VOC Emission Prevention by AUPS

It is estimated that 4 to 7 million of tons of cargo is lost due to VOC emission yearly versus 25,000 tons due to spillage. Economically as well environmentally it is unacceptable. Emissions produced by venting during loading and transit are in the range of 0.1 to 0.3 percent, depending on tanker design and cargo characteristics. Losses can reach two percent or more when the cargo has not been stripped of its most volatile components before loading aboard the tanker. Double hull tankers may produce higher VOC emission than single hull tankers, because of the insulation of the hot oil from the surrounding cooler temperatures.

The American Underpressure System (AUPS) will contain the vapor emissions that occur during transit by simply returning them to their tanks. Essentially, the system provided for the prevention of fluid cargo loss can also serve as the means for reducing vapor losses. The AUPS configuration limits collision and grounding losses by using a simple closed loop inert gas distribution system, with components and controls independent of the tanker and other ship services.

VOC Emission Control using the AUPS System

AUPS is configured to contain both VOC emissions, and spillage of oil due to hull rupture. Emissions are contained safely during transit by circulating and returning the ullage gas back to their tanks, within a closed-loop gas distribution system. The primary control objectives are to maintain the pressure and oxygen content in the closed system from the effects of minor air leaks, radiant sun loads and the potential for crude oil outgassing.

The design features for emission control are evaluated in the following sections and are based on experimental data from a full scale tanker test for air leaks and out-gassing results from the lab tests.

Design Features

The VOC emission loss by venting to maintain the desired underpressures from the effects of out-gassing, air leaks and diurnal sun's radiant heat are investigated for a 75,000 dwt tanker on a 15 day voyage, as well as the change in oxygen content.

The data suggests that in most cases the pressure and oxygen rise is modest and it would be unnecessary to vent any gas, and in any extreme case a small loss of small loss of cargo. The AUPS closed-loop features and operational modes and design schematics are also described.

Loss Due to Out-Gassing

Preliminary studies explored experimentally and theoretically the vapor pressure of various types of crude oil as a function of temperature at the underpressure conditions present in AUPS. The test results provide the mole fractions of the constituents of the saturated gases and show the hydrocarbon content of the saturated gases "are at a level of 40-50% by volume." The analysis of these results using the Claussius-Clayperon equation indicates that the VOC will achieve an equilibrium vapor pressure within the cargo hold of AUPS. The Claussius-Clayperon equation demonstrates the relationship between the vapor pressure of a liquid compared to temperature. Based on the laboratory data, it may be used to predict the boiling point of the various types of oil when exposed to the underpressures found in AUPS

The equilibrium vapor pressures for A.P.I. grades 12, 30, and 37 oil are below the total pressure of the ullage gases at the various underpressures. This result indicates that boiling of the oil will not occur at these underpressures. The vapor over the surface of the liquid crude oil will equilibrate (saturate) to this vapor pressure and will maintain the desired underpressure within the system. Once saturation, the equilibrium vapor pressure, is achieved, the VOC pressure will deviate minimally and there will be no further observable evaporation. The theoretical studies provide a good model to predict the equilibrium vapor pressure of the crude oil and may be used to provide information about the maximum attainable underpressure. Additionally, during a typical loading of crude oil, the HC content of the residual ullage gases have been measured near 30-50% by volume. The conclusion that can be drawn from these observations are:

1. AUPS ullage space after cargo loading is near saturation of HC, and if the ullage remains a closed system minimal out-gassing or evaporation will occur over the voyage in the AUPS system.
2. The test results also show that at the range of underpressures expected for AUPS operation (tested up to -5PSI) no boiling is likely.

NOTE: Laboratory tests and studies was performed by Hycal Energy Research Laboratories, LTD., of Calgary, Canada.

Loss Due to Air Leakage

The tank assembly leakage was measured during AUPS full-scale test on USNS Shoshone. The tank leakage was 1.8 inch-water during a four (4) hour test at an underpressure of -2 psi. This was the basis for sizing the leak/tank and estimating the venting requirements to maintain the initially set underpressure. A 75,000 DWT tanker with an ullage capacity per tank of 10,000 ft3, would require a venting of 8% of its ullage capacity, which results in a loss of cargo of 38 lbs/tank, or 460 lbs for the 75,000 DWT tanker.

Loss Due to Diurnal Sun's Radiant Heat

No cargo loss is expected from diurnal temperature variations in routine transit. Heat transfer analysis was performed to estimate a worst-case ullage gas temperature rise due to solar heating. The ullage pressure would increase by 7% and sharply reduce over several hours. The control procedures to mitigate these diurnal fluctuations in pressure are either to:
1. Allow the fluctuations and correct by venting when the probability of a breach is high;
2. Use a seawater heat exchanger to remove the fluctuating gas temperatures and resulting pressure.

Oxygen Level Changes Due to Air-Leaks

The air-leak rate determines the oxygen content changes of the inerted ullage spaces. The oxygen enhancement over a 15-day voyage is marginal (2% rising to 2.5% by vol.). Thus safety is assured, and no cargo loss due to VOC emission is expected.

The AUPS Closed-Loop Design

The primary purpose of the design is to maintain control over pressure and oxygen concentration. The prevention of any possibility of flammable mixtures mandates the continuous circulation of the inert gases mixture through the ullage spaces to preclude the formation of air gas pockets in these spaces. The design features:

1. Simple mechanical arrangement of piping, valves and blowers with automated controls to ensure safety and capability to provide dual protection from emissions and oil spills when operating,

2. Self-sufficiency and independence from ship services, including a separate electric supply system.

The AUPS dual containment system provides emission and spill containment when in operation. The arrangement of piping, valves and blowers to provide these functions is shown schematically in Figure 1.

Figure 1 (Click image to enlarge)

Basically, collecting the tank ullage gases into an exhaust header, and returning the gases through a separate inlet header provide the circulation of ullage gases in a closed-loop configuration. The headers are connected by two separate interconnecting ducts, one for routine circulation and one for casualty-mode circulation, each containing high head blower/valves for routine circulation. An automated control subsystem responds to critical placed sensors to perform three major functions: (1) controlling the underpressure; (2) controlling the oxygen composition of the ullage gases; and, (3) monitoring system status to assist personnel in managing the system and automatically sequencing flow during routine and casualty operational modes.

Operational Modes/Requirements

Loading is the primary source of evaporative emissions followed by in-transit emissions. The loading/unloading of cargo is performed with existing equipment and procedures. The emissions can be contained with vapor balance services or vapor recovery facilities. AUPS is enabled after these functions as follows:

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