EFFECTIVE CRUDE OIL WASHING GENERAL
Crude Oil Washing ( COW) is a requirement by Marpol for all Crude carriers. A minimum of 25% of the total number of tanks, in addition to the Heavy weather ballast tanks, needs to be washed, every voyage for sludge control, provided that all the vessels tanks are washed in a quarterly period. Ships other than SBT’s have to wash additional tanks in order to make tanks clean for taking ballast for the subsequent voyage (Departure and Arrival Ballast). Every crude oil carrier requires to carry a COW manual, which gives the details of the COW equipment and associated systems on board. The manual indicates the criteria to be used to assess the suitability of oil for crude oil washing on board. For eg. The Viscosity of the oil at the discharge temperature must not exceed 600 cst ( this is present in some charter parties as well), the discharge temp should be at least 10 deg above its pour point, and draining of the COW lines for heated crudes as it may solidify in the lines. A lot of ROB claims arise due to a pool of oil covered by sludge, poor draining back of the cargo from forward, on account of sludge accumulation at various locations. Ship owners lose a lot of money by way of deductions from the freight for the amount of cargo retained. Every vessel going to the Dry Docks have to clean their tanks and desludge for various reasons. The importance of proper COW can be appreciated, when the owner has to pay close to 500 USD/ Ton for disposal of sludge. On an average, amount of sludge disposed from a VLCC is around 400 tons. A straight loss of 200,000 USD, besides the time lost to complete the desludging operation. In order to understand the properties of the crude oil, to apply the data as provided in the MSDS, to minimize the amount of sludge generated in the tanks and to have an effective wash, it is essential to know the nature and physical behaviour of the crude oil. CLASSIFICATION OF CRUDES
Crudes can be classified into 3 categories, a) Aromatic, b) Paraffinic and c) Intermediate between a and b. Aromatic crudes are those with higher kinematic viscosity and are temperature sensitive. For optimum pumping, the temperature of the cargo should be such that the viscosity should be lower than 250 cst. These cargoes do not pose difficulties in transport, handling or COW, in the same manner as the Paraffinic crudes. Paraffinic crudes are oils with very high paraffinic wax content, which results in high Pour Point temperatures ie. In simple terms the temperature below which the oil ceases to be in a liquid state. These types of crudes must normally be heated to prevent solidification during transportation and for pumpability. Paraffinic sludge is the partially solidified hydrocarbon deposits, which are made up of various wax species from the crude oil. Once separated it is nearly impossible to get the sludge to mix homogenously with the liquid oil again. There is a certain critical temperature at which the wax contents starts to separate out from the oil and becomes a semi solid like particle in the oil. Such temperature is called as the Wax Appearance Temperature or the Cloud Point temperature. IMPLICATION OF CLOUD POINT DETERMINATION
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Knowing the cloud point temperature helps tanker officers in determining whether the oil is suitable for washing or not. For crudes with high wax content, the temperature of the wash oil must be at least 10 deg above the cloud point temperature.
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If excessive sludge is present, a fresh charge of oil should be taken after each close cycle. Crude oil washing with a liquid with high pour point would affect its efficiency, in the removal of deposited sludge. Moreover temperature reduction below the cloud point temperature might mean additional deposition in the tanks being washed.
PROCDURE TO ESHTABLISH CLOUD POINT
One of the methods of determining the cloud point of a crude with pour point of over 20-25 deg C is the BONDI test procedure although the feasibility of performing the tests on board is debatable. A representative sample of the oil is heated to about 30 deg above the pour point temp. Immerse the above container in a bath with temperature at least 15 deg below the pour point. If a graph of temperature against time is plotted at frequent intervals ( at least 30 secs ), There will be two such occasions wherein the temp loss against time will be zero. The first such point will be the cloud point and the second will be the pour point. Usually the Material Safety Data Sheet, as supplied by the terminal contains the pour point, Using such data, below mentioned formulae can also be used to determine the wax content and cloud point. 1) Approx Cloud point temp, given Pour point temp. Approx Cloud point temp deg C = 20.2 ( 10 0.00708 x – 0.1157714 ) + 8 2) Calculation of Wax content of oil given its pour point. Log (Wax content % wt) = 10 0.00708x – 0.1157714 Where x = Pour point temp. 3) Predicted/ Approx cloud point temp deg C from % wax content = 20.2 x log (Wax content % wt) + 8 Assume wax content of oil is 6%, Then, 20.2 x log6 + 8 = 15.72 + 8 C.P. = Approx 24 deg C COW
The Primary means of sludge removal is by Solvency and Dilution. Solvency is achieved by • reducing the viscosity of the sludge to a pumpable state enabling it to flow to the suction bellmouth • the property of the wash oil, in dissolving the sludge, depending on the quantity of good solvents present in the oil.
There must be a balance in effecting the above change in properties, as pre-heating the wash oil might result in reduction of solvency of the medium. Dilution relates more closely to physical reduction of the viscosity of the sludge, by introduction of liquid crude oil under pressure, into a tank containing sludge and thereafter to stir it into the liquid introduced by the jet and the combined slurry to flow towards the suction bellmouths. The amount of crude oil required to blend into the sludge depends on the volume of the sludge in the tank. If significant amount of sludge is present in the tank, a volume of the original cargo should be preferably retained in the tank, rather than stripping it dry. In order to have an effective crude oil wash, three factors must be present, which are stated as below: • The crude oil washing medium must have an adequate final fluidity, which is a temperature dependant factor. • The wash jet must introduce sufficient kinetic energy into the sludge. Washing pressure should be monitored closely. • The vessel needs a good stern trim to induce a good flow towards the suction bellmouth. Further details about the types of wash, the amount of charge required, number of fixed top and bottom machines fitted in each tank, number of tanks which can be washed simultaneously, elevations and period of the washing machines for one full cycle, requirement and procedures to do COW, shadow sectors in each tank, list of crudes not suitable for COW can be referred to, from the COW manual. The above details together with the COW manual, will assist the Deck officers, in understanding the concept of Cloud point and in performing an effective crude oil wash, thereby reducing the sludge levels in the tanks.
