4.2 Methods and systems for the control of VOC during Transit
Clasification Society 2024 - Version 9.40
Statutory Documents - IMO Publications and Documents - Circulars - Marine Environment Protection Committee - MEPC.1/Circular.680 – Technical Information on Systems and Operation to Assist Development of VOC Management Plans – (17 July 2009) - Annex – Technical Information on Vapour Pressure Control Systems and Their Operation to Assist Development of VOC Management Plans for Tankers Carrying Cruide Oil - Section 4 – Methods and systems for the control VOC - 4.2 Methods and systems for the control of VOC during Transit

4.2 Methods and systems for the control of VOC during Transit

  4.2.1 Best Practices/Design

  • .1 Manual pressure relief procedures (tank pressure control);

  • .2 P/V valve condition and maintenance;

  • .3 Condition of gaskets for hatches and piping;

  • .4 Inert gas topping up procedures;

  • .5 Partially filled tanks;

  • .6 Loading sequence and rate; and

  • .7 COW procedures (closed cyclefootnote).

  4.2.2 VOCON procedure

By reference to Figure 4.2 below, this procedure requires the monitoring and the recording of the pressure drop during a release of gas from the cargo tank vapour system. This can be undertaken with the use of the Inert Gas pressure gauge in the cargo control room or, as available, located on the Inert Gas pipeline on deck. Figure 4.2 shows a pressure drop profile using the mast riser and the inflection in the pressure drop where the mast riser valve should be shut.

Figure 4.2 A mast riser release

  The VOCON operational procedure

  (1) Before opening the mast riser, note the pressure in the Inert Gas pipeline system.

  (2) Open the pressure release valve and record/monitor the pressure within the Inert Gas pipeline at regular short intervals (every 30 seconds for a mast riser release).

  (3) Plot the pressure drop profile. This can be achieved either manually or by use of the Inert Gas Oxygen and Pressure Recorder in the Cargo Control Room but an increase in the Recorder paper feed rate will be required to achieve definition of the plot.

  (4) When the rate of pressure drop becomes constant (after the initial rapid pressure drop) then the gas release should be stopped and the valve closed.

  (5) Monitor the Tank Gas Pressure after completion of the controlled release in order to check the final pressure obtained within the Vapour/Inert Gas system.

  Advice Notes

  (A) A review of Figure 4.2 shows a clear change in the rate of pressure drop during the release period. If the gas release continues after this point then the pressure in the Inert Gas system will be quickly restored to the pressure associated with the point where the rate of pressure drop changes.

  (B) If there is a straight line drop of pressure observed and no inflection observed by 800 mmWG, then close the release valve anyway.

  (C) By reference to the ISGOTT Publication, all safety measures should be taken to minimize the hazards associated with vented gases from the vessel’s cargo tank system.

  4.2.3 Recovery of excess VOC and tank absorption (Venturi system)

The Venturi system involves a process where evolved VOC is reabsorbed back into the cargo. The system typically consists of a pressure controlled pump, feeding oil to a unit with Venturi(s). The Venturi draws VOC, H2S and inert gases (IG) from the common cargo tank venting/inert gas main line. The Venturi unit is designed to generate a bubble size optimal for their collapse in the crude oil cargo and rapid absorption. Released near the tank bottom, the soluble compounds are kept dissolved by the pressure head there. Inert gas will eventually surface.

For ships that have been provided with a Venturi type system, the VOC emissions will be controlled when the system is in operation.

The VOC control system efficiency as well as any operational limitations related to, e.g., applicability for different cargo handling modes (loading, transit, COW), maximum allowable loading rates or crude vapour pressures, are to be specified in the VOC management plan.

Parent topic: Section 4 – Methods and systems for the control VOC
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