The American Underpressure System (AUPS) test on June 11, 2001, on board the USNS Shoshone in Richmond, California, was flawlessly concluded.
    The AUPS test clearly showed the underpressure can be maintained and controlled with the following major safety considerations:
    • Structural integrity -- The Shoshone is a 45 year old vessel which was subjected to over 3 PSI of negative pressures.
    • Retention and maintenance of desired negative pressures
    • Retention and control of inert gas blanket and desired oxygen/inert gas ratio.
    • Retention and control of inert gas ratio and desired underpressures in spite of deliberate air leaks inserted during testing


    MH Systems, Inc., with the support of the Office of Naval Research (ONR) conducted the full scale test of the American Underpressure System from June 6th through June 11th, 2001. The testing was accomplished aboard the USNS SHOSHONE; a Reserve Fleet Navy tanker, while the ship was moored at the Port of Richmond, California. USNS SHOSHONE was modified to equip one tank with a basic AUPS and the necessary monitoring and data recording equipment. A valve to simulate a near bottom rupture was installed between this Test Tank and the adjacent tank which is designated the "sea tank".

    The major portion of the test program was designed to prove the validity of AUPS to mitigate the loss of cargo from a tank (or tanks) during grounding or collision incidents which lead to broaching the integrity of a vessel's hull below the waterline while maintaining a safe oxygen concentration in the ullage space. The test program also collected other data of interest to system designers; of particular interest was the validation of the mixing equations developed regarding the mixing and dilution of the aforementioned air leakage. Data also was obtained to facilitate proceeding with the detailed design development of complete operational systems.

    AUPS functions by maintaining a controlled underpressure (ie. subatmospheric pressure) in the ullage space of the vessel's cargo tanks. While this underpressure is applied a constant circulation of inert gas is flowing through the ullage space of the cargo tanks to allow instrumentation to sample the tank effluent vapor for any sign an increasing oxygen level. This increasing oxygen level is indicative of a leak of atmospheric air into the tank through defective fittings serving the top of the tank.

    To facilitate the use of controlled leakage rates to test the detection and mixing of air leaks it was necessary to have a tight and structurally sound test tank. To that end the initial steps of the test were intended to verify the structural adequacy and tightness of the tank. The tightness tests were also used to collect structural stress data for evaluation of major stresses on the ship's main structural members.


    While complete and detailed data analysis is proceeding the following highlights from the initial "quick look" analysis may be presented. Understand that these data are preliminary at this stage.

    The test tank was essentially "leak free". Starting from an underpressure of about -55 inches of water pressure gauge, (in H2OG) the test tank lost only 1.6 in H2OG during the 4 hour drop test. This was well lower than expected and indicated that there were no stray leaks to slew the mixing data.

    The stress data collected on structural elements of the ship during the tank positive and negative static pressure tests indicates the stress levels imposed on a tank due to the underpressure application are no greater than those imposed during positive pressure operations.

    The action of the underpressure to mitigate cargo outflow was validated. During 2 tests runs, when the valve simulating a rupture was opened, the system maintained the underpressure and held the liquid level in the cargo tank at the same level it was prior to the initiation of the leak. At the conclusion of the 2nd test run the test tank was vented to atmospheric pressure and the liquid level dropped to the level of the adjacent "sea" tank. This confirmed the validity of the test and the operation on the AUPS.

    The results of the controlled air leakage tests validate the predictions in the mixing equations and indicate that air leakage into the tanks at underpressure can be detected and dealt with; within the limits of experimental error. Figure below graphically shows the major parameters in the test tank during the 60 CFM circulation - 24 CFM leakage test.

    Figure One: Mixing Test
    (click to view full size)

  4. The ability of the system to maintain the required underpressure during the air leakage excursions was validated.

    Figure Two: Underpressure System
    (click to view full size)

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