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--> APPENDIX K Comparative Study of Double-Hull and Single-Hull Tankers1 Introduction Prior to 1990, most crude oil carriers were built with single hulls. Design, construction, and operational experience of double-hull tankers was limited primarily to product and parcel tankers under 40,000 tons deadweight. The stability and strength characteristics of double-hull crude oil carriers are quite different from single-hull tankers and product carriers, and designers and operators of double-hull tankers found themselves confronted with a new set of issues to consider. This appendix examines the design characteristics of double-hull tankers built since 1990. Four of the areas in which double-hull tankers perform differently as compared to single-hull tankers have been identified and investigated. These are: environmental performance with regard to oil outflow from collisions and grounding survivability characteristics after experiencing a collision or grounding intact stability during load and discharge operations hull girder strength and draft considerations for the ballast condition For comparative purposes, both single-hull and double-hull configurations have been investigated. Double-hull ships are selected to be representative of the tankage arrangements and proportions typically built since 1990. The size of a tanker has a significant influence on the stability and survivability characteristics of the vessel, and therefore the designs studied are divided into the following five groups: 1 Prepared for the Committee on Oil Pollution Act of 1990 (Section 4115) Implementation Review by Herbert Engineering Corporation, San Francisco, California, April 15, 1996.
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--> tankers of 35,000 DWT-50,000 DWT tankers of 80,000 DWT-100,000 DWT tankers of 135,000 DWT-160,000 DWT tankers of 265,000 DWT-300,000 DWT oceangoing barges Subdivision Nomenclature The following terms are used to describe the ship's subdivision: Cargo block. The cargo block is the portion of the ship extending from the forward boundary of the forward-most cargo tank to the aft boundary of the aft-most cargo tank. OPA '90 as well as the 1992 Amendments to Annex I of MARPOL 73/78 require that all oil tanks within this space be segregated from the side and bottom shell. Cargo tanks. All tanks arranged for the carriage of cargo oil. Unless noted otherwise, the term ''cargo tanks" shall be assumed to include the slop tanks. Slop tanks. Slop tanks are provided for storage of dirty ballast residue and tank washings from the cargo tanks. Annex I of MARPOL 73/78 requires that tankers be arranged with slop tanks. Cargo tank arrangements. Figure K-1 shows cross-sections of typical cargo tank arrangements for double-hull tankers. The "STA" or single-tank-across arrangement has a single center cargo tank spanning between wing tanks. This design is frequently arranged with upper hopper tanks in way of the outboard wings, in order to reduce the free surface when the cargo tanks are nearly full. The two-tanks-across arrangement has a centerline bulkhead and port and starboard cargo tanks. Vessels under 160,000 DWT are typically arranged as single tank across, two tanks across, or a combination thereof. Most larger tankers are arranged with three tanks across as required to satisfy the MARPOL requirements for tank size and damage stability. FIGURE K-1 Cargo tank arrangements.
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--> FIGURE K-2 Ballast tank arrangements. Ballast tank arrangements. Figure K-2 shows typical ballast tank configurations. —"L" tanks are the most commonly used configuration. L tanks are usually aligned with the cargo tanks, although they will occasionally extend longitudinally over two cargo tanks. —"U" tanks reduce asymmetrical flooding, and are generally used when L tank arrangements fail to meet damage stability requirements. U tanks extend over the full breadth of the ship, and have a significantly higher free surface as compared to a pair of L tanks. —"S" or side tanks are located entirely in the wing tanks. S tanks improve the survivability characteristics of a vessel as they normally will not be penetrated when bottom damage is incurred. Methodology and Assumptions Oil outflow, survivability, intact stability, ballast draft, and strength evaluations have been carried out for 27 tankers. These are all vessels that have either been delivered or are currently under contract. Oil outflow and survivability calculations have also been carried out for nine barges. All calculations have been done using HECSALV (Herbert Engineering Corporation, 1996) software. The calculation methodology and assumptions are described below. Evaluating Oil Outflow All cargo oil tanks on a double-hull tanker built to OPA 90 requirements are protectively located. Many of the damage cases that would result in oil spillage on single-hull tankers will not penetrate the cargo tanks of double-hull tankers. Double-hull tankers will have fewer accidents involving oil spillage. The mean or expected oil outflow from a casualty will usually be less with a double-hull tanker as compared to a single-hull tanker of the same size. The arrangements of double-hull tankers vary. The vessel proportions, the wing tank and double bottom dimensions, and the number and location of longitudinal and transverse bulkheads all influence the outflow performance. As a
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--> consequence, the likelihood of oil spillage and the mean or expected oil outflow will vary significantly even among double-hull tankers of the same size. The International Maritime Organization (IMO) guidelines (1995) for evaluating alternatives to double-hull tankers have been applied in this report for assessing oil outflow performance. Although originally intended for evaluating alternatives to the double-hull concept, these guidelines are also well suited for comparing the outflow performance of single-hull and double-hull tankers. The guidelines take a probabilistic approach based on historical statistical data, and provide a methodology for assessing both the likelihood of a spill and the expected outflow. The IMO guidelines account for factors such as varying wing tank widths and double bottom heights, the influence of internal subdivision, the effects of tide, and the influence of dynamic effects on outflow. Principles of Oil Outflow The following provides a brief description of the fundamental principles affecting oil outflow. More extensive discussions are contained in Tanker Spills, Prevention by Design (NRC, 1991) and the USCG report, Probabilistic Oil Outflow Analysis of Alternative Tanker Designs (DOT, 1992). Hydrostatic Balance. In the event of bottom damage, oil outflow will occur until the internal pressure exerted by the entrapped oil and flooded water within a tank equals the external pressure exerted by the seawater. If the ullage space is under pressurized such that the pressure on the oil surface is less than the atmospheric pressure acting on the seawater, outflow will be reduced. Conversely, higher ullage space pressures as might be introduced by the inert gas system will result in larger outflows. For groundings, the external pressure is reduced as the tide drops, and outflow will occur until equilibrium is once again attained. For lightly loaded tanks, the initial pressure head from the cargo oil is less than the external seawater pressure. When bottom damage is sustained, seawater enters the bottom of the tank until equilibrium is achieved. Provided the damage does not extend up the side of the tank and currents or vessel motions do not induce mixing of seawater and oil in the vicinity of the damage, no oil will be lost. Oil Entrapment in Double-Hull Tankers. When a tanker experiences bottom damage through the double bottom tanks and into the cargo tanks, a certain portion of the oil outflow from the cargo tanks will be entrapped by the double bottom tanks. This phenomenon was investigated through model testing at the David Taylor Research Center (DTRC, 1992) and the Tsukuba Institute, Ship & Ocean Foundation (Tsukuba Institute, 1992), and through numerical analysis. These studies indicate that oil entrapment is influenced by many factors, including the size and location of openings, the magnitude of the pressure imbalance, and whether the double bottom tank is flooded with water at the time the oil tank
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--> is ruptured. For conditions in which the double bottom initially floods and then the cargo tank is breached, a viscous jet is formed resulting in minimal retention of oil in the outer hull. The Marine Environmental Protection Committee (MEPC) concluded that "if both outer and inner bottoms are breached simultaneously and the extent of rupture at both bottoms is the same, it is probable that the amount of sea water and oil flowing into the double-hull space would be the same." In its regulations, IMO assumes that double bottoms below oil tanks retain a 50:50 ratio of oil to sea water. Where tidal changes introduce a slowly changing pressure differential, higher retention rates can be expected. Dynamic Oil Losses. Oil losses in excess of those predicted from hydrostatic balance calculations may result due to the initial impact when a vessel runs aground, and subsequently, from the effects of current and ship motions. These losses primarily influence single-hull vessels and alternative designs whose oil tanks contact the outer hull. Model tests at David Taylor Research Center (1992) and the Tsukuba Institute (1992) were carried out to assess the influence of initial impact and current on oil outflow. Dynamic losses are influenced by the speed of the ship, the extent of damage, the magnitude of the current, and the sea state. Under extreme weather conditions, losses up to 10 percent of the tank volume can be encountered, although dynamic oil losses of 1 percent to 2 percent are more typical. In its regulations, IMO assumes a minimum outflow of 1 percent of the volume for all breached cargo tanks which bound the outer hull. Side Damage. The location and size of the damage opening influences the amount of expected oil outflow from side collisions. If the lower edge of the damage opening lies above the equilibrium waterline, the oil level in the tank will drop to the height of the opening and the vessel will heel away from the damage. When the damage extends below the waterline, outflow of oil will occur until hydrostatic balance is achieved. Over time, all oil located below the level of the upper edge of the damage opening will be replaced by the denser seawater. In its regulations, IMO assumes that 100 percent of the oil in breached side tanks is lost. Methodology for Evaluating Oil Outflow Each of the designs has been evaluated using the conceptual analysis approach (without consideration of survivability) as defined in the IMO Interim Guidelines for Approval of Alternative Methods of Design and Construction of Oil Tankers under Regulation 13F(5) of Annex I of MARPOL 73/78 (IMO, 1995). An overview of the methodology is described below. Further details on application of these regulations can be found in Michel and Moore ( 1995). The IMO guidelines call for the calculation of three parameters: the probability of zero outflow, mean outflow, and extreme outflow. The calculation method-
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--> ology assumes the vessel experiences a collision or grounding, and that the outer hull is breached. The assumed extent of penetration, and therefore the probability that the inner hull of a double-hull tanker will be pierced, are based on the application of probability density functions as described in the following paragraphs. The probability of zero outflow is the likelihood that such an encounter will result in no cargo oil spillage into the environment, and is an indicator of a design's tendency towards avoiding oil spills. The mean outflow is the weighted average of the cumulative oil outflow, and represents the expected or average outflow. This mean outflow provides an indication of a design's effectiveness in mitigating the amount of oil loss due to collisions and groundings. The extreme outflow is the weighted average for the most severe damage cases, and provides an indication of a design's effectiveness in reducing the number and size of large spills. Historical data from collisions and groundings of tankers were collected by a number of classification societies under the direction of IMO (Lloyds Register of Shipping, 1991), and reduced into probability density distribution functions. The area under the probability density curve between two points on the horizontal axis is the probability that the quantity will fall within that range. The density distribution scales are normalized by ship length for location and longitudinal extent, by ship breadth for transverse location and transverse extent, and by ship depth for vertical location and vertical extent. Statistics for location, extent, and penetration are developed separately for side and bottom damage cases. Figure K-3 shows the probability density distribution for the longitudinal extent of grounding damage. The histogram bars represent the data collected by the classification societies, and the linear plot represents IMO's piece-wise linear fit of the data. The area under the curve up to a damage length/ship length of 0.3 equals 0.75. Based on these statistics, there is a 75 percent likelihood that the longitudinal extent of damage for a ship involved in a grounding incident will not exceed 30 percent of the ship's length. FIGURE K-3 Longitudinal extent of grounding damage.
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--> Through application of these functions to the hull and compartmentation of a particular vessel, all possible combinations of damaged compartments are determined, together with their associated probabilities of occurrence. Calculations are then performed to determine the oil outflow associated with each of these incidents. For the vessels analyzed in this study, the number of unique damage cases ranged between 100 and 350 for side damage, and between 300 and 700 for bottom damage. For side damage incidents, 100 percent oil loss is assumed for each breached cargo tank. Therefore, if a given damage incident damages only a ballast wing tank, zero outflow occurs. If a damage incident involves breaching of the ballast wing tank and the adjacent cargo oil tank, the full contents of the cargo oil tank are assumed to be lost. For bottom damage, outflow is determined by performing hydrostatic pressure balance calculations. A reduction in tide after the incident of 0.0 meters, 2.0 meters, and 6.0 meters (or one-half the draft, whichever is less) is assumed. Other assumptions applicable to bottom damage calculations are: An inert gas pressure of 0.05 bar is applied to all cargo oil tanks. This is a positive pressure and augments the oil outflow. If a double bottom ballast tank or void space is located immediately below a breached cargo tank, the flooded volume of the double bottom tank is assumed to be a 50:50 mixture of oil and seawater. The oil entrapped in the double bottom is not included in the assumed spill volume. For breached cargo tanks bounding the bottom shell, oil outflow equal to 1 percent of the tank volume is assumed as the minimum outflow. For tanks which are hydrostatically balanced in the intact condition, outflow analysis based on hydrostatic-balance principles will indicate zero outflow for grounding cases not subject to tidal change. In these circumstances, the minimum outflow value accounts for oil loss due to initial impact and the effects of current and waves. Independent calculations are carried out for side and bottom damage, and the three outflow parameters computed. For the grounding evaluation, the 0.0 meter, 2.0 meter, and 6.0 meter tidal change results are combined in a 40 percent:50 percent:10 percent ratio. The side and bottom damage results are then combined in a 40 percent:60 percent ratio. A pollution prevention index is developed by substituting the outflow parameters for the actual design and the IMO reference double-hull design into the following formula provided in the IMO Guidelines. If the Index E is greater than or equal to 1.0, the alternative design is considered at least equivalent to the IMO reference design. E= (0.5)(P0) + (0.4)(0.01+OMR) + (0. 1) (0.025 +OER) POR 0.01 + OM 0.025 + OE
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--> P0 = probability of zero outflow for the alternative design. OM = mean oil outflow parameter for the alternative design = (mean outflow)/C. OE = extreme oil outflow parameter for the alternative design = (extreme outflow)/C. C = total cargo oil onboard. POR, OMR, and OER are the corresponding parameters for the reference double-hull design of the same cargo oil capacity. The IMO reference double hulls are shown in Figure K-4. These reference designs do not represent the minimum subdivision acceptable under current MARPOL regulations. Rather, it was IMO's intent to select designs which "exhibit a favorable oil outflow performance." For instance, the 150,000 DWT reference ship has a 6 × 2 cargo tank arrangement, whereas a 5 × 2 arrangement is permissible under current rules. Similarly, the assumed double bottom depth on the VLCC is in excess of the rule requirements. The IMO Guidelines specify that C, the cargo oil onboard, be taken at 98 percent of the total cargo tank volume, and that the density of the cargo oil be as required to bring the vessel to its subdivision draft. For this analysis, it is assumed that each vessel is loaded to its summer load line with crude oil at a density of 0.90 metric tons/m3. This typically means that one tank or pair of tanks is partially full. The partially loaded tank or tanks were selected in order to maintain a trim in the intact condition between zero and 0.5 meters by the stern. In all other respects, the analysis has been carried out in strict conformance with the IMO guidelines. Survivability Evaluation Most single-hull tankers have excellent damage stability characteristics. When cargo oil tanks are breached, the oil is displaced by seawater of comparable or slightly higher density, resulting in relatively small heeling moments. For MARPOL 78 tankers, the side ballast tanks will introduce an asymmetric heeling moment. However, these tanks are arranged adjacent to cargo tanks. MARPOL 78 tankers are designed to withstand damage to a ballast tank, or to the ballast tank and an adjacent cargo tank. Breaching two ballast tanks would require damage extents longer than the length of a cargo tank, and the probability of such extents is extremely small. Double-hull tankers are arranged with wing ballast tanks along the length of the cargo block. When breached, these tanks introduce asymmetric loading which will tend to heel the vessel in the direction of the damage. In addition, the double bottom raises the height of the cargo oil, which translates into a higher center of gravity for the intact condition as compared to a single-hull tanker. Free surface effects may also be higher, as single-tank-across arrangements of cargo tanks are not uncommon in double-hull tankers. These effects all tend to increase the heeling moment. Excessive asymmetrical flooding will lead to immersion of down flooding points, and eventually the vessel will sink or capsize. IMO recognized the potential survivability problems with double-hull tankers.
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--> In Regulation 13F of the 1992 Amendments to Annex I of MARPOL 73/78 (IMO, 1992), the two compartment damage stability criterion contained in Regulation 25 of Annex I of MARPOL 73/78 was supplemented with raking bottom damage requirements. Regulation 13G and Regulation 25 both use a deterministic analysis approach in which fixed damage extents are assumed. Such calculations do not provide a clear picture of the survivability characteristics of a vessel. In this report, survivability is evaluated by applying the probabilistic density distribution functions for side damage as contained in IMO guidelines (IMO, 1995) for evaluating alternative tanker designs together with the damage survival requirements defined in Regulation 25. Methodology for Evaluating Survivability The principles affecting damage stability and survivability calculations are well documented in the literature (SNAME, 1988; IMO, 1993). The vessel is assumed to sustain damage which breaches the outer hull. Damaged compartments are assumed to be in free communication with the sea. The vessel sinks lower, trims, and heels until equilibrium is reached. A reiterative calculation approach is applied to determine the equilibrium draft and trim conditions over a range of heel angles. The computed heeling moment at each angle is then divided by the original intact displacement of the vessel less any fluid outflow, in order to develop the righting arm or "GZ" curve. From the GZ curve, the equilibrium heel angle can be determined. Properties of the GZ curve, such as its maximum value, positive range, and the area under the curve provide an indication of the reserve stability of the damaged vessel. Current analytical techniques do not provide a means for accurately determining the probability that a damaged ship will not capsize or sink. The assessment of survival or non-survival for a given damage case is therefore done on a deterministic basis. For instance, the IMO damage stability criteria for passenger ships, dry cargo ships, and tankers all contain minimum requirements regarding immersion of down flooding points, maximum heel angles, and residual stability. When these values are attained, survival is assumed. It is generally recognized that the IMO criteria reflect survival rates in a relatively moderate sea state, perhaps Beaufort force 3 or 4. For this study, the probability of flooding each combination of compartments has been determined from the probability density functions defined in the IMO guidelines. Only side damage from collisions has been considered when evaluating survivability. The vessel is assumed to be fully loaded to the summer load line draft. Consumables are assumed to be 50 percent full, and all cargo tanks 98 percent full. Where breached tanks are filled or partially filled, it is assumed that 100 percent of the fluid in the tank is displaced by seawater.
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--> FIGURE K-4 IMO reference double hulls. (a) IMO Double-Hull Reference Design No. 1 5,000 DWT. (b) IMO Double-Hull Reference Design No.2, 60,000 DWT. (c) IMO Double-Hull Reference Design No.3, 150,000 DWT. (d) IMO Double-Hull Reference Design No.4, 283,000 DWT.
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--> TABLE K-22 Principal Particulars for Oceangoing Barges #B35-S1 Single Hull #B35-D1 Double Hull #B90-S1 Single Hull #B90-S2 Single Hull #B90-S3 Single Hull #B90-D1 Double Hull #B90-D2 Double Hull #B 170-S1 Single Hull #B170-D1 Double Hull Longitudinal Bulkhead in Cargo Tanks All All All All All All All All All Longitudinal Bulkhead in Ballast Tanks None All All All Deadweight (MTons) 5.500 5.100 11.100 11,100 12.900 11,500 12.800 23,700 22.800 Type Flush Dk Flush Dk Flush Dk Flush Dk Flush Dk with Trunk Flush Dk Flush Dk with Trunk Length/Beam 3.29 5.55 3.36 3.36 4.86 13.28 5.40 13.70 13.70 Length/Depth 15.00 17.03 13.76 13.76 12.25 2.96 13.03 2.40 2.4 Beam/Depth 4.56 3.07 4.09 4.09 2.52 0.85 2.41 0.86 0.86 Loadline Draft/Depth 0.81 0.85 0.84 0.84 0.88 4.49 0.68 5.71 5.71 Number of Longitudinal Bulkheads 1 3 2 1 1 3 3 1 3 Number of Cargo Tanks (excl. slops) 4×2 4×2 3×3 4×2 7×2 5×2 7×2 6×2 6×2 Number of Ballast Tanks 0 0 1 1 0 0 0 0 13 Wing Tank Width/Required Width — 1.22 — — — 1.13 1.32 — 1.24 Wing Tank Width/Beam — 0.074 — — — 0.054 0.066 — 0.079 Double Bottom Height/Required Height — 0.694 — — — 1.062 1.047 — 1.081 Double Bottom Height/Depth — 0.039 — — — 0.042 0.034 — 0.031 Cargo Oil at 98% (m3) 6,080 5.690 12,320 12,320 16,040 12,980 14,210 28.520 26,570 Segregated Ballast (MTons) 38,242 35,789 77,490 77,490 100,888 81,642 89.378 179,385 167,120
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--> TABLE K-23 Oil Outflow Evaluation for Oceangoing Barges #B35-S1 Single Hull #B35-D1 Double Hull #B90-S1 Single Hull #B90-S2 Single Hull #B90-S3 Single Hull #B90-D1 Double Hull #B90-D2 Double Hull #B170-S1 Single Hull #B170-D1 Double Hull Side Damage Probability of zero outflow .24 .87 .19 .19 .03 .80 .85 .12 .87 Mean outflow (m3) 727 113 1,024 1,558 1,580 334 229 2,666 352 Extreme (1/10) outflow (m3) 1,566 921 2,409 3,153 3,048 2,100 1,830 5,307 3,215 Combined Bottom Damage [40% 0m: 50% 2m: 10% 6m tide] Probability of zero outflow .23 .78 .11 .11 .03 .90 .87 .05 .87 Mean outflow (m3) 581 135 1,046 1,040 648 140 152 1,249 284 Extreme (1/10) outflow (m3) 1,697 956 2,662 2,702 1,679 1,388 1,394 3,244 2,611 Combined Side and Bottom Damage [40% Side: 60% Bottom] Probability of zero outflow .24 .81 .14 .14 .03 .86 .86 .08 .87 Mean outflow (m3) 639 126 1,037 1,247 1,021 217 183 1,816 311 Extreme (1/10) outflow (m3) 1,645 942 2,561 2,882 2,226 1,673 1,568 4,069 2,852 Pollution Prevention Index 98% cargo volume (m3) 11,627 10,888 23,568 23,568 27,463 24,809 27,171 50,297 48,425 Index E .40 1.13 .39 .35 .37 1.25 1.33 .39 1.31 TABLE K-24 Survivability Evaluation for Oceangoing Barges #B35-S1 Single Hull #B35-D1 Double Hull #B90-S1 Single Hull #B90-S2 Single Hull #B90-S3 Single Hull #B90-D1 Double Hull #B90-D2 Double Hull #B170-S1 Single Hull #B170-D1 Double Hull Side Damage Survivability Index 95.0% 96.7% 92.9% 92.9% 99.7% 99.5% 99.9% 99.0% 95.0%
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--> Summary and Observations Observations on Oil Outflow Analysis of Tankers The probability of zero outflow is a measure of a tanker's ability to avoid oil spills. In this regard, double-hull tankers perform significantly better than single-hull tankers, as the protective double skin reduces the number of casualties that penetrate into the cargo tanks. As shown in Figure K-14, the probability of zero outflow is four to six times higher for double-hull tankers, indicating single-hull tankers involved in a collision or grounding will be four to six times more likely to spill oil. The probability of zero outflow is a function of the double bottom and wing tank dimensions, and is not affected by the internal subdivision within the cargo tanks. Therefore, centerline or other longitudinal bulkheads within the cargo spaces have no influence on the probability of zero outflow. The mean outflow is a measure of the ability of a design to mitigate the amount of oil outflow. Again, double hulls perform significantly better than single-hull vessels, with double-hull mean outflow values averaging one-third to one-fourth of the single-hull values. The double-side vessels (#40-DS3 and #80-DS3) perform reasonably well with respect to collisions, but have higher outflows for bottom damage. These vessels have single-tank-across arrangements for cargo tanks, which significantly FIGURE K-14 Probability of zero outflow for single-hull and double-hull tankers.
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--> increase outflow as compared to the more extensive cargo tank subdivision incorporated into the pre-MARPOL and MARPOL 78 designs. The light line on Figure K- 15 represents a curve-fit of the single-hull mean outflow data. We find that the two double-side vessels evaluated in this study fall slightly above this trend line, indicating these double-side vessels will have comparable outflow volumes to the typical single-hull vessel. For double-side vessels with oil-tight longitudinal bulkheads, improved performance as compared to single hulls can be expected. Mean outflow is influenced by the double-hull dimensions as well as the extent of internal subdivision within the cargo tanks. There is little variation in the arrangement of VLCCs, with most single-hull and double-hull designs incorporating a 5 × 3 cargo tank arrangement. Wing tank and double bottom dimensions for VLCCs typically fall between 3.0 and 3.5 meters. As a result, mean outflow values for VLCC are relatively consistent. In contrast, there is considerable scatter in the outflow values for tankers under 165,000 DWT. Figure K-16 shows the side and bottom damage contributions to mean outflow for the 150,000 DWT tankers evaluated in this study. The projected outflow is consistently lower for designs #150-D3, #150-D4, and #150-D5, all of which have an oil-tight centerline bulkhead over the length of the cargo block. Design #150-D1, with all single-tank-across cargo tanks, has the highest mean outflow. Design #150-D2 has an oil-tight centerline bulkhead arranged over about 40 percent of the cargo block, with single-tank-across cargo tanks arranged elsewhere. It is interesting FIGURE K-15 Mean outflow for single-hull and double-hull tankers.
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--> FIGURE K-16 Mean outflow data for 150,000 DWT double-hull tankers. to note that the bottom damage outflow are relatively consistent, but the single-tank-across designs perform less effectively when subject to side damage. The closer spacing of transverse bulkheads on these designs increases the probability of breaching multiple cargo tanks. Once a cargo tank is breached, oil outflow is no longer limited to one side of the vessel. As shown in Figure K-17, double-hull tankers without centerline bulkheads typically have twice the expected outflow of designs with oil-tight longitudinal bulkheads in way of the cargo block. FIGURE K-17 Mean outflow for double-hull tankers with and without centerline bulkheads.
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--> FIGURE K-18 Extreme outflow for single-hull and double-hull tankers. Extreme outflow is a measure of a design's propensity to spill large volumes of oil in the event of a very severe collision or grounding. The extreme outflow parameters are plotted in Figure K-18. Whereas double hulls were shown to be 3 to 6 times more effective in avoiding spills and reducing mean outflow, double hulls are somewhat less effective in controlling large spills. There is considerable scatter in the data points, indicating that such parameters as internal subdivision and draft/depth ratio have a significant impact on extreme outflow. With regard to extreme outflow, the double-hull vessels with single-tank-across arrangements performed more poorly than both pre-MARPOL and MARPOL 78 vessels of comparable size. The IMO Pollution Prevention Index E provides an overall picture of the outflow performance of a tanker. See Figure K-19 below. Single-hull tanker values generally fall between 0.3 and 0.4, whereas double-hull tanker values lie between .9 and 1.1. Sixty percent (9 of 15) of the double-hull designs had indices greater than 1.0, indicating equivalency to IMO's reference ships. In general, the ships with longitudinal oil tight subdivision in the cargo holds attained the highest indices. Of interest is design #150-D2, which has an Index E of 0.99, roughly equivalent to the IMO reference ship. Although approximately half the cargo oil capacity of this design is contained in single-tank-across cargo tanks, the detrimental effect of these tanks is offset by the contributions from the relatively wide wing tanks and deep double bottom tanks.
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--> FIGURE K-19 IMO pollution prevention Index E for single-hull and double-hull tankers. Observations on the Survivability of Tankers There is no discernible difference between survivability characteristics of single-hull and double-hull tankers, with the survivability indices generally falling between 99 percent and 100 percent. Two of the ships in the 35,000 to 50,000 DWT range had values of 87.2 percent and 92.5 percent, respectively. However, these values are more heavily influenced by the level of compartmentation within the engine room and adjacent spaces than to the differences between single-hull and double-hull arrangements. For ships under 225 meters in length, MARPOL damage stability requirements do not require evaluation of conditions which breach the fore or aft engine room bulkheads. For certain designs, such damages result in nonsurvival conditions. It should be noted that the the survivability index has been computed assuming a full cargo load, with all cargo tanks 98 percent full. Partial load conditions will likely have lower survival rates. Observations on the Intact Stability of Tankers With regard to intact stability, all single-hull designs are inherently stable. That is, for the worst possible combination of cargo and ballast tank loading, these vessels all maintained a GMt not less than 0.15 meters. For the double-hull vessels, 73 percent (1 of 15) were inherently stable. The designs which have the potential of instability (#40-D1, #80-D1, #150-D1, and #150-D2) all have single-tank-across cargo tanks. Designs #80-D1, #150-D1 and #150-D2 all had angles of loll below 8 degrees for the worst case loading situation, with no possibility of capsize. The load restrictions required to assure positive stability for these vessels are quite straightforward, requiring monitoring of any two ballast tanks. With all ballast tanks
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--> 2 percent full, the designs maintain positive stability through all possible cargo load conditions. Design #40-D1 incorporates a single-tank-across arrangement for the cargo tanks and some U type ballast tanks. These tanks introduce large free surface effects when they are partially full. Also, the beam/depth ratio of 1.79 is relatively low. Although the vessel is in no danger of capsizing, an angle of loll of 16 degrees will occur for the worst case loading situation. This loll angle could be further increased if the vessel is asymetrically loaded due to efforts to correct heel through counter-balancing. The load restrictions to assure positive stability for this vessel are quite complex, requiring monitoring of both ballast and cargo tanks. Observations on the Ballast Condition Analysis for Tankers The double bottom and wing tank dimensions for existing double-hull tankers generally exceed the rule requirements, providing ballast capacity in excess of that required to achieve the minimum IMO drafts. All of the designs evaluated have forward drafts at least 19 percent deeper than the IMO minimum requirements, and most designs had drafts more than 50 percent in excess of the rule minimum. Most of the double-hull designs evaluated have still-water bending moments in the ballast condition approaching the maximum permissible value assigned by the classification society. Exceptions are designs #40-D3 and #280-D3. Design #40-D3 has scantlings and consequently a permissible still-water bending moment significantly above rule requirements. Design #280-D3 has additional hull girder strength and deep ballast tanks located in the midships region. As shown in Table K-25, the average double-hull design has a permissible still-water bending moment 9 percent in excess of the ABS standard value. This is 13 percent above the average for single-hull vessels analyzed. It should be recognized, however, that rule requirements for longitudinal strength have been liberalized since many of the single-hull tankers were built. Although the relative permissible bending moments are higher, it is possible that this may be a result of higher permissible stresses rather than increased structural strength. TABLE K-25 Allowable Still-Water Bending Moments as a Percentage of the ABS Standard Value Single Hull Double Hull 35,000-50,000 DWT Tankers 106 124 80,000-100,000 DWT Tankers 98 100 135,000-160,000 DWT Tankers 89 106 265,000-300,000 DWT Tankers 93 110 Average (all tankers) 96 109
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--> Observations on the Oil Outflow Analysis and Survivability Analysis for Barges Parallel to the findings for tankers, double-hulled barges exhibited significant improvements with regard to the likelihood of avoiding spills (larger values for the probability of zero outflow) and the mitigation of the amount of oil spillage (smaller mean outflow values). Although the analysis for double-hull tankers did not extend to sizes below 25,000 DWT, it is expected that the mean outflow for tankers will be somewhat higher than for barges. This is because the reduced freeboard requirements for barges allow higher draft/depth ratios, which tends to reduce outflow from groundings. It is important to remember that this study investigates the relative performance of a design to mitigate outflow, assuming that it has experienced a collision or grounding which breaches the outer hull. The overall outflow performance must also consider the likelihood that a given vessel will experience such an accident. Therefore, a comparison of barges and tankers cannot be made on the basis of the outflow parameters alone. FIGURE K-20 Mean outflow for single-hull and double-hull barges.
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--> Cautionary Notes on the Assumptions and Limitations of this Study It is important to recognize that, due to both technical and practical limitations, there are many simplifications inherent in these calculations. The quantities of oil outflow do not represent a quantitatively accurate estimate of oil outflow, nor does the survivability index represent an exact determination of the probability that a certain design will survive a collision. Rather, these calculations provide a rational comparative measure of merit. Some of the assumptions and simplifications in the development of damage case probabilities are: The IMO statistical database (Lloyd's, 1991) used for developing the probability density functions is based on 50 to 60 incidents involving tankers above 30,000 DWT. The probability density functions are ''marginal" distributions. Locations, extent and penetrations are treated independently. Although some degree of correlation is expected, the correlated statistics are not currently available. It is believed that this approach is conservative in the sense that it tends to over-predict the amount of expected outflow. The historical casualty data primarily involve older, single-hull vessels. It is expected that extents of damage will be somewhat less for double-hull vessels. The 19 double-hull vessels analyzed in this study represent about 5 percent of the double-hull tanker fleet operating today. Efforts were made to select representative vessels. However, there are some double-hull vessels built for specific trades which have quite different characteristics as compared to these representative vessels. REFERENCES American Bureau of Shipping. 1995. Rules for Building and Classing Steel Vessels. Part 3, Hull Construction, and Equipment. New York: American Bureau of Shipping. David Taylor Research Center (DTRC). 1992. Summary of Oil Spill Model Tests. OTD 5/10, Annex 4. February. Washington, D.C.: U.S. Navy. Herbert Engineering Corporation. 1996. HECSALV Salvage Engineering Software, Version 5.08. Houston, Tex.: Herbert Engineering Corporation. International Maritime Organization (IMO). 1992. New Regulations 13F and 13G and Related Amendments to Annex I of MARPOL 73/78. London: IMO. IMO. 1993. Explanatory Notes to the SOLAS Regulations on Subdivision and Damage Stability of Cargo Ships of 100 Metres in Length and Over. London: IMO. IMO. 1995. Interim Guidelines for the Approval of Alternative Methods of Design and Construction of Oil Tankers under Regulation 13F(5) of Annex I of MARPOL 73/78. London: IMO. Lloyd's Register of Shipping. 1991. Statistical Analysis of Classification Society Records for Oil Tanker Collisions and Groundings. Report No. 2078-3-0. London: Lloyd's Register of Shipping.
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--> Michel, K., and C. Moore. 1995. Application of IMO's Probabilistic Oil Outflow Methodology. Paper presented at SNAME Cybernautics '95 Symposium. New York: SNAME. National Research Council (NRC). 1991. Tanker Spills: Prevention by Design. Marine Board. Washington, D.C.: National Academy Press. Society of Naval Architects and Marine Engineers (SNAME). 1988. Principles of Naval Architecture. Vol. 1, Stability and Strength. Jersey City, N.J.: SNAME. Tsukuba Institute. 1992. Model Tests by the Tsukuba Institute. Paper submitted to the IMO Marine Environment Protection Committee (MEPC). MEPC 32/7/1, Annex 6. London: IMO. U.S. Department of Transportation (DOT). 1992. Alternatives to Double-Hull Tank Vessel Design, Oil Pollution Act of 1990. Report to Congress, Washington, D.C.: U.S. Department of Transportation.
Representative terms from entire chapter: