Double-Hull Tanker Legislation An Assessment of the Oil Pollution Act of 1990 (1998) / Chapter Skim
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6 Design, Construction, Operation, and Maintenance of Double-Hall Vessels
6 Design, Construction, Operation, and Maintenance of Double-Hall Vessels
Pages 115-141
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From page 115... ...
confronted naval architects with new design issues; existing national and international design regulations had been developed with single-hull tankers in mind. This new challenge stimulated creativity in the design process, as illustrated by the varied hull arrangements of iThe comparative study of double-hull and single-hull designs was performed under subcontract to the committee by Herbert Engineering Corporation, whose president is committee member Keith Michel.
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From page 116... ...
Double-Hull Tank Arrangements The arrangement of tank vessel cargo tanks and ballast tanks has a major influence on a vessel's effectiveness in reducing oil outflow after an accident as well as its damage and intact stability. In particular, the subdivision of cargo and ballast tanks by centerline bulkheads can have important implications for oil outflow in the event of a collision or a grounding.
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From page 117... ...
guidelines for evaluating alternatives to double-hull tankers were used to assess the relative oil outflow of different designs. Although intended for evaluating the outflow performance of alternative arrangements to the double-hull concept, these guidelines are also well suited to comparing the outflow performance of single-hull and double-hull tankers.
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From page 118... ...
The likelihood that a double-hull tanker involved in a collision or grounding will spill oil is therefore largely influenced by the dimensions of the doublebottom and wing tanks. The amount of oil spillage is also impacted by the internal subdivision of the cargo tanks, which dictates tank sizes and the spacing of bulkheads forming tank boundaries.
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From page 119... ...
The probability of zero outflow is a function of the double-bottom and wing tank dimensions and is not affected by internal subdivision within the cargo tanks. In other words, centerline or other longitudinal bulkheads within the cargo spaces or ballast tanks have no influence on the probability of zero outflow.
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From page 120... ...
Source: Herbert Engineering Corporation, 1996. Mean Outflow The mean outflow values for tankers evaluated in the comparative study are plotted in Figure 6-2.
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From page 121... ...
- Double-Hull Tankers with Centerline Bulkhead in Some Cargo Tanks x Double-Hull Tankers with Centerline Bulkhead in All Cargo Tanks x ol l 0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 Deadweight Tons FIGURE 6-3 Variation in mean outflow with longitudinal subdivision for double-hull tankers. Source: Herbert Engineering Corporation, 1996.
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From page 122... ...
There is considerable scatter in the data points, indicating that such characteristics as internal subdivision and draft-to-depth ratio have a significant impact on extreme outflow. A1though the comparative analysis indicated that double hulls are very effective in reducing both the number of spills and the mean outflow values, their effectiveness in preventing large spills is less pronounced.
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From page 123... ...
Outflow Performance of Tank Barges The probability of zero outflow and the mean outflow values for tank barges evaluated in the comparative study are plotted in Figures 6-6 and 6-7, respectively. The results are similar to those for tankers.
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From page 124... ...
x Double-Hull Barges o - 1 0 5,000 10,000 15,000 20,000 25,000 Deadweight Tons FIGURE 6-7 Mean outflow for single-hull and double-hull tank barges. Source: Herbert Engineering Corporation, 1996.
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From page 125... ...
Single Hull Double Hull 35,000-50,000 95.0 95.9 80,000-100,000 99.6 99.7 135,000-160,000 100.0 99.7 265,000-300,000 99.9 100.0 125 It is important to remember that this study investigated the relative performance of different designs to mitigate outflow if they experienced a collision or grounding that breached the outer hull. The overall outflow performance must also take account of the likelihood that a given vessel will experience such an accident.
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From page 126... ...
The committee's review of recent tanker construction showed that most double-hull tankers are designed with double-bottom and wing tank dimensions in excess of the minimum requirements. This is probably the result of a desire to provide better access to ballast tanks for inspection and construction purposes, owner requirements for deeper ballast drafts than the IMO minimum 7Free surface effects are cargo or ballast movements that occur in partially loaded tanks as a vessel heels.
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From page 127... ...
Most of the double-hull designs evaluated in the comparative study had still-water bending moments in the ballast condition approaching the maximum permissible value assigned by the classification society. It is difficult to say what impact this may have on a ship's structure because fatigue is influenced primarily by cyclic loading.
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From page 128... ...
However, some operators expressed concern that procedures for the safe operation of double-hull tankers are left to the i2The committee acknowledges that a survey of operational experience on liquid natural gas, liquid propane gas, and ammonia carriers could provide some useful information, but such a survey would require an extensive evaluation of the operational differences between the different types of vessels before any comparisons could be made with oil tankers. i3The number of vessels in different size ranges was as follows: 36 vessels of less than 60,000 DWT; 11 vessels of 60,000 to 100,000 DWT; 34 vessels of 100,000 to 200,000 DWT; and 10 VLCCs (200,000-350,000 DWT)
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From page 129... ...
Cleaning of cargo tanks is also easier and faster on double-hull tankers. These advantages accrue because most of the ship's framing is located in the ballast tanks, leaving smooth surfaces in the cargo tanks.
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From page 130... ...
, which is composed of classification societies, oil companies, and independent tanker operators, has developed guidelines that provide information on critical areas in double-hull tankers and include a historical catalogue of failures in structural details. Coatings The ballast tanks of double-hull tankers are coated to protect the steel against corrosion that can accelerate fatigue cracking.
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From page 131... ...
The committee's economic analysis indicated an increase in capital costs for double-hull tankers of between 9 and 17 percent. Problems encountered in double-hull construction include difficulties in painting narrow double-hull spaces, in providing adequate ventilation of doublehull spaces, and in providing adequate access to double-bottom ballast tanks when the side tanks and double bottom are divided to provide damage stability.
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From page 132... ...
In addition, some companies have inspections conducted by independent surveyors at two- to five-year intervals. Many of the companies monitor ballast tanks on double-hull tankers more closely and more frequently than those on single-hull tankers.
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From page 133... ...
Safe access to these spaces is essential to monitor ballast tanks, conduct surveys required by classification societies, and maintain ballast piping. In addition, access may be needed to rescue an injured person from a double bottom in the event of an accident.
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From page 134... ...
According to TSCF guidelines, this is not effective for all ballast tanks. Some operators have had dedicated purge pipes installed in the tanks or use ballast pipes to provide air circulation in ballast spaces.
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From page 135... ...
DESIGN OF DOUBLE-HULL TANK VESSELS Design Standards The regulations governing tanker design were developed primarily with single-hull vessels in mind, although the stability and strength characteristics of double-hull vessels are quite different from those of the traditional single-hull tanker. Existing and proposed regulations pertaining to oil outflows, intact stability, and survivability of double-hull tankers are summarized in Table 6-4.
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From page 136... ...
MARPOL Draft Regulation I/25A calls for assurance of positive intact stability, both in port and at sea, through design onlyb Specifies extent of damage tanker must be able to survive Defines raking bottom damage criterion that supplements Regulation 25 Damage stability criteria for all types of ships being harmonized by IMO based on probabilistic methodology aIMO code on intact stability for all types of ships is covered by IMO instruments. bThe Marine Environment Protection Committee of IMO will circulate Draft Regulation I/25A with a view toward adoption in September 1997.
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From page 137... ...
. Structural research is proving beneficial in providing improved design tools to incorporate fatigue and structural performance in accident scenarios into doublehull tanker designs.
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From page 138... ...
The approaches used today ensure that tankers have sufficient strength to withstand the loads encountered in regular operation, but there are no provisions for the loads encountered in accidents.20 Similarly, outflow performance is based on tank subdivision only, and no consideration is given to the performance of the ship's structure in collisions and groundings. The development of tools that can be used to design tanker structures for good performance in accidents will be an important advance.
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From page 139... ...
The committee's analysis indicated that outflow performance is heavily influenced by the extent of subdivision within the cargo tanks of a doublehull tanker. Vessels with single-tank-across cargo tank arrangements (i.e., without longitudinal bulkheads)
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From page 140... ...
Operational guidelines for double-hull tankers issued by the Tanker Structure Cooperative Forum in conjunction with the International Association of Classification Societies and the International Safety Guide for Oil Tankers and Terminals are considered adequate by most operators. These guidelines address issues such as access, inspection, maintenance, ventilation, and making ballast spaces inert.
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From page 141... ...
Pp. 11-112 in Proceedings of Conference on Predictions Methodology of Tanker Structural Failure and Consequential Oil Spill, Tokyo, April 1995.
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