Safety precaution during oil handling, heating planning for oil tanker

Expansion Allowance

Before loading commences, consideration must be given to the temperature of the cargo being loaded and to the maximum temperature, which may be expected on the voyage. Where there is a possibility that the cargo temperature will rise during the voyage, sufficient ullage space must be left to allow, with safety, for the increased volume of cargo resulting from expansion.
Spot checks of the cargo density and temperature must be taken with the ship’s equipment using a sample drawn from the ships manifold and the result recorded in the Cargo Log Book. These checks are essential to provide early warning of problems with density, temperature, or even grade of the cargo.

Cargo heating planning and optimisation

Different oil cargoes such as special oil products, special crude oils, heavy fuel oils, etc. may require heating in particular in winter and cold climate regions. Some of this heat required can be supplied by exhaust gas economiser. However, in many cases an additional auxiliary boiler is needed to supply sufficient steam. Steam from exhaust gas is generally sufficient to heat the heavy fuel oil that is used on most ships; in port, however, steam from an auxiliary boiler may be needed.

For cargo heating purposes and in order to reduce fuel consumption and the heating costs, a voyage-specific cargo heating plan should be developed by the shipboard team with support from operation department at head office. For a proper plan, the following should be considered :

• Vessel tank configuration.
• Whether deck heater or tank heating coils are provided.
• Number of heating coils and surface areas.
• Cargo details including specific heat, pour point, cloud point, viscosity, and wax content.
• Weather en route including ambient air and sea water temperatures.
• Estimated heat loss and drop in temperatures.
• Recommended return condensate temperatures.
• Estimated daily heating hours and consumption.

Various parameters such as daily air/sea temperatures, weather, cargo temperatures at three levels, steam pressures, return condensate temperature, actual against estimated consumptions and temperatures are discussed between shipboard team and head office. The heating plan should be reviewed and revised appropriately throughout the voyage.

The optimum temperature to which cargo should be heated for carriage and discharge largely depends on the following factors :

1. Pour point : It is the lowest temperature at which the liquid will pour or flow under prescribed conditions. It is a rough indication of the lowest temperature at which cargo is readily pump-able. General principle is to carry cargo at 10 0C above pour point temperature.

2. Cloud point : It is the temperature at which dissolved solids are no longer completely soluble, precipitating as second phase and is synonymous with wax appearance temperature. Once separated, it requires temperature over 80 0C to dissolve the wax. Cargo temperature should not be allowed to fall below the cloud point.

3. Wax content : High wax crude tends to deposit sludge, and therefore require to be maintained at a higher temperature to prevent wax fall out.

4. Viscosity : High viscosity oils do not necessarily deposit sludge and may be carried at lower than the discharge temperatures. However, for discharge purposes, the heating will be done to reduce the viscosity to acceptable levels for cargo pumps.

5. Ambient weather and sea conditions : This will also influence the cargo carriage and discharge temperatures as these impacts the level of heat transfer from cargo tanks or fuel tanks.

The cargo heating plan would need to take into account the above parameter. As part of cargo heating planning, relevant instructions will be developed. Heating instructions should be reviewed after loading cargo, based on charterer requirement. Permission to carry and discharge the cargo at optimum temperatures should be requested from charterer or cargo owner. The heating plan should be made soon after loading cargo and reviewed/updated on daily basis considering the various factors that affect the heating and customer requirements.

A review of the heating log abstract with the following will help with better future planning and identifying the gaps :

• Actual vs. planned temperature.
• Actual vs. planned fuel oil consumption.
• Actual vs. planned heating hours.

Vessels should complete the heating abstract (daily basis) after completion of each voyage and send it ashore along with the Cargo Heating Log, also identifying any gaps. Figure shows a typical cargo heating patterns graph.

Operational control and best practice

For best practice cargo heating planning, the following should be noted :

1. Vessels should have a greater understanding of the voyage manager/charterer’s heating instructions.

2. Seek the receiver/charterer’s permission for allowable range of cargo temperatures.

3. Avoid heating during adverse weather period.

4. Create and follow the proper cargo heating plan to verify the effectiveness of actual heating progress.

5. Closely monitor and analyse cargo heating reports. Monitor heating daily to address deviations from the heating plan.

6. Do not heat for short frequent periods and running boiler at low loads.

7. Follow the recommended condensate temperature and optimum boiler settings for efficient cargo heating. Heating instructions, accompanying the heating plan, should further highlight these points.

8. Maintain efficient and good communication between the vessel and the voyage manager/charterer about the plan and execution.

Cargo heating may also benefit from the use of effective insulation. For example, using lagging on heating coil water / condensate return pipes as well as steam, thermal oil and hot-water lines on deck area. This could be significant energy saving option as it has been observed that some ships lack insulation of branch lines and cargo tanks. It is important that the insulation material is of good quality. A poor quality of insulation material is likely to rot or lose its effectiveness.

Loading And Carriage Of Low Temperature Cargo

Increasingly export of products from ports exposed to extremely low ambient air temperatures is occurring. Due to the transport and storage time the cargo temperature can be close to the ambient temperature and the terminal may have no means to heat up the cargo.

The loading and carriage of low temperature cargo requires careful Risk Assessment which should be commenced when the vessel is proposed for loading at such a terminal.

Discharge ports may also have a minimum acceptable cargo temperature which could be close to 0 deg C.

Oil tankers are not generally specifically designed to load cargo at extreme low temperatures being generally constructed of grade A steel which is not charpy tested / of low toughness and could be subject to increased risk of brittle fracture at low temperatures. Three factors have to come into play simultaneously to experience a brittle fracture :-

1. Initial crack / defect.
2. Temperatures low enough such that the material is brittle.
3. Loads on the structure.

Cargo of temperature to 0 deg C is considered acceptable to load by most classification societies. Loading of cargo of temperature down to -10 deg C should not normally present an unacceptable risk, however it is outside the criteria specified by class, and therefore class must be consulted. Loading cargo with a temperature below – 10 deg C shall not be undertaken without careful consideration of all the risks due to the increased possibility of brittle fracture. In all cases where cargo below 0 deg C is to be loaded the Ship Management Technical Office is to be advised.

Loads on the Structure

If the cargo temperature is below -10° C and the vessel is not specifically built for carriage of cargo at this temperature great care must be taken to ensure that loads on the structure during cargo operations and during passage are minimised. This may require the vessel to consider waiting in sheltered waters for a weather window on the loaded passage.

Equipment

All equipment shall be in good order and suitable for the expected cargo loading temperature, checks are to be made with manufacturers where required – Cargo / Ballast tank coating temperature tolerance.

Cargo tank temperature and high level / overfill sensors.
All instrumentation associated with deck heaters.

• Ballast valves where not submerged in ballast must be regularly worked to prevent freezing.

• The material of the piping system, couplings, cargo pumps, valves, seals and instrumentation. Valves on deck should be thermally insulated / trace heated to prevent them becoming inoperative due to ice formation and clogging.

• Cargo tank pressure relief arrangements must be suitable for the temperatures involved, trace heating applied / procedures in place to ensure continuous freedom to operate put in place.

The risk of fracture of equipment due to volumetric expansion of ice is always present.

All the above shall be considered as part of the risk assessment for the operation. Also consider safety precautions for Ballast operations in sub–zero temperatures.

Cargo system preparation

The cargo tanks, pipelines, heating system and all associated instrumentation shall be completely free of water prior to entering the cold weather area, any water present in cargo tanks will freeze immediately the cargo loading is commenced.

Cargo heating

If loading of low temperature cargo is to proceed then measures to increase the cargo temperature need to be implemented to reduce the vessels exposure to brittle fracture during loading and on route and further to make the cargo temperature acceptable to the receiving terminal.

The Reid Vapour Pressure, Flashpoint and Boiling Point of the cargo must also be considered in relation to cargo volatility. Overheating of the cargo due to insufficient circulation, cargo pump failure etc. must be avoided and shall be considered in the risk assessment. Other hazards which must be controlled are freezing of the heating medium and excessive ice build-up in ballast tanks.

Reduced / low cargo filling rate

A low tank filling rate will result in reduced cargo thermal inertia and resulting effects on the structure and allows any cargo heating up from ballast to have more effect. Depending on the ballast water temperature the cargo temperature rise can by 0.5 deg C / hour, however obviously the rate slows down as cargo mass increases.

Use of ballast water to slowly heat the cargo

This method is preferred and safest for a cargo with a low flashpoint and boiling point < 140 deg C. Where possible, and as far as practicable with respect to stability and stress considerations, ballast should be retained to the draft level to avoid freezing which will occur if the level is kept above the waterline. Ballast tanks are acceptable as a heating source provided that ice accretion on the ballast tank bulkheads does not exceed 10% of the ballast tank width or height for double bottom tanks. If the 10% is exceeded the ice layer will act as an insulator. If ballast tanks are provided with a heating system / or bubbling system this must be kept in operation to reduce icing. To assist the process the cargo shall be circulated within the tank.

Ship’s Experince Factor (SEF OR VEF)

Surveyors at a load port must always be given the information to calculate the Ship’s Experience Factor or Vessel Experience Factor.

Fire-Fighting Precautions at the Manifold

Before any loading or discharging operations are commenced, the following fire-fighting equipment is to be placed on deck in a position approximately 10 metres forward or aft, but always upwind of the manifold :

1. One portable foam-making branch pipe with eductor, connected to a fire hose and hydrant.

2. Not less than 75 litres of foam compound.

3. An additional hose with jet/spray nozzle attached, and connected to a hydrant.

4. An International shore connection and fire plan/wallet is to be available, situated in the vicinity or point of access to the vessel.

5. A semi-portable Foam or Dry Powder extinguisher is to be positioned close to the manifold or if semi-portable extinguishers are not available then two 9 litre foam extinguishers are to be used. The oil spill materials are to be kept available for use in the event of a spillage, but are not to be placed adjacent to the fire-fighting equipment.

6. The above mentioned equipment is to be provided from the nearest Supplementary Equipment Station and Oil Spill Response Centre.

7. 75 litres of foam compound will be consumed by any one branch pipe in 5 minutes. If the Master considers that the Emergency Organisation will require more than 5 minutes to supply additional foam compound to the manifold area, then additional 25 litre canisters are to be positioned in readiness.

8. The fire main must be pressurised throughout cargo operations.

Bunkering Operation

The Shore Facility’s permission must be obtained prior to carrying out bunkering during cargo oil transfer operations.

Preparation of Main Engine

The main engine must always be ready for immediate use in case of an emergency during cargo oil transfer operations.

Measures during Extreme Weather

The criteria for stopping of cargo operations and dis-connection should be agreed and listed out in the “Ship / Shore Safety Checklist” as to be within the standards as mentioned in the terminal hand book. When the vessel is moored alongside a pier or a buoy, and extreme weather with a wind velocity exceeding 40 knots, adverse currents or the like, the Master should consider shifting the vessel into safe waters. Such should be carried out well in advance, after consultation with the terminal.

Related article :

• Risk of gas freeing operation and precautions for oil tankers
• Bulk Oil Cargo
• Completion of Documentation For Bulk Oil Cargoes
• Shortage and Contamination Claims For Bulk Oil Cargoes
• Oil Discharge Monitoring Equipment For Ship