AMERICAN UNDERPRESSURE SYSTEM Cargo Outflow
Demonstration Program
INTRODUCTION
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 worstcase
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 singletank
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 singletank 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
