runs aground in a given scenario depends on its draft, and the effect of possibly different drafts of alternative designs is taken into account in the methodology. Accident factors, which define the condition of the tanker before the accident and the type of colliding vessel, are described in more detail in Chapter 4.
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An example of the distribution for speed is shown in Figure 3-2. Once the initial distributions are known, the factors are sampled from the distributions to generate accident scenarios.
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The behavior of these contact forces depends on the initial speed and mass, as well as other properties of the ship, and on the behavior of the affected structural members. The forces will change in an irregular manner with time as the ship decelerates and as different structural members undergo deformation and rupture.
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Methods that have been developed and used for this purpose include the following: Statistical analysis of previous casualty data, Detailed nonlinear finite-element methods, and Macroscopic finite-element or superelement methods. In general, the statistical methods inherently involve the behavior of structures typical of the ships forming the database and may include simplifying assumptions regarding the structural arrangements.
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(More details on these methods can be found in Alternative Tanker Designs Collision Analysis on the accompanying CD.) SIMCOL was selected because of its availability and applicability to analyzing a large number of collision scenarios.
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The systematic errors are of more concern since they are characteristic of the analysis methodology. In a complex analysis such as that using DAMAGE or SIMCOL, especially when applied repeatedly to a large number of cases, simplifying assumptions must be made to keep the computational effort within reasonable bounds.
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(For details see Alternative Tanker Designs Grounding Analysis on the accompanying CD.) The SIMCOL collision model uses modified procedures derived from the statistical work of Minorsky (1959)
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(More detailed descriptions of the two programs, including the required input, can be found in Alternative Tanker Designs Collision Analysis and Alternative Tanker Designs Grounding Analysis on the accompanying CD.) The accident factors and the description of the vessel are the inputs into both DAMAGE and SIMCOL, and they determine the extent and the location of the damage on the ship's hull.
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The oil outflow from tanks is governed by the hydrostatic balance between oil and water and by the hydrodynamic behavior of the two liquids. The calculation of oil outflow in a bottom-damage case (grounding)
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The submission for approval should be expected to cover the mechanics, operation, and reliability of the system, and the documentation should include, but not be limited to, the following: Naval architecture design; Structural drawings; Identification of those accident scenarios in which the active system would be engaged; Stability; Compliance with U.S. and international rules and regulations (USCG and IMO)
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An analysis of the performance of the system should include an assessment of the operational aspects. Either the active system constantly maintains a state that is designed to ameliorate the effects of an accident, or the system is required to respond to an accident scenario in some remedial fashion to reduce the accident's impact.
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1. Selection of an existing oil fate and transport model and its application to generate estimates of the expected physical consequences of a broad range of hypothetical spills (e.g., meters of shoreline oiled, area of slick)
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Second, the parameters of this model needed to be defined, including the case study locations for which the model would be run, spill sizes, oil type, and weather. Finally, the metrics that would be used to describe the physical consequences of each modeled release event had to be identified.
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to generate the equivalency ratios, as described in Chapter 4. Response Functions of Spill Size to Consequence The ultimate implications of an oil spill are defined in terms of the environmental, economic, financial, and social consequences of the event.
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Therefore, the results generated by SIMAP for these measures of damage are generally less well accepted by the professional community than are physical impact measures. Second, in selecting case study sites for use in modeling spill consequence, the committee's goal was not to obtain precise measures of loss 5Environmental consequences refer to the impact of an oil spill on the affected habitat, whereas physical consequences refer to physical measures of oil concentration in the water or on the shore.
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Response Costs The committee expended a great deal of effort generating a relationship that would describe oil spill response and cleanup cost as a function of spill size. The committee considered historical spill cost data and existing models of spill cleanup and response costs, as well as primary estimates of spill response and cleanup costs generated for each of the hypothetical spill locations.
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Since the best predictive models are linear transformations of physical consequence measures, the committee decided that these measures are sufficient proxies for response and cleanup costs. As noted below, the committee recommends that additional consideration be given to the development of a robust spill cleanup and response cost function, and that the consequence model developed here be tested for sensitivity to inclusion of alternative measures of consequence.
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, it was necessary to combine the physical consequence measures used into a single measure that could be employed to compare the relative consequences of spills of different sizes. The approach selected involves the use of "equivalency ratios," which define the expected consequence of a spill of a given size relative to the expected consequence of a standard reference spill.
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Site-Specific Factors The committee's approach involves modeling a large number of hypothetical releases from tanker accidents at given case study sites. To the extent that these case study sites are not representative of typical spill locations, this approach will produce biased results.
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That is, the physical consequence measures used by the committee are likely to understate third-party and response and cleanup costs for small spills. Further investigation of this limitation could lead to more confidence in the final outcomes.
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1996. The CERCLA Type A Natural Resource Damage Assessment Model for Coastal and Marine Environments (NRDAM/CME)
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1995. Application of IMO's Probabilistic Oil Outflow Methodology.
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The application involved structural damage and outflow calculations for each tanker, oil-spill fate and transport simulations in four geographic locations, combining of the outflow and spill simulation results into a consequence measure, and design comparisons. The application and its results are described in this chapter: the selection of vessels and collision and grounding scenarios, the collision and grounding analyses and their results, the selection of hypothetical spill scenarios and consequence measures, limitations of the consequence analysis, and finally the design comparison.
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Oil outflow from damaged cargo tanks is a part of the SIMCOL output, whereas DAMAGE provides information on structural damage only. A program was written to allow batch runs of a large number of grounding cases and to add an outflow calculation based on the structural damage output from DAMAGE.
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41.6 m 41.6 m 41.6 m 41.6 m 41.6 m 14.8 m 31.2 m 31.2 m 31.2 m 31.2 m 31.2 m 31.2 m 41.6 m 41.6 m 41.6 m 41.6 m 41.6 m 20 m 29 m 20 m 14.8 m 31.2 m 31.2 m 31.2 m 31.2 m 31.2 m 31.2 m 20 m 20 m Cargo VCG = 2.42 m Cargo VCG = 3.78 m below WL below WL All ballast tanks are Note that DB hull has of `J' type DB height = 3.34 m .34-m greater draft FIGURE 4-1 Profile, plan, and midship section for 150,000-DWT ships (VCG vertical location of the center of gravity; WL waterline)
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23.3 m 23.3 m 23.3 m 23.3 m 23.3 m 23.3 m 17.5 m 17.5 m 17.5 m 17.5 m 17.5 m 17.5 m 17.5 m 17.5 m 23.3 m 23.3 m 23.3 m 23.3 m 23.3 m 23.3 m 8.1 m 11.1 m 8.1 m 17.5 m 17.5 m 17.5 m 17.5 m 17.5 m 17.5 m 17.5 m 17.5 m 12.6 m 12.6 m Cargo VCG = 2.48 m Cargo VCG = 3.3 m DS width = 2.44 m below WL below WL All ballast tanks are Note that DB hull has of 'L' type DB height = 2.1m .57-m greater draft FIGURE 4-2 Profile, plan, and midship section for 40,000-DWT ships ( VCG vertical location of the center of gravity; WL waterline)
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Four hypothetical spill locations were used in the application (their selection is discussed later in this chapter) : Big Stone Anchorage, Delaware Bay; Galveston lightering area, Gulf of Mexico; Carquinez Strait Bridge, San Francisco Bay; and Farallon Islands, offshore San Francisco.
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COLLISION AND GROUNDING ANALYSES The accident factors described above were sampled using Monte Carlo simulation to generate 10,000 collision and grounding events. The structural damage and oil outflow were analyzed for each vessel using the same 20,000 scenarios.
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minimum outflow aPressure valves preset at 1500 mm water gauge (WG) , but to represent industry practice and allow the use of a uniform distribution, the range of 400 to 1000 mm WG was applied.
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Strike location 0 1 Beta (1.25, 1.45) relative position from bow Speed of 0 20 Distribution based on historical struck ship data, approximately (knots)
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Cargo tanks (ex slops) 98 98 98 98 Slop tanks 66 89 N/A N/A FO 96 96 96/95 96 FO settling, service, overflow 20 20 N/A N/A DO 96 96 N/A N/A DO service 20 20 N/A N/A Fresh water 98 98 98 100 Forepeak ballast tank 3.5 11 0 100/11.2 Other ballast tanks 0 0 0 0 NOTE: DO = diesel oil, FO = fuel oil, LCF = longitudinal center of flotation, MS = midships, N/A = not applicable, VCG = vertical center of gravity.
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Similar to the assumptions in the IMO guidelines (IMO 1996) , some oil was assumed to be captured in the ballast tanks, and a minimum outflow was assumed from damaged cargo tanks adjacent to seawater as a result of dynamic effects.
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Once the accident scenarios have been defined, determination of the outflow volume in each scenario must be adapted to the particular active system, but the basic principles of the methodology apply in the same way to analysis of active and passive systems. RESULTS OF COLLISION AND GROUNDING ANALYSES Collision and grounding analyses provided oil outflow for each of the 10,000 collision and grounding events.
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Average given a spill 0.84 0.66 3.91 4.28 Average over all scenarios 0.05 0.12 1.06 2.53 Maximum 5.04 3.25 19.35 17.44 Minimum given a spill 0.16 0.07 0.87 0.86 Number of spills 612 1,833 2,724 5,911 Probability of no spilla (%) 94.0 81.7 72.8 40.9 aProbability of no spill refers to the likelihood of no spill in all potential grounding scenarios, whereas probability of zero outflow in the IMO methodology refers to the likelihood of no spill when the outer shell of a tanker has been ruptured.
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The details of the collision and grounding studies can be found in the Alternative Tanker Designs Collision Analysis and Alternative Tanker Designs Grounding Analysis reports provided on the accompanying CD. HYPOTHETICAL SPILL SCENARIOS AND CONSEQUENCE MEASURES The committee conducted an analysis to generate a single consequence function describing the relationship of spill size to spill consequence for 8These differences in spill sizes are due, in part, to the subdivision differences between single- and doublehull tankers.
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Hypothetical Spill Scenarios To generate the required model runs, values had to be selected for several modeling parameters. These included the case study locations, weather at the time of and following the spill, oil type, and spill volume.
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As discussed below, additional locations could be run and added to the analysis. As noted earlier, the following locations were modeled: Big Stone Anchorage, Delaware Bay; Galveston lightering area, Gulf of Mexico; 600 500 400 Events of 300 200 Number 100 0 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 Outflow (million gallons)
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. Carquinez Strait Bridge, San Francisco Bay; and Farallon Islands, offshore San Francisco.
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Selection of Spill Volumes For each oil type, seven spill sizes were modeled to bound the range of estimated quantities released from tankers operating in U.S. waters.
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Historically, only a small percentage of oil spilled in the marine environment is recovered (OTA 1990) , and there is no clear pattern 800 700 600 Events 500 of 400 300 Number200 100 0 10.0 8.0 6.0 4.0 2.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Difference in Outflow (million gallons)
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More important, the committee did not expect that recovery percentages would differ across tanker designs. If a proposed design offered an advantage over the conventional double-hull tanker in terms of oil recovery, this factor would need to be taken into account in comparing the proposed design with the double hull.
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Consequence Measures As noted in Chapter 3, the committee considered a range of physical consequence measures available from SIMAP and did not include environ TABLE 4-6 Modeled Spill Sizes Crude Carrier (gallons) Product Carrier (gallons)
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Each of these measures was calculated for each hypothetical event across a range of thresholds. As described in Chapter 3, four measures were chosen for use in generating the consequence function: area of oil on water surface, oil on shoreline -- area and length, and toxicity in the water column.11 The six thresholds modeled for each physical consequence measure reflect six order-of-magnitude intervals, generally starting one order of magnitude below a conservative estimate of a biological-effects threshold for the measure, and stepping four orders of magnitude above that threshold.
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concen thresholda eding exce of probability column: water the in hydrocarbons) (total oil fuel2 No.
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9 9 9 9 9 9 9 9 9 9 9 9 10× 10× 10× 10× 10× 10× 10× 10× 10× 10× 10× 10× Sum · · · Oil 2.61 3.01 3.20 1.96 2.93 2.50 1.21 2.00 2.37 3.48 2.98 2.24 Fuel2 No. 6 microns)
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Table 4-7 provides an example of the tabular output provided by SIMAP for one case study location/oil type/spill size combination and one consequence measure. The case study site is the Galveston lightering area, the oil type is North Cape No.
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Figure 4-16 presents a set of consequence ratios for the Galveston lightering area, considering both oil types that were modeled and all four selected consequence measures. As shown, for this site the average con
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A total of 168 average consequence ratios were generated. These ratios represent the four modeled locations, two oil types, four physical consequence measures, and six spill sizes modeled (the seventh spill size, 500,000, was used as the reference spill)
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Inversely, the consequence of each gallon of oil spilled in the largest modeled spills was much smaller than the consequence of each gallon spilled in a reference 500,000-gallon spill event. Figure 4-18 presents a subset of these data points and an alternative consequence function, considering modeled spills of less than 25 million gallons.
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The Carquinez Strait Bridge equation most closely matches the null hypothesis of a one-to-one relationship of consequence to spill size, probably reflecting the closed nature of San Francisco Bay. Figure 4-21 provides separate consequence functions for each of the four physical consequence metrics considered.
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quence across threshold measures. This figure considers three alternative weightings of modeled consequence across the six thresholds considered for each physical consequence metric: consequence measures based on summing of consequence across each of the thresholds, with equal weight applied to each threshold's value (the committee's selected approach)
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/ 100 Spill ) Modeled Spill 10 of Reference of 1 (Consequence Ratio 0.1 m shore Consequence m2 shore slick pah Consequence 0.01 0.01 0.1 1 10 100 1000 Ratio of Modeled Spill Size to Reference Spill Size FIGURE 4-21 Physical consequence function sensitivity analysis: comparison of different consequence metrics.
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1000 ) 100 Modeled Spill of Reference 10 of (Consequence 1 Ratio #2 Threshold Only Spill/Consequence 0.1 10x Weight Consequence Equal Weights 0.01 0.01 0.1 1 10 100 1000 Ratio of Modeled Spill Size to Reference Spill Size FIGURE 4-23 Physical consequence function sensitivity analysis: alternative weighting of consequence across threshold measures.
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From page 101... ...
The best-fit model was generated using a simple regression through the average consequence ratios for all spills of less than 25 million gallons. The upper bound was a function that incorporated the uppermost average values, while the lower bound represented a function that incorporated the lowermost values (as shown in Figure 4-22)
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From page 102... ...
In no case did the double-hull spill and the single-hull not spill. For the 612 cases in TABLE 4-9 Demonstration of the Consequence Function Outflow in Gallons 500,000-Gallon Spill Equivalents Design 1 Design 2 Difference Design 1 Design 2 Difference 0 100,000 100,000 0.000 0.563 0.563 500,000 600,000 100,000 1.000 1.067 0.067 1,000,000 1,100,000 100,000 1.281 1.325 0.044 5,000,000 5,100,000 100,000 2.275 2.291 0.016 10,000,000 10,100,000 100,000 2.914 2.924 0.010 20,000,000 20,100,000 100,000 3.732 3.739 0.007
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which both spilled, the single-hull spilled more in 273 cases, and the double-hull spilled more in 339.14 Also shown in the table are the average spill sizes (in millions of gallons) and average consequence (in 500,000gallon spill equivalents)
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From page 104... ...
The total number of 500,000-gallon spill equivalents saved because of using the double hull was 1,250. In the 339 scenarios in which the single-hull design performed better: On average, the benefit of having a single hull was 0.36 million gallons per scenario.
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From page 105... ...
Table 4-11 indicates that, for the 40,000-DWT tankers involved in grounding incidents, use of a double-hull design in the 10,000 scenarios saved 1,189 500,000-gallon spill equivalent units over use of a single-hull design. Note that all of the other measures in the table also show a considerable advantage for the double-hull design.
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From page 106... ...
Figure 4-24a shows the results for the 40,000-DWT tankers involved in grounding scenarios using the best-fit consequence function. The light gray region in the lower left corner of the figure represents the 339 scenarios in which the single-hull design outperformed the double-hull.
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From page 107... ...
FIGURE 4-24 Comparison of 40,000-DWT vessels in grounding scenarios using three models: (a) best fit, (b)
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From page 108... ...
FIGURE 4-25 Comparison of 150,000-DWT vessels in grounding scenarios using three models: (a) best fit, (b)
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From page 109... ...
lower bound, and (c) upper bound.
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From page 110... ...
lower bound, and (c) upper bound.
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From page 111... ...
SUMMARY By performing the applications described in this chapter, the committee has demonstrated that its methodology can be used to compare tanker designs and determine their relative environmental performance. The applicability of available computational tools to the prediction of structural damage and resulting oil outflow in multiple accident scenarios has also been shown.
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From page 112... ...
1998. Review and Improvement of the IMO Probabilistic Methodology for Evaluating Alternative Tanker Designs.
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From page 113... ...
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 considering alternative designs and represents a significant improvement over existing methods.
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From page 114... ...
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.
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From page 115... ...
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 crashworthiness of ship structures, calculating oil outflows in specific accident scenarios, and modeling spills and their complex behaviors while reducing the results to numerical values.
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From page 116... ...
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.
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From page 117... ...
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.
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From page 118... ...
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.
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From page 119... ...
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 recommendations 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.
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From page 120... ...
Background on methodologies for establishing equivalency and evaluation of their strengths and weaknesses Keith Michel, Herbert Engineering Status of Society of Naval Architects and Marine Engineers' (SNAME) assessment of research on crashworthiness of tank vessel structures and damage statistics Alan Brown, Virginia Polytechnic Institute and State University Overview of available spill cost data and analyses, and what is needed to develop a spill cost metric for evaluating relative performance Dagmar Etkin, Environmental Research Consulting 120
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From page 121... ...
The following presentations were given by guest speakers: Information on alternative tanker design proposals Paul Cojeen, USCG Key issues from 1997 SNAME paper "A Framework for Assess ing Environmental Performance of Tankers in Accidental Groundings and Collisions" Jaideep Sirkar, USCG, and Wayne Willis, ICF Kaiser Inter national Oil spill cost models Robert Unsworth, Industrial Economics; Heidi Schuttenberg, Applied Science Associates Tanker industry perspective John Burke, Mobil Shipping and Transportation Company (retired) International tanker fleet's adoption of double hulls and consid eration of alternatives William O
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From page 122... ...
John Garrick, Garrick Consulting Long-term effects of oil spills in marine and coastal habitats Stan Rice and Ron Heintz, Auke Bay Laboratory, National Marine Fisheries Service/National Oceanic and Atmospheric Administration, Juneau, Alaska Fourth Committee Meeting, September 2526, 2000, Washington, D.C. The following presentations were given by guest speakers: Modeling of damage and oil outflow in collisions and groundings Alan Brown, Virginia Polytechnic Institute and State Uni versity; Kirsi Tikka, Committee Chair Spill consequence modeling Robert Unsworth and Paul Fischbeck, Committee Members Fifth Committee Meeting, January 1719, 2001, Irvine, California This was a closed session with no guest speakers.
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From page 123... ...
application of the proposed index to double hull tank vessels and alternative designs currently under consideration.
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From page 124... ...
developed the Interim Guidelines for the Approval of Alternative Tanker Designs under Regulation 13F of Annex I of MARPOL 73/78 (IMO 13F Guidelines)
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From page 125... ...
Oil outflow from the damaged compartments is calculated based on hydrostatic balance principles, i.e., oil outflows from a compartment until the hydrostatic pressure of the fluid in the tank is equal to the hydrostatic pressure of sea water at the bottom of the compartment. The survivability method requires damage stability calculations.
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From page 126... ...
. The selection of tidal heights was arbitrary, the one percent minimum oil outflow was partially supported by model tests and the 50 percent capture of oil by ballast tanks below cargo tanks was investigated by model tests, but not conclusively.
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From page 127... ...
. The factors 0.5, 0.4, and 0.1, which weigh the contribution of the probability of zero outflow, mean oil outflow and extreme outflow to the index, were chosen rather arbitrarily as a compromise that would assure the equivalency of the double-hull and mid-deck concepts (Sirkar et al.
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From page 128... ...
. Since the design of a double-hull tanker has an effect on its outflow performance in the case of a grounding or collision, it is important to be familiar with current design practices when selecting a double-hull tanker for use as a reference in comparing alternative designs.
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From page 129... ...
In general, the experience has been good, but there have been some areas in which either guidance found in the work of TSCF has not been followed or unexpected structural problems, such as microbial corrosion in cargo tanks, have developed. ARRANGEMENTS OF DOUBLE-HULLTANKERS Most new tanker designs have complied with the double-hull requirements of Regulation 13F and also meet USCG's criteria for double-hull designs under OPA 90.
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From page 130... ...
FUTURE IMPROVEMENTS IN DOUBLE-HULLTANKER DESIGNS Although classification societies have adequate tools to assess designs and ensure the safe performance of double-hull tankers, there is room for im 1This is a requirement to consider a long extent of bottom damage (about 60 percent of the ship's length) as one possible condition for damage stability calculations.
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From page 131... ...
Efforts now under way within many organizations to improve design details are expected to bring greater safety and reliability to tankers of the future. If and when newer alternative designs are considered, it will also be necessary to ensure that any new structural details are adequately evaluated.
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From page 132... ...
Bontadelli is a private consultant in natural resources policy issues. From 1992 through March 1999, he served as Administrator of the Office of Oil Spill Prevention and Response of the California Department of Fish and Game, where he directed prevention, removal, abatement, response, containment, and cleanup efforts related to oil spills in the marine waters of California.
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From page 133... ...
committee that produced the 1991 report Tanker Spills: Prevention by Design. He participated in the United Nations' 1992 IMO Comparative Study on Tanker Design and is a member of the Society of Naval Architects, the Royal Institute of Naval Architects, and the American Bureau of Shipping.
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From page 134... ...
Ms. Lentz served on the Marine Board committees that produced the 1991 report Tanker Spills: Prevention by Design and the 1998 report Double Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990.
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From page 135... ...
Dragos Rauta is Technical Manager of INTERTANKO, Oslo, Norway, an independent association of owners and operators of tank vessels involved in oil and chemical transportation operations worldwide, representing the majority of the world tanker fleet. He is responsible for technical issues related to ship design, operations, regulatory issues, pollution prevention, maintenance and inspection, and accident investigations.
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From page 136... ...
His specialties include natural resource economics, damage assessment, environmental benefits assessment, and policy analysis. He is a nationally recognized expert in natural resource damage assessments for oil spills and hazardous waste sites and is a leader in efforts to apply regulatory costbenefit analysis to reduce the effects of environmental pollution.
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Key Terms
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