AMERICAN UNDERPRESSURE SYSTEM Cargo Outflow Demonstration Program


INTRODUCTION

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The objective of this Excel program is to determine the cargo loss for a single tank or for a tanker of specified design when subjected to specified conditions of damage. The analysis develops a simplified estimate of cargo loss, which reveals the general characteristics of the cargo containment problem, including the effects of ship proportions, tank arrangement and dynamic underpressure.

A number of different design alternatives are considered. The conventional tanker hull has effectively no provision for cargo containment beyond the hull plating itself. A double bottom or double sidewall can be incorporated in the design to reduce the probability that a hit will result in rupture of the tank. An additional design option is to provide a horizontal intermediate deck, which divides the cargo vertically. This approach reduces loss under some circumstances. There are also options in the general arrangement of the cargo tanks, which affect the loss characteristics of the ship.

Since the mechanism of cargo loss is hydrostatic, manipulation of the prevailing hydrostatic pressures can be used to reduce damage. One option is to arrange for the ullage space to be maintained at a predetermined sub atmospheric pressure. This underpressure can be selected to minimize cargo loss in case of grounding, historically the most likely type of accident. This damage control policy will be identified as the active underpressure system. Alternatively, the underpressure can be set to prevent loss under the worst-case condition, namely a rupture just below the waterline. Finally, it is possible to adjust the underpressure at the time of the accident to meet the actual conditions of damage; this action would be carried out by estimating the physical location of the rupture and adjusting the underpressure to minimize it. This approach will be identified as the dynamic underpressure system.

These design options can be applied to all tanks or only to selected tanks, leading to a very large number of potential designs. The analysis of this problem treats each tank separately, and is therefore capable of analyzing any given design.

TECHNICAL APPROACH

The analysis first determines, using hydrostatic principles, the cargo loss that will occur from a single tank when it experiences a point rupture. This calculation allows for ullage pressure levels other than atmospheric so that underpressure operation can be studied. The single-tank analysis is then extended to include probabilistic factors that allow for the relative likelihood of collision and grounding events, and to allow for structural enhancements such as the double hull. Two measures of containment effectiveness are used to compare alternative design concepts. The first of these is the expected, or average, outflow in a grounding or collision incident. The second is the probability of zero outflows. Finally, the analysis is extended to cover all of the cargo tanks in a specific vessel, introducing additional parameters to allow for tank arrangement and tank size.

The calculation for each tank determines outflow for any point rupture as a basis of further analysis. The outflow is divided into direct and displacement components. The assumption is then made that rupture locations are uniformly distributed over the vertical extent of the tank, making it possible to determine two statistical measures of containment effectiveness, the expected, or average, outflow from the tank and the probability of zero outflow. Other secondary phenomena, such as tidal effects, entrainment losses and ship list and trim, are not considered in this preliminary analysis. For the ship as a whole, and for a single incident, the probabilities of grounding and collision are identified as analysis inputs. These quantities are important only in their relative values so as to properly weight the capabilities of the design in each type of incident. If these quantities add up to one, the result of the outflow calculation is on a "per incident" basis without regard to occurrences other than grounding or collision, for example, fire. These event probabilities, together with structural vulnerability parameters to represent the loss reduction afforded by the double hull, allow the development of effectiveness measures for the single tank. These single-tank calculations are presented in the "Single Tank" sheet.

Turning now to the analysis of the set of tanks comprising a tanker vessel, another component must be added to the calculation to represent the hit probability for each tank. For a grounding incident, the hit location is assumed to be equiprobable over the bottom area of the hull; therefore the hit probability is the effective horizontal area of the tank divided by the total bottom area. Similarly, collision hits are assumed equiprobable over the area of the two sides of the vessel, so the hit probability is the area of the tank in the profile view divided by the total side area of the ship. In either case, of course, these probabilities apply only when the tank is adjacent either to the bottom or the side. Location vulnerability parameters are therefore defined to reflect the differing vulnerability of individual tanks. Finally, an additional factor in the calculation is the capacity of the tank expressed as a fraction of total cargo volume. This factor assures that the vulnerability of the tank is weighted by its size; the expected total cargo outflow is thus expressed as a fraction of total cargo. The results for the whole vessel are presented in the "All Tanks" sheet.

This excel program is based on the original Visual Basic program called "MH Single" which was developed by MH Systems, Inc. To receive your free version of the Visual Basic program along with its documentation, please contact MH Systems.

Your further inquiries are invited. Write to: Corporate@mhsystemscorp.com

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