Alternative Double Hull Tanker Design
Double Side Hull + Sub-Atmospheric Ullage

Technical Paper Presented at
World Maritime Technology Conference (WMTC) 2009
Click here to download Technical Paper

This paper presents an alternative to the double hull configuration now being constructed and used for crude oil transport service.

As currently being built, these vessels meet their design objective of reducing oil spills resulting from collision or grounding. However, double hulls are structurally complex and expensive, and appear to be developing operational disadvantages as well. This paper proposes to have double side hulls on a single bottom hull (without an inner-bottom) for new construction or to simply retrofit a single hull tanker with double side hulls and add on the sub-atmospheric pressure in the ullage.

The underpressure approach in this paper uses a closed loop vapor control system that prevents or drastically minimizes emission of crude oil vapor. This paper is presented in three parts:(1) Introduction to the underpressure system (2) Emission containment of crude oil vapor in the underpressure system and (3) Comparative probabilistic outflow analysis of double hull versus double side hull +underpressure system. Analysis to date indicates that this design concept can provide equivalent cargo loss protection at reduced cost without introducing risk elements or uncertainties.

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HusainWMTC2009 Paper No 42 012309.pdf
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Completion of the American Underpressure System (AUPS) Test Program
Final Report to the Office of Naval Research (ONR) - 2004
Click here to download Executive Summary of Final Report


The U.S. Congressional Oil Pollution Act of 1990 allowed for the evaluation and approval of alternatives that can be determined to have equal or better performance than the double hull in protecting the marine environment.

In April 2004, MH Systems completed the final part of the test, validation and analysis of the American Underpressure System (AUPS) under the aegis of the Office Naval Research (ONR). This is culmination of a decade of long work to find alternative solution(s) to prevent or reduce catastrophic oil spills. AUPS is an active inert gas controlled system utilizing a vacuum technique, which drastically reduces or totally prevents spillage from a rupture to the hull, accidentally or from enemy action. It has been tested in full scale on the USNS Shoshone also under contract from ONR in 2002.

The EXECUTIVE SUMMARY of the Final Report, prepared at the completion of the comprehensive analysis of the tests and risk-based design, is shown herein.

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MHS Final Report 2004 - Exec Summary.pdf
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TECHNICAL PAPER
Crude Oil Under Negative Pressures and Hydrocarbons Emission Containment

Paper presented to the World Maritime Technology Conference October 2003 in San Francisco, CA. Event co-sponsored by SNAME - Society of Naval Architects and Marine Engineers.

Authors:
M. Husain
, (M) President, MH Systems, Inc., San Diego, CA
H. Hunter, MH Systems, Inc. San Diego, CA
D. Altshuller, MH Systems, Inc., San Diego, CA
E. Shtepani, Hycal Energy Research Labs Ltd., Calgary, AB, Canada
S. Luckhardt, Energy Research Center, University of California, San Diego, CA

ABSTRACT

This paper presents the results of experimental and theoretical analysis of crude oil under negative (sub-atmospheric) pressures. The use of negative pressure in the ullage space of tankers has been proposed for the mitigation of cargo losses resulting from accidental hull rupture. This potential application resulted in detailed laboratory tests of various crudes over a range of temperatures and pressures. The findings were then applied to prevent or minimize VOC (Volatile Organic Compound) emissions occurring during transport of crude oil.

Equilibrium between liquid and gas/vapor components is reached at moderate hydrocarbon partial pressures. Predictions of liquid-vapor transformation can be made very accurately from known relationships, in the present case, the Peng-Robinson equation of state. The amount of material vaporized in reaching equilibrium is small, and the composition of the source liquid is changed only slightly. It can be shown that the partial pressures of hydrocarbon components are low, and can be accommodated by a closed system.

The adverse environmental and economic losses caused by VOC emissions from crude oil tankers are significant. A brief discussion on the design of a closed, negative pressure system for VOC emission minimization is discussed. The closed loop system, along with a seawater heat exchanger, mitigates the effects of temperature and pressures as well and will significantly reduce VOC venting to the atmosphere...


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A17(R52) MH Systems Paper SNAME 2003.pdf
A17(R52) MH Systems Paper SNAME 2003.zip

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AUPS DEFINED
A State-of-the-Art Spill Avoidance System

The American Underpressure System (AUPS) is a dynamic system which creates a slight vacuum (two to four pounds per square inch) in each cargo tank.  This vacuum, assisted by the outside hydrostatic pressure of the surrounding water, prevents or minimizes cargo loss in the event of hull rupture.  In case of a bottom rupture caused by grounding, nearly all of the cargo can be protected.  In the case of side hull damage, cargo below the level of the damage will be lost, while the cargo above the side hull rupture will be protected.

State-of-the-art hardware and software are used that can easily be installed in tankers.   This system can be used in conjunction with existing inert gas systems that are mandatory on most tankers to prevent explosions. 

"In the case of a bottom rupture caused by grounding, nearly all of the cargo can be protected."

AUPS consists essentially of exhaust blowers with their isolation and control valves tapping into the inert gas system seal/non-return valves.  A negative pressure of inert gas is created in the ullage space - the volume of gas above the oil.  This negative pressure or underpressure is continuously adjusted and prevents oil from spilling if the tanker is ruptured.  Stated simply, the oil is held in the tank by the slight underpressure. 

The retrofitting of this technology on existing tankers and barges, small and large shipyards worldwide could realize about $5 billion in new business, a significant portion of which could be accomplished in U.S. facilities.

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CONCEPT MADE SIMPLE
Imagine a Straw...simple.jpg (28316 bytes)

The underpressure concept is best understood by making an analogy with which most people can identify.  Imagine sipping water half-way up through a straw and sealing off the top of the straw with your fingertip.  A simple principle of hydraulics allows the water to be held in the straw at this level until the finger is released.

This same principle allows oil to be contained within the hull should a rupture occur in the tank.  When a tanker is loaded, the oil level inside the tank is higher than the surrounding seawater level.  This causes a higher pressure to exist inside the tanker due to hydrostatic pressure.  Normally, when a hull is ruptured, the pressure inside forces oil to flow out up to the level of the surrounding seawater (discounting density).  Using AUPS, this complete loss would not occur. 

The system equalizes the pressure inside and outside the tanker at the rupture point by applying a slight underpressure of 2 to 4 psi in the ullage space of the tank.  As oil flows out, it is replaced by seawater up to the rupture point only.  Oil loss is held to a minimum as all oil above the rupture point will remain in the tanker.    In the event of an accidental grounding bottom rupture, AUPS would prevent virtually all oil spillage from the tanker.
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AUPS SYSTEM RELIABILITY

The American Underpressure System utilizes the existing Inert Gas system in its intact configuration and functions. The positive pressure function of the existing I.G. system remains unchanged. The underpressure system simply adds a modular system to draw down the inerted ullage gas mixture to a predetermined underpressure level, and maintains the underpressure using a simple control system. Thus a major portion of the underpressure system is already present in existing ships. Some current inert gas systems lack redundant functionality, and may be upgraded as part of the underpressure system installation. Further investigation to establish reliability baselines is needed in this area as the current systems are often not tracked for failure history.

The underpressure system is an active system, that is, it functions through the operation of components such as valves, fans and controls. In designing the system for seagoing service, consideration must be given to the reliability of the system so that its ability to control cargo loss is not impaired. This objective is achieved through three mechanisms, (1) the selection of high reliability components, (2) the use of parallel redundancy for key components and (3) the inherent ability of the system to function in degraded modes of operation.

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