| OTM-7.1 |
Oil Tanker Manual OTM Hazards of Oil Cargoes |
Doc No.: OTM 7.1
Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD |
1.Hazards of Oil Cargoes
Hydrocarbon Vapours:
- Hydrocarbon gases are heavier than air (1.5 to 3 times) and tend to accumulate in the vicinity of the area where they are generated. A large amount of gas might exist sometimes in unpredictable locations. The bottom of the pump room is a typical example.
- Gases flow to the leeward side, and are dangerous in that they may cause explosion at spaces other than where they are generated.
- The explosion limits or flammable limits (LEL / UEL or LFL / UFL) varies according to the type of Hydrocarbon gas in question.
- Their proportionate mixtures present in the petroleum vapour in question. This is generally around 1.8% Vol. in Air (Min.) to 9.5% Vol. in Air (Max); whereas International Chamber of Shipping recommends a range of 1.0% to 10.0 Vol. %, to assume safer standards.
- The danger for explosion is far greater in a lightly laden ship, while loading / unloading cargo, during ballasting operations or during tank cleaning, rather than when fully laden. This is because on a loaded vessel, the tank atmosphere contains hydrocarbons concentrations of well over above UEL.
Toxic Hazards of H.C. Vapours
- Petroleum gas is noxious and harmful to the body. The table K-01-D-1 shows the concentration of petroleum gas and its effects on the human body.
- Toxicity can be greatly influenced by the presence of some minor components such as aromatic hydrocarbons (e.g. Benzene and Hydrogen Sulfide i.e. H2S).
- Even if a tank is empty, gases might be regenerated from sludge in the tank. Precautions are also necessary when working in tanks because Petroleum Gas or Inert Gas from other tanks might enter the tank due to leakage from valves.
HC Gas concentration
(Volumetric Proportion in Air)
| Concentration | Effect on the Human Body |
|---|---|
| 0.02% (300ppm) | Industry Permissible concentration (OEL-TWA for 8 Hrs.) or 2% LEL |
| 0.1% (1,000ppm) | Irritation in the eyes within an hour |
| 0.2% (2,000ppm) | Irritation in the eyes, nose, or throat within 30 minutes, dizziness, and unsteadiness |
| 0.7% (7,000ppm) | Signs of giddiness within 15 minutes |
| 1.0% (10,000ppm) | Sudden giddiness occurs, and if the body is exposed to the same conditions continuously, unconsciousness results, and can sometimes lead to death |
| 2.0% (20,000ppm) | Sudden giddiness, unconsciousness, resulting in death |
Hydrogen Sulphide (H2S)
Characteristic of Hydrogen Sulphide (H2S)
- H2S is a highly toxic, corrosive and flammable gas that in low levels will smell like rotten eggs.
- It may be present in bunkers in dissolved state or as a gas. It may also be found in certain Natural gases, Crude oils, and certain refined products such as Naphtha.
- It is colourless and heavier than air, having a relative vapour density of 1.189.
- Exposure to high levels of H2S can be fatal after a very short period of time.
- H2S is a liquid-soluble gas and produces vapour when the liquid is agitated or heated. It is not possible to predict the likely H2S vapour concentration present above a liquid in a tank, from any given liquid concentration, but as an example, oil containing 70 “ppm by weight” concentration of H2S in liquid has been shown to produce 7000 “ppm by volume.”
- TWA: 1 ppm, STEL: 5 ppm.
H2S Gas Concentration
(PPM by Vol. in air)
(Source - Occupational Safety and Health Administration (OSHA))
| Concentration | Effect on the Human Body |
|---|---|
| 0.00011-0.00033 | Typical background concentrations |
| 0.01-1.5 ppm | Odour threshold (when the rotten egg smell is first noticeable to some). Odour becomes more offensive at 3-5ppm. Above 30ppm, odour described as sweet or sickeningly sweet. |
| 2-5 ppm | Prolonged exposure may cause nausea, tearing of the eyes, headaches, or loss of sleep. Airway problems (bronchial constriction) in some asthma patients. |
| 20 ppm | Possible fatigue, loss of appetite, headache, irritability, poor memory, dizziness. |
| 50-100 ppm | Slight conjunctivitis (gas eye) and respiratory tract irritation after one hour. May cause digestive upset and loss of appetite. |
| 100 ppm | Coughing, eye irritation, loss of sense of smell after 2-15 minutes (olfactory fatigue). Altered breathing, drowsiness after 15-30 minutes. Throat irritation after one hour. Gradual increase in severity of symptoms over several hours. Death may occur after 48 hours. |
| 100-150 ppm | Loss of sense of smell (Olfactory fatigue or paralysis). |
| 200-300 ppm | Marked conjunctivitis and respiratory tract irritation after one hour. Pulmonary oedema may occur from prolonged exposure. |
| 500-700 ppm | Staggering then collapse in five minutes. Serious damage to the eyes in 30 minutes. Death after 30-60 minutes. |
| 700-1000 ppm | Rapid unconsciousness. 'Knockdown' or immediate collapse within one to two breaths. Breathing stops. Death within minutes. |
| 1000-2000 ppm | Nearly instant death. |
Precautions for Hydrogen Sulphide (H2S)
- In cases where H2S concentrations are known to be greater than 100 ppm in the vapour space and likely to be present in the atmosphere, Emergency escape Breathing Apparatus shall be made available to personnel working in the hazardous area, who should already have a Personal (pocket-able) H2S gas monitoring alarm / instrument. Personnel should always carry personal monitors whenever H2S concentrations could exceed the OEL-TWA. Examples of these occasions include but are not limited to:
- Gauging.
- Sampling.
- Entering a pump room.
- Connecting and disconnecting lines/hoses.
- Cleaning filters.
- Draining to open containment.
- Mopping up spills.
- The presence of H2S in bunkers should not be ruled out. Empty bunkers tanks shall be tested for the presence of H2S prior to bunkering.
Benzene
Health Concerns in Connection with Benzene
- Benzene is present in varying concentrations in some crude oils, and the MSDS shall be consulted each time before cargo handling.
- Benzene gas has poor warning qualities, as its odour threshold is well above the TLV-TWA limits. Exposure to concentrations in excess of 1000 ppm can lead to unconsciousness and death. Benzene can also be absorbed through the skin and becomes toxic when ingested.
- If there is evidence that dissolved benzene is present in the liquid cargo in quantities of 0.5% by volume or more, respiratory protection is required when conducting tank “de-mucking” or standing upwind from tanks containing benzene that are being vented.
- Benzene is known as a strong carcinogen and is known to cause leukemia. When handling cargoes with a benzene concentration of more than 0.5%, the Master is to ensure that all personnel involved are aware of the hazards and proper precautions are taken.
- Ships carrying mixtures with a benzene content of 0.5% or more should carry safety equipment equivalent to what is required in paragraph 14.2 of the IBC Code.
TLV (Threshold Limit Value) – TWA (Time Weighted Average)
The airborne concentration of a toxic substance averaged over an 8-hour period, usually expressed in parts per million (ppm).
TLV - STEL (Short Term Exposure Limit)
The airborne concentration of a toxic substance averaged over any 15-minute period, usually expressed in parts per million (ppm).
Gas IACS RECOMMENDATION
Gas IACS RECOMMENDATION TWA 8 Hour work shift (ppm)/ (STEL) Limit 15 min working (ppm) Benzene (C6H6) 1/5 Hydrogen Sulphide (H2S) 1/5 Carbon Dioxide (CO2) 5/30 Carbon Monoxide (CO) 25 /50 Nitrogen Dioxide (NO2) 1 /3 Nitrogen Monoxide (NO) 25/50 Sulphur Dioxide (SO2) 2/5
Mercaptans
Health Concerns of Mercaptans
- Mercaptans are colourless gases, having a smell similar to rotting cabbage.
- They are generated by the degradation of natural organisms.
- They can be detected by smell at concentrations below 0.5 ppm, although health effects are not experienced until the concentrations are several times higher than this.
- The initial effects and precautions of Mercaptans on persons are similar to those caused by H2S exposure.
- Mercaptans may be found in the following conditions:
- They may occur on ships where seawater has remained beneath oil or where oil residues are left in tanks that contain water. (Such as dirty ballast tanks, after they have been completely drained.)
- They may also be found in water treatment plants and ballast treatment facilities.
- They are also present in vapours of pentane plus cargoes and in some crude oils.
- They are also used as an odorizing agent in natural gas.
Inert Gas
Composite of Inert Gas
After efficient scrubbing of the inert gas (to reduce the content of sulphur dioxide), the typical constituents of a flue gas are shown in the table:
| Inert Gas | Percent Present (after scrubber) |
|---|---|
| Nitrogen (N) | 83% |
| Carbon Dioxide (CO2) | 12-14% |
| Oxygen (O2) | 2-4% |
| Sulphur Dioxide (SO2) | 50 ppm |
| Carbon Monoxide (CO) | Trace |
| Nitrogen Oxides (NOx) | 200 ppm |
| Water Vapour (H2O) | Trace (high if not effectively dried) |
| Ash and Soot (C) | Traces |
| Density | 1.044 (heavier than air) |
Health Concerns of Inert Gas
The main hazard of inert gas is its low oxygen content.
The subsequent hazards, such as the presence of traces of toxic gases in inert gas, inside cargo tanks and spaces, are reduced and controlled by following the company’s designated “Procedures for Entry into Enclosed Spaces”.
By gas freeing from a “Purged condition (HC=2% VOL)” to the “Gas-free condition (HC=<1% LEL)” and until steady 21% by volume oxygen reading is achieved, sufficient dilution of such toxic gases to below their OEL-TWA limits will have been achieved.
Oxygen-Deficient Atmosphere
The health effects and consequences due to lack of oxygen are listed in the table below. These effects will take place without any warning such as odour or physical symptoms.
In tanks and/or voids with complicated geometry and high possibility of "pockets of atmosphere" with low O2 content, where rescue operations may be difficult, the use of a portable oxygen meter with audible alarm is strongly recommended.
Health Effects from Lack of Oxygen
| Oxygen Level | Effect |
|---|---|
| 22% | Oxygen enriched atmosphere |
| 20.8% | Normal level (±0.2%) |
| 19.5% | Oxygen deficient atmosphere |
| 16% | Impaired judgment and breathing |
| 14% | Rapid fatigue and faulty judgment |
| 11% | Difficult breathing and death in a few minutes |
Personal protective equipment
Eye / face protection: Wear safety glasses. If splash potential exists, wear full face shield or chemical goggles.
Skin protection: Wear chemical-resistant, impervious gloves. Full body suit and boots are recommended when handling large volumes or in emergency situations. Flame retardant protective clothing is recommended.
General hygiene: Avoid contact with skin. Keep away from food and drink. Provide eyewash station and safety shower. Handle in accordance with good industrial hygiene and safety practice.
Respiratory protection: Use a properly fitted, breathing apparatus of approved standard if a risk assessment indicates this is necessary. Breathing apparatus selection should be determined by adequately trained personnel, based on the contaminants, the degree of potential exposure and published respiratory protection factors. This equipment should be available for non-routine and emergency use.
Precautions during Cargo Operations
Cargo loading, tank cleaning and gas-freeing are those procedures on board a tanker that expose the crew to the largest risk of exposure to vapours from the products carried, both in the accommodation and on open deck. It is, therefore, essential during these operations that all:
All doors leading from the outside to the accommodation and to the engine room should be closed and kept closed during these operations. Only one door on the windward side/nearest to the cargo control room is to be used as an access.
All doors inside the accommodation shall be kept closed during the operation.
The ventilation to the accommodation shall be stopped/re-circulated and the fire flaps kept closed.
Vapor concentrations on deck shall be measured prior to any work being undertaken.
The crew working on deck shall wear appropriate protective equipment.
Only work related to cargo handling is allowed on deck.
Presence of personnel in the engine room shall be kept to a minimum during these operations.
Controlled Tank Venting System
Vapours displaced from the tank during loading, tank cleaning, tank breathing and gas-freeing should be emitted through a controlled tank venting system complying with either SOLAS regulation II- 2/16.3.2, or paragraph 8.3.2 of the IBC Code, or paragraph 2.14.2 of the BCH Code, as applicable.
Whenever a vapour emission control system is available ashore, vapours displaced from the tank during loading should be returned to that system (vapour return).
Cargo Measurements and Sampling
All cargo related measurements (e.g., ullage, temperature and sampling) should be carried out in a closed mode to minimize the risk of exposing the crew and shore personnel to harmful vapours. When this is not possible, personal protection equipment should be worn.
Medical Monitoring
Crewmembers potentially exposed to benzene vapour inhalation should be submitted to a program of regular suitable medical checks on their health. The results of such checks should be kept on record under normal confidential practices in the medical profession.
Hazards Involved in the Handling of the Products
All petroleum products are highly flammable.
All petroleum products are marine pollutants.
All petroleum products are eye and skin irritants.
Petroleum Hydrocarbon vapours may cause breathing difficulty. When inhaled, even small quantities of petroleum gas can cause symptoms of diminished responsibility and dizziness similar to intoxication, along with headaches and irritation to the eyes. It can be fatal if inhaled in sufficient quantities. These symptoms can occur at concentrations well below the LFL. However, petroleum gases vary in their effects and people vary in their tolerance of those effects. Even if the conditions can be tolerated, do not assume the gas concentration is within safe limits. The smell of petroleum gas mixtures is variable and, in some cases, may dull the sense of smell. This is especially likely, and serious, if the mixture contains hydrogen sulphide (H2S).
Gasolines containing tetraethyl lead or tetramethyl lead. The amounts of tetraethyl lead (TEU or tetramethyl lead, TMU normally added to gasolines are insufficient to make the gases significantly more toxic than those from unleaded gasolines.
Biofuels are produced from renewable organic materials and include ethyl alcohol. Fatty Acid Methyl Esters (FAME), vegetable oils (triglycerides) and alkanes (C10-C26). They may have a flashpoint either above or below 60' C.
Loading Very High Vapor Pressure Cargoes
Gas Evolution
The natural gasoline curve in figure 2.4 is for a series of blends with different TVP. The crude oil curve is for a series produced by adding increasing amounts of butane to a crude oil. At lower TVP, the dependence of depth on TVP is not marked for either type of cargo. At greater TVP, the curve becomes progressively steeper, indicating that in this range a small increase in TVP can cause a large increase in gas evolution.
Boiling starts when the TVP exceeds one bar. In the case of the natural gasoline blends, this coincides quite closely with the steep increase in gas layer thickness. However, with the crude oil/butane blends, the steep increase does not occur until a TVP significantly above one bar is reached. Crude oils may be stabilized so that their TVPs are near, or somewhat above, one bar as they enter the tanker. In practice, some boiling may occur even without butanisation, but the gas evolution is not necessarily excessive.
In boiling, gas bubbles form below the surface of the liquid, but only down to a depth at which the total pressure (atmospheric plus hydrostatic) is equal to the TVP. The consequent loss of gas in this region may lead to a local fall in TVP. In addition, the latent heat required to evaporate the gas results in cooling, which also reduces the TVP. The reduction in TVP in the liquid near the surface from both these causes tends to delay boiling, even though the TVP of the bulk of the liquid is above one bar. This is why crude oils can be handled with their TVPs somewhat above one bar. It does not apply to the same extent to the natural gasoline type of product because the gaseous constituents in a crude oil are only a small proportion of the total, whereas a natural gasoline usually consists almost entirely of potentially gaseous compounds. This means that the availability of gas, where boiling, is far greater with the natural gasolines than with crude oils.
Natural gasolines suffer hardly any decrease of TVP due to gas depletion when they begin to boil, and boiling is much more likely to continue than with crude oils.
Volatile Cargoes with High Vapor Pressure
Volatile cargoes with a high vapour pressure are most likely to result in flammable atmospheres outside the tanks. Examples are:
Crude oil.
Motor and aviation gasolines.
Natural gasolines.
Light distillate feedstocks and naphtha.
The volume and concentration of gas forming this layer at the beginning of loading depends on many factors, including the:
TVP of the cargo.
Amount of splashing as the oil enters the tank.
Time required to load the tank.
Occurrence of a partial vacuum in the loading.
Precautions while Loading Very High Vapor Pressure Cargoes
Cargoes with high vapour pressure introduce problems of cargo loss from excessive vapour release. They can also gas-up cargo pumps, causing discharge difficulties. Special precautions may be necessary, e.g.:
Permitting only closed loading.
Avoiding loading when the wind speed is less than five knots.
Using very low initial flow rates into tanks.
Using very low topping-off rates.
Avoiding a partial vacuum in the loading line.
Avoiding loading oil that is hot due to lying in shore pipelines exposed to the sun. If unavoidable, this oil should be loaded to tanks that vent well clear of the superstructure, e.g. forward tanks.
Providing extra supervision to monitor gas dispersion and to ensure compliance with all safety requirements.
Monitoring IG main pressure where this indicates the cargo tank pressure. A maximum pressure of around 1,000mm WG should be used and the loading rate adjusted accordingly.
To prevent gassing-up of cargo pumps, the expected TVP of the cargo at the discharge port should, under normal circumstances, not exceed 0.7 bar either for crude oil or products. A TVP of up to 0.8 bar may be considered if the ship is fitted with an IG system, or if some other acceptable method of pressurisation is to be used during discharge.
References: For further details refer to ISGOTT (6th Edition) Chapter 12.1.8 Loading very high-pressure cargoes.
STATIC ACCUMULATOR CARGOES
General
• Certain cargoes have static generating and accumulating characteristics (An oil with an electrical conductivity of less than 50 pico Siemens/metre (pS/m)), capable of retaining a significant electrostatic charge. As such a product moves through a pipe, it leaves a static charge on the pipe and picks up the opposite charge itself. It also picks up a static charge when it is sprayed or splashed against a metal surface, such as when cargo first enters a tank. The rate of charge generation increases with a rate of flow agitation in the tank and the amount of entrained water.
• A charged product could be a source of ignition if:
A static charge was generated.
Enough static charge was accumulated to cause an incendiary spark.
A mean of discharging the spark existed.
An ignitable air-vapour mixture was present.
By reducing the initial and maximum loading rates, the accumulated static charge is kept too small to cause an incendiary spark. By rigorously excluding objects from the tank, and by delaying sampling, thieving, etc. until after the static charge is dissipated, the spark gap is eliminated. On vessels equipped with an inert system an ignitable air-vapor mixture is eliminated by reducing the oxygen concentration.
Precautions while Loading Static Accumulator Cargoes
Planning
At the pre-transfer conference, the officer in charge should determine the static generation precautions necessary giving consideration to Volatility Rating, Flash Point and Reid Vapor Pressure, Loading rates based upon pipeline diameters and lengths and any restrictions in pipeline diameter.
Rate Restrictions
Advise terminal representatives of any flow rate restrictions and make appropriate logbook entries to the effect that the rate was reduced for “cushioning” the product, i.e. safety reasons.
A loading rate based on a linear velocity of 1m/sec at the tank inlet for the initial loading of static accumulator cargoes into non-inerted tanks.
A loading rate based on a linear velocity of 7m/sec for bulk loading of static accumulator cargoes into non-inerted tanks.
A loading rate based on a linear velocity of 12m/sec for loading non-static accumulator cargoes and for loading static accumulator cargoes into inerted tanks. This velocity is for guidance only and is generally considered as a rate above which pipeline erosion may occur at pipe joints and bends.
Relaxation Time
Relaxation time is the time necessary to dissipate a static charge. This is about 30 minutes in a cargo tank.
Prohibit the use of conductive (metal) ullaging, dipping, gauging or sampling equipment during product transfer into a tank and for 30 minutes after completion of operations to allow the settling of gas bubbles, water or particulate matter in the liquid and the relaxation of any electrical charge. After the 30 minute settling time, metal ullaging, dipping, gauging or sampling equipment may be used but it must be effectively bonded and securely earthed to the structure of the ship before it is introduced into the tank and must remain earthed until after removal.
Prohibit the use of all non-conductive (non-metal) containers of more than one litre capacity for dipping, ullaging and sampling during loading and for 30 minutes after completion of product transfer into a tank.
Inerted Vessels
Vessels fitted with inert gas systems are not required to abide by these precautions provided that all cargo tanks are maintained in an inert condition throughout the entire loading. In this context, an inert condition is defined as O2 content below 8%.
Spread Loading
The practice of loading a number of tanks simultaneously to reduce the flow rate of oil or product into a tank to reduce static electricity generation when loading static accumulator cargoes.
Anti-static Additives
If the oil contains an effective anti-static additive, it is no longer a static accumulator. While this suggests the precautions applicable to a static accumulator can be relaxed, it is still advisable to observe them. The effectiveness of anti-static additives depends on how long since the additive was introduced to the product, proper mixing, other contamination and the ambient temperature. It can never be certain that the product's conductivity is above 50pS/m unless it is continuously measured.
References
For further details refer to ISGOTT (6th Edition) Chapter 12.1.7 for Cargo operations/loading procedures for Static Accumulator oils. Chapter 12.1.7.5 for Examples on initial loading rates for static accumulator cargoes. Figure 12.2 for Control Hazards associated with initial loading of static Accumulator cargoes.