APPENDIX K Comparative Study of DoubleHull and SingleHull Tankers^{1}
Introduction
Prior to 1990, most crude oil carriers were built with single hulls. Design, construction, and operational experience of doublehull tankers was limited primarily to product and parcel tankers under 40,000 tons deadweight. The stability and strength characteristics of doublehull crude oil carriers are quite different from singlehull tankers and product carriers, and designers and operators of doublehull tankers found themselves confronted with a new set of issues to consider.
This appendix examines the design characteristics of doublehull tankers built since 1990. Four of the areas in which doublehull tankers perform differently as compared to singlehull 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 singlehull and doublehull configurations have been investigated. Doublehull 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:
 tankers of 35,000 DWT50,000 DWT
 tankers of 80,000 DWT100,000 DWT
 tankers of 135,000 DWT160,000 DWT
 tankers of 265,000 DWT300,000 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 forwardmost cargo tank to the aft boundary of the aftmost 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 K1 shows crosssections of typical cargo tank arrangements for doublehull tankers. The "STA" or singletankacross 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 twotanksacross 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.

 Ballast tank arrangements. Figure K2 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 doublehull tanker built to OPA 90 requirements are protectively located. Many of the damage cases that would result in oil spillage on singlehull tankers will not penetrate the cargo tanks of doublehull tankers. Doublehull tankers will have fewer accidents involving oil spillage. The mean or expected oil outflow from a casualty will usually be less with a doublehull tanker as compared to a singlehull tanker of the same size.
The arrangements of doublehull 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
consequence, the likelihood of oil spillage and the mean or expected oil outflow will vary significantly even among doublehull tankers of the same size.
The International Maritime Organization (IMO) guidelines (1995) for evaluating alternatives to doublehull tankers have been applied in this report for assessing oil outflow performance. Although originally intended for evaluating alternatives to the doublehull concept, these guidelines are also well suited for comparing the outflow performance of singlehull and doublehull 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 DoubleHull 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
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 doublehull 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 singlehull 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
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 doublehull 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 K3 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 piecewise 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.
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 onehalf 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 hydrostaticbalance 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 doublehull 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)(P_{0}) + (0.4)(0.01+O_{MR}) + (0. 1) (0.025 +O_{ER}) P_{OR} 0.01 + O_{M} 0.025 + O_{E}
P_{0} = probability of zero outflow for the alternative design. O_{M} = mean oil outflow parameter for the alternative design = (mean outflow)/C. O_{E} = extreme oil outflow parameter for the alternative design = (extreme outflow)/C. C = total cargo oil onboard. P_{OR,} O_{MR,} and O_{ER} are the corresponding parameters for the reference doublehull design of the same cargo oil capacity.
The IMO reference double hulls are shown in Figure K4. 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/m^{3}. 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 singlehull 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.
Doublehull 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 singlehull tanker. Free surface effects may also be higher, as singletankacross arrangements of cargo tanks are not uncommon in doublehull 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 doublehull tankers.
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 nonsurvival 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.
The assessment of survivability is based on a comparison with the IMO regulation 25 (3) of Annex I of MARPOL 73/78. These limits are as follows:
 Equilibrium heel angle. Maximum 30 degrees, or 25 degrees if the deck edge is immersed.
 Righting arm. Maximum residual righting lever of at least 0. 1 meters.
 Range of positive stability. Range of positive stability beyond the equilibrium heel angle of at least 20 degrees.
 Progressive flooding. Down flooding points such as overflows and air pipes for all nonbreached compartments shall not be immersed at the equilibrium waterline.
An index of survivability has been determined by summing the probabilities for each damage case which satisfies these survival criteria. Typically, index values fall between 97 percent and 100 percent.
Intact Stability Evaluation
Singlehull tankers are inherently stable. The MARPOL regulations for hypothetical outflow, tank length, and damage stability dictate the tank size, and tend to encourage an arrangement of the longitudinal bulkheads such that wing tanks and center tanks have comparable widths. Furthermore, singlehull tankers built to MARPOL 73 and MARPOL 78 requirements typically have only two and four ballast tanks, respectively, within the cargo block. These ships have relatively small free surface effects, even when all cargo and ballast tanks are slack simultaneously. Since it is not possible to create an unstable situation for most singlehull tankers, IMO did not institute intact stability requirements for tankers.
In contrast, doublehull tankers have ballast tanks covering the entire cargo block. Structural and cost optimization under current MARPOL regulations tend to encourage larger tanks and a minimization of longitudinal bulkheads. For tankers under 120,000 tons deadweight, the lowcost solution is to have minimum wing tank widths (1 to 2 meters), with single cargo tanks spanning between wing tank bulkheads. The increase in the number of ballast tanks and the tendency towards wider ballast and cargo oil tanks means increased free surface effects, and a reduction in stability. This reduction in stability is exacerbated by the rise in the center of gravity of the cargo oil due to the double bottoms. As a result, some doublehull designs are unstable for certain combinations of ballast and cargo loading. There have been a number of incidents in the last few years in which tankers have become unstable during cargo operations. Although no tankers have capsized at the pier, angles of loll up to 15 degrees have been reported.
Principles of Intact Stability
The stability of a ship is influenced by a number of factors: the vertical center
of gravity of the ship, the free surface of liquids within tanks, and the righting moment developed as the vessel heels.
The vessel shown in Figure K5 (a) exhibits positive transverse stability. As the vessel heels, the center of buoyancy shifts from B to B 1. The buoyancy force acts upward through the center of buoyancy B 1, and the weight of the vessel acts downward through the vertical center of gravity G. The distance GZ is the righting arm. As the buoyancy force is tending to right the vessel, the ship is stable, and the righting arm GZ is positive.
The vessel in Figure K5 (b) illustrates the impact of the rise in the center of gravity on stability. The heeling moment has increased to where it now exceeds the buoyancy moment, and the vessel has negative stability. The weight force is now acting outboard of the buoyancy force, and the righting arm GZ is negative.
As the vessel heels, liquids in partially full tanks shift towards the low side. This moves G in the direction of heel, reducing the righting arm GZ. This phenomenon is called the free surface effect. For a rectangular tank, the free surface varies as the cube of the width of the tank.
Figure K6 shows typical tanker designs with different degrees of internal subdivision. When an oil tight centerline bulkhead is introduced into a doublehull tanker design, the free surface effect is reduced by a factor of four. That is, the combined free surface of the port and starboard tanks is onefourth of the free surface of the single tank. The threetankacross arrangement is typical of many of the small and midsize singlehull tankers. For a vessel with the proportions shown, the free surface effect is about oneseventh of the ''single tank across" arrangement.
For nonrectangular tanks, the free surface effect will vary with the level of the liquid in the tank. For instance, Figure K7 shows a ballast U tank which is 35 percent full with the water level at onehalf the double bottom height, and a tank in which the water level is increased so that the ballast extends into the wings. For the arrangement shown, the free surface effect changes by a factor of three. During cargo handling operations, relatively small changes in ballast can have a dramatic effect on the overall stability.
A plot of the GZ values provides a picture of the stability characteristics of a vessel (Figure K8). The stable vessel shown in Figure K8 (a) has a positive GZ though 60 degrees heel. The unstable vessel shown in Figure K8 (b) has a negative GZ and will capsize. Designers and operators often refer to the metacenter height, GM, for an indication of the stability of a vessel in its upright condition. The GM is equal to the slope of the GZ curve at zero degrees heel. The condition shown in Figure K8 (a) has a positive GM, whereas the condition shown in Figure K8 (b) has a negative GM.
It is possible for a vessel to be unstable in the upright condition, but attain positive stability as the vessel heels. This phenomenon is illustrated by the GZ plot shown in Figure K8 (c). The GM is negative and the vessel will tend to heel to one side. It will come to rest at the point when the GZ becomes positive, in this case at 15 degrees. This equilibrium heel angle is referred to as the "angle of loll." If the operator mistakenly assumes that the heel angle is caused by offcenter loads rather than negative stability, the operator may decide to add ballast or cargo to the uphill side. The vessel will then abruptly flop to the opposite side, generally assuming an even greater heel angle.
Methodology for Evaluating Intact Stability
For each design, the GM has been calculated for a matrix of load conditions. Uniform loading is assumed at a step size of 1 percent for both cargo tanks and ballast tanks. The free surface correction to GM for the tanks is based upon moment transference for 1 degree heel with 0.9 specific gravity cargo oil and 1.025 specific gravity ballast. Consumable and miscellaneous tanks, such as fuel oil and portable water, are about 50 percent full.
If none of the GM values is less than 0.15 meters, the vessel is assumed to be inherently stable. That is, the vessel will always remain stable regardless of the sequencing of ballast and cargo transfer operations.
If GM values less than 0.15 meters are possible, then the following additional conditions are evaluated:
 The extreme (worst case) load condition stability calculations are performed for the worst case scenario of ballast and cargo loading. Rather than applying free surface effects, liquid transference for each tank is computed at each heel angle. This provides a more accurate assessment of stability at large heel angles. From this calculation, it is determined whether the vessel has any risk of capsize. If the vessel cannot capsize, then the largest possible angle of loll is computed.
 The number of cargo tanks which can be partially full with all ballast tanks at 2 percent filling. The double bottom tanks generally have flat lower surfaces supported by a grillwork of floors and stiffeners, making it difficult to completely strip the tanks of ballast water. Two percent filling has been selected as a readily attainable level of stripping. All ballast tanks are set to 2 percent filling, and all cargo tanks to the level which minimizes GM.
 Even at 2 percent filling, the free surface effects can have a significant impact on stability. If a significant number of cargo tanks must be either empty or 98 percent full in order to maintain positive stability with all ballast tanks at 2 percent filling, then the operating restrictions become
 complicated and the risk of operator error increases. Therefore, this condition provides a good indication as to whether satisfactory stability can be maintained through reasonably simple operational restrictions.
 Evaluation of load restrictions to maintain positive stability. A load restriction that would assure positive stability throughout cargo handling operations is developed.
Evaluating Ballast Condition
The international requirements for doublehull tankers are contained in Regulation 13F of the 1992 Amendments to Annex I of MARPOL 73/78 (IMO, 1992). Minimum dimensions for wing tanks and double bottom tanks are specified. This regulation also states that wing tank and double bottom tanks used to meet the IMO ballast draft requirements "shall be located as uniformly as practicable along the cargo tank length."
This requirement tends to produce doublehull tankers with a relatively homogeneous longitudinal distribution of ballast. As compared to most MARPOL 78 tankers where ballast is concentrated closer to amidships, the doublehull tankers can be expected to have higher hogging moments.
In practice, most doublehull tankers are designed with double bottom and wing tank dimensions in excess of the minimum requirements. This is in response to a number of factors: the desire to provide better access into the ballast tanks for inspection and construction purposes, owner requirements to have deeper ballast drafts than the IMO minimum values, and for structural and oil outflow considerations.
Methodology for Evaluating Ballast Condition Longitudinal Strength and Drafts
The fore and aft drafts and the maximum stillwater bending moments and shear forces have been computed for the heavy ballast condition. Consumables such as fuel oil and fresh water have been assumed 50 percent full. When allocating ballast, an effort has been made to maximize the forward draft, subject to the following:
 For both the MARPOL tankers and the doublehull tankers, ballast is allocated to segregated ballast tanks only.
 Stillwater shear forces and bending moments are maintained within allowable values.
 At least 110 percent propeller immersion is maintained.
The drafts are presented as a percentage of the IMO minimum requirements and as a percentage of propeller immersion. Strength results are presented as a percentage of the allowable values assigned to the vessel by the classification
TABLE K1 Sizes and Hull Types of Tank Vessels Evaluated

Single Hull 
Double Side 
Double Hull 
35,00050,000 DWT tankers 
2 
1 
3 
80,000100,000 DWT tankers 
2 
1 
4 
135,000160,000 DWT tankers 
3 
— 
5 
265,000300,000 DWT tankers 
3 
— 
3 
5,00025,000 DWT barges 
5 
— 
4 
Total 
15 
2 
19 
society. For comparative purposes, the class assigned permissible stillwater bending moment amidships is presented as a percentage of the value obtained by computing a stillwater bending moment based on the minimum section modulus, permissible stresses, and assumed wave bending moments contained in Part 3, Section 6 of the American Bureau of Shipping Rules (ABS, 1995). These baseline values are referred to as the ABS standard values in this study.
Evaluating Design
Table K1 lists the hull types and numbers of vessels analyzed in this study. Designs have been selected to be representative of the ships and barges trading in U.S. waters. Singlehull tankers in each group include both preMARPOL (without segregated ballast tanks) and MARPOL 78 (with segregated ballast tanks in protective locations) vessels. A number of doubleside tankers are currently used for lightering services, and therefore a 40,000 DWT and an 85,000 DWT double side tanker have been evaluated. Doublehull tankers in the 35,000 DWT to 160,000 DWT range include vessels with singletankacross cargo tank arrangements, as well as vessels fitted with tight centerline bulkheads through the cargo block.
Oil outflow, survivability, intact stability, ballast draft, and strength evaluations have been carried out for each tanker. Oil outflow and survivability calculations have also been carried out for each barge.
Evaluating 35,000 DWT50,000 DWT Tankers
Design Characteristics
Tankers in this size range are often product carriers, with many of the designs having extensive internal subdivision to allow for carriage of a variety of cargoes and grades. Designs above 40,000 DWT generally have a breadth of about 32.2 meters, which is the maximum permitted for normal transit through the Panama Canal. Typical dimensions are as follows:
• 
Lbp 
168.0 m200.0 m 
• 
beam 
27.4 m32.2 m 
• 
depth 
14.8 m19.1 m 
• 
scantling draft 
10.9 m12.7 m 
The cargo blocks for singlehull tankers under 50,000 DWT have historically been arranged three tanks across, and five to eight tanks long. The doublehull tankers built since 1990 have been either one cargo tank across, two cargo tanks across, or a combination thereof. Typical arrangements are shown in Figure K9.
The singletankacross designs are usually arranged with seven to nine cargo tanks plus two slop tanks. The twotanksacross arrangement is generally constructed with twelve (6 × 2) to sixteen (8 × 2) cargo tanks plus two slop tanks.
Three doublehull tanker designs have been evaluated. Design #40D 1 has a singletankacross arrangement for all cargo oil tanks, and a combination of U and L ballast tanks. Design #40D2 and #40D3 are arranged with an oil tight centerline bulkhead fitted over the entire length of the cargo block, and L type ballast tanks. Design #40D2 has the highest degree of internal subdivision, with an 8 × 2 cargo tank arrangement.
Evaluating 80,000 DWT100,000 DWT Tankers
Design Characteristics
Typical dimensions for tankers in this size range are as follows:
• 
Lbp 
210.0 m242.0 m 
• 
beam 
38.8 m44.2 m 
• 
depth 
19.2 m23.2 m 
• 
scantling draft 
12.2 m16.6 m 
The cargo blocks for singlehull tankers between 75,000 DWT and 110,000 DWT have been typically arranged three tanks across, and four or five tanks long. Doubleside tankers, primarily used as shuttle tankers, generally have singletankacross cargo tank arrangements, and 4.5 to 6.0 meterwide wing tanks.
Most of the doublehull vessels between 75,000 DWT and 110,000 DWT are singletankacross designs, with seven to nine cargo tanks plus two slop tanks. Only a few tankers in this size range have been fitted with oil tight longitudinal bulkheads. Recent designs include arrangements with twelve (6 × 2), fourteen (7 × 2), and eighteen (6 × 3) cargo tanks plus slop tanks.
Four doublehull tanker designs have been evaluated. Design #80D1 and #80D2 have singletankacross arrangements for all cargo oil tanks. Design #80D3 is a hybrid with a combination of singletankacross and port and starboard cargo tanks. Design #80D4 has an oiltight centerline bulkhead fitted over the entire length of the cargo block. All four designs have L type ballast tanks over the entire length of the cargo block.
TABLE K2 Principal Particulars for 35,000 DWT50,000 DWT Tankers

#40S1 preMARPOL 
#40S2 MARPOL 78 
#40DS3 Double Sides 
#40D1 Double Hull 
#40D2 Double Hull 
#40D3 Double Hull 
Longitudinal Bulkhead in Cargo Tanks All 
All 
All 
None 
None 
All 
All 
Longitudinal Bulkhead in Ballast Tanks 



Some 
All 
All 
Deadweight (MTons) 
39,000 
36,000 
40,000 
47,000 
40,000 
46,000 
Length/Beam 
7.01 
5.53 
5.72 
5.40 
6.51 
5.41 
Length/Depth 
12.92 
10.37 
12.45 
9.67 
12.50 
9.08 
Beam/Depth 
1.84 
1.88 
2.18 
1.79 
1.92 
1.68 
Loadline Draft/Depth 
0.75 
.68 
.66 
.68 
.74 
.64 
Number of Longitudinal Bulkheads 
2 
15 
6 
8 
16 
14 
Number of Cargo Tanks (excl. slops) 
13 
15 
6 
8 
16 
14 
Number of Ballast Tanks 
7 
6 
10 
9 
22 
12 
Wing Tank Width/Required Width 
— 
— 
2.24 
1.13 
1.22 
1.00 
Wing Tank Width/Beam 
— 
— 
0.139 
0.070 
0.083 
0.062 
Double Bottom Height/Required Height 
— 
— 
— 
1.00 
1.09 
1.08 
Double Bottom Height/Depth 
— 
— 
— 
0.111 
0.140 
0.112 
Cargo Oil at 98% (m^{3}) 
44,000 
44,000 
47,000 
52,000 
43,000 
54,000 
Segregated Ballast (MTons) 
11,000 
13,000 
20,000 
23,000 
22,000 
20,000 
TABLE K3 Oil Outflow Evaluation for 35,000 DWT50,000 DWT Tankers

#40S1 preMARPOL 
#40S2 MARPOL 78 
#40DS3 Double Sides 
#40D1 Double Hull 
#40D2 Double Hull 
#40D3 Double Hull 
Side Damage 

Probability of zero outflow 
.31 
.54 
.84 
.85 
.82 
.85 
Mean outflow (m^{3}) 
2,180 
741 
771 
1,467 
618 
803 
Extreme (1/10) outflow (m^{3}) 
6,194 
4,571 
7,509 
12,166 
4,402 
6,739 
Combined Bottom Damage [40% 0m: 50% 2m: 10% 6m tide] 

Probability of zero outflow 
.13 
.10 
.12 
.83 
.84 
.84 
Mean outflow (m^{3}) 
1,817 
2,868 
4,156 
646 
397 
560 
Extreme (1/10) outflow (m^{3}) 
6,296 
8,885 
10,406 
5,987 
3,344 
4,914 
Combined Side and Bottom Damage [40% Side: 60% Bottom] 

Probability of zero outflow 
.20 
.28 
.41 
.84 
.83 
.84 
Mean outflow (m^{3}) 
1,962 
2,017 
2,802 
974 
485 
657 
Extreme (1/10) outflow (m^{3}) 
6,255 
7,160 
9,247 
8,458 
3,767 
5,644 
Pollution Prevention Index 

98% cargo volume (m^{3}) 
41,433 
37,929 
43,017 
50,708 
42,764 
50,467 
Index E 
.36 
.38 
.43 
.91 
1.06 
1.02 
TABLE K4 Survivability Evaluation for 35,000 DWT50,000 DWT Tankers

#40S1 preMARPOL 
#40S2 MARPOL 78 
#40DS3 Double Sides 
#40D1 Double Hull 
#40D2 Double Hull 
#40D3 Double Hull 
Side Damage Survivability Index 
92.5% 
97.5% 
99.2% 
87.2% 
100.0% 
97.1% 
TABLE K5 Intact Stability Evaluation for 35,000 DWT50,000 DWT Tankers
TABLE K6 Ballast Condition Evaluation for 35,000 DWT50,000 DWT Tankers

#40S1 preMARPOL 
#40S2 MARPOL 78 
#40DS3 Double Sides 
#40D1 Double Hull 
#40D2 Double Hull 
#40D3 Double Hull 
IMO Draft Requirements 

Minimum draft forward (m) 
4.4 
4.1 
4.3 
4.2 
4.4 
4.2 
Heavy Ballast Condition 

Number of cargo oil tanks used 
3 
3 
none 
none 
none 
none 
Draft aft (m) 
8.0 

8.0 
7.6 
8.5 
8.9 
Propeller immersion 
117% 
141% 
124% 
138% 
143% 
121% 
Draft forward (m) 
5.0 
4.5 
4.4 
6.3 
6.6 
6.9 
Draft forward as % of IMO required 
113% 
110% 
102% 
151% 
150% 
165% 
Allowable Bending Moment (Hog) 

As % of ABS minimum value 
98% 
123% 
97% 
100% 
118% 
153% 
Heavy Ballast Condition 

Maximum bending moment 
39% 
75% 
54% 
98% 
96% 
56% 
Maximum shear force 
43% 
30% 
24% 
82% 
40% 
11% 
TABLE K7 Principal Particulars for 80,000 DWT100,000 DWT Tankers

#80S1 preMARPOL 
#80S2 MARPOL 78 
#80DS3 Double Sides 
#80D1 Double Hull 
#80D2 Double Hull 
#80D3 Double Hull 
#80D4 Double Hull 
Longitudinal Bulkhead in Cargo Tanks 
All 
All 
None 
None 
None 
Some 
All 
Longitudinal Bulkhead in Ballast Tanks 



All 
All 
All 
All 
Deadweight (MTons) 
97,000 
81,000 
85,000 
97,000 
94,000 
96,000 
96,000 
Length/Beam 
5.75 
5.93 
5.09 
5.57 
5.57 
5.60 
5.57 
Length/Depth 
11.62 
12.46 
12.38 
11.65 
12.00 
12.05 
11.65 
Beam/Depth 
2.02 
2.10 
2.43 
2.09 
2.15 
2.15 
2.09 
Loadline Draft/Depth 
0.75 
0.64 
0.67 
0.68 
0.70 
0.69 
0.69 
Number of Longitudinal Bulkheads 
2 
2 
2 
2 
2 
3 
3 
Number of Cargo Tanks (excl. slops) 
12 
11 
7 
7 
7 
10 
12 
Number of Ballast Tanks 
4 
8 
10 
10 
12 
13 
14 
Wing Tank Width/Required Width 
— 
— 
3.00 
1.00 
1.35 
1.45 
1.23 
Wing Tank Width/Beam 
— 
— 
0.136 
0.048 
0.064 
0.069 
0.059 
Double Bottom Height/Required Height 
— 
— 
— 
1.05 
1.05 
1.05 
1.23 
Double Bottom Height/Depth 
— 
— 
— 
0.105 
0.108 
0.108 
0.123 
Cargo Oil at 98% (m^{3}) 
119,000 
100,000 
98,000 
108,000 
106,000 
108,000 
108,000 
Segregated Ballast (MTons) 
13,000 
38,000 
40,000 
41,000 
40,000 
39,000 
42,000 
TABLE K8 Oil Outflow Evaluation for 80,000 DWT100,000 DWT Tankers

#80S1 preMARPOL 
#80S2 MARPOL 78 
#80DS3 Double Sides 
#80D1 Double Hull 
#80D2 Double Hull 
#80D3 Double Hull 
#80D4 Double Hull 
Side Damage 

Probability of zero outflow 
.22 
.32 
.92 
.81 
.85 
.87 
.83 
Mean outflow (m^{3}) 
9,508 
6,180 
1,411 
4,369 
3.170 
2,253 
2,013 
Extreme (1/10) outflow (m^{3}) 
20,750 
13,311 
14,105 
30,634 
25,586 
19,988 
15,071 
Combined Bottom Damage [40% 0m: 50% 2m: 10% 6m tide] 

Probability of zero outflow 
.09 
.09 
.11 
.82 
.81 
.80 
.82 
Mean outflow (m^{3}) 
4.564 
6,814 
8.939 
1,485 
1.706 
1,497 
1,093 
Extreme (1/10) outflow (m^{3}) 
13.813 
17,097 
24,155 
12,726 
13,348 
11,524 
8,828 
Combined Side and Bottom Damage [40% Side: 60% Bottom] 

Probability of zero outflow 
.14 
.19 
.44 
.82 
.82 
.83 
.83 
Mean outflow (m^{3}) 
6,542 
6,560 
5,928 
2,639 
2,292 
1,799 
1,461 
Extreme (1/10) outflow (m^{3}) 
16.588 
15,583 
20,135 
19,889 
18,243 
14,910 
11,325 
Pollution Prevention Index 

98% cargo volume (m^{3}) 
104,749 
81.227 
92,276 
105,826 
102,081 
104,169 
105,269 
Index E 
.29 
.28 
.46 
.85 
.88 
.96 
1.03 
TABLE K9 Survivability Evaluation for 80,000 DWT100,000 DWT Tankers

#80S1 preMARPOL 
#80S2 MARPOL 78 
#80DS3 Double Sides 
#80D1 Double Hull 
#80D2 Double Hull 
#80D3 Double Hull 
#80D4 Double Hull 
Side Damage Survivability Index 
99.2% 
100.0% 
100.0% 
99.2% 
99.7% 
99.9% 
99.9% 
TABLE K10 Intact Stability Evaluation for 80,000 DWT100,000 DWT Tankers
TABLE K11 Ballast Condition Evaluation for 80,000 DWT100,000 DWT Tankers

#80S1 preMARPOL 
#80S2 MARPOL 78 
#80DS3 Double Sides 
#80D1 Double Hull 
#80D2 Double Hull 
#80D3 Double Hull 
#80D4 Double Hull 
IMO Draft Requirements 

Minimum draft forward (m) 
4.9 
5.0 
4.8 
4.9 
4.9 
4.9 
4.9 
Heavy Ballast Condition 

Number of cargo oil tanks used 
3 
3 
none 
none 
none 
none 
none 
Draft aft (m) 
9.4 
8.7 
8.3 
8.3 
9.1 
8.9 
8.9 
Propeller immersion 
124% 
114% 
112% 
117% 
113% 
113% 
127% 
Draft forward (m) 
6.3 
5.7 
6.3 
6.1 
6.0 
5.9 
6.0 
Draft forward as % of IMO required 
129% 
115% 
131% 
124% 
122% 
119% 
123% 
Allowable Bending Moment (Hog) 

As % of ABS minimum value 
106% 
91% 
96% 
91% 
98% 
120% 
91% 
Heavy Ballast Condition 

Maximum bending moment 
37% 
77% 
84% 
100% 
100% 
86% 
100% 
Maximum shear force 
3% 
61% 
64% 
67% 
67% 
35% 
62% 
Evaluating 135,000 DWT160,000 DWT Tankers
Design Characteristics
Tankers in this size range are generally designed to the maximum proportions suitable for passage through the Suez Canal. Typical dimensions are as follows:
• 
lbp 
258.0 m265.0 m 
• 
beam 
43.0 m50.0 m 
• 
depth 
22.8 m25.8 m 
• 
scantling draft 
15.2 m17.2 m 
The cargo blocks for singlehull SUEZMAX tankers have historically been arranged three tanks across, and five or six tanks long. The doublehull SUEZMAX tankers built since 1990 have been either one cargo tank across, two cargo tanks across, or a combination thereof. Typical arrangements are shown in Figure K11.
The singletankacross designs are usually arranged with nine cargo tanks plus two slop tanks, which is the maximum tank size meeting the IMO tank size and outflow requirements as defined in Regulations 2224 of Annex I to MARPOL 73/78.
The twotanksacross arrangement is generally constructed with ten (5 × 2) or twelve (6 × 2) cargo tanks plus two slop tanks. Although the 5 × 2 arrangement satisfies IMO requirements, damage stability requirements impose some operating restrictions with regard to deep draft conditions with partially full cargo tanks. This, together with considerations for greater segregation of cargoes, has led many shipowners to opt for the 6 × 2 cargo tank arrangement.
Five doublehull tanker designs have been evaluated. Design # 150D1 has a singletankacross arrangement for all cargo oil tanks. Design #150D2 is a hybrid, with four singletankacross cargo tanks and three pairs of port and starboard cargo tanks. Designs #150D3 through #150D5 all have an oiltight centerline bulkhead fitted over the entire length of the cargo block. Design # 150D5 has relatively wide wing tanks and a deep double bottom. In order for design #150D5 to meet the IMO two compartment and raking bottom damage stability requirements, approximately 60 percent of the ballast capacity within the cargo block length is arranged in U tanks.
TABLE K12 Principal Particulars for 135,000 DWT160,000 DWT Tankers

#150S1 preMARPOL 
#150S2 MARPOL 78 
#150S3 MARPOL 78 
#150D1 Double Hull 
#150D2 Double Hull 
#150D3 Double Hull 
#150D4 Double Hull 
#150D5 Double Hull 
Longitudinal Bulkhead in Cargo Tanks 
All 
All 
All 
None 
Some 
All 
All 
All 
Longitudinal Bulkhead in Ballast Tanks 



All 
Some 
All 
All 
Some 
Deadweight (MTons) 
156,000 
149,000 
155,000 
151,000 
136,000 
150,000 
150,000 
157,000 
Length/Beam 
5.00 
5.78 
5.22 
5.50 
5.08 
5.61 
5.52 
5.22 
Length/Depth 
13.40 
11.81 
10.40 
11.48 
9.75 
10.79 
11.58 
10.40 
Beam/Depth 
2.68 
2.04 
1.99 
2.09 
1.92 
1.92 
2.10 
1.99 
Loadline Draft/Depth 
0.77 
0.66 
0.67 
0.70 
0.67 
0.71 
0.70 
0.69 
Number of Longitudinal Bulkheads 
2 
2 
2 
2 
3 
3 
3 
3 
Number of Cargo Tanks (excl. slops) 
12 
9 
11 
9 
10 
12 
12 
12 
Number of Ballast Tanks 
4 
8 
6 
12 
16 
14 
14 
15 
Wing Tank Width/Required Width 
— 
— 
— 
1.15 
1.88 
1.35 
1.28 
1.67 
Wing Tank Width/Beam 
— 
— 
— 
0.048 
0.078 
0.059 
0.053 
0.067 
Double Bottom Height/Required Height 
— 
— 
— 
1.25 
1.65 
1.40 
1.40 
1.67 
Double Bottom Height/Depth 
— 
— 
— 
0.109 
0.131 
0.117 
0.123 
0.133 
Cargo Oil at 98% (m^{3}) 
187,000 
177,000 
180,000 
165,000 
159,000 
164,000 
167,000 
179,000 
Segregated Ballast (MTons) 
24,000 
64,000 
66,000 
59,000 
63,000 
56,000 
57,000 
67.000 
TABLE K13 Oil Outflow Evaluation for 135,000 DWT160,000 DWT Tankers

#150S1 preMARPOL 
#150S2 MARPOL 78 
#150S3 MARPOL 78 
#150D1 Double Hull 
#150D2 Double Hull 
#150D3 Double Hull 
#150D4 Double Hull 
#150D5 Double Hull 
Side Damage 

Probability of zero outflow 
.24 
.40 
.34 
.79 
.87 
.82 
.80 
.81 
Mean outflow (m^{3}) 
10,868 
9,175 
8,404 
6,436 
3,674 
3,182 
3,481 
3,252 
Extreme (1/10) outflow (m^{3}) 
27,712 
20,406 
18,161 
43,106 
30,789 
23.699 
23,837 
23,524 
Combined Bottom Damage [40% Om: 50% 2m: 10% 6m tide] 

Probability of zero outflow 
.08 
.08 
.08 
.80 
.83 
.82 
.82 
.83 
Mean outflow (m^{3}) 
7,965 
11.600 
11.725 
2,652 
2,009 
1,663 
1,605 
1.520 
Extreme (1/10) outflow (m^{3}) 
20,945 
29,795 
29.367 
21,183 
16,049 
13.060 
12,638 
12,623 
Combined Side and Bottom Damage [40% Side: 60% Bottom] 

Probability of zero outflow 
.14 
.21 
.18 
.80 
.84 
.82 
.81 
.82 
Mean outflow (m^{3}) 
9.126 
10,630 
10.396 
4.166 
2,675 
2,271 
2.355 
2,213 
Extreme (1/10) outflow (m^{3}) 
23,652 
26,039 
24,885 
29,952 
21.945 
17,316 
17,118 
16.983 
Pollution Prevention Index 

98% cargo volume (m^{3}) 
186,692 
162,063 
168.926 
164,895 
148,206 
163,619 
162,225 
170,961 
Index E 
.36 
.34 
.34 
.87 
.99 
1.06 
1.04 
1.09 
TABLE K14 Survivability Evaluation for 135,000 DWT160,000 DWT Tankers

#150S1 preMARPOL 
#150S2 MARPOL 78 
#150S3 MARPOL 78 
#150D1 Double Hull 
#150D2 Double Hull 
#150D3 Double Hull 
#150D4 Double Hull 
#150D4 Double Hull 
Side Damage Survivability Index 
99.9% 
100.0% 
100.0% 
99.8% 
99.2% 
99.9% 
100.0% 
99.5% 
TABLE K15 Intact Stability Evaluation for 135,000 DWT160,000 DWT Tankers
TABLE K16 Ballast Condition Evaluation for 135.000 DWT160,000 DWT Tankers

#150S1 preMARPOL 
#150S2 MARPOL 78 
#150S3 MARPOL 78 
#150D1 Double Hull 
#150D2 Double Hull 
#150D3 Double Hull 
#150D4 Double Hull 
#150D5 Double Hull 
IMO Draft Requirements 

Minimum draft forward (m) 
5.4 
5.5 
5.3 
5.3 
5.1 
5.2 
5.3 
5.3 
Heavy Ballast Condition 

Number of cargo oil tanks used 
3 
none 
none 
none 
none 
none 
none 
none 
Draft sft (m) 
11.5 
10.3 
10.2 
9.0 
11.0 
9.9 
7.9 
10.9 
Propeller immersion 
147% 
126% 
126% 
110% 
139% 
115% 
106% 
135% 
Draft forward (m) 
6.9 
6.9 
8.3 
6.9 
9.2 
7.1 
7.3 
8.2 
Draft forward as % of IMO required 
130% 
126% 
158% 
130% 
181% 
135% 
138% 
156% 
Allowable Bending Moment (Hog) 

As % of ABS minimum value 
94% 
89% 
83% 
102% 
109% 
127% 
94% 
99% 
Heavy Ballast Condition 

Maximum bending moment 
85% 
100% 
56% 
100% 
100% 
100% 
100% 
100% 
Maximum shear force 
60% 
89% 
69% 
53% 
56% 
90% 
48% 
53% 
Evaluating 265,000 DWT300,000 DWT Tankers
Design Characteristics
Typical dimensions for VLCCs are as follows:
• 
Lbp 
315.0 m326.0 m 
• 
beam 
53.0 m68.0 m 
• 
depth 
26.0 m32.0 m 
• 
scantling draft 
19.0 m23.0 m 
A majority of the singlehull VLCCs have a 5 long × 3 wide cargo tank arrangement. The preMARPOL designs typically have one or two ballast tanks within the cargo block, whereas the MARPOL 78 designs usually have wing ballast tanks port and starboard at the No.2 and No.4 positions. Variations include a few tankers with 4 × 3 cargo tank arrangements at the lower end of the size range, and some vessels with 6 × 3 cargo tank arrangements.
Most of the doublehull designs built since 1990 are arranged with 5 × 3 cargo tanks plus slop tanks. The double bottom depth is typically about 3 meters, and the wing tank widths vary from 3 to 4 meters. A typical arrangement is shown in Figure K12.
Three doublehull tanker designs have been evaluated. All three have a 5 × 3 cargo tank arrangement. Design #280D1 has all L ballast tanks, design #280D2 has predominantly L ballast tanks with one U tank. Design #280D3 has predominantly fullbreadth double bottom ballast tanks with independent side tanks port and starboard, together with midship ballast tanks arranged inboard of the longitudinal bulkheads.
TABLE K17 Principal Particulars for 265,000 DWT300,000 DWT Tankers

#280S1 preMARPOL 
#280S2 MARPOL 78 
#280S3 MARPOL 78 
#280D1 Double Hull 
#280D2 Double Hull 
#280D3 Double Hull 
Longitudinal Bulkhead in Cargo Tanks 
All 
All 
All 
All 
All 
All 
Longitudinal Bulkhead in Ballast Tanks 



All 
Some 
None 
Deadweight (MTons) 
277,000 
268,000 
292,000 
280,000 
300,000 
298,000 
Length/Beam 
5.87 
5.97 
5.50 
5.47 
5.52 
5.37 
Length/Depth 
12.31 
12.12 
10.13 
10.10 
10.32 
10.06 
Beam/Depth 
2.10 
2.03 
1.84 
1.85 
1.87 
1.87 
Loadline Draft/Depth 
0.81 
0.78 
0.69 
0.66 
0.71 
0.70 
Number of Longitudinal Bulkheads 
2 
2 
2 
4 
4 
4 
Number of Cargo Tanks (excl. slops) 
16 
12 
13 
15 
15 
15 
Number of Ballast Tanks 
3 
6 
6 
13 
14 
17 
Wing Tank Width/Required Width 
— 
— 
— 
1.97 
3.52 
3.15 
Wing Tank Width/Beam 
— 
— 
— 
0.068 
0.061 
0.053 
Double Bottom Height/Required Height 
— 
— 
— 
1.55 
1.50 
1.60 
Double Bottom Height/Depth 
— 
— 
— 
0.099 
0.097 
0.101 
Cargo Oil at 98% (m^{3}) 
309,000 
314,000 
338,000 
343,000 
338,000 
244,000 
Segregated Ballast (MTons) 
37,000 
41,000 
114,000 
108,000 
102,000 
111,000 
TABLE K18 Oil Outflow Evaluation for 265,000 DWT300,000 DWT Tankers

#280S1 preMARPOL 
#280S2 MARPOL 78 
#280S3 MARPOL 78 
#280D1 Double Hull 
#280D2 Double Hull 
#280D3 Double Hull 
Side Damage 

Probability of zero outflow 
.15 
.19 
.33 
.82 
.81 
.72 
Mean outflow (m^{3}) 
23,448 
21,617 
15,342 
4,778 
4,474 
6,134 
Extreme (1/10) outflow (m^{3}) 
56,162 
49,224 
35.107 
34,226 
31,393 
35,325 
Combined Bottom Damage [40% 0m: 50% 2m: 10% 6m tide] 

Probability of zero outflow 
.08 
.07 
.07 
.80 
.79 
.81 
Mean outflow (m^{3}) 
9,392 
10,668 
15,956 
2,949 
2,420 
2,704 
Extreme (1/10) outflow (m^{3}) 
26,254 
27,190 
50,624 
22,731 
19,283 
23,039 
Combined Side and Bottom Damage [40% Side: 60% Bottom] 

Probability of zero outflow 
.11 
.12 
.17 
.81 
.80 
.78 
Mean outflow (m^{3}) 
15,014 
15,047 
15,710 
3,681 
3,242 
4,076 
Extreme (1/10) outflow (m^{3}) 
38,217 
36,003 
44,417 
27,329 
24,127 
27,953 
Pollution Prevention Index 

98% cargo volume (m^{3}) 
299,561 
267,062 
319,218 
306,572 
328,112 
325,272 
Index E 
.30 
.28 
.33 
1.04 
1.09 
1.01 
TABLE K19 Survivability Evaluation for 265,000 DWT300,000 DWT Tankers

#280S1 preMARPOL 
#280S2 MARPOL 78 
#280S3 MARPOL 78 
#280D1 Double Hull 
#280D2 Double Hull 
#280D3 Double Hull 
Side Damage Survivability Index 
100.0% 
100.0% 
99.7% 
100.0% 
100.0% 
100.0% 
TABLE K20 Intact Stability Evaluation for 265,000 DWT300,000 DWT Tankers

#280S1 preMARPOL 
#280S2 MARPOL 78 
#280S3 MARPOL 78 
#280D1 Double Hull 
#280D2 Double Hull 
#280D3 Double Hull 
Minimum GMt (m) 
6.94 
7.40 
6.76 
2.40 
4.08 
0.15 
SWB (% capacity) 
2% 
2% 
2 
9% 
5% 
39% 
Cargo oil (% capacity) 
80% 
97% 
98% 
98% 
98% 
98% 
Minimum GMt w/ 2% SWB (m) 
6.94 
7.4 
6.76 
4.05 
4.84 
5.30 
Possibility of Capsize 
none 
none 
none 
none 
none 
none 
Maximum Angle of Loll 
none 
none 
none 
none 
none 
none 
Load restrictions 
none 
none 
none 
none 
none 
none 
TABLE K21 Ballast Condition Evaluation for 265,000 DWT300,000 DWT Tankers

#280S1 preMARPOL 
#280S2 MARPOL 78 
#280S3 MARPOL 78 
#280D1 Double Hull 
#280D2 Double Hull 
#280D3 Double Hull 
IMO Draft Requirements 

Minimum draft forward (m) 
6.0 
6.0 
6.0 
6.0 
6.0 
6.0 
Heavy Ballast Condition 

Number of cargo oil tanks used 
3 
3 
none 
none 
none 
none 
Draft aft (m) 
11.6 
10.7 
12.2 
12.5 
12.9 
13.1 
Propeller immersion 
110% 
104% 
111% 
119% 
129% 
126% 
Draft forward (m) 
8.6 
8.4 
9.6 
8.4 
7.9 
9.4 
Draft forward as % of IMO required 
144% 
140% 
161% 
141% 
132% 
158% 
Allowable Bending Moment (Hog) 

As % of ABS minimum value 
86% 
87% 
106% 
105% 
106% 
119% 
Heavy Ballast Condition 

Maximum bending moment 
70% 
90% 
59% 
100% 
100% 
90% 
Maximum shear force 
64% 
85% 
100% 
53% 
79% 
57% 
Oceangoing Barges
Design Characteristics
Oceangoing barges operating in U.S. waters tend to be smaller than tankers, with few barges exceeding 25,000 DWT. Barges are subject to less stringent loadline requirements than selfpropelled tank ships, and will generally have a lower freeboard. When barges are carrying lighter crudes and products, it is not usual for the cargo oil to be in hydrostatic balance relative to the sea.
Singlehull barges above 5,000 DWT are generally arranged with one and sometimes two longitudinal bulkheads. The cargo tank arrangement will vary depending on the extent of cargo segregation required. Common arrangements include (4 × 2) up to (8 × 2) cargo tanks, with a few vessels featuring threewide cargo tank configurations.
Barges may be of the flush deck type, or fitted with a raised trunk as shown in Figure K13. Voids are arranged fore and aft within the rake. Oceangoing barges are generally pushed or pulled by tugs, and are often constructed with a notch aft. Ballast tanks may be of the L or U type. They are generally left as void spaces.
Four doublehull tank barges have been evaluated. Designs #B35D1, B90D1, and B90D2 are new barges constructed in the last five years. Design #B179D1 is a proposed conversion of #B179S1, an existing 23,700 DWT singlehull barge.
TABLE K22 Principal Particulars for Oceangoing Barges

#B35S1 Single Hull 
#B35D1 Double Hull 
#B90S1 Single Hull 
#B90S2 Single Hull 
#B90S3 Single Hull 
#B90D1 Double Hull 
#B90D2 Double Hull 
#B 170S1 Single Hull 
#B170D1 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% (m^{3}) 
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 
TABLE K23 Oil Outflow Evaluation for Oceangoing Barges

#B35S1 Single Hull 
#B35D1 Double Hull 
#B90S1 Single Hull 
#B90S2 Single Hull 
#B90S3 Single Hull 
#B90D1 Double Hull 
#B90D2 Double Hull 
#B170S1 Single Hull 
#B170D1 Double Hull 
Side Damage 

Probability of zero outflow 
.24 
.87 
.19 
.19 
.03 
.80 
.85 
.12 
.87 
Mean outflow (m^{3}) 
727 
113 
1,024 
1,558 
1,580 
334 
229 
2,666 
352 
Extreme (1/10) outflow (m^{3}) 
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 (m^{3}) 
581 
135 
1,046 
1,040 
648 
140 
152 
1,249 
284 
Extreme (1/10) outflow (m^{3}) 
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 (m^{3}) 
639 
126 
1,037 
1,247 
1,021 
217 
183 
1,816 
311 
Extreme (1/10) outflow (m^{3}) 
1,645 
942 
2,561 
2,882 
2,226 
1,673 
1,568 
4,069 
2,852 
Pollution Prevention Index 

98% cargo volume (m^{3}) 
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 K24 Survivability Evaluation for Oceangoing Barges

#B35S1 Single Hull 
#B35D1 Double Hull 
#B90S1 Single Hull 
#B90S2 Single Hull 
#B90S3 Single Hull 
#B90D1 Double Hull 
#B90D2 Double Hull 
#B170S1 Single Hull 
#B170D1 Double Hull 
Side Damage Survivability Index 
95.0% 
96.7% 
92.9% 
92.9% 
99.7% 
99.5% 
99.9% 
99.0% 
95.0% 
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, doublehull tankers perform significantly better than singlehull tankers, as the protective double skin reduces the number of casualties that penetrate into the cargo tanks. As shown in Figure K14, the probability of zero outflow is four to six times higher for doublehull tankers, indicating singlehull 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 singlehull vessels, with doublehull mean outflow values averaging onethird to onefourth of the singlehull values.
The doubleside vessels (#40DS3 and #80DS3) perform reasonably well with respect to collisions, but have higher outflows for bottom damage. These vessels have singletankacross arrangements for cargo tanks, which significantly
increase outflow as compared to the more extensive cargo tank subdivision incorporated into the preMARPOL and MARPOL 78 designs. The light line on Figure K 15 represents a curvefit of the singlehull mean outflow data. We find that the two doubleside vessels evaluated in this study fall slightly above this trend line, indicating these doubleside vessels will have comparable outflow volumes to the typical singlehull vessel. For doubleside vessels with oiltight longitudinal bulkheads, improved performance as compared to single hulls can be expected.
Mean outflow is influenced by the doublehull dimensions as well as the extent of internal subdivision within the cargo tanks. There is little variation in the arrangement of VLCCs, with most singlehull and doublehull 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 K16 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 #150D3, #150D4, and #150D5, all of which have an oiltight centerline bulkhead over the length of the cargo block. Design #150D1, with all singletankacross cargo tanks, has the highest mean outflow. Design #150D2 has an oiltight centerline bulkhead arranged over about 40 percent of the cargo block, with singletankacross cargo tanks arranged elsewhere. It is interesting
to note that the bottom damage outflow are relatively consistent, but the singletankacross 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 K17, doublehull tankers without centerline bulkheads typically have twice the expected outflow of designs with oiltight longitudinal bulkheads in way of the cargo block.
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 K18. 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 doublehull vessels with singletankacross arrangements performed more poorly than both preMARPOL 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 K19 below. Singlehull tanker values generally fall between 0.3 and 0.4, whereas doublehull tanker values lie between .9 and 1.1. Sixty percent (9 of 15) of the doublehull 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 #150D2, 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 singletankacross cargo tanks, the detrimental effect of these tanks is offset by the contributions from the relatively wide wing tanks and deep double bottom tanks.
Observations on the Survivability of Tankers
There is no discernible difference between survivability characteristics of singlehull and doublehull 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 singlehull and doublehull 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 singlehull 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 doublehull vessels, 73 percent (1 of 15) were inherently stable. The designs which have the potential of instability (#40D1, #80D1, #150D1, and #150D2) all have singletankacross cargo tanks.
Designs #80D1, #150D1 and #150D2 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
2 percent full, the designs maintain positive stability through all possible cargo load conditions.
Design #40D1 incorporates a singletankacross 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 counterbalancing. 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 doublehull 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 doublehull designs evaluated have stillwater bending moments in the ballast condition approaching the maximum permissible value assigned by the classification society. Exceptions are designs #40D3 and #280D3. Design #40D3 has scantlings and consequently a permissible stillwater bending moment significantly above rule requirements. Design #280D3 has additional hull girder strength and deep ballast tanks located in the midships region.
As shown in Table K25, the average doublehull design has a permissible stillwater bending moment 9 percent in excess of the ABS standard value. This is 13 percent above the average for singlehull vessels analyzed. It should be recognized, however, that rule requirements for longitudinal strength have been liberalized since many of the singlehull 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 K25 Allowable StillWater Bending Moments as a Percentage of the ABS Standard Value

Single Hull 
Double Hull 
35,00050,000 DWT Tankers 
106 
124 
80,000100,000 DWT Tankers 
98 
100 
135,000160,000 DWT Tankers 
89 
106 
265,000300,000 DWT Tankers 
93 
110 
Average (all tankers) 
96 
109 
Observations on the Oil Outflow Analysis and Survivability Analysis for Barges
Parallel to the findings for tankers, doublehulled 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 doublehull 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.
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 overpredict the amount of expected outflow.
 The historical casualty data primarily involve older, singlehull vessels. It is expected that extents of damage will be somewhat less for doublehull vessels.
The 19 doublehull vessels analyzed in this study represent about 5 percent of the doublehull tanker fleet operating today. Efforts were made to select representative vessels. However, there are some doublehull vessels built for specific trades which have quite different characteristics as compared to these representative vessels.
REFERENCES
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