VOC Management Plan Frequently asked Questions (FAQs) Training Guidance and and understanding the Principles

What is a VOC Management Plan? and, Why do you need it?  A VOC Management Plan is a plan that is Crude Oil Tanker specific and compiled in accordance with IMO Guidelines (MEPC 185 (59) and MEPC Circ 680) to supply guidance for the limitation of VOC emissions from Crude Oil cargoes during its transportation and handling (loading or discharge (COW)).  In accordance with MEPC Resolution 181 (59) the Plan will be inspected by Port State Control officers pursuant to the requirements of Regulation 15.6 of MARPOL Annex VI.

What is NMVOC? and Why control it?  Non Methane Volatile Organic Compounds (NMVOC) evolve in differing proportions from Crude Oil cargoes and, subject to conditions, are released to the atmosphere during loading and carriage.  VOC gases are harmful and a pollutant to the atmosphere and Maritime Environment. They also act a precursor gas to the formation of OZONE which is a recognised Greenhouse Gas.

What are the Critical Criteria impacting Tanker Operations?  There are two main criteria impacting the necessity for NMVOC control for tanker operations. They are:  Pressure – necessitating a release for control  Temperature – generating vapour pressure  There are other operational criteria that will impact the generation of NMVOC such as:  Any other energy input into the cargo – Rolling and pitching at sea; rate of loading and turbulence in the cargo tanks.

Cargo Tank Gas Phases  Crude Oil Cargo Tanks contain two gas/vapour phases that individually and together have an impact upon Pressure generation in the Cargo Tank gas phase system. These are: –The Inert Gas (I.G.) component of the total –The NMVOC component of the total  Each of these gases should be considered separately and together for NMVOC Control

The Impact of Pressure  Cargo Tanks have a limiting pressure controlled by the P/V valve setting.  How is Pressure generated by I.G.?  IG Pressure is dependent upon the temperature of Gas and follows the “Ideal Gas” Law  It varies throughout the Day and Night with heating from sunlight  How is Pressure generated by NMVOC?  NMVOC pressure is dependent upon the Liquid temperature of the crude oil and the amount of volatile components in the crude oil cargo which evolve into the vapour space at varying concentrations (temperature dependent). This varies with differing Crude Oil types.

NMVOC concentrations during loading

The Impact of Temperature  The Impact of temperature is different for the two types of gases present in the vapour phase system onboard a crude oil tanker.  IG temperature will causes daily fluctuations in the pressure contribution from this gas.  NMVOC liquid phase temperature determines the amount of evolved vapour and associated concentration to that found in the liquid phase – Henry’s Law or Raoult’s Law and Saturated Vapour Pressure (SVP).

A Pressure/Temperature Profile of a crude oil during a voyage

Cargo Tank Pressure (necessitating release)  The cargo tank pressure shown on pressure gauges on the IG system reflects the total pressure in the system and is the combination/addition of the IG and NMVOC pressures – Dalton’s Law of Partial Pressure  Cargo Tank P/V valves are typically set at around 1400 mmWG (16.53 psia).  When the total or combined pressures exceed the opening pressure a proportion of NMVOC will be released.

Where Do you get the information from to determine Pressure?  The NMVOC pressure is found on the Material Safety Data Sheet (MSDS) for the cargo. When carrying more than one grade of cargo a rough average of the vapour pressures will supply guidance.  The MSDS could supply data as: –The Reid Vapour Pressure (RVP) but this is measured at approx 38 Deg C –Saturated Vapour Pressure (SVP) but here the standardised conditions for measurement must also be known.

Tanker Operations covered by the Management Plan  The Crude Oil Tanker operations required to be covered by the VOC Management Plan are:  Control of Emissions during Loading  Control of Emissions during the Voyage  Control of Emissions during Discharge – Crude Oil Washing  Control of Emissions using Specialist Equipment – if fitted  Additional considerations should be given to Maintenance routines for:  P/V valves and other vapour control systems  Tank openings and vapour leakage

Control of Emissions on Loading (1)  Prior to loading the cargo tanks need to be fully inerted. To limit the extent of emissions the over pressure in the cargo tanks should be set to the lowest deemed safe. –This will allow the NMVOC to evolve to generate its own Partial Pressure at its Saturated Vapour Pressure which will contribute to the Total Pressure within the tank vapour system  During loading NMVOC is evolved from the incoming cargo and combined with existing NMVOC in the cargo tanks. There are two pressure control methods:  Release to atmosphere via mast riser but maintaining the highest possible pressure within the tank vapour system  Using the Vapour Return Manifold connection and return the vapour ashore as per IMO MSC Circ again maintaining the highest possible tank pressure.

Control of Emissions on Loading (2)  Evolution of NMVOC during loading is mainly caused by pressure drops in the loading pipeline system – throttled valves etc.  The main cause of vapour evolution during loading is the pressure drop at the top of the loading drop line. At this point vapour bubbles evolve and are carried into the cargo tanks where they “boil off” until the “Equilibrium Pressure” (Saturated Vapour Pressure of the crude oil) in the cargo tank vapour system is achieved. This pressure is independent of the Inert Gas pressure.

Control of Emissions during the Voyage  NMVOC will evolve from the crude oil cargo during the voyage. This will cease when the pressure in the vapour phase pressure is in “equilibrium” with the liquid phase SVP. Any change in cargo temperature will impact the equilibrium pressure. Higher temperature will increase the pressure and vice versa.  During the voyage the SVP should be maintained in the cargo vapour system and any required manual vapour releases should not go below this pressure – reference VOCON Procedure. LOW CLOSING PRESSURES SHOULD BE AVOIDED after a release.

Control of Emissions during Discharge  Vapour emissions should not occur during a discharge programme unless heavy weather ballast is to be placed in a cargo tank.  During discharge care should be exercised with the supply of IG into the cargo tanks and avoiding excessive pressure in the system. A low pressure cargo tank IG regime will assist in efficient discharging/stripping.  During Crude Oil Washing (COW) selection of crude oil type (multi parcel loadings) with lowest SVP is preferred. Closed Cycle COW will limit VOC generation during this process and avoid excessive VOC for displacement at next loading.

Knowledge of Operation of Special Control Equipment  There are numerous differing types of VOC control equipment being marketed. Become acquainted with the operation of this equipment through it instruction manuals.  One popular method being employed is the increased opening pressure settings of P/V valves from 1400 mmWG to around 2000 mmWG together with avoidance of manual releases during a voyage.

Record Calculation and Sheet  The VOC Management Plan requires the recording and estimation of the extent of VOC emissions during a voyage.  This can only be done with manually controlled releases – see next slide for proposed spreadsheet calculation/recording method.  The extent of release during loading can be estimated from the volume of the loaded cargo and, thus, the extent of displaced vapour.

Record of Releases – Manually controlled discharge The main shipboard input criteria for this spreadsheet are the accurate recordings for: The Start and Stop Times for the manual release The Tank Pressure at the start and stop of the release A measurement or estimation of the % NMVOC in the vapour phase of the tanks