Volatile organic compounds (VOCs) evaporate from crude oil and other volatile petroleum cargoes throughout tanker loading, sea passage, and cargo-handling operations. MARPOL Annex VI Regulation 15 requires crude oil tankers to carry an approved, ship-specific VOC Management Plan when calling at ports that regulate VOC emissions, and it provides the framework for shore-side vapor emission control systems (VECS) that collect and process the displaced vapor during loading. The safety architecture connecting a tanker to a shore VECS, including detonation arresters, oxygen monitors, and the inert-gas interface, is detailed in OCIMF’s ISGOTT 6th Edition (2020) and, for US waters, in USCG 33 CFR Part 154 Subpart E.
What VOCs Are and Why They Matter
Volatile organic compounds are carbon-containing molecules with high vapor pressure at ambient temperature. The US EPA defines a VOC as any compound with a vapor pressure above 0.1 mm Hg at 20°C for regulatory purposes (40 CFR 51.100(s)), though the MARPOL Annex VI definition in Regulation 15.1 is cargo-sector specific: VOCs are the hydrocarbon gases that escape from crude oil and similar petroleum cargoes.
Crude oil is not a single compound. It’s a mixture of hundreds of hydrocarbons, from dissolved methane and ethane through pentanes, hexanes, and heavier fractions. The lighter components, those below roughly C5, have vapor pressures high enough to reach the ullage space as gas at ambient conditions. Reid Vapor Pressure (RVP) is the standard measure of this tendency: a light Arabian crude might carry an RVP of 8 to 12 psi at 37.8°C, while a heavy Merey blend from Venezuela runs 3 to 5 psi. A higher RVP means more vapor displaced per tonne of cargo loaded.
VOCs from crude oil loading include methane, ethane, propane, butane, pentane, and lighter aromatic compounds including benzene and toluene. Benzene is a known human carcinogen (Group 1, International Agency for Research on Cancer), and toluene causes central nervous system effects at sustained exposure above 50 ppm. Ethane, propane, and butane contribute to ground-level ozone formation through photochemical reactions with nitrogen oxides. Methane is a potent greenhouse gas, with a 20-year global warming potential 82.5 times that of CO2 (IPCC AR6, 2021), though its proportion in crude oil vapor is much smaller than in LNG boil-off.
The economic dimension is direct: vapor that leaves the cargo tank is cargo that doesn’t arrive at the discharge port. A VLCC loading 250,000 tonnes of crude at a terminal without vapor control can lose 200 to 500 tonnes of cargo equivalent as evaporated vapor, depending on crude volatility and loading rate. At 100,000 to $250,000 in lost product value per loading operation. Vapor recovery, by returning that vapor to the shore processing plant, partially recovers the loss and provides the terminal with a saleable fuel gas or liquid product stream.
MARPOL Annex VI Regulation 15: the Framework
MARPOL Annex VI was adopted in September 1997 and entered into force on 19 May 2005. Regulation 15 within that Annex addresses VOC emissions specifically. Its scope and requirements are often misread, so the text matters.
Who Regulation 15 Applies To
Regulation 15.2 applies to tankers carrying crude oil. It does not apply to product tankers, chemical tankers, or gas carriers under this regulation (though those ship types may face national or port-state VOC controls). The regulation uses “tanker” in the sense of a vessel carrying crude oil in bulk as cargo.
Regulation 15.3 requires that every such tanker, when operating in a port or terminal where the port state has notified the IMO that VOC emissions from tankers are to be controlled, must have an approved Volatile Organic Compound Management Plan on board. The obligation to have the Plan is triggered not by the ship’s equipment but by the port state’s decision to regulate: a tanker that never calls at a regulated port technically has no Regulation 15 obligation, though in practice the Plan is treated as a standard document by most major flag states.
Regulation 15.4 permits a port state to require ships to connect to shore vapour emission control systems during loading. This is the enabling provision for mandatory VECS connection at terminals in Norway’s North Sea ports, US facilities under 33 CFR 154, and the EU-regulated loading terminals.
Regulation 15.5 requires that any VECS installed on a tanker be approved by the Administration (the flag state) taking into account safety standards developed by the IMO. Those standards are contained in the VOC Management Plan guidelines issued by MEPC.
The IMO Guidelines: MEPC.1/Circ.680 and MEPC.1/Circ.719
IMO MEPC.1/Circ.680, issued in 2009 and superseded by MEPC.1/Circ.719 in 2012, provides the format and minimum content requirements for the VOC Management Plan. MEPC.1/Circ.719 remains the current reference. The guidelines specify that the Plan must be:
- Ship-specific, written in the working language of the ship’s crew and in English.
- Approved by or on behalf of the Administration (the flag state). In practice, approval is delegated to Recognized Organizations, meaning the classification societies.
- Regularly reviewed and updated, with each amendment countersigned by the flag administration or its RO.
The Plan’s content, per MEPC.1/Circ.719, must address: a description of the vessel’s cargo piping, vapor header, and inert gas system as they relate to VOC control; cargo loading procedures including maximum loading rates and the relationship between loading rate and vapor generation rate; measures to minimize VOC emissions during sea passage through cargo tank pressure and temperature management; crude-oil washing procedures where applicable and how they affect VOC generation; and the record-keeping log that documents each loading operation’s VOC-related parameters.
The guidelines also require the Plan to include a port-specific annex for each regularly visited terminal that mandates VECS connection, covering the terminal’s system capacity limits, the connection and disconnection procedure, and emergency shutdown protocols.
Sources of VOC Emissions on Crude Tankers
Understanding where VOC comes from shapes every technical and operational control measure. The sources aren’t equal in magnitude.
Vapor Displacement During Loading
Loading is by far the largest VOC event in a tanker’s operating cycle. As liquid crude enters the bottom of a cargo tank, it displaces the gas in the ullage space upward and out through the venting system. The volume of vapor displaced equals, approximately, the volume of liquid loaded. At a loading rate of 10,000 m³/hr across a VLCC’s manifold, roughly 10,000 m³/hr of ullage gas is pushed out, carrying a hydrocarbon concentration that can range from 20% to over 60% by volume, depending on crude volatility and initial tank atmosphere composition.
The vapor displaced during loading is a mixture of the original tank atmosphere (inert gas, or air on a non-inerted tank) and vapor released from the incoming cargo’s liquid surface. The richer the crude, the higher the concentration of flammable and toxic components in the displaced stream. This is the stream that a VECS captures and returns to the shore plant.
Tank Breathing During Sea Passage
A crude tanker at sea doesn’t stop generating VOC after loading. During voyage, cargo tank temperatures fluctuate with ambient temperature and solar radiation. Cargo temperature changes shift the vapor-liquid equilibrium at the cargo surface, releasing additional gas into the ullage space. When tank pressure rises above the pressure-vacuum (P/V) valve set point, the valve opens and releases vapor to atmosphere, unless a high-velocity vent or controlled venting system routes it differently.
MARPOL Annex VI Reg 15 addresses this through the VOC Management Plan’s sea-passage section: the plan specifies inert gas top-up procedures to maintain slight positive pressure (typically 500 to 1,500 mm H2O above atmospheric) which delays P/V valve opening, and defines the maximum acceptable tank temperature if temperature-controlled holds are available.
Crude-Oil Washing Operations
Crude-oil washing (COW) strips waxy residues from tank walls using hot crude cargo as the washing medium. During COW, the rotating jet machines spray crude against tank surfaces, generating substantial vapor from the heated crude. COW regulations under MARPOL Annex I Regulation 33 require this operation during discharge on crude tankers fitted with the COW system, and the associated vapor generation must be addressed in the VOC Management Plan. The inert gas system maintains an inert atmosphere during COW to prevent explosive conditions; the VOC management implication is that inert gas consumption is higher during COW operations, and the tank exit vent stream contains elevated hydrocarbon concentrations.
See the companion article on marine tank cleaning and crude-oil washing for the detailed COW operational framework.
The Vapor Emission Control System (VECS)
A vapor emission control system is the technical hardware that collects vapor displaced from cargo tanks during loading, transports it from the ship manifold to a shore processing unit, and either recovers or destroys the hydrocarbons. The ship side and the shore side must be compatible and safety-interlocked before loading can begin at a VECS-equipped terminal.
Ship-Side Components
The ship’s vapor return arrangement consists of a vapor collection header running along the cargo tank top, connected to individual tank venting pipes through isolation valves; a vapor return manifold located adjacent to or within the cargo manifold area; and a vapor return hose or spool connecting the ship manifold to the terminal’s vapor collection header.
Vapor return manifold: The ship-side vapor return manifold is typically a flanged connection of 250 mm to 400 mm nominal bore on a VLCC, sized to handle the vapor volume generated at maximum loading rate. The flange standard is most commonly ANSI 150# raised face, though some regional terminals use different standards, making the pre-connection compatibility check a procedural requirement in ISGOTT Chapter 7.
The vapor header runs from the manifold forward and aft, with branch connections into each cargo tank’s ullage space through pressure-actuated inlet valves. On most crude tankers, the vapor header doubles as the inert gas distribution line: inert gas travels from the IG plant forward and aft through the same header during inerting operations, and vapor flows in reverse (outward) during loading when the VECS is connected.
Isolation valves and P/V valves: Between each cargo tank and the vapor header sits an isolation valve, usually a remotely operated butterfly valve, that can close individual tanks out of the loading sequence. The pressure-vacuum valves on each tank provide final protection: they’re set to open at positive pressures of 1,400 to 2,000 mm H2O (approximately 1.4 to 2.0 kPa above atmospheric) and close when pressure returns to normal. If the vapor return line becomes obstructed or the shore VECS shuts down, the P/V valves open and vent to atmosphere rather than allow the tank to over-pressurize.
Shore-Side VECS Components
The shore facility’s vapor emission control system connects to the ship’s vapor return manifold and processes the incoming vapor stream. A complete shore VECS includes:
| Component | Function | Key Design Standard |
|---|---|---|
| Vapor collection header | Receives vapor from ship manifold via hose/arm | USCG 33 CFR 154.2100; ISGOTT Ch.7 |
| Detonation (flame) arrester | Prevents flame from propagating back toward the ship | ATEX / USCG 33 CFR 154.2107 |
| Pressure-vacuum relief valve | Limits header pressure swing; protects ship tanks | Set per terminal design limits |
| Oxygen analyzer | Continuous O2 monitoring; triggers shutdown >8% O2 | USCG 33 CFR 154.2101; ISGOTT 7.4 |
| Vapor blower/compressor | Moves vapor from header to processing unit | Explosion-proof motor, ATEX Cat 2 minimum |
| Vapor processing unit | Recovery (condensation, adsorption) or destruction (flare, thermal oxidizer) | EPA 40 CFR 63 Subpart Y (US terminals) |
| Emergency shutdown system (ESD) | Closes vapor block valves and stops loading on detected fault | Linked to ship-shore ESD per ISGOTT App.C |
How Loading With VECS Connection Works
Before the manifold hose connection is made, the ship’s officer-in-charge and the terminal representative conduct a pre-transfer safety inspection per ISGOTT Ship/Shore Safety Checklist, Part B (vapor operations). Both parties verify: the ship’s vapor return valve is closed, the ship’s P/V valves are set correctly, the shore detonation arrester is in place and undamaged, and the oxygen analyzer is calibrated and showing a reading.
After the vapor hose is connected and leak-tested, the shore operator opens the shore block valve and starts the VECS blower, pulling the vapor header into slight negative pressure relative to the ship’s tank pressure. The ship’s officer opens the vapor return valve progressively as loading begins. Vapor flow goes from ship tanks through the vapor header, through the vapor return manifold, down the vapor hose, through the shore detonation arrester, into the shore vapor collection header, through the blower, and to the processing unit.
The loading rate is limited by the shore VECS capacity, expressed in the terminal information as a maximum vapor loading rate in cubic meters per hour. USCG 33 CFR 154.2103 requires shore facilities to determine and post this maximum rate; the ship must not exceed it without shore authorization, since over-rate vapor flows can exceed the processing unit’s capacity and cause a pressure buildup that trips the system’s high-pressure shutdown.
Safety Interlocks and the Inert Gas Interface
The intersection of vapor recovery and inert gas operations is where the most serious tanker incidents have originated. Getting this interface wrong creates either an oxygen-enriched explosive atmosphere inside the shore VECS, or an over-pressurized cargo tank on the ship. Both are catastrophic failure scenarios.
Oxygen Content: the Critical Variable
On an inerted crude tanker, the tank ullage space contains a mixture of inert gas (predominantly nitrogen and carbon dioxide from the flue gas generator or inert gas generator), cargo vapor, and traces of the original air atmosphere pushed out during inerting. A well-inerted tank atmosphere runs at 2% to 5% oxygen by volume with hydrocarbon concentrations of 20% to 80% of LEL. This mixture is non-explosive because the oxygen level is below the limiting oxygen concentration (LOC) for crude oil vapors, which is approximately 11% O2 at atmospheric pressure.
When this gas mixture flows into the shore VECS vapor processing unit (which may use a thermal oxidizer, a condensation recovery unit, or an adsorption unit), the oxygen concentration in the incoming stream must stay below 8% by volume. USCG 33 CFR 154.2101(a) requires automatic shutdown of the vapor collection system if the oxygen concentration in the vapor header exceeds 8% O2. ISGOTT 7.4.2 sets the same threshold. The 8% limit provides a margin below the LOC, accounting for any possible air ingress into the vapor line from the ship side.
If the ship’s cargo tanks are not properly inerted before loading begins, air in the tank ullage space will mix with the incoming cargo vapor. That mixture, displaced through the vapor header into the shore system, can carry 10% to 21% oxygen. Inside a thermal oxidizer or condensation unit, an oxygen-rich hydrocarbon mixture that reaches an ignition source can detonate. This is precisely the failure mode that the oxygen analyzer interlock prevents.
Detonation Arresters
A detonation arrester (also called a flame arrester or deflagration arrester depending on its tested rating) is installed at the ship-shore vapor connection on the shore side, at the entry point to the shore vapor header. Its function is to quench any flame front that might propagate from the shore processing unit back toward the ship’s cargo tanks.
USCG 33 CFR 154.2107 requires end-of-line detonation arresters at the vapor collection point for US facilities. ISGOTT 7.4.6 requires the arrester to be approved for the vapor mixture characteristics of the cargo being transferred. Crude oil vapor arrester elements use a matrix of metal foil or sintered metal that absorbs heat from a propagating flame, dropping the gas temperature below the autoignition point within millimeters of travel distance.
The key operational requirement is that the arrester element must be inspected and certified as clean before each loading operation. Wax, asphaltene, or particulate buildup from crude oil vapor can block the arrester element, reducing its flow area and potentially causing excessive pressure drop across the vapor line. A blocked arrester that causes tank overpressure during loading is a well-documented failure mechanism in VECS incident reports.
Pressure Monitoring and High-Pressure Shutdown
The vapor header pressure on the ship side must be monitored continuously during VECS-connected loading. ISGOTT 7.4.3 and USCG 33 CFR 154.2105 require pressure transmitters with automatic shutoffs set to close the vapor block valve and signal emergency shutdown if header pressure rises above a preset limit, typically 600 to 800 mm H2O above the tank design positive pressure limit. High pressure in the vapor header is most commonly caused by a blockage in the vapor hose or arrester, or by a VECS blower failure.
Conversely, if the VECS blower develops excessive suction (negative pressure) in the vapor header, it can pull tank pressure below the P/V valve vacuum set point, causing atmospheric air to be drawn into the tank through the P/V valve. That air ingress raises oxygen content and destroys the inerted condition. Pressure monitoring provides the shutdown signal in both over-pressure and under-pressure scenarios.
Ship-Shore Emergency Shutdown (ESD) Integration
ISGOTT Appendix C defines the requirements for ship-shore ESD linkage. At VECS-equipped terminals, the vapor return system ESD is integrated with the cargo loading ESD so that a vapor high-pressure or high-oxygen trip simultaneously closes the ship-side vapor return valve, trips the shore VECS blower, and signals the cargo loading arm or hose to close. The sequence matters: vapor isolation before cargo pump shutdown prevents the pressure transient that could otherwise spike the vapor header on abrupt cargo stoppage.
The ship must designate a responsible officer to monitor the vapor header pressure and oxygen readings throughout loading. ISGOTT 7.1.2 requires this as a continuous watch function, not a periodic check, during VECS-connected operations.
The Relationship With the Inert Gas System
The inert gas system and the vapor management system share cargo tank piping and interact at every stage of tank preparation, loading, and discharge. A full treatment of IGS hardware and principles is in the marine inert gas systems article; the specific VOC management intersections are covered here.
Pre-Loading Tank Inerting
Before loading can begin at a VECS terminal, the cargo tanks must be inerted to a verified oxygen content below the terminal’s acceptance threshold. Most VECS terminal procedures require the ship to certify oxygen below 5% O2 in all tanks before the vapor return hose is connected. MEPC.1/Circ.719 requires the VOC Management Plan to include the inerting procedure as a standard pre-loading step.
Inerting is achieved by running the inert gas plant (a flue gas scrubber-type IG plant on most VLCCs and Suezmaxes, or a nitrogen membrane or generator on smaller vessels) and displacing the tank atmosphere through the P/V valves or a high-velocity vent mast. When oxygen at the vent outlet drops below the target, the tank is declared inerted and the P/V valve is closed to maintain the inert atmosphere. The sequence of multiple tanks requires coordination between the cargo control room operator and the IG plant operator; inert gas supply rate and distribution valve settings determine which tanks are inerted in what order.
Inert Gas Top-Up During Loading
As cargo enters the tanks during loading, the ullage space volume shrinks. The inert gas plant runs in parallel with the VECS connection to replenish the inert gas consumed by the shrinking ullage space, maintaining positive pressure. This sounds counterintuitive: why add gas to a system that’s already flowing gas to shore? The answer is pressure control. Without IGS top-up, the flow of vapor out through the VECS connection would gradually deplete the ullage pressure, potentially pulling it into the negative range. Negative pressure in a cargo tank during loading draws in air through any unsealed opening, raising oxygen content.
In practice, the IGS is run at a low delivery rate (perhaps 10% to 20% of maximum capacity) during loading to maintain the vapor header at 500 to 1,000 mm H2O positive pressure. The P/V valves remain closed throughout. The VECS blower on the shore side controls the actual vapor flow rate by its speed and suction setting.
Gas-Freeing Interaction
Gas-freeing (removing cargo vapor from tanks to prepare for inspection or dry-dock) is the reverse process: air is blown into the tanks and cargo vapor is pushed out. ISGOTT Chapter 11 covers this in detail. During gas-freeing, a VECS connection is not used because the tank atmosphere transitions from inert/vapor through the explosive range on its way to safe (air). MARPOL Annex VI Regulation 15 doesn’t require vapor recovery during gas-freeing; the operation is conducted through the high-velocity vent masts at open sea, far from populated areas and away from ignition sources.
US Regulatory Framework: USCG 33 CFR 154 Subpart E
The United States applies the most detailed vapor control system regulations of any national regime through 33 CFR Part 154 Subpart E (Sections 154.2100 to 154.2160). These rules apply to marine transfer facilities (terminals) that handle petroleum and volatile petroleum mixtures in the lower 48 states and apply to every ship calling at those facilities, regardless of flag.
What the Rules Cover
33 CFR 154 Subpart E sets design and performance standards for shore-side VCS equipment: the vapor collection hose or arm design, detonation arrester specifications, oxygen analyzer performance (response time within 30 seconds, calibrated daily), pressure monitoring and alarm setpoints, vapor processing unit minimum destruction efficiency (95% by weight for thermal oxidizers, per 33 CFR 154.2141), and the operational approval process.
The rules also define what ships must have to connect to a shore VCS: 33 CFR 154.2200 (cross-referenced from Subpart E) requires ships to have a vapor connection with a compatible flange, a vapor isolation valve, and pressure monitoring capability on the ship-side vapor header. Ships without these features can’t legally connect to a US shore VCS and therefore can’t load at US terminals that mandate VECS connection.
Maximum Allowable Transfer Rate (MATR)
One of the practical fleet-operations implications of 33 CFR 154 is the Maximum Allowable Transfer Rate (MATR) concept. Under 33 CFR 154.2103, the terminal calculates its MATR based on the rated capacity of its vapor processing unit. The ship can’t exceed the MATR without written authorization from the terminal operator, and the terminal must post the MATR in its information-to-mariners documentation. Ships loading crude at US Gulf Coast terminals regularly encounter MATR limits that constrain loading rates below what the ship’s cargo pumps could otherwise achieve, particularly on large VLCCs where the vapor generation rate at maximum loading rate can exceed 50,000 m³/hr of vapor flow.
California Air Resources Board (CARB) Rules
California applies state-level vapor control requirements through the California Air Resources Board Marine Tank Vessel Loading Operations regulation (17 CCR § 93108). California’s rules apply to tankers loading in California ports and are more prescriptive than 33 CFR 154 in some respects, including requiring enhanced vapor collection efficiency (95% capture rate versus the federal 90% minimum) and specific reporting obligations. California’s terminal operators are required to maintain CARB-certified VCS systems, and ships calling at California terminals must comply with both federal and state requirements simultaneously.
Operational VOC Management: Crude-Oil Cargo Practices
Beyond the VECS connection, the VOC Management Plan addresses operational practices that reduce VOC generation throughout the loading and sea-passage cycle.
Loading Rate Optimization
There’s a direct relationship between loading rate and vapor displacement rate. Doubling the loading rate doubles the volume of vapor displaced per unit time. The VOC Management Plan sets out the maximum loading rate compatible with the ship’s vapor return arrangement capacity and the terminal’s VECS rated capacity. In practice, loading rate is agreed between the ship’s chief officer and the terminal operator at the pre-loading conference, documented in the ship-shore safety checklist.
Some crude terminals with older or smaller VECS installations can’t accept the maximum VECS-connected loading rate of modern VLCCs. The agreement to load at a reduced rate must be documented in the tanker’s logbook and in the VECS operational record required by the VOC Management Plan.
Cargo Temperature Management During Voyage
Crude oil temperature affects vapor pressure: warmer cargo releases more vapor per unit time. The VOC Management Plan’s sea-passage section addresses cargo heating (where cargo heating coils are fitted) and, where relevant, the maximum cargo temperature permitted to minimize P/V valve opening frequency. MEPC.1/Circ.719 requires the plan to specify whether cargo heating is used, at what temperature, and how tank pressure is to be monitored during voyage.
On a conventional crude tanker without cargo temperature control, the tank temperature follows the ambient and sea-water temperature. In warm climates, particularly during transit through the Arabian Sea or the Gulf of Mexico in summer, tank vapor pressures can rise enough to crack P/V valves open repeatedly during the voyage. The VOC Management Plan documents the acceptable operating envelope and the procedure for inert gas top-up if tank pressure drops below the positive-pressure target.
Pressure Management: the Inert Gas Blanket
Maintaining slight positive pressure in all cargo tanks throughout the laden voyage is the single most effective operational measure for minimizing VOC loss at sea. Positive pressure keeps the P/V valve closed, prevents air ingress, and reduces the vapor release rate from the cargo surface (because the cargo surface is exposed to a near-saturated vapor atmosphere that limits further evaporation from the liquid).
ISGOTT 7.6 recommends maintaining 100 to 500 mm H2O positive pressure in all laden tanks as a baseline. The IGS operator must check tank pressures at least twice daily during the laden voyage, top up inert gas if any tank shows declining pressure, and log the readings in the IG register. Any P/V valve opening must be noted and investigated.
Trim and List Management
Cargo trim (the difference between forward and aft draft) affects the position of the cargo surface relative to the tank ullage space and can influence vapor release patterns. More importantly, excessive list can cause cargo to contact the P/V valve flame screen on small tanks or produce ullage readings that disguise over-filling. The VOC Management Plan addresses trim and list limits from the perspective of tank vapor space geometry, complementing the stability and cargo distribution requirements.
Cargo Tanks, Vent Systems, and P/V Valve Design
The pressure-vacuum valves on crude tanker cargo tanks are the last line of protection between the tank atmosphere and the environment (when a VECS is not connected) or between the tank and the VECS header (when connected).
P/V valves on crude tankers are set to open at positive gauge pressures of 1,400 to 2,000 mm H2O (approximately 137 to 196 Pa above atmospheric) and at negative (vacuum) pressures of 350 to 700 mm H2O below atmospheric. The positive set point is high enough to maintain the inert gas blanket without constant opening, but low enough to protect the tank structure against over-pressure. The vacuum set point prevents tank structural damage if the inert gas plant fails or the tank vents are accidentally blocked.
High-velocity vents (also called high-velocity pressure valves or HVP valves) are an alternative to P/V valves on some tanker designs. They discharge vapor at velocities above 30 m/s to dilute the vapor rapidly in the wind stream and prevent it from falling back to the deck. High-velocity vents are used for gas-freeing operations and for tanks not connected to the VECS during loading. ISGOTT Chapter 5 covers vent mast design requirements and the separation distances from ignition sources.
For the full piping and valve arrangement context, see the marine cargo pumps and piping article, which covers the cargo distribution system of which the vapor piping is a component.
Chemical Tankers and Gas Carriers: Different Regimes
MARPOL Annex VI Regulation 15 applies specifically to crude oil tankers. Chemical tankers and gas carriers have separate VOC-related requirements.
Chemical Tankers
Chemical tankers carrying volatile liquid chemicals (as defined in IBC Code Chapter 1.3) are subject to cargo-specific vent requirements under the IBC Code, not Regulation 15. The IBC Code, Chapter 8, specifies the type of venting system (open, controlled, or closed) for each cargo based on its vapor pressure, toxicity, and flammability characteristics. Benzene and styrene, for example, require closed (pressurized) cargo handling systems with no atmospheric venting during loading, independent of MARPOL Annex VI.
Gas Carriers (LPG)
LPG carriers operating under the IGC Code manage boil-off gas in a fundamentally different way. LPG cargo is liquefied under pressure (propane at approximately 8 bar gauge at 20°C) or refrigerated, so there’s no “displacement during loading” mechanism in the same sense as crude oil. Vapor generated during loading or by cargo warming is either reliquefied by the cargo compressors and returned to the cargo tanks, or is burned as fuel in the ship’s main engine (dual-fuel LPG carriers). The VOC management problem for LPG carriers is boil-off rate management during sea passage, not loading vapor displacement.
LNG carriers face the same basic physics: boil-off from cargo warming is either reliquefied or used as propulsion fuel. The IMO Polar Code and IGC Code frameworks apply, not MARPOL Annex VI Regulation 15.
OCIMF, ISGOTT, and Industry Standards
The Oil Companies International Marine Forum (OCIMF) and the joint OCIMF/ICS/IAPH publication ISGOTT (International Safety Guide for Oil Tankers and Terminals, 6th Edition, 2020) are the primary industry guidance documents for tanker vapor management operations. ISGOTT is not a regulation, but it’s referenced by port state control regimes and class societies as the recognized industry standard: a terminal that doesn’t follow ISGOTT recommendations is considered sub-standard by the SIRE (Ship Inspection Report Programme) inspection system.
ISGOTT Chapter 7 (Vapor Emission Control Systems) covers: the design and testing requirements for ship-side vapor return arrangements; the ship-shore compatibility check procedure; the operational sequence for connecting and disconnecting vapor hoses; oxygen monitoring requirements; emergency procedures; and the maintenance inspection regime for vapor return equipment.
OCIMF’s SIRE inspection forms (VIQ 7th Edition, 2022) include specific questions about vapor management plan currency, oxygen analyzer calibration records, P/V valve test dates, and vapor hose condition. A SIRE deficiency in vapor management can result in a terminal rejecting the vessel for loading until the deficiency is corrected, which has direct commercial consequences for voyage planning.
Recordkeeping and Port State Control
The VOC Management Plan must be carried aboard and presented to port state control officers on demand. Under MARPOL Annex VI Regulation 15, the absence of an approved VOC Management Plan on a crude tanker calling at a regulated port is a detainable deficiency under Paris MOU, Tokyo MOU, and USCG enforcement guidelines.
Recordkeeping required by the Plan includes: a VOC Loading Log documenting date, port, terminal, cargo loaded, loading rate, vapor return connection status, P/V valve openings (if any), and oxygen readings during the operation; the maintenance log for the ship-side vapor return equipment including P/V valve testing dates and detonation arrester inspection records; and the training records for crew members assigned vapor management duties.
The USCG additionally requires US terminals to maintain a shore-side log of all vapor transfers, including the MATR in effect, the actual loading rate, and any VCS alarm activations, under 33 CFR 154.2160. Ship masters loading at US terminals should be aware that both the ship’s VOC log and the terminal’s VCS log are subject to USCG inspection and must be consistent with each other.
Limitations
Several important limitations apply to VOC management systems and the regulatory framework governing them:
Port coverage is uneven. MARPOL Annex VI Regulation 15 only binds ships calling at ports where the port state has notified the IMO of VOC regulation. As of 2025, the major regulated ports include those in Norway, EU member states, and the United States. Many crude exporting terminals in the Middle East, West Africa, and Southeast Asia have no mandatory VECS, so a tanker loading at Ras Tanura or Bonny may have no regulatory obligation to use vapor control during that call, regardless of the ship’s equipment status.
The Plan covers management, not necessarily hardware. A VOC Management Plan can be fully compliant without the ship having any vapor return equipment installed. If the ship never calls at a port that mandates VECS connection, the Plan addresses loading optimization and sea-passage pressure management using only the P/V valve and IGS system that every inerted tanker already has. Ships without installed vapor return manifolds cannot connect to VECS terminals, limiting their trading flexibility.
VECS efficiency is not 100%. Even a correctly operated shore VECS does not capture 100% of displaced vapor. Vapor can bypass the system through any unsealed tank access, through P/V valve openings if header pressure rises temporarily above tank positive-pressure set point, or through leaks in the vapor hose connection. US regulations require a minimum of 90% vapor collection efficiency (33 CFR 154.2141), acknowledging that some fugitive emissions are unavoidable.
Detonation arrester maintenance is often underweighted in fleet planning. Class societies require P/V valves to be function-tested annually and weight-tested at each intermediate and special survey, but the inspection interval for detonation arresters on ship-side vapor return arrangements is less standardized. Some flag states accept a visual inspection; others require element replacement on a fixed interval. A clogged or damaged arrester that causes excessive pressure drop across the vapor system is a safety risk that doesn’t announce itself with an obvious alarm.
The IGS-VECS interface requires active crew management. The interaction between inert gas top-up rate, loading rate, and VECS connection pressure is not a set-and-forget system. Tank pressure swings during multi-tank simultaneous loading, cargo stratification effects on vapor release rate, and changes in loading rate during the operation all require the deck officer to monitor and adjust IGS delivery, vapor return valve position, and coordination with the shore VECS operator continuously throughout loading. The SIRE inspection record for the 2022-2024 period shows vapor management as among the top-10 most frequently cited deficiency categories for tankers calling at VECS terminals.
Regulation 15 doesn’t address methane separately. The regulation covers VOCs as a category, but methane (the most potent greenhouse gas fraction of crude oil vapor) doesn’t receive specific controls under this framework. The IMO’s ongoing work under the GHG Strategy (MEPC 80, 2023) may eventually add methane-specific requirements, but as of 2025, methane from crude oil loading is addressed only through the general VOC Management Plan, not through any separate limit.
See Also
- Marine Inert Gas Systems: the IGS hardware, inerting procedures, and oxygen monitoring systems that underpin all cargo-tank VOC control
- Marine Tank Cleaning and Crude-Oil Washing: crude-oil washing procedures and their interaction with VOC generation
- Marine Cargo Pumps and Piping: the cargo piping network that includes the vapor return header
- Oil Tanker: tanker types, design features, and regulatory classification relevant to MARPOL Annex VI Reg 15 applicability
- MARPOL Annex VI: the full Annex VI framework including Regulations 14 (sulfur), 15 (VOC), 19 (energy efficiency), and their enforcement regime
- MARPOL Annex VI Reg 15 VOC Emissions: the specific Regulation 15 article with jurisdictional implementation detail
Relevant calculators for this topic: