National Academies Press: OpenBook
« Previous: Appendix J
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
  • 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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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-

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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

Single-hull 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, single-hull 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 single-hull tankers, IMO did not institute intact stability requirements for tankers.

In contrast, double-hull 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 low-cost 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 double-hull 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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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 K-5 (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 K-5 (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 K-6 shows typical tanker designs with different degrees of internal subdivision. When an oil tight centerline bulkhead is introduced into a double-hull 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 one-fourth of the free surface of the single tank. The three-tank-across arrangement is typical of many of the small and mid-size single-hull tankers. For a vessel with the proportions shown, the free surface effect is about one-seventh 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 K-7 shows a ballast U tank which is 35 percent full with the water level at one-half 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.

FIGURE K-5 Variation in intact stability. (a) Positive stability. (b) Negative stability.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

FIGURE K-6 Effect of levels of internal subdivision on free surface effect. (a) Single tank across, free surface effect = 1. (b) With centerline bulkhead, free surface effect = 1/4. (c) Three tanks across, free surface effect = 1/7.

A plot of the GZ values provides a picture of the stability characteristics of a vessel (Figure K-8). The stable vessel shown in Figure K-8 (a) has a positive GZ though 60 degrees heel. The unstable vessel shown in Figure K-8 (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 K-8 (a) has a positive GM, whereas the condition shown in Figure K-8 (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 K-8 (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 off-center 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.

FIGURE K-7 Effect of levels of liquid in tanks on free surface effect. (a) U Tank 35 percent full, free surface effect = 1. (b) U Tank 60 percent full, free surface effect = 1/3.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

FIGURE K-8 Stability characteristics of a vessel. (a) Vessel with positive stability. (b) Negative stability leading to capsize. (c) Negative stability leading to an angle of loll.

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
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
  • 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 double-hull 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 double-hull tankers with a relatively homogeneous longitudinal distribution of ballast. As compared to most MARPOL 78 tankers where ballast is concentrated closer to amidships, the double-hull tankers can be expected to have higher hogging moments.

In practice, most double-hull 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 still-water 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 double-hull tankers, ballast is allocated to segregated ballast tanks only.
  • Still-water 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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-1 Sizes and Hull Types of Tank Vessels Evaluated

 

Single Hull

Double Side

Double Hull

35,000-50,000 DWT tankers

2

1

3

80,000-100,000 DWT tankers

2

1

4

135,000-160,000 DWT tankers

3

5

265,000-300,000 DWT tankers

3

3

5,000-25,000 DWT barges

5

4

Total

15

2

19

society. For comparative purposes, the class assigned permissible still-water bending moment amidships is presented as a percentage of the value obtained by computing a still-water 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 K-1 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. Single-hull tankers in each group include both pre-MARPOL (without segregated ballast tanks) and MARPOL 78 (with segregated ballast tanks in protective locations) vessels. A number of double-side tankers are currently used for lightering services, and therefore a 40,000 DWT and an 85,000 DWT double side tanker have been evaluated. Double-hull tankers in the 35,000 DWT to 160,000 DWT range include vessels with single-tank-across 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 DWT-50,000 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:

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

Lbp

168.0 m-200.0 m

beam

27.4 m-32.2 m

depth

14.8 m-19.1 m

scantling draft

10.9 m-12.7 m

The cargo blocks for single-hull tankers under 50,000 DWT have historically been arranged three tanks across, and five to eight tanks long. The double-hull 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 K-9.

The single-tank-across designs are usually arranged with seven to nine cargo tanks plus two slop tanks. The two-tanks-across arrangement is generally constructed with twelve (6 × 2) to sixteen (8 × 2) cargo tanks plus two slop tanks.

Three double-hull tanker designs have been evaluated. Design #40-D 1 has a single-tank-across arrangement for all cargo oil tanks, and a combination of U and L ballast tanks. Design #40-D2 and #40-D3 are arranged with an oil tight centerline bulkhead fitted over the entire length of the cargo block, and L type ballast tanks. Design #40-D2 has the highest degree of internal subdivision, with an 8 × 2 cargo tank arrangement.

Evaluating 80,000 DWT-100,000 DWT Tankers
Design Characteristics

Typical dimensions for tankers in this size range are as follows:

Lbp

210.0 m-242.0 m

beam

38.8 m-44.2 m

depth

19.2 m-23.2 m

scantling draft

12.2 m-16.6 m

The cargo blocks for single-hull tankers between 75,000 DWT and 110,000 DWT have been typically arranged three tanks across, and four or five tanks long. Double-side tankers, primarily used as shuttle tankers, generally have single-tank-across cargo tank arrangements, and 4.5 to 6.0 meter-wide wing tanks.

Most of the double-hull vessels between 75,000 DWT and 110,000 DWT are single-tank-across 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 double-hull tanker designs have been evaluated. Design #80-D1 and #80-D2 have single-tank-across arrangements for all cargo oil tanks. Design #80D3 is a hybrid with a combination of single-tank-across and port and starboard cargo tanks. Design #80-D4 has an oil-tight 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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

FIGURE K-9 Typical arrangements for 50,000 DWT tanker.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-2 Principal Particulars for 35,000 DWT-50,000 DWT Tankers

 

#40-S1 pre-MARPOL

#40-S2 MARPOL 78

#40-DS3 Double Sides

#40-D1 Double Hull

#40-D2 Double Hull

#40-D3 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% (m3)

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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-3 Oil Outflow Evaluation for 35,000 DWT-50,000 DWT Tankers

 

#40-S1 pre-MARPOL

#40-S2 MARPOL 78

#40-DS3 Double Sides

#40-D1 Double Hull

#40-D2 Double Hull

#40-D3 Double Hull

Side Damage

Probability of zero outflow

.31

.54

.84

.85

.82

.85

Mean outflow (m3)

2,180

741

771

1,467

618

803

Extreme (1/10) outflow (m3)

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 (m3)

1,817

2,868

4,156

646

397

560

Extreme (1/10) outflow (m3)

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 (m3)

1,962

2,017

2,802

974

485

657

Extreme (1/10) outflow (m3)

6,255

7,160

9,247

8,458

3,767

5,644

Pollution Prevention Index

98% cargo volume (m3)

41,433

37,929

43,017

50,708

42,764

50,467

Index E

.36

.38

.43

.91

1.06

1.02

TABLE K-4 Survivability Evaluation for 35,000 DWT-50,000 DWT Tankers

 

#40-S1 pre-MARPOL

#40-S2 MARPOL 78

#40-DS3 Double Sides

#40-D1 Double Hull

#40-D2 Double Hull

#40-D3 Double Hull

Side Damage Survivability Index

92.5%

97.5%

99.2%

87.2%

100.0%

97.1%

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-5 Intact Stability Evaluation for 35,000 DWT-50,000 DWT Tankers

 

#40-S1 pre-MARPOL

#40-S2 MARPOL 78

#40-DS3 Double Sides

#40-D1 Double Hull

#40-D2 Double Hull

#40-D3 Double Hull

Minimum GMt (m)

1.96

2.93

3.68

-4.03

1.54

0.15

SWB (% capacity)

2%

2%

2%

8%

4%

98%

Cargo oil (% capacity)

98%

98%

98%

9%

97%

4%

Minimum GMt w/ 2% SWB (m)

1.96

2.93

3.68

-0.91

1.54

.96

Possibility of Capsize

none

none

none

none

none

none

Maximum Angle of Loll

none

none

none

15 degrees

none

none

Load restrictions

none

none

none

a

none

none

Note: Load Restrictions required to maintain GM greater than 0.15 meters for load/discharge operation:

a#40-D1: at least three pair of ballast tanks at 50% or more filling (or) at least two cargo oil tanks stripped.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-6 Ballast Condition Evaluation for 35,000 DWT-50,000 DWT Tankers

 

#40-S1 pre-MARPOL

#40-S2 MARPOL 78

#40-DS3 Double Sides

#40-D1 Double Hull

#40-D2 Double Hull

#40-D3 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%

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

FIGURE K-10 Typical arrangements for 80,000 DWT tankers.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-7 Principal Particulars for 80,000 DWT-100,000 DWT Tankers

 

#80-S1 pre-MARPOL

#80-S2 MARPOL 78

#80-DS3 Double Sides

#80-D1 Double Hull

#80-D2 Double Hull

#80-D3 Double Hull

#80-D4 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% (m3)

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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-8 Oil Outflow Evaluation for 80,000 DWT-100,000 DWT Tankers

 

#80-S1 pre-MARPOL

#80-S2 MARPOL 78

#80-DS3 Double Sides

#80-D1 Double Hull

#80-D2 Double Hull

#80-D3 Double Hull

#80-D4 Double Hull

Side Damage

Probability of zero outflow

.22

.32

.92

.81

.85

.87

.83

Mean outflow (m3)

9,508

6,180

1,411

4,369

3.170

2,253

2,013

Extreme (1/10) outflow (m3)

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 (m3)

4.564

6,814

8.939

1,485

1.706

1,497

1,093

Extreme (1/10) outflow (m3)

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 (m3)

6,542

6,560

5,928

2,639

2,292

1,799

1,461

Extreme (1/10) outflow (m3)

16.588

15,583

20,135

19,889

18,243

14,910

11,325

Pollution Prevention Index

98% cargo volume (m3)

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 K-9 Survivability Evaluation for 80,000 DWT-100,000 DWT Tankers

 

#80-S1 pre-MARPOL

#80-S2 MARPOL 78

#80-DS3 Double Sides

#80-D1 Double Hull

#80-D2 Double Hull

#80-D3 Double Hull

#80-D4 Double Hull

Side Damage Survivability Index

99.2%

100.0%

100.0%

99.2%

99.7%

99.9%

99.9%

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-10 Intact Stability Evaluation for 80,000 DWT-100,000 DWT Tankers

 

#80-S1 pre-MARPOL

#80-S2 MARPOL 78

#80-DS3 Double Sides

#80-D1 Double Hull

#80-D2 Double Hull

#80-D3 Double Hull

#80-D4 Double Hull

Minimum GMt (m)

5.27

5.66

8.77

-2.79

0.41

0.15

2.98

SWB (% capacity)

2%

2%

2%

4%

4%

5%

4%

Cargo oil (% capacity)

97%

98%

98%

72%

90%

91%

98%

Minimum GMt w/ 2% SWB (m)

5.27

5.66

8.77

0.49

1.05

3.03

3.54

Possibility of Capsize

none

none

none

none

none

none

none

Maximum Angle of Loll

none

none

none

3 degrees

none

none

none

Load restrictions

none

none

none

a

none

none

none

Note: Load Restrictions required to maintain GM greater than 0.15 meters for load/discharge operation:

a #80-D1: at least one pair of ballast tanks at 50% or more filling.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-11 Ballast Condition Evaluation for 80,000 DWT-100,000 DWT Tankers

 

#80-S1 pre-MARPOL

#80-S2 MARPOL 78

#80-DS3 Double Sides

#80-D1 Double Hull

#80-D2 Double Hull

#80-D3 Double Hull

#80-D4 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%

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Evaluating 135,000 DWT-160,000 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 m-265.0 m

beam

43.0 m-50.0 m

depth

22.8 m-25.8 m

scantling draft

15.2 m-17.2 m

The cargo blocks for single-hull SUEZMAX tankers have historically been arranged three tanks across, and five or six tanks long. The double-hull 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 K-11.

The single-tank-across 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 22-24 of Annex I to MARPOL 73/78.

The two-tanks-across 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 double-hull tanker designs have been evaluated. Design # 150-D1 has a single-tank-across arrangement for all cargo oil tanks. Design #150-D2 is a hybrid, with four single-tank-across cargo tanks and three pairs of port and starboard cargo tanks. Designs #150-D3 through #150-D5 all have an oil-tight 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 #150-D5 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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

FIGURE K-11 Typical arrangements for 150,000 DWT tankers.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-12 Principal Particulars for 135,000 DWT-160,000 DWT Tankers

 

#150-S1 pre-MARPOL

#150-S2 MARPOL 78

#150-S3 MARPOL 78

#150-D1 Double Hull

#150-D2 Double Hull

#150-D3 Double Hull

#150-D4 Double Hull

#150-D5 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% (m3)

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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-13 Oil Outflow Evaluation for 135,000 DWT-160,000 DWT Tankers

 

#150-S1 pre-MARPOL

#150-S2 MARPOL 78

#150-S3 MARPOL 78

#150-D1 Double Hull

#150-D2 Double Hull

#150-D3 Double Hull

#150-D4 Double Hull

#150-D5 Double Hull

Side Damage

Probability of zero outflow

.24

.40

.34

.79

.87

.82

.80

.81

Mean outflow (m3)

10,868

9,175

8,404

6,436

3,674

3,182

3,481

3,252

Extreme (1/10) outflow (m3)

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 (m3)

7,965

11.600

11.725

2,652

2,009

1,663

1,605

1.520

Extreme (1/10) outflow (m3)

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 (m3)

9.126

10,630

10.396

4.166

2,675

2,271

2.355

2,213

Extreme (1/10) outflow (m3)

23,652

26,039

24,885

29,952

21.945

17,316

17,118

16.983

Pollution Prevention Index

98% cargo volume (m3)

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 K-14 Survivability Evaluation for 135,000 DWT-160,000 DWT Tankers

 

#150-S1 pre-MARPOL

#150-S2 MARPOL 78

#150-S3 MARPOL 78

#150-D1 Double Hull

#150-D2 Double Hull

#150-D3 Double Hull

#150-D4 Double Hull

#150-D4 Double Hull

Side Damage Survivability Index

99.9%

100.0%

100.0%

99.8%

99.2%

99.9%

100.0%

99.5%

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-15 Intact Stability Evaluation for 135,000 DWT-160,000 DWT Tankers

 

#150-S1 pre-MARPOL

#150-S2 MARPOL 78

#150-S3 MARPOL 78

#150-D1 Double Hull

#150-D2 Double Hull

#150-D3 Double Hull

#150-D4 Double Hull

#150-D5 Double Hull

Minimum GMt (m)

11.7

6.37

6.70

-0.53

-2.51

2.74

3.86

0.77

SWB (% capacity)

24%

2%

2%

5%

26%

5%

5%

14%

Cargo oil (% capacity)

88%

98%

98%

72%

10%

98%

97%

97%

Minimum GMt w/ 2% SWB (m)

11.99

6.37

6.70

0.42

0.69

3.50

4.62

2.92

Possibility of Capsize

none

none

none

none

none

none

none

none

Maximum Angle of Loll

none

none

none

5 degrees

8 degrees

none

none

none

Load restrictions

none

none

none

a

b

none

none

none

Note: Load Restrictions required to maintain GM greater than 0.15 meters for load/discharge operation:

a #150-D1: at least one pair of L tanks at 50% or more filling

b #150-D2: at least two U ballast tanks at 50% or more filling.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-16 Ballast Condition Evaluation for 135.000 DWT-160,000 DWT Tankers

 

#150-S1 pre-MARPOL

#150-S2 MARPOL 78

#150-S3 MARPOL 78

#150-D1 Double Hull

#150-D2 Double Hull

#150-D3 Double Hull

#150-D4 Double Hull

#150-D5 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%

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Evaluating 265,000 DWT-300,000 DWT Tankers
Design Characteristics

Typical dimensions for VLCCs are as follows:

Lbp

315.0 m-326.0 m

beam

53.0 m-68.0 m

depth

26.0 m-32.0 m

scantling draft

19.0 m-23.0 m

A majority of the single-hull VLCCs have a 5 long × 3 wide cargo tank arrangement. The pre-MARPOL 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 double-hull 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 K-12.

Three double-hull tanker designs have been evaluated. All three have a 5 × 3 cargo tank arrangement. Design #280-D1 has all L ballast tanks, design #280-D2 has predominantly L ballast tanks with one U tank. Design #280-D3 has predominantly full-breadth double bottom ballast tanks with independent side tanks port and starboard, together with midship ballast tanks arranged inboard of the longitudinal bulkheads.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

FIGURE K-12 Typical arrangements for 280,000 DWT tankers.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-17 Principal Particulars for 265,000 DWT-300,000 DWT Tankers

 

#280-S1 pre-MARPOL

#280-S2 MARPOL 78

#280-S3 MARPOL 78

#280-D1 Double Hull

#280-D2 Double Hull

#280-D3 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% (m3)

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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-18 Oil Outflow Evaluation for 265,000 DWT-300,000 DWT Tankers

 

#280-S1 pre-MARPOL

#280-S2 MARPOL 78

#280-S3 MARPOL 78

#280-D1 Double Hull

#280-D2 Double Hull

#280-D3 Double Hull

Side Damage

Probability of zero outflow

.15

.19

.33

.82

.81

.72

Mean outflow (m3)

23,448

21,617

15,342

4,778

4,474

6,134

Extreme (1/10) outflow (m3)

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 (m3)

9,392

10,668

15,956

2,949

2,420

2,704

Extreme (1/10) outflow (m3)

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 (m3)

15,014

15,047

15,710

3,681

3,242

4,076

Extreme (1/10) outflow (m3)

38,217

36,003

44,417

27,329

24,127

27,953

Pollution Prevention Index

98% cargo volume (m3)

299,561

267,062

319,218

306,572

328,112

325,272

Index E

.30

.28

.33

1.04

1.09

1.01

TABLE K-19 Survivability Evaluation for 265,000 DWT-300,000 DWT Tankers

 

#280-S1 pre-MARPOL

#280-S2 MARPOL 78

#280-S3 MARPOL 78

#280-D1 Double Hull

#280-D2 Double Hull

#280-D3 Double Hull

Side Damage Survivability Index

100.0%

100.0%

99.7%

100.0%

100.0%

100.0%

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-20 Intact Stability Evaluation for 265,000 DWT-300,000 DWT Tankers

 

#280-S1 pre-MARPOL

#280-S2 MARPOL 78

#280-S3 MARPOL 78

#280-D1 Double Hull

#280-D2 Double Hull

#280-D3 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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

TABLE K-21 Ballast Condition Evaluation for 265,000 DWT-300,000 DWT Tankers

 

#280-S1 pre-MARPOL

#280-S2 MARPOL 78

#280-S3 MARPOL 78

#280-D1 Double Hull

#280-D2 Double Hull

#280-D3 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%

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
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 self-propelled 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.

Single-hull 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 three-wide cargo tank configurations.

Barges may be of the flush deck type, or fitted with a raised trunk as shown in Figure K-13. 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 double-hull tank barges have been evaluated. Designs #B35-D1, B90-D1, and B90-D2 are new barges constructed in the last five years. Design #B179-D1 is a proposed conversion of #B179-S1, an existing 23,700 DWT single-hull barge.

FIGURE K-13 Typical arrangement for double-hull oceangoing barges.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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%

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×

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.

Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 191
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 192
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 193
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 194
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 195
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 196
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 197
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 198
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 199
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 200
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 201
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 202
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 203
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 204
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 205
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 206
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 207
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 208
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 209
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 210
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 211
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 212
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 213
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 214
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 215
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 216
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 217
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 218
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 219
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 220
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 221
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 222
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 223
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 224
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 225
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 226
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 227
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 228
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 229
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 230
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 231
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 232
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 233
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 234
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 235
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 236
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 237
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 238
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 239
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 240
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 241
Suggested Citation:"Appendix K." National Research Council. 1998. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Washington, DC: The National Academies Press. doi: 10.17226/5798.
×
Page 242
Next: Appendix L »
Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990 Get This Book
×
Buy Paperback | $64.00 Buy Ebook | $49.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

The passage of the Oil Pollution Act of 1990 (OPA 90) by Congress and subsequent modifications of international maritime regulations resulted in a far-reaching change in the design of tank vessels. Double-hull rather than single-hull tankers are now the industry standard, and nearly all ships in the world maritime oil transportation fleet are expected to have double hulls by about 2020.

This book assesses the impact of the double hull and related provisions of OPA 90 on ship safety, protection of the marine environment, and the economic viability and operational makeup of the maritime oil transportation industry. The influence of international conventions on tank vessel design and operation is addressed. Owners and operators of domestic and international tank vessel fleets, shipyard operators, marine architects, classification societies, environmentalists, and state and federal regulators will find this book useful.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!