Chemical tankers are among the most complex ships ever constructed. The cargoes they carry often, present tremendous challenges and difficulties from a safety point of view and many chemicals are also a far greater pollution threat than crude oils.
Hazard evaluation of chemicals is in itself a complex problem. It stems from the combination of the flammability and toxicity characteristics of the chemicals themselves as well as from design and operation hazards. Here in this chapter we shall discuss the routine operations and the precautions a crew of a chemical tanker must take in order to carry out various operations safely.
Given below is a sequence of various operations, which need to be carried out as a routine on a chemical tanker
Fig: sequence of chemical tanker cargo operations
Prior planning for loading cargo information must be exchanged between the ship and shore authorities. Operations concerning cargo handling, tank cleaning, pre-wash, ballasting and bunkering must be discussed. Each ship / terminal should have procedures for this exchange of information. (See ICS publication on chemical carriers for information exchange format)
For a safe operation every aspect of loading must be planned. The planning includes distribution, sequence etc.
Scantlings of an Oil tanker correspond to a cargo of specific gravity of 1.0 (water) plus an additional scantling to take care of the vapour pressure above the liquid of 0.25 kp/cm2 (2.5 m water head).
Many chemical cargoes have far higher specific gravities (e.g. caustic soda 1.4, phosphoric acid 1.7, sulphuric acid 1.8, molasses 1.50, and trichloroethylene 1.45). Some of the centre tanks in chemical tankers are therefore often strengthened for specific gravities up to 1.4 – 1.85. Tanks for heavy cargoes such as heavy liquids also have their top parts dimensioned for heads higher than 0.25 kp/cm2.
As centre tanks do not provide internal damping due to absence of web frames, as compared with wing tanks, slack centre tanks with filling ratios between 20 and 90% should be avoided when carrying heavy cargoes.
The ship / shore or ship / ship checklist must be filled in each time. Each item must be checked by both ship and shore personnel. The completed checklist is of no value if it is merely regarded as a paper exercise. This topic is discussed in detail in the later chapters.
Ship checks prior arrival for loading / discharging
- Tank instrumentation like level gauges, temp alarms, high level alarms must be checked for proper working and accuracy.
- Hatches and lids of tanks to be firmly closed
- Cargo pipelines, crossover and drain valves closed
- All unused lines secured and blanked off.
- Cargo deck lighting checked for adequacy.
Ship checks after arrival but prior operations
- Weather forecast information must be sought.
- Compatibility of ship – shore vapour return facility if used must be reviewed.
- Characteristics of the product handled available through MSDS.
- If cargo is liable to self react, then arrangements for using inhibitor and its limitations must be known keeping the duration of the voyage into account.
- Enclosed space procedures in place for inspection of tanks if needed including ventilation.
- Tanks cleared for loading must be closed immediately.
- Flame screens to be in position if some ullage ports are to be kept open.
- Sea suction and overboard discharge valves if connected to cargo lines must be closed and kept sealed.
- All scuppers and open drains onto the jetty must be plugged. Accumulated rain water must be effectively drained off from time to time. Contaminated water must be transferred to a slop tank or other suitable receptacle.
- Correct personal protective equipment clothing and breathing apparatus, appropriate to the cargo should be immediately available and should be worn as necessary.
- Just prior to cargo transfer the responsible officer must ensure proper lining up. (I.e. only required valves are open and all others valves are shut).
- In the Liaison meeting with ship / shore staff an agreement regarding load/discharge rates, emergency stopping methods must be agreed.
Cargo loading precautions
Commencement of cargo loading must be slow and before achieving a full rate both ship and shore must be satisfied that there are no leaks in the system. Quantities must be checked and exchanged at regular intervals with the ship and shore. During routine ullaging and testing a responsible officer must ensure that the proper procedures are being followed as per local / international regulations. Proper personal protection equipment must be used.
Compressed gas is used sometimes by terminals to press out products out of shore tanks into the ship resulting in over pressurisation of ships cargo tank. The pressures range is between 2.5 to 5 bars. Change from liquid supply to gas supply may cause abrupt increase in tank filling rates and over pressurisation of a closed tank can occur in seconds. A crewmember stationed at the manifold will be best placed to detect and react to any indication that a flow in the system has changed from liquid to gas.
Ships undergo a risk of overflow when topping off tanks. Close monitoring must be done at this stage and high level alarms must never be bypassed. Loading must be stopped should one suspect a malfunction. Tanks, which have already been topped, must be shut and checked at regular intervals to ensure that no more cargo is entering the tank. When nearing completion of loading the terminal authorities must be notified and if necessary loading rate reduced.
After completion of loading the shore line sometimes need to be cleared by use of air, inert gas or pigging (line scraper). A responsible officer must ensure that there is sufficient space in the tank to receive the line content. Over pressurisation may also result when the gas used for blowing causes dramatic fluctuations of pressure. If nitrogen is used to clear the cargo hose after loading a cargo treated with an inhibitor that depends upon oxygen, care must be taken to minimise the entry of nitrogen into the cargo. Not only the nitrogen will deplete the oxygen in the cargo but will find its way into the ullage space in the tank causing further reduction in oxygen levels.
Precautions upon completion of Loading
After completion of loading sampling must be done in a safe manner and all the relevant valves must be shut. Storage of samples must be in accordance with the IBC code.
Precautions during the voyage
Attention must be paid to the shipper’s instructions and MSDS for a safe carriage of chemicals on board ship. Sufficient inert gas must be carried or capable of being generated during the voyage. Regular checks on contents of the cargo in the tank must be made with respect to ullage, temperature and pressure. Cargoes which may self react must be monitored daily to detect any abnormal behaviour. Unexpected changes are an early indication of a possible self reaction. With inhibited cargoes the precautions and limitations as mentioned in the inhibitor certificate must be carefully observed. Polymerisation may occur where the cargo vapours condense, areas like vent valves, flame arrestors should be regularly inspected for blockages due to formation of solid polymers.
Precautions during discharge of Cargo
Ship/Ship/Shore checklist: The ship / shore or ship / ship checklist must be filled in each time. Each item must be checked by both ship and shore personnel. The completed checklist is of no value if it merely regarded as a paper exercise.
Having discussed with the shore authorities on discharging procedures, rates and sequence attention must be paid to setting of lines, and venting procedures.
If a vapour balance is to be used by returning inert gas displaced from the shore tanks to the ship, the pressure in the ships tanks must be carefully monitored and over and under pressurisation of tanks must be avoided. Checks on cargo quantities discharged and received by shore must be compared at regular intervals.
When discharging cargoes under a blanket of nitrogen, it is necessary to ensure that no air is drawn into the tank. Nitrogen must be supplied under pressure either from storage receptacles or generated on board, into the ullage space. If it is necessary to obtain nitrogen from shore then prior agreement on its flow and pressures is necessary. Though the requirement of nitrogen is only 0.2 bar the nitrogen supplied by shore is in excess of 5 bars to 7 bars. Now this is a lot of pressure, which can cause permanent damage to cargo tanks. Due care must be exercised to keep the pressure in the tanks within limits especially when the ullage space in the tank is low. A safe procedure is to use a pressure-reducing device on the nitrogen supply line. There should also be a regular communication with the terminal and the ship must always monitor the pressure in the tanks.
After completion of bulk discharge of certain cargoes like animal and vegetable oils and fats, it is necessary to manually sweep the cargo towards the pump suction to complete the discharge. This process is called “puddling” or “squeegeeing”. It is necessary that safety procedure must be followed for entering spaces that are “enclosed” and “slippery”. Ventilation must be continued during this operation keeping a responsible person at the hatch entrance to monitor the activities inside.
Part discharge of chemicals may create highly stressed part-loaded conditions for the hull structure. Consult the loading manual for the vessel and take appropriate action such as ballasting. At the same time one should avoid using cargo tanks for ballast as estuary or harbour water may contaminate them unduly.
Some cargoes present difficulties with final stripping of individual tanks. Typical cargoes are:
- “light” products such as naphtha and gasoline;
- viscous but still light and gassing “spiked” crude oil (high contents of butane); and
In order to improve suction to the pumps one sometimes pressurises the cargo tanks by admitting compressed air, steam, or, strongly preferable, inert gas under overpressure from fans. (Do not use steam with flammable cargoes, and cargoes which may become contaminated by the steam condensate). Steam may produce electric charges, which may trigger off an explosion. If steam is used when carrying molasses remember to keep the tank lids somewhat open during and after steaming. If not, there might be some sub pressure developing, which may damage the tank. (Note: In many instances the charterer may not permit steaming of molasses during discharge).
If pressurising of the tank is to be used, remember the tanks are dimensioned for a vapour pressure of 0.25 kp/cm2.This means that one must not exceed 0.15 kg/cm2 (1.5 m in water head) in service. Never use manometers to measure such low pressures, those available on board are far too insensitive. Instead use a plastic hose as water filled U-tube, and let the difference in water levels not exceed 1.5 m between the two columns. A U-tube can also be arranged to act as a safety valve so that the water blows out if the desired pressure is exceeded. Many instances of damaged tanks have occurred.
Precautions upon completion of discharge
It is essential that the cargo remains must be reduced to a minimum. Tanks must be stripped according to the requirements of the ship’s Procedures and Arrangements manual. All relevant manifolds must be closed and secured. All openings for the cargo tank for that product must be secured shut. Cargo hoses and arms must be disconnected only after they have been drained of all cargo residues and relieved of all pressure.
Precautions during ballasting and deballasting
Most chemical tankers meet the segregated ballast requirements of MARPOL. These tanks must clearly be marked. Sometimes however cargo tanks may be used to carry additional ballast and before doing so the responsible officer must ensure that the cargo tank is clean or does not contain cargoes, which may react with water. Ballasting a tank, which has not been cleaned, may give rise to inflammable vapours being expelled in the surrounding cargo deck areas. A cargo tank containing “dirty” ballast must be marked and must never be allowed to overflow. Discharge of such ballast must take place in accordance of MARPOL and local regulations. Checks must be made when discharging ballast to ensure that cargo residues are not entering the sea. When deballasting, it must be ensured that cargo vapours are not being drawn into the ballast tanks especially when done during the loading is going on. The ballast tank lids therefore must be kept closed at al times. Upon completion of ballast all openings and valves used for the operation are closed.
Tank cleaning is essential on chemical tankers and must be recognised as a potentially hazardous operation. The ships P&A Manual (Procedures and Arrangements Manual) explains the MARPOL implementations under different conditions which must be studied prior carrying out this operation.
Different techniques are required for cleaning from each cargo – there may be a Tank Cleaning Guide on board. (Dr. Verwey’s Guide)
These guides describe:
PRE-WASH: Removes pools or piles of residue from the tank and corresponds to the whole washing operation in a petroleum tanker.
MAIN-WASH: Removes traces of the last cargo from the tank coating etc.
SPOT-WASH: Local cleaning.
AFTER-WASH: Removes traces of the cleaning agents, e.g. using Salt Water or Tank washing chemicals, etc.
Use of tank cleaning chemicals
These have an affinity for water as well as for oils or certain chemicals. Hence the oil etc. can be pumped away rather than remaining as a film on the surfaces.
These chemically convert animal or vegetable oil into soap, which is soluble, for example Caustic Soda.
May also include surfactants (wetting agents, dispersants), which reduce surface tension and give better wetting properties.
Precautions when using chemicals for tank cleaning
- Check tank coating compatibility,
- Check compatibility with previous and next cargo.
- Do not exceed recommended strength.
- Mix in same or another tank and recirculate.
- Direct injection into deck wash line with air-driven pump.
- Spray undiluted and then wash.
- During the tank washing operation gases are being released from the tanks and the designed venting system is probably not in use.
- Use protective clothing
- Test atmospheres.
- Dispose slops as per rules.
- Take into account the cargo following and the standard of cleanliness required.
- May be easier in the long run to wash with fresh water if low chloride content required for next cargo.
Some chemical cleaning jobs
|Water-soluble products Alkalis Alcohols||Pre-wash cold water, usually fresh|
|Volatile Products Acetone, M.E.G, Toulene||Ventilation may be sufficient|
|Mineral Oils, Less volatile chemicals||Emulsifiers|
|Vegetable or animal oils Drying Oils Linseed, Cottonseed oil||(Strict attention to temperatures recommended) Cold pre-wash Saponifiers|
|Non-Drying Oils Palm oil, coconut oil, tallow||Hot water (75°C), Saponifiers or emulsifier|
|Polymerisable products Styrene||Immediate washing, flooding and flushing, with fresh water|
|Water Reactive cargo TDI Sulphuric acid||Solvent Wash (e.g. Diesel) Large quantity of water|
Precautions during tank cleaning
Risks from gasses expelled from tanks cannot be over emphasised. Depending on the recent cargo carried in tanks that are to be cleaned, vapours that are toxic, flammable and corrosive would be released during the operation. In view of this, it is therefore of outmost importance that these operations must be carried out using the approved procedures and arrangements for the ship. The personnel involved must fully understand the dangers and take necessary precautions, because of the consequences of an inadvertent error can be very serious and sometimes fatal.
A responsible officer must supervise the operation. A pre cleaning conference must be held with all concerned with the operation. A written cleaning schedule must be in place. Duties must be allocated and properly understood by all concerned.
Preparations include adequate checks to equipment, which need to be used. All personnel must be informed that the operations are about to begin and only those involved in operation must be allowed on the cargo deck.
Water being the most common cleaning medium, is generally used with detergents mixed as additive to increase the cleaning quality. Water must not be used to clean tanks in case of those chemicals, which react with it. It may be permissible to vent out all the cargo residues of a highly volatile cargo from a tank. In every case due safety aspects of the operations should be considered.
After carrying a low flash point cargo, a flammable mixture vapour should always be suspected until tests have established that the atmosphere is non-inflammable. Toxic vapour in harmful concentrations must be assumed after unloading cargoes unless otherwise recorded by instruments. Toxic or flammable cargo vapour should be suspected in cofferdams or any other spaces within the cargo area into which such cargo may have leaked.
Tank washing must be carried out in inert atmosphere. Although the atmosphere is inerted, portable washing machines must be connected along with all hose connections prior introducing them into the tank. Connections must not be broken until after the machine has been completely removed from the tank. The tank must be continuously stripped and water must not be allowed to accumulate in the tank being washed. The purity of inert gas must be maintained and monitored. Oxygen content not exceeding 8% must be maintained at mid level and 1 meter below the deck level of the tank being washed. Washing should be stopped if the level of oxygen in the tank goes beyond 8%.
Some non-inflammable cargoes, even though carried in an inert atmosphere to maintain quality do not pose a hazard even if these tanks are washed in non inerted condition. In this case a gradual decay of the inerted condition during washing is acceptable. In all other cases if the inert atmosphere is not maintained then the procedures for carrying out cleaning must be followed as if tanks are being washed in an undefined atmosphere as mentioned below.
When tanks are being washed in an undefined atmosphere after carrying inflammable cargo, the atmosphere in an empty tank should be treated as flammable. The only way to guarantee safety during washing is to ensure that there can be no source of ignition. Good tanker practise will avoid all normal sources but additionally the following must be done:
- Flush tank bottom with water and strip well. Piping system including pumps, crossovers, discharge lines etc must also be flushed. Flushing water must be drained into slops.
- Free fall of washing water into slop tanks must be avoided unless the tank is inerted.
DO NOT DO THIS WITH CARGOES WHICH REACT WITH WATER
- Connect the hose (one or multiple) to the tank cleaning machine before introducing into the tank. Similarly remove machine from tank before disconnecting it from the hose(s)
- Do not use synthetic ropes to support machines
- Do not use machines having a throughput of > 60 M3 / hr and do not use nozzles having a throughput of > 17.5 M3 / hr.
- Total water throughput per cargo tank should be as low as possible and in no case > 110 M3 / hr.
- Tank should be kept drained during washing and if necessary washing should be stopped to clear any accumulated liquid in tank.
- Recirculated water should not be used since it increases generation of static electricity.
- Sounding rods and other equipment must not be introduced through a sounding pipe reaching close to the bottom of the tank and earthed to it.
- No other material that may create spark or static electricity should be lowered into the tank.
- Steam should not be injected into the tank.
If a sounding pipe is not used it is essential that any metallic components of a sounding rod or other equipment are bonded and securely earthed until removal from the tank. This procedure must be used during washing and for five hours afterwards (this period may be reduced to 1 hour if the tank is continuously mechanically ventilated after washing).
Within the five hour or one hour period as stated above
- An interface detector of metallic construction may be used if earthed to the ship.
- A metal rod could also be used at the end of a metal tape, which is earthed to the ship.
- A metal rod attached to a natural fibre rope must never be used.
- Non metallic equipment made entirely of non metallic materials may be used without earthing.
- No ropes made of synthetic polymers or chains should be used for lowering equipment into a cargo tank.
Water washing may be inadequate and therefore special cleaning methods are employed in cases of certain kind of cargoes. Local authorities may impose special requirements and additional safety or environmental requirements. The choice of tank cleaning agent must be made with full knowledge of the cargo characteristics. If the only means of tank cleaning is to involve entry in to the tank, precautions must strictly be followed. No one must enter the tank without the express permission of a responsible person. Tank atmosphere must be checked and chemical absorption detectors should be used to detect the presence of specific gases and vapour at TLV (Threshold Limit Value) levels.
In exceptional circumstances chemicals may be required to wipe down product traces and residues from tank walls using chemicals in local areas. This poses additional risks to persons carrying out the job especially in confined spaces. Such risks must be evaluated before starting the operation and must not be undertaken unless the personnel involved can be safely protected from the hazards. MSDS for these chemicals must be available on board. In addition the manufacturer’s instructions or recommendations on use of these chemicals must be fully understood.
Precautions during discharge of dirty ballast
Dirty ballast must be discharged into sea in accordance with the MARPOL and local requirements. Discharge must be made below the water line though an underwater outlet on the side of the ship away from the essential water inlet valves. When discharge is made above the water line care should be taken to ensure that cargo vapours or liquid does not get blown back on board. If such a risk exists then discharge must be made below the water line and if this is not possible then an alteration of course, speed must be considered and the personnel must wear protective equipment on deck.
Precautions during slop management
Compatibility of cargo residues and cleaning agents must be considered just as carefully when handling slops as when handling the cargoes themselves. Compatibility with cargoes must be considered when washings need to be put into slops especially when cleaning multiple tanks, which have different cargoes.
The following should be avoided:
- Mixing Annex I (oil) slops with Annex II (chemicals) slops.
- Mixing of slops of incompatible cargoes
- Mixing of slops from vegetable oils or fats with chemical slops or petroleum oil slops.
If the ship’s cargo tanks are to be used as slops, care should be taken to ensure that the cargo is compatible with the cargo tank coating. It may be noted that some cargoes, which are compatible, may, when mixed with water, form acids and thus damage a tank coating, e.g. slops from hydrolytic cargoes in zinc coated tank.
Precautions during gas freeing
- Venting of toxic vapours must be made from the vessels approved venting system. Escape of gas must not occur at deck levels.
- Tank openings must be kept closed if portable blowers are used to vent a tank. These openings must only be opened just before the work is about to commence. This is to avoid accidental entry into the tank whilst the tank is being gas freed.
- When gas freeing is done by permanently fitted fans through cargo lines the lines must be drained properly. Valves on system other than those required for ventilation must be secured shut. The fans must be blanked off or disconnected from the system when not in use.
- Fixed gas freeing must not be used for gas freeing a tank while simultaneously being used to ventilate another tank in which tank cleaning is in progress.
- Portable fans used must be either water, pneumatic driven. Their constructions must be such that no incendiary spark can be produced in the event of the turbine blades touch the casing. Guards to prevent contact with the fan blades must be provided.
- Portable fans must be arranged in such a way such that ventilation is effective and all the parts of tanks are ventilated.
- Portable fans must be properly earthed to the ships deck.
- Wind direction must be noted and due care must be taken to ensure that the gases do not enter or come any where near air intakes to accommodation / engine room. Central air-conditioning units must preferably be set to recirculation mode prior and during the entire gas freeing operation.
- If it is suspected that gas is being drawn into the accommodation block, the air conditioning unit and mechanical ventilation systems must be stopped and the intakes covered or closed.
- If the tanks are connected by a common venting system then each tank should be isolated to prevent transfer of gas from one tank to another.
- When a tank appears to have been gas freed and all mechanical ventilation has been stopped, a period of 10 minutes should elapse before taking final gas measurements.
- Upon completion of all gas freeing and tank washing the gas venting system should be carefully inspected paying attention to the PV (Pressure Vacuum) valves and HV (High Velocity) valves. Spark arrestors must be checked and cleaned.
- On completion of all gas freeing attention must be paid to all equipments used and to all enclosed and partly enclosed spaces that can retain or contain cargo residues or vapour so that no unsuspected dangerous pockets remain. These spaces include cargo lines, cargo valves, vapour return lines, cargo pumps, stripping lines and valves, venting lines, PV valves and lines, ullage and sounding arrangements, heating coils, cargo handling equipment store rooms, protective clothing store rooms and cargo sample stores.
Removal of sludge, scale and sediment
Physical demucking of cargo tanks may sometimes become necessary. The usual procedures for man entry must be followed. Equipment used for sludge removal must be made from materials, which do not give rise to risk of ignition. Removal of sludge must only be considered after gas freeing is completed and tank certified fit for man entry.
Opening of cargo lines and cargo handling equipment
Cargo pipelines, Manifold crossovers, vent lines etc. should be cleared of cargo residues and should be cleaned and gas freed at the same time as the cargo tank. It should be always assumed that these lines will contain pockets of cargo residues and vapour. Precautions must be taken accordingly.
If it becomes necessary to open up lines and handling equipment on deck or in a cargo tank or a cargo pump room the following precautions must be taken.
- The task must be assessed and approved by a responsible officer and a work permit issued before starting any work.
- Isolate the associated pipeline after gas freeing the space.
- The atmosphere of the space must be tested for flammability and toxicity.
- When opening up air measurement for flammability and toxicity should be made in vicinity of the work area.
- Fire fighting equipment must be kept in readiness.
Following precautions should be taken;
- Heating medium should be compatible with the cargo.
- Normally water (steam) is used for heating cargo. If water is unacceptable, heating oil may be used.
- Care is necessary to prevent transfer of heat through safety barrier,
- When heating the cargo, pressure inside the heating system should be less than maximum rated pressure of the system.
- Cargo temperature to be measured by restricted or closed instruments.
- Toxic parcels (e.g. Phenol) require the heating circuit to be outside the engine room, with heat exchanger circuit external to tank (heating ducts outside).
- Sampling equipment in cargo area before the fluid is returned to the engine room.
NOTE: Purge heating coils before use. When not in use, toxic cargo may leak in.
- It is better to keep heating coils blanked off when not in use.
- If cargo is more than 75°C (140°F) hotter than sea temperature, check builders / owners / classification society’s requirements regarding precautions to be taken before loading.
- The contents of next tank should be taken into consideration before the cargo is heated
- Observe shipper’s requirements and maintain records of cargo temperatures.
- Always heat up slowly for discharge – risk of scorching on heating coils.
Precautions when taking temperatures manually
- Protective clothing with goggles.
- Release pressure carefully (spray of condensed cargo).
- Do not let water in (wet thermometer line).
- Deck conditions safe in heavy water
- Cargo may be kept cool by hosing down above tank (e.g.: Styrene).
Some chemicals have much higher coefficients of thermal expansion than hydrocarbon oils. Cargo may be warmed by a heated cargo in the next tank leading to expansion of the cargo.
Inert Gas systems
Flue gas I.G. system
This system is used commonly in conventional tankers. This gas contains appreciable amounts of Carbon and acidic sulphur compounds and more or less saturated with water vapour, therefore not suitable for most chemicals.
This system burns gas-oil hence less sulphur to scrub out. It may have dryers. Systems are required to produce and maintain a positive pressure in the system. Hence when in service but switched off, there is a positive pressure in the tank with a risk of being connected back to a source of ignition. We must therefore maintain an effective cargo / gas barrier. Lack of effectiveness has resulted in a serious and in a catastrophic explosion in petroleum tankers.
Shippers may not accept this I.G. in practice for fear of contaminating sensitive cargoes.
In this system, inerting is done of the tank using shore supply (truck) of nitrogen before loading. However ships need to carry bottles of N2 to maintain pressure during the voyage (sometimes near tank, sometimes up forward with built-in distribution line). The main drawback is that additional Nitrogen is needed from shore while discharging. Modern ships do have nitrogen gas generators fitted to generate / supplement the inert gas.