Tanker Spills Prevention by Design (1991) / Chapter Skim
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5 Design Alternatives
5 Design Alternatives
Pages 101-157
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From page 101... ...
For this chapter, the alternatives were grouped according to common physical principles. This resulted in the following three categories: secondary "barriers" to oil intermingling with water; the mitigation of pollution via "outflow management" techniques; and the reduction of pollution potential through "increased penetration resistance." Operational options for "accident response" were grouped separately; these options can be employed 101
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From page 102... ...
Column 2 (Effectiveness) indicates, in a general sense, the type of accident in which the proposed alternative will be effective, in terms of controlling cargo outflow.
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From page 103... ...
A dot (~) in the appropriate section indicates that the design alternative, to some extent, creates that concern.
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From page 104... ...
2. Double bottom A non-cargo space between the cargo tank bottom plating and the ship's hull bottom plating.
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From page 105... ...
in D.B. will reduce amount of protectively located segregated ballast.
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From page 106... ...
8a. Mechanically Driven Vacuum The ullage space is subject to a mechanically induced vacuum such that the combined vacuum plus cargo head favors water inflow in the event of bottom damage.
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From page 107... ...
Would mitigate pollution in all but the most catastrophic accidents. Mitigates pollution outflow from groundings.
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From page 108... ...
8c. Imaginary Double Bottom A passive vacuum system coupled with a water layer below the oil cargo.
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From page 109... ...
Mitigate pollution outflow for groundings. Smaller tanks reduce pollution for a specified amount of hull damage.
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From page 110... ...
11. Grinding Bow Forebody bottom structure to be double bottom with internal transverse structure designed to act in a grounding as a "rock rasp".
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From page 111... ...
All of the negatives of double hull structure plus a quantum increase in risks associated with uninspected voids adjacent to cargo tanks. Single barrier defense which, once breached, offers no improvement over conventional single hull design.
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From page 112... ...
19. Distressed-Ship Cargo Transfer System The cargo tanks are fitted with a high suction dedicated auxilliary pipe/ valve system that could transfer oil out of the upper part of any damaged tank to undamaged ballast tanks.
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From page 113... ...
DESIGN ALTERNATIVES 4. Concerns Safety issues raised by introducing hydrocarbons into noninerted spaces.
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From page 114... ...
For example, while a double bottom generally is combined with some doubleside tankage, a double-sided vessel seldom features a double bottom because sufficient ballast can be carried in the side tanks to meet regulatory requirements. Double bottoms, double sides, and double hulls are in use today?
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From page 115... ...
2. Double Bottom Double bottoms (see Figure 5-2)
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From page 116... ...
Double bottoms do not increase oil outflow in grounding, although some critics have made this claim. The committee's reasoning was explained in Chapter 4, in the discussion of perceptions about salvage of double-hull tankers.
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From page 117... ...
can be found in some crude oil and product tankers. As with double bottoms, the minimum MARPOL-required width is 2 meters or B/15, whichever is less; this requirement may not provide sufficient ballast capacity.
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From page 118... ...
After incurring side damage, a double-side vessel is highly susceptible to asymmetric flooding. However, as with double bottoms, the extent of settling and heeling depends on several factors, principally ballast tank arrangement.
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From page 119... ...
Some product tankers, combination carriers, and a few crude carriers already have a double hull, which essentially is a combination of a double bottom and double sides. The width of the side tanks may be less than in the doubleside design however, because the required ballast can be divided among the side and bottom spaces.
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From page 120... ...
At the same time, however, double hulls magnify all the risks noted for double bottoms and double sides related to construction, maintenance, inspection, and safety. The large structural area subject to cracks, for example, puts high demand on access for inspection.
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From page 121... ...
But the committee could find no evidence that this concept has been utilized successfully in a cargo tank. Its total absence in the tank vessel industry likely is due to practical obstacles, which have been insurmountable so far.
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From page 122... ...
This concept involves less-than-full loading of cargo tanks. An accident then would result in the damaged tank's oil/water interface hydrostatics favoring water inflow, or, at least, limiting cargo oil outflow.
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From page 123... ...
A ship would carry 15 to 20 percent less cargo, depending on its design and on the level of cargo required for hydrostatic control. As a result, up to 15 percent more ships (and/or larger ships)
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From page 124... ...
Hydrostatics favoring water inflow can be achieved by dividing cargo tanks into separate upper and lower chambers (see Figure 5-6~. This greatly reduces the cargo head acting on the lower chamber and the hull bottom plating (the lower chamber's cargo head equates to its height, rather than the full depth of the vessel)
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From page 125... ...
The following discussion compares the theoretical performance of the IOTD w/DS with that of other design options, particularly the double hull. To comply with MARPOL space requirements for segregated ballast tanks, the double side-voids in an IOTD w/DS vessel would be about twice the width of those in a double-hull vessel; therefore, the IOTD w/DS would offer greater protection to cargo tanks in some collision scenarios.
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From page 126... ...
Conceptually, retrofit of an intermediate deck to an existing MARPOL vessel would require less steel than retrofit of a double bottom. However, a major modification for the cargo handling, ventilation, and cleaning systems would be required.
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From page 127... ...
7be Convertible Tanks Convertible tanks are a variation on the intermediate oil-tight deck. The cargo tanks have an intermediate deck, which contains remote-activated, securable openings connecting the upper and lower chambers.
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From page 128... ...
while simultaneously withdrawing air from the ullage space. Depending on the cargo type, the vacuum system design characteristics, and the contribution of hydrostatic control (if any)
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From page 129... ...
This system also would require modification of inert gas systems (IGS is discussed in Chapter 3) through a change in international conventions, because this safety system depends on overpressure of cargo tanks.
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From page 130... ...
8c. "Imaginary" Double Bottom A variation on the hydrostatically driven vacuum principle or hydrostatic control alternatives is the "imaginary" double bottom, where chemically treated water is placed in the bottom of the cargo tank to a depth of 1 to 2.5 meters (see Figure 5-9~.
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From page 131... ...
Proposals for manually closing a ship's vents when the ship is in shallow waters, or for connecting IGS branches to cargo tanks, may have merit. However, there is still a concern with additional cargo vapor pressure build-up.
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From page 132... ...
9a. Service Tank Location While pollution-control efforts have focused on restricting outflow from cargo tanks, most tankers carry 2 to 5 percent of their cargo deadweight in heavy fuel oil.
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From page 133... ...
of an ice breaker. The outer hull, made of normal-grade steel plating, would be shaped (as now)
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From page 134... ...
In a grounding, the most vulnerable cargo tanks would be protected by a double bottom. In moving over the obstacle, the rasp-like transverse structure would grind down the material, thus creating a clear and unobstructed passage for the remaining 70 to 80 percent of the vessel (or a smooth bed for the ship to come to rest on)
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From page 135... ...
While this concept is intended to provide both a high-yield and an energy-absorbing hull structure, the committee found no evidence that the energy dissipation involved in the displacement and crushing of the honeycomb structure would provide significantly increased resistance to failure in comparison to conventional structures, such as the double hull. This concept would require extensive research and testing.
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From page 136... ...
14. High-Yield Steel Bottom Structure The behavioral characteristics of steel can be controlled by its composition and manufacturing process.
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From page 137... ...
17. Enhanced Information Processing In this concept, the ship's hull, tank, and pipe structure would incorporate extensive sensing devices connected to a central processing facility.
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From page 138... ...
Towing fittings and their arrangements may enhance the ability of tugs to save disabled tankers before grounding or foundering. These fittings also assure that damaged tankers can be attached to positioning equipment, thus reducing the risk associated with uncontrolled movement of a stranded tanker.
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From page 139... ...
one proposal would have the m~d-he~ght suction line extended aft into the ship's pump room, where it could be installed so as to line up with the suction side of the segregated ballast system. A spool piece, not in place during normal operations, could be fitted following an accident, allowing the ship's ballast system to handle nearly immediate evacuation of oil from damaged cargo tanks into intact segregated ballast tanks.
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From page 140... ...
· The committee has eliminated from further consideration, regarding pollution control and cost, six design alternatives that, though promising in concept, lack the basic technical supporting data and involve major design and operational uncertainties. These include the resilient membrane, convertible tanks, deflecting hull, honeycomb hull, imaginary double bottoms, and ceramic-clad hull.
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From page 141... ...
· Based on results of its technical assessment, the committee determined that the following design alternatives should be assessed on the basis of their pollution-control effectiveness: MARPOL tanker (as the reference vessel) double bottom double sides double hull hydrostatic control intermediate oil-tight deck smaller tanks · In conducting a technical assessment of the above designs, it was apparent that the following three compound design alternatives may offer improved outflow reduction potential for a range of accident scenarios.
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From page 142... ...
The intermediate oil-tight deck and smaller tanks might be applicable. Hydrostatic control appears to have limited, if any, application to towed barges.
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From page 143... ...
, the void space in a double hull must maintain a clearance of 2 feet. This width is considered the minimum for construction, inspection, and maintenance.
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From page 144... ...
. In addition, DnV assessed the implications of the design arrangements for outflow performance of an 80,000 DWT tanker; this assessment was based on extrapolations of the results for VLCCs and 40,000 DWT vessels, where appropriate.
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From page 145... ...
The double sideldouble bottom (i.e., double hull' VLCC designs (arrangements lA,lBJ have the smallest potential oil outflow in collision and grounding, given the assumptions regarding damage location and extent. Compared with a modern conventional MARPOL VLCC, the double sidel double bottom tanker is likely to spill, on the average, only about 33 percent of the standard amount of oil.
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From page 146... ...
The amount of oil likely to escape from a VLCC with a double bottom for ballast and segregated ballast side tanks (arrangements 4, 4A and 4B', is about 40 percent of the amount that would be spilled from a modern conventional MARPOL VLCC. The positive influence of locating the side ballast tanks forward, with regard to potential oil outflow in collisions, is evident comparing arrangements 4B and 4A.
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From page 147... ...
Double Bottom Double Sides Double Hull Intermediate Oil Tight Deck Hydrostatic Control Small Tanks 147 · Modern Conventional VLCC (OR) · Double bottom in whole cargo area (2)
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From page 148... ...
Table 5-3 links the DnV arrangements to the general design alternatives considered by the committee. The combined ratings for collision and grounding (using the 40/60 weighting)
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From page 149... ...
MARPOL Tanker (reference standard) Double Bottom Double Sides Double Hull Intermediate Oil Tight Deck · Modern 40,000 DWT SBT (1)
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From page 150... ...
, 40,000 DWT. Reference DnV Report, Figure 4.18, Appendi 1300 120U 110T 100F 9O 0 80W 7060I 50N 40 D 30 X 2010 O x F
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From page 151... ...
· Double bottoms protect against oil outflow in groundings, particularly low-energy groundings. · The intermediate oil-tight deck, when combined with double sides, should provide protection against both groundings and collisions.
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From page 152... ...
The committee used DnV data to derive outflow estimates for three combinations: double sides with hydrostatic control; double hull with hydrostatic control; and intermediate oil-tight deck with double sides. The outflow ratings for these combinations were not evaluated directly by DnV.
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From page 153... ...
3. Four cases (double hull, double hull with hydrostatic control, double sides with the intermediate oil-tight deck, double sides with hydrostatic control)
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From page 154... ...
; furthermore, this combination only improves performance in collision as a result of cargo tanks being less than full. · In this analysis, double sides with the intermediate oil-tight deck is better than a double hull in collision as a result of wider side ballast tanks, and is presumed to prevent nearly all outflow in low-energy groundings and some outflow in high-energy groundings (because the analysis takes no account of factors such as tides, current, or ship motion)
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From page 155... ...
Similarly, protecting cargo tanks with ballast spaces outboard, or beneath them, does not mean that internal cargo tanks can be made larger than is common in single-skin ships without possible increase in oil outflow. This suggests that, for smaller ships, both damage stability assumptions and hypothetical oil outflow should be reconsidered by IMO and the Coast Guard.
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From page 156... ...
double bottom on the EXXON VALDEZ would have reduced oil outflow by 60 percent at most, and by a minimum of 25 percentstill a significant figure (U.S. Coast Guard, Marine Safety Center internal memorandum, May 25, 1989)
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From page 157... ...
Report prepared for IMO Marine Environment Protection Committee, received by Committee on Tank Vessel Design, NRC, Washington, D.C., November, 1990. Toyko.
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