Dr Moses Amweelo
President Nangolo Mbumba has emphasised the crucial need to protect marine ecosystems, and address climate change for the prosperity of present and future generations.
In the meantime, the President urged all citizens to join hands in protecting the ocean, emphasising its intrinsic value and indispensable role in sustaining life on Earth.
The threat that oil tankers pose to the marine environment can best be appreciated by considering normal tanker operations.
A cycle of tanker operations can be broken down into separate stages of loading, loaded voyage, unloading, ballast voyage, and tank cleaning.
During loading, the oil is pumped into the cargo tanks through pipelines. The sequence of filling the tanks is given in the ship’s loading orders, which also contain an estimate of the quantity of cargo to be load and the tanks in which it will be stowed. Unloading is accomplished by discharging the cargo directly into a terminal (refinery) or tank storage area, or into a tank barge for onward transportation. After the cargo has been discharged, the ship rides high in the water, making it difficult to manoeuvre.
To compensate for this condition, the ship takes on ballast. Ballasting is processing whereby seawater is taken aboard into the cargo tanks or segregated ballast tanks to ensure proper propeller immersion, and to provide good handling and sea-keeping characteristics.
The amount of ballast taken aboard depends upon the anticipated weather conditions, the distance and route of the ballast voyage, the vessel’s lightship displacement (weight), length-to-depth ratio, and other characteristics.
The amount of ballast taken aboard generally varies from 20 to 50% of the vessel’s total cargo-carrying capacity, but may be greater during periods of severe bad weather. The question is: what are the techniques for the separation of oil and water? The technique traditionally used for the separation of oil and water is based on the difference in density between the two substances.
This is still the principle used in most gravimetric separators, and it also plays an important role in the techniques commonly used for purifying water to the 15-ppm standard.
As oil cannot dissolve in water unless it is in very small amounts, a mixture of oil and water consists of drops of one fluid dispersed in the other. The component present in the smaller quantity generally appears as drops suspended in the other, but conditions are also affected by surface tension characteristics, which in turn depend on the type of oil, the temperature, etc.
Agitation of an oil-water mixture causes the drops to be divided into smaller droplets, and an emulsion is formed. If the droplets are sufficiently small in size and large in number, a stable emulsion is created, in which the drops are so small that gravimetric forces have no influence, and the behaviour of the mixture is controlled solely by surface tension.
Such stable emulsions can be created when the oil content lies between approximately 25 and 75%. The emulsion will consist of an oil-in-water emulsion at the lower end of the range and a water-in-oil emulsion at the upper end. Ballast that is put directly into cargo tanks immediately after cargo discharge comes into contact and mingles with the oil that has remained in the tanks. This oily (or dirty) ballast must be disposed of prior to arrival at the loading port, unless the loading port has suitable reception facilities like Walvis Bay port.
It is usual to dispose of dirty ballast at sea and replace it with clean ballast suitable for direct discharge into the harbour at the loading port. Unless all ballast is segregated or clean ballast, empty cargo tanks must therefore be washed during the ballast voyage to remove the residual oil (clingage), and provide space for the clean ballast. As with the amount of dirty ballast taken aboard, the number of tanks washed is a function of the vessel’s proportions, the weather, the route, and the need to clean tanks periodically for internal inspection, repair at a shipyard, or sludge buildup control. Generally, between one-third and one-half of the ship’s tanks will be washed on each ballast voyage.
If all tank washings and dirty ballast were to be discharged directly into the sea, a considerable quantity of oil (up to 0.5% of the oil carried) would be released, which is unacceptable under international rules.
A procedure has been developed that results in most oil being retained on board. After departure on the ballast voyage, the oily ballast is allowed to settle, resulting in oil and water separation.
Meanwhile, a number of dirty cargo tanks are being water-washed to accept clean ballast for arrival.
All tank washings are transferred to the slop tanks. After ballasting the newly-cleared tanks, the dirty ballast is discharged by pumping the comparatively clean ballast underneath the oily top layer directly into the sea, and adding the top layer to the slop tank. The slop tank is allowed to settle so that the oil and water separate, a process that can be assisted by heating the tank.
Upon arrival at the loading port, the clean ballast can be discharged into the harbour, and the ship can be loaded. Crude oil and some black products can also be loaded into the slop tanks on top of the oil and water already there.
The whole procedure has become known as the Load on Top (LOT) procedure. In the case of most other products, the slop tanks would have to be discharged to reception facilities before loading. Since product tankers are likely to change products, they have to clean not only the tanks for clean ballast, but all other tanks where product contamination may occur.
Tank washing can never be eliminated, however, owing to the need for carrying out repairs in tanks and for dry-docking. Product tankers and smaller crude oil tankers (i.e. less than 20,000 tonnes of deadweight) are not equipped with crude oil washing installations ,and require tank washing for product changes and sludge control.
*Dr Moses Amweelo is a former Minister of Works, Transport and Communication. He is currently a lecturer at IUM and UNAM on a part-time basis. He earned a doctorate in Technical Science, Industrial Engineering and Management from the International Transport Academy (St Petersburg, Russia).