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4.2 CARE OF THE CARGO

Vessels must fulfil their obligations as stipulated in the bill of lading and in the charter party with respect to cargo. Most carriage contracts contain 'due diligence' and 'proper care of cargo' clauses, which apply during all phases of cargo handling. Proper care requires constant vigilance, routine checks and preventive maintenance. However, since the question of 'proper care' is the most common ground for cargo disputes (both in and out of court), maintaining accurate records of inspections and conditions found is essential. Securing and recording evidence of cargo surveillance and care will provide the proof needed to protect the owner against claims for lack of due diligence and assist in the speedy resolution of many cargo claims.

4.2.1 Cargo heating
Cargo heating may be required in order to maintain the cargo in a fluid and pumpable condition and/or to reduce waxy residues remaining on board after discharge.
A viscous oil with a low pour point (ie. BCF crude oils), will require heating to maintain fluidity and enable it to be pumped efficiently. A lighter oil with a high wax content (ie. FLOTTA crude oil), may require heating on a long voyage to prevent phase separation of the cargo and resulting high ROB quantities after discharge.


Case study
A 100,000 DWT tanker experienced extensive cargo heating difficulties during a long, winter, trans-Atlantic passage with two parcels of FLOTTA and NINIAN crude oil. Following the discharge in air temperatures of -25 C, the calculated ROB quantity was in excess of 17,000 barrels of un-pumpable sludge and/or wax.

Case analysis
There is no evidence of lack of effort on the part of the crew of this ship. The problem was with the level of maintenance and readiness of the cargo heating system. The owner must have in place a program of regular tests of all critical cargo systems and must take immediate action to correct any defects noted. The owners or managers should have been aware of the condition of the heating system when considering this charter and either refused the charter or made necessary repairs before loading the cargo. Having failed to do either, they exposed themselves and their P&I club to a large ROB claim.


The heating program for the voyage will be determined by the type of crude oil cargo. Aromatic crude oils contain little or no wax. They can be permitted to cool during the voyage, provided the vessel's heating system is capable of raising the cargo temperature to the required pumping temperature before discharge. Paraffinic crude oils must be maintained above a certain critical temperature (the cloud point), to avoid phase separation of the wax they contain.
Charterers must provide accurate heating instructions and accurate cloud point, pour point and viscosity information for the cargo to permit the ship to provide proper care. The charter party should indicate the temperature to which the cargo is to be heated and/or the temperature at which it is to be maintained. It is important to know whether the cargo can be permitted to cool during the voyage and if so, to what temperature it can cool before heating is required to raise it to the required discharge temperature. The minimum carriage temperature is determined by the oil's cloud point, the heating capacity of the ship and the sea temperature at the time the oil is being heated. Many charter party clauses however, are vague on the matter of cargo heating, for example: 'Maintain average temperature of 75 F' could be interpreted to be met if half of the cargo is at 95 F and the other half is at 55 F. Clearly, that is not what is intended by the instruction!
A more precise heating instruction would be:
Ensure that the average cargo temperature of each tank is a minimum of 75 F.
The minimum temperature of 75 F is to be maintained throughout the voyage, that is, do not allow the cargo temperature of any tank to drop below the minimum with a view of re-heating the cargo during the later stages of the voyage.
Keep voyage cargo heating records and provide them for charterers inspection at the discharge port, if requested.
If the load port cargo temperature is below the minimum required, advise charterers immediately and begin heating the cargo to the minimum temperature.
If there is any doubt about the charterer's heating instructions, the master should telex the charterer requesting clarification.

If no reply is received, then the owners should be consulted and a message similar to that below sent to charterers:
"Please refer to our recent telex regarding cargo temperatures. Since no reply has been received, we presume that you require the temperature to be regarded as a target. We will endeavour to maintain the temperature within two or three degrees of the target. This will minimise the loss of light ends and keep the arrival viscosity in the light range. If you do not agree, or if the temperature indicated in the voyage orders is required to be a minimum temperature, please confirm without delay, bearing in mind that a higher temperature may result in fall out of sediment and water, as well as the loss of light ends. Otherwise please advise the upper and lower temperatures acceptable as normal tolerance."

The master should understand what cargo heating costs are included in the freight, and what heating costs are additional to freight and for whose account. Detailed records of cargo heating and bunker consumption must be maintained to permit proper cost allocation at the end of the voyage. Cargo should be heated to a temperature slightly above the lowest allowable temperature indicated in the voyage instructions or charter party.
Although the pour point of the cargo may be known, the critical element governing heating requirements is the cloud point of the oil. Unfortunately, the cloud point temperature is usually not available to the vessel at the load port and is rarely stated on the certificate of quality.
The cloud point is the temperature at which wax crystals are precipitated as solids from the liquid oil. In distillate fuels, this can be visually observed and measured, however is not possible to do so with crude oils.
A crude oil cloud point can be ascertained with higher wax content crudes, by plotting graphically the observed density of the oil, at 5 C intervals, over a range of 40 C. The density-temperature plot will have a gradient change (upward or downward), at the cloud point temperature. The measurements necessary to prepare such a plot can be made on a tanker suitably equipped with hydrometers and a thermometer.

A useful approximation of a crude oil cloud point can be calculated using the oil's pour point (PP) and the following formula:
Approx. cloud point C = 20.2 (10^(0.00708 x ppl - 0.1157714) + 8

For the usual range of crude oil pour points, this formula gives the cloud points indicated in the lower part of the following graph:


Curve of crude oil cloud point as a function of pour point. Where no cloud point information is provided, the curve above may be used to determine a crude oil's approximate cloud point, for cargo heating purposes.

The cloud point is the critical temperature at which a phase separation of paraffinic crude oils begins. The waxy material and its associated oil components which begin to separate when the bulk oil reaches its cloud point are significantly different from the bulk cargo. The separated material is exceptionally difficult to return to suspension in the bulk oil by heating alone, or even by an effective COW program. Even a successful COW programme which takes into account the dynamic behaviour of the phase separated material will only succeed in moving the problem from the ship tanks to the shore tanks. To avoid these difficulties, the temperature of a paraffinic crude oil cargo should never be permitted to fall below its cloud point!
Too much cargo heating can also create problems for the owner. Heating the cargo to a higher temperature than required will:
Increase light-end vapour losses, with resulting increases in air and environmental pollution.
Reduce the solvency of the crude oil for crude oil washing.
Accelerate the fall out of sediment and water otherwise suspended in the crude.
Waste fuel and may (on turbine ships) reduce the voyage speed.
Increase the possibility of vapour lock problems with the cargo pumps while offloading.

Cargo temperatures, measured at three points in each tank, must be taken and recorded daily. Cargo which has been allowed to cool from loading temperature (but still above indicated minimum limit) and cargo which is being actively heated may have a temperature difference of 10 - 15 C between the coolest and warmest oil layers in the tank.
In those rare cases where the chief officer may be left without any guidance, he should refer to the pour points and viscosities indicated in the cargo certificate of quality and manage the heating program to achieve the following guidelines:
The minimum carriage temperature during transit should be:
For paraffinic crudes, a temperature which maintains all parts of the cargo above its cloud point, or
For other oils, the cloud point or a temperature from which the oil can be readily heated to final discharge temperature, whichever is higher, or
A temperature at which the oil has an apparent viscosity of 25 poise (2.5 Pa s) at a rate of shear of 9.7 s^(-1).

Charterers should recognise that it is physically impossible to heat the entire contents of a cargo tank to the same temperature. This is particularly true of wing cargo tanks in cold climates, where the cargo is being vigorously heated at the bottom of the tank and vigorously cooled at the side.
In some cases, the ship will be unable to maintain the required
transit temperature or attain the desired discharge temperature. If this occurs, the owners must be advised immediately, indicating any suggested remedies. One possible remedy (with turbine tankers) is to reduce the vessel's speed, making more steam available for heating cargo. Such a measure should not be undertaken without first consulting owners and charterers.
The required discharge temperature for a cargo can be determined by using the greater of:
1 For products: add 20 C (minimum) to the pour point.
2 For all oils, the temperature required to reach a cargo Kinematic viscosity of 250 Centistokes.
3 For crude oil washing, the temperature required to reach a cargo Kinematic viscosity of 60 Centistokes and/or a temperature 10 C above the cloud point.

COWing of tanks containing bottom residues of phase separated material requires a Kinematic viscosity of 60 Centistokes or less, a cargo temperature greater than 40 C and maximum acceptable aft trim. Back-flooding and dilution of heavily sedimented tanks may be necessary. COW oil should not be recirculated. Refer to Section 9 of the vessel's COW manual for guidance on crude oils which are unsuitable for crude oil washing.
The chief officer must carefully evaluate the weather and sea temperature in deciding when to begin heating the cargo to discharge temperature. Cargo must be raised progressively in the latter part of the voyage, until the required discharge temperature is reached. It is important that the cargo be heated gradually to avoid possible sedimentation/deposition of heavier hydrocarbons in the vicinity of the heating coils due to either chemical reaction or migration of lighter components caused by 'boiling' of the oil in contact with the steam coils. Records of past voyages under similar conditions should be consulted to determine the amount of time needed for the necessary heating. Heating is more efficient at anchor than underway, so if the ship will anchor for several days before discharging, the tanks need not be at required temperature when arriving. Heating should be applied to bring each cargo tank to desired discharge temperature 12 - 24 hours prior to the start of off-loading. Where both heated and unheated cargo parcels are carried on the same voyage, care must be exercised to avoid raising the temperature of the unheated product above 40 C. This may happen if the unheated tank shares two bulkheads with heated tanks and the sea temperature is above 25 C. It can also occur when three or four bulkheads of the unheated product tank adjoin heated tanks.
To protect the ship structure from excessive thermal stress, cargo should never be heated above:
65 C if the sea temperature is 0 C or below, 75 for SBT tankers in sea temperatures 5 or above, and 85 C in sea temperatures 5 C and above.
Asphalt carriers are not subject to this limitation.

4.2.2 Dehumidification
Some lubricating oil or lube oil feed stock cargoes require dehumidification during transport. This can only be accomplished if the ship carries a dehumidification plant and the necessary supply piping to provide dry air or nitrogen to the lube oil cargo tank. Proper care of dehumidified cargoes requires the necessary care and maintenance of the dehumidification equipment, including recording the machinery instrument readings and maintaining a log of the dates and times the unit is used to service the cargo tanks.

4.2.3 Water contamination
Water contamination of cargoes in transit occurs principally through leaking heating coils or by sea water entry through tank washing openings on deck. If heating coil failure occurs after loading, shut down the steam to the affected coil immediately and notify the engine room (so that they can take extra precautions to prevent oil contamination of the boilers).
Tightness of tank cleaning openings should be verified by the leading pumpman and spot checked by the chief officer. On critical product tanks, such as alcohol and lube oil tanks, the chief officer should personally check the tightness of every nut on every tank cleaning cover plate during the first day of the voyage.
Suspected water contamination, such as might result from storm damage, must be reported immediately to the owners and charterers.
The navigation log should record the incidence of 'seas on deck' associated with boisterous weather and storms.

4.2.4 Cargo cross contamination
If the precautions indicated in section 2.20.3 have been properly employed in making the loading plan, there should be little opportunity for cross-contamination of the cargoes during a multi-grade voyage. The principal opportunity will occur if cargo is transferred during the voyage to adjust the trim or stress of the ship. The precautions indicated in Section 4.3 must be carefully observed for such transfers.
Contaminations may be detected in transit by changes in the ullages of the tanks involved. If changes in the relative ullages of two tanks are noted, the chief officer and master must immediately attempt to determine the cause of the changes and take all possible action to reduce any leakage. The master must immediately notify the owner and charterer of the facts of the case.

4.2.5 Vapour losses in transit
Whilst the reid vapour pressure (RVP), of crude oils is generally between 3.0 and 6.0 psi., some crude oils RVP's can be much higher. An example is Brent Blend with an RVP of 10.5, close to that of gasoline.
Vapour losses during the loaded passage can be the result of:
Slack cargo tanks.
Cargo or vapour heating.
Cargo agitation.
Excessive injection of inert gas.
Defects in cargo tank opening seals/closures.

Excessive heating of heated crude oil cargoes can cause vapour losses as light ends are 'distilled' off into the ullage space in the tank and subsequently lost.
The rate of evaporation in crude and light products is dependent on the temperature of the oil at the atmosphere/oil interface inside the tank. The oil surface can be warmed by radiant heating of the main deck plating as when the deck is being heated by bright sunlight. A dark coloured main deck (black or dark red), increases radiant heating of the cargo. To reduce vapour generation and losses from this cause, the decks of crude and light product carriers should be painted a light grey colour. On warm days with strong sunlight, the decks over the cargo tanks can be cooled by directing a flow of water from the ship's fire stations across the deck.
Cargo motion and sloshing in the tank produces pockets of elevated pressure in the tank atmosphere. If one of these pressure pockets is adjacent to the tank vent opening, it may activate the pressure relief valve, releasing tank hydrocarbon vapours from the tank. Such motions can only be reduced by effective weather routing of the vessel to reduce ship motions and by managing the loaded stability of the ship to produce a lower, but acceptable, metacentric height and slower roll rates.
During the loaded passage, the inert gas content of the cargo tanks should be regularly checked and the pressure of the inert gas main continuously monitored. Positive IGS pressure in the cargo tanks must be maintained either automatically or manually. The automatic mode will be more reliable and give lower losses due to excess IGS injection provided that the performance of the system is carefully checked to confirm that it is stopping IGS flow at the desired 'high pressure limit'. Once the tanks are suitably inerted, only the minimum pressure necessary (100 mm water gauge), to prevent the entry of outside air and oxygen into the tank need be maintained.
Defects in P/V valves, ullage cap gaskets, tank hatch gaskets, vent line gaskets or packing glands, tank cleaning opening gaskets, or ullaging fitting valves can provide an exit path for cargo tank vapours, producing unnecessary vapour losses. During the loaded passage, a regular program of soap testing one or two tank openings for tightness should be carried out and the results logged to provide proof of due diligence in proper care of the cargo.
If the vessel is loading only a part cargo, then it should be stowed in an arrangement which minimises slack tanks. Slack tanks permit evaporation of a greater volume of light ends into the ullage spaces and permit sloshing, which increases the rate of evaporation and tank breathing, during the voyage.

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