to The Marine Technology Society, November 1991
ADVANCED SPILL AVOIDANCE SYSTEM FOR
Mo Husain President MH Systems, Inc.
Del Mar, CA
ASSESSMENT AND HAZARD CONTROL
This paper describes SPILLSTOPTM-II
Advanced Spill Avoidance System For Unmanned Barges. The Oil
Pollution Act of 1990 mandates "double containment"
for all barges less than 5,000 gross tons. Double containment
is not restricted to double-hulls; rather, it means any system
that will effectively prevent spillage of oil from an accidental
rupture of the barges.
Barges under 5,000 gross tons are typically
unmanned and without propulsion; i.e. not self-propelled.
The lack of these assets is compatible with SPILLSTOP-II that
can be retrofitted to existing barges with minimum modifications.
The SPILLSTOP-II is an adaptation of SPILLSTOP - Advanced
Spill Avoidance System for Oil Tankers.
modified SPILLSTOP system for oil carrying-barges has these
and unmanned for the duration of the voyage.
ullage underpressure (0.5 to 1.5 psi) to equalize forces internal
and external to the rupture location, thus preventing or minimizing
topping of the ullage space to account for negligible vapor
the ullage gases for additional safety.
the ullage space either to the atmosphere or, if required,
into an on-board "storage container".
These features are controlled by an advanced
and automated computer system utilizing stored, compressed
inert gas as the energy source for an inert gas ejector to
evacuate the ullage space either to the atmosphere or to a
storage container. The power source to operate valves/computer
are rechargeable, marinized batteries.
Presently barge ullage spaces are not
inerted, unlike tankers over 20,000 tons that have mandatory
inerting requirements. The SPILLSTOP-II system can be configured
for either inerted or non-inerted ullage space requirements,
although it is strongly recommended that the ullage space
be inerted and that the ejector be driven by stored inert
gas. The required amount of compressed inert gas is small
and the price of safety - eliminating explosion possibilities
- is worth the negligible increase in cost.
It should be noted that a barge tank structurally
enhanced for air tightness is more likely to retain its underpressure
than a large oil tanker, primarily because there are less
"leaky candidate" penetrations. Therefore, it is
very likely that once underpressure is introduced in the ullage
space there will be no additional augmentation of underpressure
for voyage duration of less than a week.
The SPILLSTOP system for oil tankers dynamically
controls an underpressure in the ullage space of tank(s) during
the voyage of the vessel and, in the event of a rupture of
the tank, equalizes the hydrostatic pressure at the rupture
location to prevent or minimize oil spillage. SPILLSTOP-II
for unmanned barges is different from systems designed for
tankers with an inert gas system already installed. The barge
has no provisions for human intervention after the voyage
starts. SPILLSTOP-II for barges is totally automated and consists
of inert gas storage bottle(s) coupled to an ejector that
is controlled by simple sensors and a scaled computer. Presently
barges do not have any requirement for inerting the ullage
space with inert gas for preventing explosion. Although the
system proposed herein can be used with a compressed air ejector,
for safety it is strongly recommended that inert gas be used.
Therefore, the spill avoidance system for barges described
herein utilizes inert gas (or nitrogen) storage bottle(s).
The plan view and sectional schematic, shown
in Figure 1, basically consist
of a stored gas such as nitrogen coupled to a single stage
ejector that will compress and transfer the ullage gas from
its underpressure condition (typically 13.2 psia) to either
the atmosphere or to storage container(s). The ejector (Figure
2) is a static pump with no moving parts that uses the
high velocity jet of stored gas to mix with the low velocity
ullage gas. Therefore, through conservation of the momentum
of the streams, a resultant mixture of stored gas/ullage gas,
streams out at an intermediate velocity that can be converted
by means of a diffuser to a pressure higher than the original
ullage gas pressure (Figure
2). The ejector envisioned for this application throttles
down the stored pressure so that at the end of the convergent
nozzle, the stored gas exits at Mach 1 or sonic velocity.
The areas of the conic throat and ullage stream passage are
sized to permit a weight flow from each stream so with the
exchange of momentum between two streams (mass x velocity),
an adequate velocity is available to the mixed streams that
will pass through divergent diffuser passage. The diffuser
converts its kinetic energy of the mixed stream into pressure
energy and thus meets the exit pressure requirements (atmospheric
pressure or container pressure). In addition, if inerting
is a requirement, SPILLSTOP-II is configured to inert the
ullage space during the draw-down of pressure by the ejector
and during the topping operations performed to account for
negligible tank vapor leakage. This will ensure that the ullage
gas remains out of the range of flammability no matter what
operations are in progress.
All of these operations are automatically monitored
and controlled by simple sensors/computer. The power to operate
the valve(s)/computer can be drawn from marinized batteries.
System Reinstallation Survey
It is generally believed that smaller,
and most barges carrying petroleum products are not rigidly
maintained. Therefore, it is near mandatory that a thorough
structural inspection be performed prior to installation of
SPILLSTOP system on barges, and any structural members found
to be deficient due to rust, lack of maintenance, or simply
due to normal wear and tear, must be replaced. All tanks must
be made air tight and tested for air tightness by applying
Structural foundation may be required
to support the inert gas bottle(s) and the ejector. The housing
for the scaled computer and batteries must by weather proof.
All control valves and relief valves must be in duplicate,
installed in parallel for increased reliability and safety.
- Barge is loaded with cargo, the ullage
gas is monitored and inert gas or nitrogen is supplied to
ensure the inert condition of the ullage gases.
- Underpressure is initiated prior to
starting the voyage. The reduction of ullage pressure is
performed by the ejector exhausting the inerted ullage gas
down to the desired value, and pumping the exhausted gas
either to atmosphere (non-containment) or, if required,
to a storage container (containment).
- Topping with inert gas or adjustment
to the ullage pressure is done automatically during the
voyage by the combination of SPILLSTOP sensors/computer/inert
gas or nitrogen source. This includes the assurance of ullage
gas inertness at all times.
- The containment requirement can be
phased in by the later introduction of a small special storage
compartment in the barge for the inerted hydrocarbons.
In a typical barge configuration, a puncture
at the bottom shell usually means the hydrostatic pressure
at the level of puncture inside the tank is higher than the
surrounding column of water. The application of slight underpressure
at the tank ullage effectively equalizes the pressure inside
and outside the tanker at the ruptured area. When a rupture
occurs, the water displaces the oil up to the top of the ruptured
area. If pressure equalization at this point of the rupture
is undertaken no further spillage occurs.
The hydrostatic overpressure depends on
many factors such as cargo density, loading conditions, barge
configuration, and the height of the tank rupture from the
tank bottom. Generally, the underpressure required to equalize
forces inside and outside a tanker barge is a moderate 0.4
to 1.5 psi. However, the equalization of these forces across
the height of the tank rupture area (side shell rupture) does
not prevent leakage up to that point because of forces between
the dissimilar fluids at the hole, resulting in stratified
Tank Underpressure Requirement
An analysis was performed based on a sample
barge as shown in Figure 3. A typical tank
cross-section is shown in Figure
4 with a grounding rupture such that the distance from
the water line to the top of the rupture is he.
and the height of the rupture contained oil Above the top
of the rupture is Hi. The ullage as depicted has
a controlled underpressure Pv. The underpressure
can be set to balance the forces internal and external to
the tank at the highest point of rupture so that oil spillage
will only occur up the highest point of the ruptured opening.
The forces that predominate are the hydrostatic fluid pressures
and the ambient and underpressure forces, as follows:
External Pressure (Pe) = Atm.
Pressure (Pa) + Hydrostatic Water Press. (Dw
Internal Pressure (Pi) = Controlled
Ullage Press. (Pv) + Hydrost. Oil Press. (Do
H is hydrostatic head
D is density of fluids
Pe - PI = 0 or Pv
= Pa + Dw x He Do
x Hi ..(1)
He = 9.4 ft.
Hi = 12.25 ft.
Dw = Density of Water = 64
Do = Density of Oil = 57 lb/ft3
Pa = atmospheric pressure = 14.7 psia
Then, from equation 1..
Pv = 14.7 x 144 + 64 x 9.4
- 57 x 12.25 = 2020 (psf) or 14.02 psia
The under pressure as defined is the pressure
change required below atmospheric pressure (14.7) to sustain
equilibrium of forces:
Underpressure = Pa - Pv
= 14.7 - 14.02 = 0.68 psia
To repeat, the underpressure to satisfy
the pressure forces external to and internal to the tank at
the location of the ruptured hole is dependent on many factors,
including cargo density, tank loading conditions, draft and
the height of the rupture from the waterline.
Stratified Out-Flow of Oil (In
Case of Side Hull Damage)
The pressure equilibrium as shown in the
equations works nicely in all cases as long as the viscosity
of the contents inside the tank and the liquid outside the
tank is the same. However, based on experimental evidence
it appears that due to forces between the two dissimilar viscous
fluids, in this case (crude) oil and saltwater, the water
is drawn into the tank through the lower part of the hole,
as shown in Figure 5. A stratified flow
results, even though equilibrium of pressure is maintained
throughout. Once the water reaches the top part and the viscosity
of liquid inside the tank is the same as outside, the flow
stops and the equilibrium state is reached again. Stratified
outflow of oil occurs only in the event of side hull rupture;
bottom shell rupture does not induce stratified flow.
A preliminary assessment of accident risk
factors, hazard causes and controls for each identified hazard
with a catastrophic level of concerns is indicated below.
It is concluded that these hazards can be controlled to an
Structural Integrity Under Reduced Pressure
The structural strength of barges as based on
the current design criteria to withstand negative pressures
(0.5 psi to 1.5 psi) has been analyzed, and is found to be
more than adequate. A study to determine the "Effect
of Negative Pressures on Tanker Structure" performed
at the University of Michigan has concluded that increases
in structural stresses, over and above the normal stresses
due to loading and ship motions, are insignificant The percent
increase in the structural stress level ranges from 2% to
5% for 2 to 4 psi underpressures.
Evaporation and Volatility of the Cargo
The SPILLSTOP-II system for barges under
5,000 tons requires underpressures in the range of 0.5 psi
to 1.5 psi, or the equivalent total ullage space pressure
of 14.2 psia to 13.2 psia. True vapor pressures of a range
of highly volatile cargoes such as motor gasoline, aviation
gasoline and light naphtha are at most about 5 psia at stock
temperature of 60° F and up to
10 psia at stock (cargo) temperature of 100°
F. SPILLSTOP-II underpressure requirement (approximately 13
psia ullage pressure space) will not cause boiling to occur
even with the extremely volatile fuels of vapor pressure approaching
10 psia. Boiling occurs when the temperature of the liquid
surface reaches the point at which the true vapor pressure
of the liquid is equal to or exceeds the total pressure of
the tank. Therefore, under normal cargoes with vapor pressure
of less than 2 psia or extremely volatile cargoes of vapor
pressure near 10 psia, there is sufficient safety margin to
eliminate concerns for this phenomenon.
The object of the Emergency Transfer System
(ETS) is to transfer the cargo oil from the ruptured vessel
to a receiving vessel.
A schematic diagram of the Emergency Transfer
System is shown below in Figure 6. The concept of
ETS is essentially straightforward. Assume that the tank ullage
space has a differential pressure of Pv; then suction
pressure at the discharge end of the pipe is set at PETS
which must be less than Pv. The required suction
pressure PETS is dependent on the underpressures
Pv and the net head of oil in the transfer piping. The SPILLSTOP
system control software will automatically set the required
The primary resources required to implement
the SPILLSTOPTM-II for barges are as follows:
a. Inert Gas Bottle(s) (Pressurized)
b. Ejector (Air)
c. Tank Level Indicators
d. Draft Level Indicators
e. Gauges & Sensors
f. Sealed Computer
g. Air tubing
i. Control Valves (air)
j. Emergency Transfer System Discharge
k. Relief Valves
l. Rechargeable Batteries (Explosion Proof)
n. Shut off Valves
By law under OPA 90, a double containment
system for barges under 5,000 gross tons is inevitable. The
SPILLSTOPTM-II system for barges virtually has
no moving parts, and consumable items (Figure
8) in the system are very low-air or inert gas and rechargeable
batteries are the only two consumable items. Therefore, it
is expected to be a rugged, low cost, and highly reliable
system. It is entirely automated and unmanned for the duration
of the voyage.
Your further inquiries are invited.
Write to: Corporate@mhsystemscorp.com