Shipping is a cornerstone of international trade, and the safe and environmentally responsible management of onboard operations is paramount. Two critical areas of focus are the Ozone-Depleting Substances (ODS) and compliance with MARPOL Annex VI, which deals with the prevention of air pollution from ships. Effective management practices onboard, including the handling of Volatile Organic Compounds (VOCs), play a crucial role in ensuring safety, efficiency, and environmental sustainability.
Volatile Organic Compounds (VOCs) are organic chemicals that evaporate at room temperature, contributing to air pollution. VOCs are significant precursors to tropospheric ozone formation, commonly known as smog. This pollution not only degrades air quality but also poses health risks to humans and animals.
Four factors influence the rate of VOC evolution and their release into the atmosphere:
Volatility or Vapor Pressure of Crude Oil
Higher volatility means crude oil is more prone to vaporizing and releasing VOCs.
Temperature of the Liquid and Gas Phases
Increased temperatures accelerate the release of VOCs due to higher vapor pressures.
Pressure Settings in Cargo Tanks
Lower pressure control leads to increased vapor release as the cargo tank vents excess gas.
Volume of the Vapor Phase
Larger vapor spaces in tanks allow more VOC accumulation and potential release.
MARPOL Annex VI sets strict guidelines to prevent air pollution from ships. It includes limits on sulfur oxide (SOx) and nitrogen oxide (NOx) emissions, as well as regulations for managing ODS, VOCs, and shipboard incineration.
Minimizing Inert Gas Usage: When inerting cargo tanks, limit the volume of inert gas to reduce VOC and inert gas mixture release. This requires adherence to the Inert Gas System Manual.
Pressure Management: Open the mast riser isolation valve (if applicable) during loading to prevent overpressure unless port regulations dictate otherwise.
Using Vapor Emission Control Systems (VECS): Direct evolved vapors back to the shore terminal where VECS is available, minimizing atmospheric emissions.
Thermal Breathing: During voyages, temperature fluctuations cause vapor release through thermal expansion in cargo tanks.
Pressure Relief System Settings: Maintain the pressure relief system at the highest safe level to minimize vapor release.
Deck Cooling: For excessive vapor release, cooling cargo tanks using water sprinkling on the deck can help stabilize temperatures.
Heating Waxy Cargoes: Avoid excessive cooling of waxy crude oils to prevent solidification.
Controlled Inerting: As in loading, minimize the inert gas used during unloading to reduce unnecessary VOC emissions.
Efficient VECS Usage: Utilize VECS to return vapors to the terminal, reducing atmospheric discharge.
Shorten Washing Duration: Minimizing the duration of crude oil washing reduces VOC emissions.
Closed Cycle Washing: Adopt a closed-loop crude oil washing program for better VOC management.
Structural Health Monitoring: Regularly monitor tank pressure to prevent over- or under-pressure events. Pressure relief systems should allow full flow relief to avoid structural damage.
Alarms and Action Plans: Ensure pressure sensors are operational and alarm systems are active. Crews must respond promptly to prevent mishaps.
Recordkeeping is critical for demonstrating compliance with VOC and other air pollution regulations. Proper documentation includes:
Loading and unloading logs.
Pressure relief system settings.
Instances of vapor release during voyages.
Maintenance of systems like VECS and crude oil washing.
ODS, such as refrigerants and halons, are regulated under MARPOL Annex VI due to their potential to deplete the ozone layer. To ensure compliance:
Maintain an inventory of ODS equipment onboard.
Use alternative refrigerants with low global warming potential (GWP) where possible.
Conduct regular maintenance to prevent leaks.
Properly handle and dispose of ODS during repairs or decommissioning.
To ensure effective management onboard a ship:
Training and Awareness: Crewmembers must understand the environmental regulations and the rationale behind procedures for handling VOCs and ODS.
System Maintenance: Periodic inspection and maintenance of inert gas systems, VECS, and pressure relief systems ensure their efficiency and compliance with MARPOL requirements.
Integrated Approach: Combine emission control measures with overall voyage planning to optimize fuel use and reduce emissions.
Despite current measures, VOC and ODS management onboard faces challenges:
Technological Barriers: Not all vessels are equipped with VECS or advanced systems to mitigate emissions effectively.
Regulatory Compliance: Varying port regulations may complicate uniform adherence to emission controls.
Operational Costs: Implementing advanced systems can be financially demanding.
Future solutions could include:
Widespread adoption of closed-loop systems for washing and gas handling.
Use of advanced predictive systems to monitor and manage VOC evolution dynamically.
Transition to low-carbon or alternative fuels to reduce the overall environmental footprint.
The management of VOCs, ODS, and other emissions onboard ships is a critical aspect of maritime operations that demands a combination of technical, procedural, and regulatory compliance measures. Adherence to MARPOL Annex VI and other international guidelines ensures that shipping continues to play a pivotal role in global trade without compromising environmental integrity. Enhanced training, investment in advanced systems, and a commitment to sustainability will be key to meeting future challenges and protecting the maritime environment.
