Tanker Spills Prevention by Design (1991) / Chapter Skim
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4 Engineering Considerations
4 Engineering Considerations
Pages 77-100
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From page 77... ...
Further, these discussions are intended to provide an understanding of issues that are basic to any evaluation of tank vessel designs. The overall assessment of specific designs mentioned in the following pages can~be found in Chapter 5.
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From page 78... ...
, as well as side structure and longitudinal bulkheads; · hull bending moments caused by unbalanced longitudinal distribution of buoyancy and weights; · wracking (twisting) loads, tending to distort the hull's shape, caused by oblique seas, asymmetrical loading, etc.; impact loads such as the slamming of the ship's forward body during heavy pitching; · the weight of water on deck caused by heavy seas; · sloshing loads on bulkheads caused by impact loads of cargo motions in partially filled tanks; and · thermal loads caused by unequal temperatures such as from heated cargo.
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From page 79... ...
Knowledge of the principles of fracture mechanics grew with the awareness that the combination of long pipeline sections between connections and the cold weather environment made ideal conditions for fracture propagation. With that image as a prelude, the engineering difficulties posed by advances in tanker design will be discussed in the following pages.
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From page 80... ...
Compounding this concern is the fact that MARPOL tankers, due to the volume of segregated ballast tankage, have larger areas susceptible to the maximum corrosion rates than pre-MARPOL ships. Tankers with double bottoms, double sides, or double hulls have even more area in void/ballast tanks susceptible to corrosion, as shown in Table 4-1.
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From page 81... ...
Classification societies are examining their strength standards (Ferguson/ Lloyd's Register, 1990) , and some shipbuilders and owners are reconsidering their extensive use of high-tensile steel (Royal Institute of Naval Architects, 19904.
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From page 82... ...
TANK PROPORTIONS, ARRANGEMENTS, AND STABILITY Size and arrangement of cargo tanks are significant issues in design of double bottoms, double sides, and double hulls (the alternative designs proposed most frequently to reduce pollution risk)
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From page 83... ...
One effect is that these tankers tend to have deeper hulls, with cargo extending higher above the water line to compensate for the volume lost to ballast tanks. Bulkhead and Ballast Tank Dimensional Considerations Most conventional tankers in the 50,000 to 250,000 DWT range have two longitudinal bulkheads, dividing the cargo space into three tanks across.3 The bulkheads typically are spaced at one-third to one-fifth the beam (B/3 to B/5)
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From page 84... ...
Neither minimum segregated ballast requirements nor damage-stability assumptions are altered for ships with double bottoms, double sides, or double hulls. The obvious design approach is to make use of the ship's required segregated ballast volume in arrangements that minimize additional impact on cargo capacity.
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From page 85... ...
For larger ships, where a B/15 double bottom is so high that inspection and maintenance become difficult, a designer might choose to limit the inner bottom to a height such as 3 meters.4 This approach has the further advantage of offsetting any reduction in cargo carrying capacity, and/or creating additional side protection (to fulfill ballast requirements) without further impact on cargo capacity.
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From page 86... ...
The foregoing description addresses the more conventional design approaches to double bottoms, double sides, and double hulls. Another tank arrangement scheme receiving considerable attention is the intermediate oil-tight deck (IOTD)
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From page 87... ...
\ 1 e. Double Bottom Double Side C = Cargo B = Ballast FIGURE 4-2 Tanker ballast tank arrangements.
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From page 88... ...
Stability of Double-Hull Tankers A range of factors must be weighed in assessing double bottoms, double sides, and double hulls. These designs have operational and in some cases survivability advantages, but they also involve penalties in such areas as ease of inspection and maintenance, and possibly safety.
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From page 89... ...
Some of the doublebottom and double-side cases have a greater equilibrium heel, and lower range of stability, than is present with the single-skin case. The foregoing discussion suggests that the damage-stability requirements for double sides, double bottoms, and double hulls should be increased, to approximate the stability achieved by comparable single-hull MARPOL tankers.
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From page 90... ...
3, 4, 5, and 6 12 5.2 50 Stbd Wing Tanks No protectively located segregated ballast tanks were damaged to avoid running an inordinate number of damage conditions.
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From page 91... ...
For example, if a ship grounds on a pinnacle in the open ocean or on a coastline exposed to waves and ocean swells, then these forces can cause ship motions that can result in additional damage, or even loss of the ship in bad weather. If the hull is breached, then flooding of voids such as inner-bottom or side-ballast tanks will make the ship heavier, leaving it harder aground.
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From page 92... ...
Is remote. In summary, there is insufficient evidence to refute the committee's opinion that the great majority of tanker groundings at service speeds will leave the tanker firmly stranded, and in need of significant outside salvage assistance whether or not the ship has a double bottom.
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From page 93... ...
If flooded, they immediately make the ship heavier and assist in stabilizing the hull on the strand; (2) The bottom voids likely will have protected at least some of the cargo tanks, not only limiting cargo loss but also preserving those cargo tanks for easy cargo removal and/or ballasting; (3)
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From page 94... ...
Intact voids can be completely filled with ballast, and if necessary can be cleared relatively quickly, with submersible pumps, if the ballast system is inoperative. The committee could not identify salvage-related concerns that should limit the use of properly designed double hulls.7 Cargo Systems One factor influencing oil outflow in an accident, and subsequent salvage activity, is the tanker cargo system.
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From page 95... ...
Cargo piping should not be located in a double bottom void. To do so risks cargo loss even when the inner bottom remains intact.
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From page 96... ...
Source: Society of Naval Architects and Marine Engineers, 1980. Another concern with conventional cargo systems is that once hydrostatic equilibrium is achieved, the cargo system, even if operable, will pump only water, once water reaches the suction level (assuming water flows in faster than oil can be pumped out)
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From page 97... ...
This section addresses these aspects of alternative designs and their potential impact on the lives of the men and women who work on these ships. Explosions and Fires Segregated ballast tanks, mandated for new ships over 20,000 DWT, are by definition either empty or filled with seawater ballast.
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From page 98... ...
These designs increase significantly the void areas adjacent to cargo tanks. Assuming the risk of leaks is proportionate to the bulkhead area of ballast tanks shared with cargo tanks, double bottoms and sides increase significantly the risk of encountering explosive atmospheres.
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From page 99... ...
In addition, the outer shell of double-bottom, double-side, and double-hull ships should be no more vulnerable to penetration than conventional single-hull MARPOL tankers. In ships with double bottoms and/or double sides, cargo piping systems should be kept out of the voids, to improve damage resistance and to reduce risk of oil leakage and explosion.
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From page 100... ...
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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