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Tanker Spills: Prevention by Design 5 Design Alternatives This chapter will introduce and explore tank vessel designs and operational alternatives that are intended to mitigate pollution in an accident. The committee's technical assessment is based on how the various characteristics of these designs influence safety, operation, and cargo outflow in an accident. The physical phenomena and engineering issues underlying this analysis were discussed in the previous two chapters. The designs considered in this chapter were chosen from several dozen alternatives gathered or solicited from various sources and, in a few cases, suggested by committee members. Proposals ranged from the conceptual to the tested and operational. Some originated in the United States, others in one of several foreign countries; some were suggested by individuals, others by major international corporations or industry associations. All of the suggestions were reviewed, but not all were included in the formal evaluation. The exclusion criteria are explained in the following paragraphs. Proposals were screened to eliminate the clearly impractical esoteric visions, as well as those judged beyond the bounds of industry compatibility. The surviving proposals then were rendered into 17 broad examples—designs generalized enough to encompass the key concepts and details. For this chapter, the alternatives were grouped according to common physical principles. This resulted in the following three categories: secondary ''barriers" to oil intermingling with water; the mitigation of pollution via "outflow management" techniques; and the reduction of pollution potential through "increased penetration resistance." Operational options for "accident response" were grouped separately; these options can be employed
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Tanker Spills: Prevention by Design with most hull design alternatives. An itemized matrix listing all of these alternatives and operational options can be found in the next section of this chapter. The committee evaluated the 16 alternatives based on the following criteria: (1) previous studies or documented experience with the design; (2) concerns about engineering, safety, and practicality derived from the experience of committee members; and (3) theoretical effectiveness of the design in preventing/mitigating pollution in collisions and groundings. As part of its analysis, the committee judged each design according to its developmental status; those not ready for immediate use were evaluated in terms of future promise. The first half of the chapter covers general technical aspects of each design. Based on these considerations, some of the alternatives were eliminated from further committee consideration. The second half of the chapter mathematically assesses the pollution-mitigation potential of the remaining alternatives, and several possible combinations of these designs, in regard to groundings and collisions. For this section, the committee made use of a study developed by Det norske Veritas (DnV), contained in Appendix F. (This study will be referred to as the DnV analysis.) The methodology is explained in relevant sections of that report. The designs and combinations assessed for pollution-mitigation potential also are subjected to cost-effectiveness analysis, described in Chapter 6. THE MATRIX The matrix presented on the following pages was prepared by the committee to combine, in one document, all of the technical considerations discussed in Chapters 3, 4 and 5. The matrix is intended to apply to both tankers and barges. The following description is intended to assist in understanding of the document. Column 1 (Alternative Description) lists the design alternative considered in that row. Column 2 (Effectiveness) indicates, in a general sense, the type of accident in which the proposed alternative will be effective, in terms of controlling cargo outflow. If an alternative is effective in the accident type noted, then a dot (•) appears in the proper column. The terms "high" and "low" damage severity reflect the relative speed of the vessel prior to the accident. For collisions, speed refers to the ramming vessel; effectiveness applies to the vessel that is struck. The matrix does not grade the relative effectiveness of the various designs; that subject is taken up in the latter half of this chapter. In Column 3 (Implementation), the committee has indicated technology status and technical constraints. Regulatory and financial constraints are
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Tanker Spills: Prevention by Design not considered in the first two sections. The last three sections of Column 3 indicate whether the alternative, in the committee's view: (1) could be applied to new tankers at the present time (within the next two years); (2) should be considered at some time in the future; or (3) could be applied (retrofitted) to existing vessels. A dot (•) in the column indicates applicability, and a question mark (?) indicates technical applicability but economic difficulties. A blank space (no dot) indicates no applicability, from a technical standpoint. Retrofitting involves some unique complications, which will be discussed in the text as applicable. In Column 4 (Concerns), the headings follow the format of Chapter 4 through the subject of explosions. A dot (•) in the appropriate section indicates that the design alternative, to some extent, creates that concern. A general explanation of these concerns can be found in Chapter 4. The last four sections refer to the following concerns: Safety Downgrade—A dot in this section means that, to some extent, existing safety practices or requirements are diminished. (An example is an alternative that prevents the use of inert gas systems.) In all cases, safety downgrade probably could be overcome, but the committee felt it important to note the concern so that alternatives would not be viewed as panaceas. Operations Complexity—Because crew size has been reduced and crew fatigue has played a role in some pollution incidents, the committee felt that alternatives requiring extra vigilance in operation should be identified. Design Integration—A dot indicates alternatives involving either new technology or new design practices. This concern is highlighted to ensure that the advantages afforded by an alternative are not offset by some additional problem. Rules and Regulation—A dot indicates alternatives requiring interpretation or revision of existing rules. The last section of Column 4, entitled "Comments," is intended to summarize the major technical or operational concerns to give the reader a sense of the priorities. Column 5 (Pivotal Argument) is the committee's summary of all preceding columns; the comments represent the major arguments for and against each design alternative. These arguments are restricted to technical and operational matters. Finally, Column 6 (Warrant Committee's Economic Evaluation) indicates which alternatives will be pursued in the benefit/cost assessment (Chapter 6). Those alternatives requiring significant time and/or research and development to implement, and those lacking sufficient information for a benefit/cost assessment, are indicated by dots in the "No" section.
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Tanker Spills: Prevention by Design TANK VESSEL DESIGN ALTERNATIVES 1. Alternative Description 2. Effectiveness 3. Implementation CONTROL METHOD: Grounding Collision TECHNOLOGY STATUS (YEARS TO DEVELOP) Constraints Applicability Barrier Damage Severity Damage Severity CONCEPT RESEARCH NEW CONSTRUCTION RE-FITS HIGH LOW HIGH LOW DEVELOPMENT EXISTING NOW FUTURE 1. Protectively Located Segregated Ballast; (Marpol Tanker); Ballast tanks isolated from cargo tanks. Located to restrict possible outflow. This is current regulation. • • Existing No constraints - present standard • • • 2. Double bottom; A non-cargo space between the cargo tank bottom plating and the ship's hull bottom plating. • • • Existing Structural and weight- complications w/refit • • ? 3. Double Sides; A non-cargo space between the cargo tank side plating and the plating of the ship's hull. • • • Existing Structural and weight complications w/refit • • ? 4. Double Hull; A non—cargo space between the cargo tank and the hull. • • • • Existing Structural and weight-complications w/refit • • ? 5. Resilient Membrane; A tough, pliable, nonstructural barrier separating the cargo from the ship's structure and acting to maintain separation of cargo and water in the event of being breached. • • Concept - minimum 10 years to develop Technology not sufficiently developed to support even a 'proof of concept' case.
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Tanker Spills: Prevention by Design 4. Concerns 5. Pivotal Argument 6. Warrants Committee Economic Assessment STRENGTH MAINTENANCE STABILITY SALVAGEABILITY EXPLOSION SAFETY DOWNGRADE OPERATIONS COMPLEX DESIGN INTEGRATION RULES & REGULATIONS Comments For Against YES NO • • • Damaged stability concerns if ballast tank is ruptured. Has eliminated a major cause of world's oil pollution (ballasting of oil tanks). Inexpensive and in existence. Does not effectively handle damage conditions. Will not reduce oil outflow unless only ballast tank is ruptured. • • • • • Explosion concerns due to gas in voids. Will prevent pollution in groundings where inner hull is not breached. It can be effective in limiting or preventing pollution in case of low to moderate damage. Does not assist in collision damage. Possible increased pollution potential due to collision because salt water ballast (SWB) in D.B. will reduce amount of protectively located segregated ballast. Explosion concerns due to gas in voids. • • • • • Explosion concerns due to gas in voids. Will prevent pollution outflow in collisions where inner hull is not breached. It can be effective in limiting or preventing damage in low to moderate damage cases. Does not assist in grounding damage. Loss of buoyancy and resultant heel may cause increased ground reaction. Increased structural maintenance Explosion concerns due to gas in voids. • • • • • Design depth of double hull could provide structural and capacity restraints. Will prevent pollution or mitigate extent of immediate pollution in the event of all but the most severe accidents. Increased structural maintenance Explosion concerns due to gas in voids. • • • • • • • No technical support for the concept Required materials, operations, design demands not investigated. Simplicity of concept. If problem of integrating membrane into ship's structure can be overcome, this solution could be quite beneficial. The practical limitations of material properties. Impact of operational functions and the behavior of resilient membranes in contact with complex shapes and arrangements are virtually unexplored. •
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Tanker Spills: Prevention by Design 1. Alternative Description 2. Effectiveness 3. Implementation CONTROL METHOD: Outflow Management Grounding Collision TECHNOLOGY STATUS (YEARS TO DEVELOP) Constraints Applicability Damage Severity Damage Severity CONCEPT RESEARCH NEW CONSTRUCTION RE-FITS HIGH LOW HIGH LOW DEVELOPMENT EXISTING NOW FUTURE 6. Passive Control Hydrostatically Balanced Loading Concept; The establishment of the potential oil/water hydrostatic equilibrium at a height above the highest designed, or incurred for, or incurred point of damage.; Would limit (via loading criteria) the cargo's head pressure at tank bottom to equal to or less than draft's water head at tank bottom such that bottom damage results in less oil outflow. • • Existing. No modifications required. • • • 7. Intermediate Oil Tight Deck; A structural deck running the full length of the cargo area about 1/4 to 1/2 the depth above the bottom. 7a. Independent Tanks; The top and bottom tanks would be independent of each other. Would require upper and lower cargo piping. • • • Development — minimum 2 years to incorporate. Design of piping system and strength of int. deck. Outflow performance uncertain—testing needed. • ? 7b. Convertible Tanks; The top and bottom tank would be integrated through sluice valves. • • • Development — minimum 2 years to implement. Need to develop and test system. • ? 8a. Mechanically Driven Vacuum; The ullage space is subject to a mechanically induced vacuum such that the combined vacuum plus cargo head favors water inflow in the event of bottom damage. • Development — minimum 4-5 years to implement in full-size ship tests. Need to prove practicality of providing tight deck and fail safe valving. • •
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Tanker Spills: Prevention by Design 4. Concerns 5. Pivotal Argument 6. Warrants Committee Economic Assessment STRENGTH MAINTENANCE STABILITY SALVAGEABILITY EXPLOSION SAFETY DOWNGRADE OPERATIONS COMPLEX DESIGN INTEGRATION RULES & REGULATIONS Comments For Against YES NO • • • • • Operating with all slack tanks could create a free surface problem. Mitigates pollution in the first few hours after the accident; instantaneously on-line. Low cost option to retrofit. Reduces deadweight of vessel which increases cost to transport and requires more vessels. Given sufficient time after accident and no other response, the pollution effect would be the same as a vessel carrying the same amount of cargo. Vessel may be subject to high sloshing loads, which is an especially critical problem in retrofit application. May be difficult to administer operationally• • • • • • • Magnitude of piping produces operational concerns. Would mitigate pollution in high-energy accidents except for the most catastrophic ones. Complex. Cracks in intermediate deck, or open cargo valves would void the function of the intermediate deck. • • • • • Magnitude of valving produces operational concerns. Would mitigate pollution in all but the most catastrophic accidents. Operationally burdensome and complex. Hydrostatic isolation of upper and lower tanks could be in jeopardy. • • • • • • • • • Major operations hazard. Existing deck structure insufficient. Requires constantly maintained tight ship. Mitigates pollution outflow from groundings. Adversely affects vessel and personnel safety. Generates serious operational problems (i.e. vapor disposal, maintenance & reliability). •
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Tanker Spills: Prevention by Design 1. Alternative Description 2. Effectiveness 3. Implementation CONTROL METHOD: Outflow Management (cont.) Grounding Collision TECHNOLOGY STATUS (YEARS TO DEVELOP) Constraints Applicability Damage Severity Damage Severity CONCEPT RESEARCH NEW CONSTRUCTION RE-FITS HIGH LOW HIGH LOW DEVELOPMENT EXISTING NOW FUTURE 8b. Hydrostatically Driven Vacuum (Passive); A vacuum which occurs due to the run out of oil cargo. • Development—minimum 2 years to implement. Need to prove effectiveness and environmental safety of chemicals. • • 8c. Imaginary Double Bottom; A passive vacuum system coupled with a water layer below the oil cargo. • Development—minimum 2 years to implement. Need to prove practicality of providing tight deck and fail safe valving. • • 9. Smaller Tanks; Increase compartmentalization to reduce oil outflow exposure. • • Existing • • 9a. Service Tank Location; Position all oil service and oil waste tanks clear of the vessel's hull in a defensive location relative to hull damage. • • • • Existing. • •
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Tanker Spills: Prevention by Design TANK VESSEL DESIGN ALTERNATIVES 4. Concerns 5. Pivotal Argument 6. Warrants Committee Economic Assessment STRENGTH MAINTENANCE STABILITY SALVAGEABILITY EXPLOSION SAFETY DOWNGRADE OPERATIONS COMPLEX DESIGN INTEGRATION RULES & REGULATIONS Comments For Against YES NO • • • • • • • • Major operations hazard. Existing deck structure insufficient. Requires constantly maintained tight ship. Mitigate pollution outflow for groundings. Adversely affects vessel and personnel safety. Generates serious operational problems (i.e. vapor disposal, maintenance & reliability). • • • • • • • • • Major operations hazard. Existing deck structure insufficient. Requires constantly maintained tight ship. Mitigate pollution outflow for groundings. Adversely affects vessel and personnel safety. Generates serious operational problems (i.e. vapor disposal, maintenance & reliability). • • • • Increased maintenance and piping. Smaller tanks reduce pollution for a specified amount of hull damage. Effective for smaller vessels and barges. Major capital and operating expense. Will allow slightly less pollution than existing PL/SBT in event of major hull penetration. • • • No negative impact on existing protocols. Positive pollution preventing requirement easily achievable on new buildings. Inefficient use of space. Only addresses minor pollution source of ship's outflow. •
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Tanker Spills: Prevention by Design 1. Alternative Description 2. Effectiveness 3. Implementation CONTROL METHOD: Increased Penetration Resistance Grounding Collision TECHNOLOGY STATUS (YEARS TO DEVELOP) Constraints Applicability Damage Severity Damage Severity CONCEPT RESEARCH NEW CONSTRUCTION RE-FITS HIGH LOW HIGH LOW DEVELOPMENT EXISTING NOW FUTURE 10. Internal Deflecting Hull; An internal, forward, inner lower and bottom tank structure of exceptional strength similar to existing icebreaker hulls; Conventional outer hull would combine to form partial double hull. Upon grounding the vessel rides up and the inner hull deflects the vessel away from the obstacle. Existing SBT is retained. • • • Development—minimum 2 years to develop. Need to develop structure and perform model tests. • 11. Grinding Bow; Forebody bottom structure to be double bottom with internal transverse structure designed to act in a grounding as a "rock rasp". Thereby grinding down underwater obstacles and allowing safe override by remaining structure. • • • Concept—minimum 10 years to develop. Need to develop & test both materials and structure. • 12. Unidirectionally Stiffened Bottom Structure; Bottom structure designed to have "crushable" transverses in combination with longitudinal structure such that bottom structure moves (in the event of grounding) as a unified pliable member. • • • Research—minimum 3 years to develop. Existing crushable structures would need to be reviewed and expanded. Need to model test structure. • 13. Honeycomb Hull Structure; Utilize high energy absorbing deep honeycomb steel structure, sandwiched between steel plates. • • • Research—minimum 5 years to develop. Need to develop structure and perform model tests • 14. High Yield Steel Bottom Structure; Construct bottom plating and structure using high yield type steel in conjunction with design stress limitations associated with mild steel. • • Existing. • • 15. Concrete; Construct hull using reinforced concrete internally molded to outer steel cargo tank structure. • • Concept—minimum 10 years to develop. Need to examine methods of utilizing concrete for this application. • 16. Ceramics; Place ceramic coating on the exterior of the hull. • • Concept—minimum 10 years to develop. • •
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Tanker Spills: Prevention by Design 4. Concerns 5. Pivotal Argument 6. Warrants Committee Economic Assessment STRENGTH MAINTENANCE STABILITY SALVAGEABILITY EXPLOSION SAFETY DOWNGRADE OPERATIONS COMPLEX DESIGN INTEGRATION RULES & REGULATIONS Comments For Against YES NO • • Provides both a forward secondary barrier to grounding induced damage as well as acting to divert vessel from continued damage. Provides superior protective support in the area most prone to grounding. Does not address side damage Will cause considerable damage if the vessel rams another vessel. • • • • Success largely dependent upon strength of rock. Provides secondary bottom structure which must be breached before cargo is exposed to the sea. In groundings, could provide better protection of cargo areas. Practical materials and applications seem unlikely. • • • • Could compromise bottom strength in a seaway. Provides structural support for grounding without losing cargo capacity. Does not address collision. Research needed to insure structure would protect against plate tearing. • • • • • • • Increases safety concerns without improving upon conventional double hull. Provides secondary structural barrier via high impact absorbing structure. Could be less weight and cost than conventional double hull. All of the negatives of double hull structure plus a quantum increase in risks associated with uninspected voids adjacent to cargo tanks. • • Raises the severity of bottom impact that can occur without hull rupture. Should be considered in combination with other options. Single barrier defense which, once breached, offers no improvement over conventional single hull design. Repair at yards unfamiliar with the material is a concern. • • • • • Raises the severity of bottom impact that can occur without incurring oil outflow. Unique torsion and tension properties of concrete do not match those required to manage ship structural dynamics. • • • • • Effects of adhering ceramics to the hull. Raises the severity of bottom impact that can occur without incurring oil outflow. Could conceivably provide a smoother cleaner hull surface. Torsional and impact properties of ceramics are not understood for this application. Could be cost prohibitive. •
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Tanker Spills: Prevention by Design TABLE 5-1 VLCC Tanker—List of Alternatives/DnV Arrangements Analyzed Committee Alternative DnV Arrangement (Arrangement Code) MARPOL Tanker (reference standard) • Modern Conventional VLCC (OR) Double Bottom • Double bottom in whole cargo area (2) • Double bottom with ballast in side tanks (options 4, 4A, and 4B) • Double bottom in side tanks, single bottom in center tank (6) Double Sides • Double side, single bottom (3B) • Double side in whole cargo area and partial double bottom (5) Double Hull • Double side and double bottom, with two longitudinal bulkheads (1A), with bulkhead at centerline (1B) Intermediate Oil Tight Deck • Intermediate Oil Tight Deck (8) Hydrostatic Control • Hydrostatically balanced loading (9) Small Tanks • Tank size half of MARPOL requirements (7) The double-bottom designs analyzed would spill no oil at all in about 85 percent of all groundings, whereas some oil always escapes from single-bottom designs, irrespective of tank size and the presence of a vacuum tank system. A vacuum system reduces significantly the amount of oil escaping in groundings for the single-bottom designs analyzed. However, the total amount of oil lost from the modern conventional VLCC with a vacuum system is still about twice the amount escaping from the double side/double bottom design. Comparing VLCCs with double sides to VLCCs with single sides, the former provides an effective barrier against oil outflow in 20 percent of the all collisions for arrangements 1A and 1B, and in 42 percent of all collisions for arrangement 3B. The protection is particularly effective for low-energy collisions with limited damage penetration. The influence of increasing the double-bottom height as compared with increasing the double-side width is shown in Table 5-2. For example, increasing the VLCC double-bottom height from 2.0 to 3.9 m (B/15) reduces the probability of oil outflow in groundings from 58 to 14 percent, while increasing the width of side tanks from 2.0 to 3.0m only reduces the probability of outflow in collision from 88 to 80 percent. The specific conclusions drawn by DnV can be found in Appendix F (page 275); the committee's commentary can be found on page 300.
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Tanker Spills: Prevention by Design TABLE 5-2 Probability for NO Oil Leakage as Function of Double Bottom Height and Distance Between Double Sides* GROUNDING (VLCC) DISTANCE BETWEEN INNER/OUTER BOTTOM PROBABILITY FOR NO LEAKAGE METERS B/—** PERCENT 2.0 28.6 41.7 2.4 23.8 52.6 3.0 19.0 69.0 3.9 14.7 86.0 6.6 8.6 99.8 COLLISION (VLCC) DISTANCE BETWEEN INNER/OUTER SKIN PROBABILITY FOR NO LEAKAGE METERS B/—** PERCENT 2.0 28.6 12.1 3.0 19.0 20.4 5.8 9.86 39.4 6.3 9.08 42.0 * Reference Table 3.3.1 in DnV Report, Appendix F. ** Ratio, beam to between hull width. Probabilistic Ranking of 40,000 DWT Tankers The 40,000 DWT designs chosen for the analysis are similar to the VLCCs. The objective in analyzing both very large (VLCC) and small (40,000 DWT) tankers was to detect trends, so the results could be applied, in a general sense, to other tanker sizes. The committee also selected the 40,000 DWT tanker for analysis by DnV because the size is typical in U.S. coastal activity. Particular attention has been directed to investigating the influence of double-side width on oil outflow, as several 40,000 DWT product tankers have been built with narrow sides. The arrangements listed in Table 5-3 were analyzed by DnV; Appendix F (page 278) provides a summary of the assumptions made regarding ballast capacity, side width, bottom height, and longitudinal bulkheads. Sketches of the various arrangements are found in Appendix F (pages 281-285). Table 5-3 links the DnV arrangements to the general design alternatives considered by the committee. The combined ratings for collision and grounding (using the 40/60 weighting) gives the combined ranking shown in Figure 5-15 for 5-knot grounding speed, and in Figure 5-16 for 10-knot ground speed.
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Tanker Spills: Prevention by Design TABLE 5-3 Small Tanker (40,000 DWT Tanker)—List of Alternatives/DnV Arrangements Analyzed Committee Alternative DnV Arrangement (Arrangement Number) MARPOL Tanker (reference standard) • Modern 40,000 DWT SBT (1) Double Bottom • Double bottom and single sides (3)* Double Sides • Double sides and single bottom (2) Double Hull • Narrow (0.76 m apart) double sides and double bottom (4)* • Double sides (1.2 m apart) and double bottom (5)* • Wide (2.0 m apart) double sides and double bottom (6)* • Wide (2.0 m apart) double sides and double bottom, short tanks (7), no centerline bulkhead Intermediate Oil Tight Deck • Intermediate Oil Tight Deck (8) * With centerline bulkhead. At 5 knots, design arrangement 6 with wide double side and low double bottom achieves a rating of 57, followed by arrangement 8 (the intermediate oil-tight deck) with an rating of -65. Only design arrangement 2, the double side without a double bottom, has an index above the reference vessel (MARPOL). The ranking does not change for the 10-knots scenario; design arrangement 6 (wide double sides with double bottom) remains the best with a rating of 38, followed by design arrangements 8 (intermediate oil-tight deck), 4 (narrow double sides and double bottom) and 5 (mid-width double sides and double bottom). Results—40,000 DWT Tanker Analysis This analysis demonstrates the value of double bottoms in preventing oil outflow. Table 5-4 shows the effect of increasing double-bottom height and double-side width on the probability of spilling oil. The probability falls rapidly as double bottom height is increased. By contrast, increasing the width of double sides does not have the same dramatic effect. Estimated Oil Outflow from a 80,000 DWT Tanker Based on the studies of VLCCs and 40,000 DWT tankers, DnV appraised the potential oil outflow from a 80,000 DWT tanker designed with several hull configurations. No supporting calculations were carried out. The conclusions drawn by DnV can be found in Appendix F (page 296); the committee's commentary can be found on page 302.
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Tanker Spills: Prevention by Design FIGURE 5-15 Combined ranking, 5 knots (low-energy), 40,000 DWT. Reference DnV Report, Figure 4.18, Appendix F. FIGURE 5-16 Combined ranking, 10 knots (high energy), 40,000 DWT. Reference DnV Report, Figure 4.19, Appendix F.
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Tanker Spills: Prevention by Design TABLE 5-4 Probability for NO Oil Outflow in Collision and Grounding* GROUNDING (40,000 DWT) DISTANCE BETWEEN INNER/OUTER BOTTOM PROBABILITY FOR NO LEAKAGE METERS B/—** PERCENT 1.83 15.0 85.1 2.0 13.7 90.0 2.6 10.5 98.4 3.9 7.0 99.9 COLLISION (40,000 DWT) DISTANCE BETWEEN INNER/OUTER SKIN PROBABILITY FOR NO LEAKAGE METERS B/—** PERCENT 0.76 36 8.6 1.2 22.8 16.3 2.0 13.7 29.2 3.0 9.1 41.9 * Reference Table 4.3.1 in DnV Report, Appendix F. ** Ratio, beam to between hull width. The Committee's Overall Conclusions from DnV Analysis As noted earlier, DnV's conclusions, along with committee comments, are provided in Appendix F. The committee drew the following overall conclusions from the analysis of all tanker sizes: Double hulls provide significant overall protection against oil outflow in low-energy collisions and groundings. Wide tanks are likely to spill more oil than long, narrow tanks. Double sides protect against oil outflow in collisions, particularly low-energy collisions. Double bottoms protect against oil outflow in groundings, particularly low-energy groundings. The intermediate oil-tight deck, when combined with double sides, should provide protection against both groundings and collisions. Hydrostatic loading and/or smaller tanks should reduce oil outflow when used in conjunction with any design concept. SUMMARY AND SIGNIFICANCE OF OUTFLOW ESTIMATES The results of the DnV probabilistic study and the committee's own estimates for oil outflow relative to a standard of reference (100%), the
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Tanker Spills: Prevention by Design TABLE 5-5 Performance of Alternative Designs for Large Tankers (VLCC)—Oil Outflow Design Alternative for VLCC (240,000 DWT) Tanker Oil Outflow Relative to MARPOL* (100%) for Composite** of Collisions (40%) and Groundings (60%) Low-Energy (5 kn) High Energy (10 kn) Double Bottom (B/15) 42 37 Double Sides 88 130 Double Hull 33 26 Hydrostatic Control (passive) 62 40 Smaller Tanks (1/2 MARPOL) 58 70 Intermediate Oil-Tight Deck with Double Sides 32*** 23*** Double Sides with Hydrostatic Control (passive) 32*** 21*** Double Hull with Hydrostatic Control (passive) 30*** 22*** * MARPOL standard tankers have protectively located segregated ballast tanks. ** Composite based on frequency of collision and groundings. *** Committee estimate (see Appendix K). single-hull tanker with protectively located segregated ballast tanks (PL/SBT—MARPOL), are listed in Tables 5-5 and 5-6. As noted earlier, the committee recognized the possibility of combining certain design alternatives to improve pollution control. The committee used DnV data to derive outflow estimates10 for three combinations: double sides with hydrostatic control; double hull with hydrostatic control; and intermediate oil-tight deck with double sides. The outflow ratings for these combinations were not evaluated directly by DnV. Therefore, these oil outflow percentages (as shown by ***) are to be considered indicative examples of performance but less rigorous than the other percentages. Outflow Performance Relationships and the Underlying Reasons To understand the significance of these performance values, it is important to identify the major differences between designs, to understand the reasons for these differences, and to understand to what extent this knowledge can be used for decision making. 1. The performance benefits from each design alternative are significantly better for the big ships (VLCCs) than for the little ships (40,000 DWT). This is true at both high and low speeds. The reason is that, in big ships, the size and numbers of cargo tanks are not greatly changed among design alternatives. This, in turn, is the result of both IMO hypothetical oil outflow requirements and structural demands in
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Tanker Spills: Prevention by Design TABLE 5-6 Performance of Alternative Designs for Small Tankers (40,000 DWT)—Oil Outflow Design Alternative for 40,000 DWT Tanker Oil Outflow Relative to MARPOL* (100%) for Composite** of Collisions (40%) and Groundings (60%) Low-Energy (5 kn) High Energy (10 kn) Double Bottom (B/15) 82 50 Double Sides 136 130 Double Hull 68 43 Hydrostatic Control (passive) 52 34*** Smaller Tanks 68 76*** Intermediate Oil-Tight Deck with Double Sides 57*** 36*** Double Sides with Hydrostatic Control (passive) 70*** 44*** Double Hull with Hydrostatic Control (passive) 61*** 39*** * MARPOL standard tankers have protectively located segregated ballast tanks. ** Composite based on frequency of collision and groundings. *** Committee estimate (see Appendix K). big tankers. In smaller tankers, the presence of a protective skin (i.e., double bottom or double side) allowed the designer of the DnV series to select a smaller number of much larger tanks. Neither hypothetical oil outflow nor structural demands impose significant limits on cargo tank size in these small ships. The result is that in any accident piercing the inner skin of a smaller tanker, a significantly larger amount of cargo is exposed to release than in the base ship. 2. The performance improvement for all alternatives appears better at high speed (10 knots) than slow speed (5 knots) for both big and little tankers. The reason is that, at higher speeds, the longitudinal extent of damage is greater in groundings (although collision, as assumed here, is independent of speed). The greater the longitudinal damage in the bottom, the more cargo potentially exposed to spillage, and accordingly protective alternatives appear to be, and are likely to be more useful at higher speeds. In this regard, it is important to recognize that the base ship performance value of 100 percent represents a very different amount of oil spillage in the two ships. For the base VLCC at 5 knots, 100 percent represents grounding outflow of about 11,000 tons while at 10 knots, 100 represents grounding outflow of about 21,000 tons—or an increase of almost twofold. 3. Four cases (double hull, double hull with hydrostatic control, double sides with the intermediate oil-tight deck, double sides with hydrostatic control) appear to provide nearly the same pollution prevention benefit. This is due to a significant assumption used in making these estimates,
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Tanker Spills: Prevention by Design that is, that hydrostatic control works nearly perfectly with no loss due to tide, swell, or ship motion. This assumption may not be valid, and these estimates thereby may underestimate oil outflow. These important apparent performance differences also can be explained, however, by recognizing that: The double hull is definitely very effective in low-energy groundings, but it cannot prevent some collision outflow due to narrow side tank voids. Because the double hull alone, in this analysis, prevents most of the outflow in groundings, adding hydrostatic control improves performance only marginally (outflow already is minimal); furthermore, this combination only improves performance in collision as a result of cargo tanks being less than full. In this analysis, double sides with the intermediate oil-tight deck is better than a double hull in collision as a result of wider side ballast tanks, and is presumed to prevent nearly all outflow in low-energy groundings and some outflow in high-energy groundings (because the analysis takes no account of factors such as tides, current, or ship motion). In this analysis, double sides with hydrostatic balance appears equivalent to double sides with the intermediate oil-tight deck for the above reasons, but, in fact, will not perform as well because tides, swells, and ship motion can be expected to ensure grounding outflow following the accidents. Applicability of Results From the preceding, the following observations can be made concerning the applicability. These particular numbers, while providing a relative performance indicator for one particular set of designs and assumptions, should be regarded only as a sample of the type of analysis that can be made for these and other design possibilities (particularly with regard to number and size of cargo tanks in small ships). Other assumptions about grounding and collision speeds, etc., also should be investigated. Accidents other than grounding and collision (i.e., fire and explosion, structural failures) should be subjected to similar analysis. Significance of Results Because the committee made substantial use of data from the DnV study, as well as the committee estimates, it is important to understand the significance, limitations, and possible implications of these numbers before using them as the basis for the cost-benefit analysis discussed in Chapter 6. These estimates are significant in that they show a substantial potential
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Tanker Spills: Prevention by Design reduction in oil outflow in collisions and groundings for several alternatives for one plausible set of designs, and grounding and collision circumstances. However, they show that a wide range of results are possible even for the same hull design concept, when these designs are applied to ships of different sizes with different cargo tank configurations and sizes. Due to a variety of limitations, the estimates clearly should be regarded as only a sample of the type of work that could be done with more comprehensive analysis. The limitations include use of one particular statistical casualty profile, a number of simplifying assumptions regarding oil outflow immediately following collisions, and the disregard for tidal and wave effects in the grounding outflow analysis. The DnV study points out the relationship between tank arrangements and oil outflow. Although the DnV study did not directly address damage stability, oil outflow in an accident and damage stability are linked directly to particular tank arrangements. The results of the DnV analysis have two important implications, which are consistent with the committee's findings regarding damage stability discussed in Chapter 4. First, for MARPOL SBT tankers (having excess freeboard and a large amount of empty ballast tank volume during loaded passage), providing improved oil outflow reduction under all plausible accident scenarios would require careful consideration of both ballast and cargo tank size and arrangements. While there is no question that the use of ballast tanks as protective spaces can reduce oil outflow, it also can have adverse consequences on stability. Similarly, protecting cargo tanks with ballast spaces outboard, or beneath them, does not mean that internal cargo tanks can be made larger than is common in single-skin ships without possible increase in oil outflow. This suggests that, for smaller ships, both damage stability assumptions and hypothetical oil outflow should be reconsidered by IMO and the Coast Guard. Second, because all the analyses submitted to the committee, by DnV and others, were based on investigations of conventional tanker designs, it is difficult to predict how combination carriers (OBOs and ore/oil, or O/Os) would be affected under similar accident scenarios and outflow limitations. While it is clear that combination carriers, when carrying oil, must comply with all tanker regulations, the committee recognizes the following difficulties that may be encountered, particularly in OBOs: The very wide tanks in OBOs may lead to difficulty in hydrostatic loading, as OBOs cannot be sailed safely with slack tanks; The lack of longitudinal subdivision of OBOs also may lead to complications if hypothetical oil outflow criteria for ships smaller than VLCCs are adjusted downward; The narrow side tanks typical of OBOs also may lead to complications in complying with criteria applied to conventional tankers.
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Tanker Spills: Prevention by Design The committee has not conducted studies of damage stability in OBOs but believes these ships must be able to comply with whatever criteria are deemed necessary for tankers; The committee questions whether hatch covers on OBOs and O/Os would be able to withstand forces imposed by a vacuum system, were that option to be applied. While it might be argued that combination carriers need not comply with each specific requirement applied to tankers, the committee feels it would be wrong to waive regulations for combination carriers when this would result in unequal protection against oil outflow from accidents of any type. Such a loophole could encourage increased construction of combination carriers, to circumvent requirements designed to protect the environment. NOTES 1. Tanker Advisory Center, Guide for the Selection of Tankers, 1990, New York. 2. Clarkson Research Studies Ltd., 1990. 3. Clarkson Research Studies Ltd., 1990. 4. The Coast Guard estimates that an 11.5' (B/14.4) double bottom on the EXXON VALDEZ would have reduced oil outflow by 60 percent at most, and by a minimum of 25 percent— still a significant figure (U.S. Coast Guard, Marine Safety Center internal memorandum, May 25, 1989). 5. The committee estimated retrofit costs based on information from shipyard sources. These costs are highly dependent on the base vessel. 6. Assuming that cargo tank lengths and the ship's beam are the same as in the equivalent deadweight MARPOL standard. 7. The committee was shown videotapes of small-scale tests using a plexiglass model of one IOTD w/DS proposal, in a simulated low-energy grounding. While the results were impressive some committee members remain skeptical about actual performance under a wide range of operating conditions. Of special concern is the dynamic condition of a vessel running aground at service speeds (14 to 16 knots). 8. On the average, it would be applicable to about 40 percent of accidents and 60 percent of outflow volume in U.S. waters—those events where major grounding damage and spillage (more than 30 tons) have been incurred (see Figure 1-11). 9. Additional parameters are detailed in Appendix F (pages 252 and 278). 10. Committee estimates for design combinations were derived using DnV data in the combined ranking values. The value in the estimates for collision performance for one design is added to the grounding performance for another alternative, producing a composite estimate for a case not shown directly by DnV. For example, to generate an estimate for double sides with hydrostatic control at 5 knots, the committee combined the performance in collision of the double-side case 3B with the grounding performance of the hydrostatic control case 9, to generate a combined ranking of about 32. A more complete explanation can be found in Appendix K. REFERENCES Clarkson Research Studies, Ltd. FAX to D. Perkins, National Research Council, Washington, D.C., August 31, 1990.
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Tanker Spills: Prevention by Design Det norske Veritas. 1990. Potential Oil Spill from Tankers in Case of Collision and/or Grounding: A comparative study of different VLCC designs. Report conducted for the Royal Norwegian Council for Scientific and Industrial Research, Oslo. DnV 90-0074. Lloyd's Register of Shipping. 1989. Maritime Overseas Corporation 64,000 DWT Product Oil Carrier Sloshing Investigation. Report prepared for MOC, New York, October 1989. CSD 89/33. Ministry of Transport-Japan. 1990. Prevention of Oil Pollution. Report prepared for IMO Marine Environment Protection Committee, received by Committee on Tank Vessel Design, NRC, Washington, D.C., November, 1990. Toyko. Tanker Advisory Center. 1990. Guide for the Selection of Tankers. New York: TAC. U.S. Coast Guard. 1989. Marine Safety Center internal memorandum, May 25, 1989. U.S. Coast Guard. 1990. Navigation and Inspection Circular 2-90. Published by the Coast Guard, Washington, D.C., September 21, 1990.
Representative terms from entire chapter: