Marine Engineering Study Materials

Regulation 15 of Annex Ⅵ of MARPOL 73/78, as revised by IMO Resolution MEPC.176(58) regulates the VOC emissions from a tanker in designated port(s) or terminal(s) of a Party regulation such emissions.

Regulation 15.6 requires that a tanker carrying crude oil shall have on board and implement a VOC Management Plan approved by the Administration. The aim of the VOC Management Plan is to identify the arrangements and equipment required to enable compliance with Regulation 15.6 of the revised Annex Ⅵ of MARPOL 73/78, and to clearly identify for the ship’s officers all operational procedures for VOC emission control.

Chapter 1 – Objectives

The purpose of the VOC management plan is to ensure that the operation of a tanker, to which regulation 15 of MARPOL Annex VI applies, prevents or minimizes VOC emissions to the extent possible.

Emissions of VOCs can be prevented or minimized by:

1. Optimizing operational procedures to minimize the release of VOC emissions; and/or
2. Using devices, equipment, or design changes to prevent or minimize VOC emissions.

Best management practices for preventing or minimizing VOC emissions during the loading and carriage of cargoes which generate VOC emissions are:

• The loading procedures should take into account potential gas releases due to low pressure and, where possible, the routing of oil from crude oil manifolds into the tanks should be done so as to avoid or minimize excessive throttling and high flow velocity in pipes;
• The ship should define a target operating pressure for the cargo tanks. This pressure should be as high as safely possible and the ship should aim to maintain tanks at this level during the loading and carriage of relevant cargo;
• When venting to reduce tank pressure is required, the decrease in the pressure in the tanks should be as small as possible to maintain the tank pressure as high as possible;
• The amount of inert gas added should be minimized. Increasing tank pressure by adding inert gas does not prevent VOC release but it may increase venting and therefore increased VOC emissions; and
• When crude oil washing is considered, its effect on VOC emissions should be taken into account. VOC emissions can be reduced by shortening the duration of the washing or by using a closed cycle crude oil washing programme.

A person shall be designated in the VOC management plan to be responsible for implementing the plan and that person may assign appropriate personnel to carry out the relevant tasks.

Ship-specific procedures should be written or modified to address relevant VOC emissions, such as the Loading, Carriage of relevant cargo; and Crude oil washing. If the ship is equipped with VOC reduction devices or equipment, the use of these devices or equipment should be incorporated into the above procedures as appropriate.

The plan should describe the training program to facilitate best management practices for the ship to prevent or minimize VOC emissions.

Chapter 2 – General Information

Contain Particulars of Registration and Principal Dimensions.

Chapter 3 – Cargo Tanks and Equipments

Contain list of cargo tanks, tank capacity, setting pressures of each pressure/vacuum valve, setting pressure of PV breaker, set value for pressure alarms, Cargo Tank Venting System (details of the inert gas system), details of pressure/vacuum valves, details of PV breaker, Vapour Emission Control System (VECS), information on maximum allowable loading rate with maximum vapour densities, Crude oil washing system, etc.

Chapter 4 – Methods for the control Volatile Organic Compound (VOC) Emissions

VOCs are a pollutant to the air and act as a precursor to the formation of Tropospheric Ozone – commonly termed Smog.

Before cargo loading, where inert gas is added for inerting in cargo tanks, the amount of inert gas added should be minimized to prevent the release of mixture gas of VOC and inert gas.

Cargo vapour (VOC) evolves during cargo loading. The quantity of this vapour depends on the loading capacity, and the vapour is emitted to atmosphere through high velocity relief/vacuum valves to protect cargo tanks and their relevant pipe lines. To prevent over pressure in the cargo tanks system, the individual tank high velocity relief/vacuum valves is to be opened during cargo loading unless special regulations are required by Port Authorities.

Where VECS is available, evolved vapour from cargo oil is to be return to shore terminal using this system and reduce the vapour release to atmosphere as far as possible.

During voyage, cargo vapours may be evolved by thermal breathing in cargo tanks.

The setting pressure of the pressure relief system should be as high as safely possible to reduce the cargo vapour release to atmosphere, but when cargo tank pressure exceeds the setting pressure, the cargo vapour (mixture of air, inert gas and VOC) is released to atmosphere through high velocity valves for each cargo tank vent.

The vessel has cargo heating system. The cooler the cargo temperature the lower will be the Saturated Vapour Pressure of the crude oil but care should be taken not to allow cooling of waxy cargoes too much. The vessel’s requirement of cargo heating is as follows; They vessels steam heating coils are blanked from the main steam line. The vessel is only engaged in carriage of white petroleum product oils with low flash point which does not necessitate heating of cargo.

Before unloading cargo, where inert gas is added for inerting in cargo tanks, the amount of inert gas added should be minimized to prevent the release of mixture gas of VOC and inert gas.

Where VECS is available, evolved vapour from cargo oil is to be return to shore terminal using this system and reduce the release to atmosphere as far as possible.

When crude oil washing is carried out, its effect on VOC emissions should be taken into account. VOC emissions can be reduced by shortening the duration of the washing or by using a closed cycle crude oil washing programme.

The setting pressure of the pressure relief system is as high as safely possible to reduce the cargo vapor release to atmosphere. A secondary means of allowing full flow relief of vapor, air or inert gas mixtures are provided onboard to prevent over pressure or under pressure in the event of failure of the pressure relief system. Where pressure sensors are fitted alternatively, the crews are to be pay attention to this monitoring system with the alarm and take appropriate actions to prevent tank over/under pressure. Regular inspections of P/V valves, hatches and pipe connections for tightness are to be carried out. Any leakages detected shall be entered into the maintenance program for rectification at first opportunity.

Chapter 5 – Training Programme

A training programme is to be developed for the persons intended to assume overall charge of the VOC management on board each ship, which should contain:

1. An introduction to the purpose of VOC emission control. (Volatile organic compounds (VOCs) may be toxic, and when they evaporate into the air they can react with Nitrogen Oxides (NOx) in sunlight and split apart oxygen molecules in air and thereby form ground-level ozone, commonly referred to as smog. The layer of brown haze it produces is not just an eyesore, but also is a source of serious illnesses. Ozone is extremely irritating to the airways and the lungs, causing serious damage to the delicate cells lining the airways. It contributes to decreased lung function, increased respiratory symptoms and illnesses.)
2. An introduction to Regulation 15 of MARPOL Annex VI
3. An introduction to the principles of VOC emission control
4. General VOC emission control options
5. Ship specific VOC emission control options
6. Monitoring and recording of VOC release
7. Hazards and Safety related to VOC emission control (The hull and its pressure limitations, Personnel safety hazards related to exposure to crude oil vapour.)

Chapter 6 – Designated Person

A person should be designated to assume overall charge of the VOC management on board the ship, with necessary experiences in cargo handling and full knowledge of the contents of the VOC management plan.

Chapter 7 – Ship Specific Drawings

Such as:

1. General Arrangement
2. Cargo and Ballast Tank Plan  
3. Diagram of Cargo Oil Line
4. Diagram of Cargo Oil Tank Vent Line
5. Diagram of Inert Gas Line
6. Diagram of Tank Cleaning System
7. Pressure/Vacuum Relief Valve, etc.

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Reference: VOC MANAGEMENT PLAN

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