Tanker Spills Prevention by Design (1991) / Chapter Skim
Currently Skimming:
3 Physical Bases of Phenomena Active in Casualties
3 Physical Bases of Phenomena Active in Casualties
Pages 55-76
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 55... ...
It is, perhaps, obvious that these discussions can be neither complete nor comprehensive. Topics to be discussed in this chapter include: momentum exchange and energy dissipation in collisions and groundings; residual strength following damage; fires and explosions; corrosion and fatigue; hydrostatic pressures in opposition; and diffusion and dynamics of fluid/vessel motion.
|
From page 56... ...
Despite the large energies involved, only a small percentage of grounding incidents lead to oil outflows Many minor groundings entail minimal hull damage and are unlikely to be detected until subsequent drydocking. This emphasizes the importance of the first three energy dissipation mechanisms in groundings.
|
From page 57... ...
Such definitions avoid categorizing collisions or groundings simply by the amount of kinetic energy to be dissipated, and also recognize that damage termed "severe" in one grounding may not be in another. In the literature, grounding incidents commonly are characterized in terms of the speed of the impacting ship.
|
From page 58... ...
This process continues until either sufficient energy is absorbed to prevent any further indenting, or the outer shell reaches its rupture strain. If the latter occurs, then the penetrating body will continue advancing, resisted only by the remaining intact structure, with some energy dissipation through friction of the penetrating and resisting structures rubbing together.
|
From page 59... ...
For all three cases striking and stricken ships in collisions and ships involved in groundings the work done by forces generated in stretching hull plating beyond the elastic limit, the so-called "membrane" energy, is a very significant contributor to the total energy absorbed. Damage to ship structure incurred in collisions and groundings can be classified in two broad categories: Sharp and localized cuts of the bottom or side plating caused by a hard wedge, or more uniform crushing and rupture resulting from excessive straining at some distance from load applications sites.
|
From page 60... ...
Light, stiff ships with hull plating resistive to rupture may ride over obstacles in groundings with minimal permanent damage; fully loaded Vet CCs of current design are likely to incur damage almost equal to the height of the rock above the vessel's bottom, and with little vertical ship motion. These situations are contrasted in a highly idealized manner in Figures 3-2a and 3-2b.
|
From page 61... ...
If this happens, and rupture occurs before the full kinetic energy is dissipated, then rupture progresses in the "steady-state" mode. Detachment of stiffeners usually follows; this
|
From page 62... ...
This method, which represents the dissipative action of specific structural arrangements and scantlings, promises to predict the extent of damage to the hull girder in groundings and collisions (Wierzbicki et al., 1990~. An earlier concept related the resisting contact forces between colliding ships only to the volume of distorted materials in the side of a stricken ship (Minorsky, 1959~.
|
From page 63... ...
FIGURE 3-3 Simple computational model of the crushing of plates under inplane concentrated loading. (The gap, G-1, illustrates considerable membrane tension in the plate.)
|
From page 64... ...
This methods accounts for fracture of bottom plating by modifying the original empirical collision formula, through use of a term related to the area of the hull plating involved. The proportionality constant is determined from experimental data.
|
From page 65... ...
The ability of the damaged structure to carry such unchanged loads can be called "residual strength." Virtually the same finite element analysis techniques used to predict load paths and stress distributions for the original structural design also can be used to estimate the residual strength of the damaged structure, with appropriate accounting for stress concentrators such as sharp cracks associated with the failure. By focusing solely on achieving reductions in oil spillage, it is possible to conceive of designs that, once damaged, could place the entire vessel, crew, and cargo at unacceptable risk.
|
From page 66... ...
Therefore, unless tanks have been certified "gas free" by a marine chemist, it must be assumed that hydrocarbons sufficient to support combustion are present in the cargo tanks. Ignition sources exist in several forms aboard tankers.
|
From page 67... ...
Figure 3-6 shows, schematically, the major components of an IGS (Gray, 1979~. Note that an IGS connects cargo tanks, which contain potentially explosive hydrocarbon vapors, to the ship's engine room, with its multiplicity of ignition sources.
|
From page 69... ...
The pump drivers, being potential ignition sources, generally are not in the pump room, but in the engine room; they are connected to the pumps by a shaft through a bulkhead. These drivers can be steam turbine, reciprocating steam engine, electric or hydraulic motor, or diesel engine power sources.
|
From page 70... ...
From this limited discussion, it follows that segregated ballast tanks, located between the outer hull and in-board cargo tanks, can be particularly susceptible to corrosion. In pre-MARPOL ships, seawater ballast was placed mostly in tanks that recently had held cargo; the oily film clinging to tank sides provided some measure of corrosion protection.
|
From page 71... ...
This section seeks to explain how this is possible, by addressing the physical mechanisms governing oil outflow. Hydrostatic pressure is an isotropic phenomenon.
|
From page 72... ...
In other words, the column of oil with a density that produces equal pressure will be 19 percent higher than the balancing height of sea water. This is a significant difference, and it suggests that tankers might adopt loading schemes or install systems that take advantage of these differences in specific gravity, to minimize the potential loss of cargo from ground· 4 ngs.
|
From page 73... ...
The hydrostatic oil pressure at the tank portion is greater than sea pressure. The right hand tank bottom is breached and oil runs out until the hydrostatic pressures are balanced.
|
From page 74... ...
This is obviously a much slower process than oil outflow driven by unbalanced hydrostatic pressure. Even without ship or water motion, some exchange of oil and water will occur as the result of differences in specific gravity.
|
From page 75... ...
Beyond such guidelines for conventional hull structures, there appears to be great potential for diminishing the extent of hull damage, and even preventing the initiation of plate rupture in higher energy groundings and collisions, through proper selection of external hull materials and/or innovative structural design. Hydrostatic balance can be achieved at some point along the hull of a cargo tank at the bottom, for example and this can reduce or eliminate oil outflow.
|
From page 76... ...
Paper presented at meeting of the Committee on Tank Vessel Design, Washington, D.C., June 5-7, 1990. Lloyd's Register of Shipping.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.