UNDERPRESSURE SYSTEM (AUPS)
ON BOARD USNS SHOSHONE
||NORTHERN CALIFORNIA SECTION
SOCIETY OF NAVAL ARCHITECTS
AND MARINE ENGINEERS
The Full Scale Test
Of The American Underpressure System (AUPS)
On Board The USNS Shoshone
Mo Husain, President MH Systems, Inc.,
San Diego, CA
Robert E. Apple, Sr. Vice-President,
MH Systems, Inc., San Diego, CA
Randy Sharpe, Sharpe Surveying, Alameda
- Consultant to MH Systems, Inc.
Gary Thompson, Vice-President, M.
Rosenblatt & Sons,
American Underpressure System (AUPS), an active inert gas
controlled system utilizing vacuum technique, drastically
reduces or totally prevents spillage from an accidental rupture
of the hull. Existing inert gas systems are modified to provide
underpressure in tank ullage space during the voyage so that
if tank rupture occurs, oil spill above the rupture is prevented.
The actively and dynamically controlled underpressure is set
to insure that pressure forces inside and outside the tank
are equalized at the line of rupture and then there is no
inducement for flow to occur.
The underpressure requirement is generally moderate (-2 to
-4psi), and is well within the structural limit of the tanker
as has already proven by the recent Full Scale test of the
AUPS on USNS Shoshone in the Bayship & Yacht Shipyard in Richmond
California on June 11, 2001. Also, the test has shown that
the AUPS can maintain an inerted atmosphere in the ullage
space at all times to prevent explosion hazards. Additionally,
the laboratory tests with three different crudes shows that
AUPS closed loop system in the operational mode can prevent
emission and loss of cargo due to evaporation even when subjected
to moderate negative pressures.
The AUPS in conjunction with the existing inert gas system
can be used as a primary containment system for existing single
hull tankers until all single hull tankers are retired or
as a backup to other costly approaches. It is a simple, practical
approach that can be easily retrofitted in a timely manner
into existing tankers at minimal costs and logistical impacts.
The desire for a system
to provide spill protection for single hull tankers was first
expressed in the U.S. Congress Oil Pollution Act of 1990.
This act mandated the exclusive use of double hull tanker
construction after 2015. The Act also specified studies on
spill prevention for existing tankers and specifically mentioned
research and development on the use of vacuum in tanks.
Systems (MHS) initiated the development of the American Underpressure
System, (AUPS), in 1989. The system reduces or virtually eliminates
oil outflow from an oil tanker damaged in grounding or collision
by applying a partial vacuum to the empty space above the
oil cargo, the ullage space. This negative pressure eliminates
the pressure difference at the rupture point to prevent outflow.
Analysis predicts that this underpressure system will reduce
average outflow per incident by approximately 65%, a reduction
approaching that predicted for the double hull. The system
is also expected to improve further the effectiveness of double
In the operation of the underpressure system, negative pressure
is applied to the ullage space, to control the pressure balance
at the point of rupture to reduce or eliminate outflow. Figure
1 shows the midship cross section of a tanker. The ullage
space above the cargo is supplied with an inert gas mixture,
derived generally from flue gases, in order to prevent the
formation of an explosive mixture. In the event of damage
at the point "X", cargo would flow out of the conventional
vessel until the pressure at "X" inside the tank is equal
to that outside the tank. The underpressure system provides
a reduced, sub-atmospheric, pressure above the cargo. The
inside and outside pressures at "X" are equalized, preventing
outflow of cargo.
OF THE AUPS TEST
for a "Full Scale Test of the American Underpressure System"
(AUPS) was released by the Advanced Research Project Agency
in May 1995 as a part of the MARITECH competition. A three-phase
test of the AUPS, administered by the Maritime Administration,
Phase I, Concept Definition,
was completed in April 1996. It consisted of extensive documentation
of the previous six years of design studies, experiments and
technical papers. Also prepared were several plans including
the preliminary master plan for the analyses, test and full
validation of the AUPS.
Phase II, Development of a Detailed Test
Plan, including a Validation Plan,
was completed in March 1997. The submittals included system
schematics, preliminary analyses, test memos for testing the
tanker with, first fresh salt water, and then crude oil, and
descriptions of the extensive analyses required for testing
and for the subsequent generic system design.
III The American Underpressure System Test and Validation
In late September 1998, in the Defense
Appropriations Bill, Congress directed the Secretary of the
Navy to provide funding to the Maritime Administration. In
1999 Congress redirected funding to the Office of Naval Research.
The contract was initiated July 11, 2000. The project involved
four elements of investigative effort. The ultimate objective
of the project was to develop the information necessary to
permit rulemaking changes so that the system can be installed
are four elements of investigative effort as shown below:.
1. Conduct full-scale tests on a tanker.
This paper, however, primarily discusses
the full-scale tests of AUPS on USNS Shoshone.
2. Conduct supporting analyses.
3. Conduct Laboratory tests.
4. Develop the Generic Design.
Full Scale Test
full-scale test was planned to be more than a simple demonstration
and validation. It afforded a means of exploring critical
design issues; particularly those in which uncertainty about
scaling effects required validation of conclusions arrived
at by model-scale or laboratory testing. The key issues investigated
were the following.
of cargo-- effectiveness of ullage underpressure in reducing
cargo outflow (spillage).
control-- the dynamic stability of ullage space pressure
control using pressure measurements, fan characteristics,
valving, and digital control algorithms.
evolution-- the rate of evolution of hydrocarbon gas/vapor
under reduced ambient pressure. Correlation with laboratory
inerting by inert gas mixing-- the ability to maintain an
inert atmosphere while simultaneously maintaining a reduced
ullage pressure. Verification of transient and steady state
loads-- validation of finite-element structural analyses
of pressure-induced loads.
test design was driven by the need to use a reasonably contemporary
tanker configuration, at full scale and using minimum modifications.
A suitable vessel, the USNS Shoshone, of about 35,000 dwt,
was made available from the reserve fleet. One tank on the
port side was designated as the nominal cargo tank for the
purposes of the test. An adjacent centerline tank was used
as a simulated ocean, thus avoiding contact with the environment.
The simulated ocean tank was filled with fresh water, rather
than salt water, so as not to contaminate the vessel.
intervening bulkhead between the two tanks was pierced about
one and one-half feet above the tank bottom, and fitted with
a pneumatically operated 12" valve. This arrangement permitted
the valve to be used to simulate a damage incident. Nitrogen
gas was drawn from a liquid supply to serve as a simulated
inerting medium. M. Rosenblatt & Sons, Inc prepared the
design of the required tanker modification and AUPS system
installation. The test setup is shown in Figure 2.
system utilized in the full-scale demonstration test on USNS
Shoshone was required in order to control the ullage space
underpressure and inertness during routine and spill containment
operations. A computer simulation was developed for the test
to verify that stable and conservative operation would prevail
during the test. These test series confirmed the control system's
of the control system arrangement for the test is shown above
in Figure 2. There are three flow streams interfacing with
the test tank. The test tank contains water as cargo and an
adjoining tank simulates the draft of the tanker. A rupture
valve 12 inches in diameter and 18 inches above the tanker
bottom is located in a common bulkhead so that spillage is
contained when the valve is opened, thereby preventing possible
impact on the environment. To preclude the formation of localized
high concentrations of oxygen in the ullage space due to potential
air leakage into an underpressured tank, a continuous supply
of inert gas is circulated through the ullage space and then
vented to the atmosphere.
supplied dockside from a cryogenic liquid. It is stored, vaporized,
and regulated to approximately 17.0 psia and 70º F. The nitrogen
enters the AUPS distribution piping through a flow meter and
a single-loop control valve. The control valve is based on
electro-pneumatic technology and operates in conjunction with
a remotely- located Remote Electro-Pneumatic Controller. The
controller is a single-loop, fluid control that compares the
ullage pressure to the desired set point. It uses air to operate
the valve opening to maintain the controlled variable. A response
speed adjustment can be used to vary the rate at which the
valve will open and close. There are three adjustments for
tuning the controller - the desired set point, the speed of
response, and the width of the dead band. Conventional pressure
and temperature transducers are used.
Air is introduced
into the ullage space at rates in excess of the expected air
leakage into a tank assembly. The test will validate the oxygen
dilution capability of the system. The piping arrangement
provides for a remotely operated manual valve followed by
a flow meter, and by pressure and temperature sensors.
steam vents the ullage gases to the atmosphere. The piping
arrangement consists of a flow meter; blower and blower trim
valve, and associated pressure and temperature sensors. The
gas vents to the atmosphere through a vent riser containing
a Pressure/Vacuum (P/V) relief valve and a flame arrestor.
The blower is a 2-stage, regenerative, radial compressor driven
by a constant-speed electric motor. The blower pressure rise
matches the overall resistance to the flow venting to the
atmosphere. The trim valve secured the required flow and underpressure
and was not used as a control variable when final perturbations
Control System Features
data acquisition console is capable of real time monitoring
displayed and data capture of the test parameters.
simulated tank rupture using a 12-inch butterfly valve with
spring return and manual override.
- Remotely located
test control centers on the ranker housing the test conductors
and data acquisition and control consoles, and a center
for the display of spill containment.
computer simulations modeled the main flow components, duct
loses, and interactions to yield the transient and steady
state test parameters versus time.
output provided assurance to proceed with the test. The test
results showed conformance
with the simulation model
performed conservatively over a range of perturbing conditions
such as air leakage and inert gas circulating rates.
- Inertness and underpressure
were maintained during routine and spill containment.
stability was present on all these operations.
of AUPS Test on USNS Shoshone
of Crude Oil
use of crude oil resulted in the requirement for many permits
and technical reviews. Due to the anticipated use of crude
oil for the test on board SHOSHONE it was necessary to involve
the Coast Guard in plan approval for the project. Meetings
were held with the local Coast Guard Marine Safety Office
for San Francisco Bay to outline the scope of the project
and determine the extent of Coast Guard involvement. It was
agreed that the SHOSHONE would not require full inspection
for issue of a Certificate Of Inspection. The Coast Guard
agreed to limit their inspections to the installation of the
test equipment. Plans were developed for the test installation
by M. Rosenblatt and Sons and sent to the Coast Guard Marine
Safety Center in Washington DC. There was some initial hesitation
on the part of the Marine Safety Center and the staff of Coast
Guard headquarters technical branch concerning the project.
After meeting with these individuals it was agreed that they
would review the plans for compliance with Coast Guard regulations
for installation of the system on board a tank vessel.
review process additional information was provided to the
Marine Safety Center of the USCG on structural calculations
for the SHOSHONE. This was required due to initial concerns
over the vacuum on the ullage causing structural damage to
the cargo tanks to be involved in the test. Elaborate finite
element analyses were carried out by Dr. Alaa Mansour of University
of California, Berkeley. These calculations, approved by the
American Bureau of Shipping (ABS), found that the cargo tank
could be subjected to 12 PSI of negative pressure without
sustaining any damage. After some modifications to the plans
to meet all of the Coast Guard regulations for installation
of the system on a tank vessel, the plans were approved by
the Coast Guard and the Navy approved the delivery of the
SHOSHONE to MH Systems to install the system and conduct the
U.S. Coast Guard (USCG), Office of Naval Research (ONR), Office
of Naval Operations (OPNAV)
commenced on board SHOSHONE the Coast Guard Marine Safety
Office inspections department was involved in inspection of
the installation of the system in accordance with the approved
plans. This involved several call outs to have an inspector
witness welding and testing of the installation. Prior to
conducting the tests the Coast Guard was satisfied that the
system was installed in accordance with the approve plans.
When the test was completed, it was required to return the
SHOSHONE to her original condition. The Coast Guard Inspector
was again called to witness fit-up and welding of the inserts
of the cargo tanks and deck where piping penetrations were
removed. Upon completion of these repairs the Coast Guard
issued a final inspection report noting that the ship was
returned to satisfactory condition for MARAD to accept it
back into the reserve fleet.
Bay Area Air Quality Management District Approval
to Coast Guard approval, approvals had to obtain from the
California Bay Area Air Quality Management District (BAAQMD).
All hydrocarbon emissions from the SHOSHONE had to be determined
and a plan approved by the BAAQMD. During the test of the
system while the AUPS system was maintaining a vacuum on the
ullage space of the #5 port cargo tank there would be a small
amount of hydrocarbon emissions from the exhaust of the vacuum
blower. Calculations were made as to the amount of the emissions
and found to be under the 15-pound threshold in the California
regulations. A permit was applied to conduct the test and
it was granted. Steve Hill of BAAQMD was very helpful in working
with us on the permitting process for this test.
Recovery System Approval
built prior to the regulation of vapor emissions for crude
oil and did not have a vapor recovery system installed. The
original plans for the installation included piping for a
vapor recovery system. However during the search for a barge
to deliver crude oil to SHOSHONE it was determined that there
were no barges capable of conducting vapor recovery while
loading the SHOSHONE. There were barges in the industry which
could reclaim their own vapors while being loaded but they
could not vapor balance with SHOSHONE. MHS approached the
BAAQMD and requested a one-time permit to load SHOSHONE from
a barge without vapor recovery. They granted MHS the one time
permit to load the crude oil. However the loading of the barge
both from the terminal and then when offloading SHOSHONE still
had to comply with the vapor recovery regulations. This somewhat
simplified the test installation on SHOSHONE, as the vapor
recovery piping was no longer required. However it limited
the choices of barges to load SHOSHONE with crude oil. The
Barge JOVALAN operated by Public Service Marine a Harley Marine
Services company was identified as being certificated by the
Coast Guard and the BAAQMD to conduct this operation. The
Coast Guard agreed to the use of this vessel for litering
the crude oil to SHOSHONE. Unfortunately scheduling problems
and cost uncertainties combined at the last minute and prevented
us from loading crude oil for this test.
When all reviews
and approvals were finally received, then the Office of Naval
Research presented the results to the cognizant offices in
naval operations for final approval.
of the Dead Ship Tow
Guard was also involved in the approval of the dead ship tow
of SHOSHONE from the Suisun Bay Reserve Fleet to the Richmond
Point facility where the work was to be carried out. A towing
plan was provided to the Coast Guard by Foss and the tow completed
using three tugs. There was a San Francisco Bay Harbor Pilot
onboard to direct the tow as well as a riding crew from Bay
Ship & Yacht. Both dead ship towing operations were uneventful.
Tow, Test Set-up Installation & Testing
a survey of the vessel, it was towed to the shipyard and modified
in accordance with the test plan. Ship modifications included
provision of the simulated rupture, the addition of gas and
fluid inlet/outlet piping, connection and test of the underpressure
fan, strain gage instrumentation, and pressure, flow and temperature
instrumentation. A data acquisition and computer display and
storage facility was set up on deck. Figure 3 shows approximate
spatial relationship of measurement parts.
were approximately two weeks of setup and testing. These operations
were open to witnessing by USN and others. The final test,
involving a formal demonstration of the simulated rupture,
was observed by a large party or representatives from a number
Approximate spatial relationship of measurement
END OF TANK
access and inert gas inlet
oxygen gauging point, Hand level indication and ullage pressure
- Inboard Oxygen measurement
point, sometimes referred to as "steam" connection
- Blower suction point,
blower oxygen concentration measured from here
- Air leak entrance
- Tank level indicator
and ullage pressure measurement point
The testing sequence consisted of the following
loads testing-- installation of strain gages and monitoring
of stress changes under varying fluid levels and underpressure
levels. Measured strains were converted to stress using
parameters obtained from a test coupon. The loads' testing
was required before further testing was permitted.
testing-- the sea tank remained at atmospheric pressure
during all tests and was not leak tested. The cargo tank,
however, had to be leak test and, as necessary, rehabilitated
to achieve a leak-tight enclosure.
mixing tests-- a series of tests was conducted with preset
inert gas inflow rates and with differing levels of simulated
air leakage. These tests were designed to demonstrate transient
mixing behavior, to provide a basis for validating theoretical
analyses of mixing effects.
tests-- two separate, but essentially identical spill tests
were carried out. Pre-calculated levels of underpressure,
chosen for outflow prevention at the observed cargo and
outflow depths, were used. In the second test, the underpressure
was discontinued after the 30 minute test period in order
to observe cargo outflow.
4 shows the simulated test set up with real time sensor inputs.
This was presented to the observers that were in the ship's
mess hall so that they could watch the activities in real
Water Substitution of Crude Oil
of the test, the design of the installation and all of the
regulatory approvals in the Bay Area and in Washington D.C.
were to allow crude oil to be used in the test. 6,300 bbls
were required. At the last moment the schedule for delivery
of the crude oil to the USNS Shoshone and the cost of delivery
and return became unacceptable uncertainties. With the approval
of the Office of Naval Research, fresh water was substituted
for crude oil. As a result some credibility was lost but fortunately
very little information was sacrificed. This was because laboratory
tests with crude oil were being conducted simultaneously.
Three crude oil samples covering most of the natural range
were (laboratory) tested at a number of temperatures and negative
pressures, and extensive analysis was performed
of Full Scale Test Results
- The spill test validated
the effectiveness of ullage underpressure in restraining
cargo loss. In two separate tests, a 30-minute exposure
to potential spillage evidenced no cargo loss.
- The simple underpressure
control system was stable and was able to set and maintain
a specified ullage pressure even with controlled leakage.
No real time adjustment was required.
- The laboratory
test, covering a wide range of crude oil density, obtained
consistent measurements of evaporant composition. As expected,
the substances evaporated were mostly C7 and below.
- The Equation
of State (EOS) model developed by the laboratory, typical
of many similar computer programs used in the petroleum
industry, correlated well with measurements. The EOS model
can be used as a design tool in the detail development of
the underpressure system.
- The transient and steady state gas
mixing predictions were reasonably confirmed in testing.
The calculated time constants were well supported. These
results suggest that the analytical approach is reasonably
good, but that some further analysis or mockup testing may
be required for some geometrical arrangements.
- The test validated that safe inert
gas level in the ullage space can be maintained in spite
of air leakage
- Strain measurements indicated low stress
increments due to pressure loading, and agreed reasonably
well with predictions.
project, the "American Underpressure System (AUPS) Test and
Validation Project", does not complete the program - it initiates
the program. The single focus of the program is to reduce
the potential for catastrophic events from casualties to single
hull tankers, as well as from double hull tankers. The project,
which has just completed, has verified that these catastrophic
events can be prevented or, at least, significantly minimized.
The technical information obtained from the full-scale test,
from the laboratory experiments, and from the accompanying
analyses was used to proceed to initiate the design installation
of AUPS in a 70,000 dwt operating crude carrier.
design and technical data must satisfy many questions and
requirements that have been set forth by the U.S. Coast Guard.
This procedure is required to allow the rulemaking change
that will permit crude carriers to maintain an inerted vacuum
in all of the ullage spaces. There are four principal areas
of effort that still must be accomplished. The design must
proceed to a level approaching in detail and definition that
of a contract design. The control system must be defined to
the same level. A sophisticated simulation program must be
developed to permit the investigation of all aspects of all
catastrophes, and the reliability, economic and other analyses
required by the Coast Guard must be made.
is a strong sense of urgency to try to protect the single
hull tankers with AUPS before any additional catastrophes
might occur. One important additional benefit to the U.S.
Maritime Industry is the additional work that these installations
will provide to the repair shipyards,
Systems is scheduled to have all of the designs and technical
data available for Coast Guard review by the Spring of 2002.
(8) photographs Test Installation on board USNS Shoshone
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