Technical Paper

Paper presented to The Marine Technology Society, November 1991

ADVANCED SPILL AVOIDANCE SYSTEM FOR UNMANNED BARGES
Mo Husain President MH Systems, Inc. Del Mar, CA

ABSTRACT
SYSTEM APPROACH
GENERAL THEORY
RISK ASSESSMENT AND HAZARD CONTROL
EMERGENCY TRANSFER SYSTEM
REQUIRED RESOURCES
SUMMARY

ABSTRACT

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.

The modified SPILLSTOP system for oil carrying-barges has these operational features:

Automated and unmanned for the duration of the voyage.

Slight ullage underpressure (0.5 to 1.5 psi) to equalize forces internal and external to the rupture location, thus preventing or minimizing cargo spillage.

Automated topping of the ullage space to account for negligible vapor leakage.

Inerting the ullage gases for additional safety.

Evacuating 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.

SYSTEM APPROACH


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 positive pressure.

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.

Operational Sequence

  1. 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.
  2. 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).
  3. 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.
  4. The containment requirement can be phased in by the later introduction of a small special storage compartment in the barge for the inerted hydrocarbons.

GENERAL THEORY


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 flow.

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 x He)

Internal Pressure (Pi) = Controlled Ullage Press. (Pv) + Hydrost. Oil Press. (Do x Hi)

Where:

H is hydrostatic head

D is density of fluids

For Equilibrium:

Pe - PI = 0 or Pv = Pa + Dw x He – Do x Hi ..(1)

Where
He = 9.4 ft.

Hi = 12.25 ft.

Dw = Density of Water = 64 Ib/ft3
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.

RISK ASSESSMENT AND HAZARD CONTROL


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 acceptable level.

Structural Integrity Under Reduced Pressure in Tanks

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.

EMERGENCY TRANSFER SYSTEM


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 PETS.

REQUIRED RESOURCES


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 Pipes

k. Relief Valves

l. Rechargeable Batteries (Explosion Proof)

m. Alarms

n. Shut off Valves

SUMMARY


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.

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