VOC Management Plan
Frequently asked Questions
(FAQs)
Training Guidance
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
60
50
% VOC Concentration
40
Orig Vapour %
30
Evolved Vapour %
Total Vapour %
20
10
0
0
10
20
30
40
50
60
% Fill of Cargo Tanks
70
80
90
100
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
30
1600
P/V Valve Opening Pressure
1400
25
1200
1000
15
800
600
10
Maximium Normal
Control Operating
Pressure before Manual
Release by Vessel's
Command
5
400
200
0
0
0
5
10
15
20
25
Day Number
30
35
40
45
50
Pressure mmWG
Temperature Deg C
20
Tank temp
Pressure
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. 585. 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
Voyage NMVOC Emission Calculation Spreadsheet
Time at
Start of
Date Release
29th Sept 1020
Tank
Tank
Mean Diameter Length of
NMVOC
Pressure at Pressure at Pressure of Release Release Mean Total Vapour
NMVOC equiv.
Time at Duration of Start of Stop of during pipeline Pipeline Velocity of Volume of
Vapour Liquid
Stop of Release release Release Release opening from Tank Release Release % of Release Release
Release (Minutes) (mmWG) (mmWG) (Metres WG) (Metres) (Metres) (Metres/sec) (Metres ^3) NMVOC (Metres^3) (Metres^3)
1028
8
1200
800
1.072
0.508
30
4.22
410.60
80 328.48 1.314
NMVOC
equiv.
Weight
Release
(Tonnes)
0.716
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