Crude oil washing (COW) is the process of washing out residue from

oil tanker An oil tanker, also known as a petroleum tanker, is a ship designed for the bulk cargo, bulk transport of petroleum, oil or its products. There are two basic types of oil tankers: crude tankers and product tankers. Crude tankers move large quant ...

s using the

crude oil Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring u ...

itself. After the oil tankers have been emptied, crude oil is pumped back and preheated in the slop tanks, then sprayed back with high-pressure

nozzle A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe (material), pipe. A nozzle is often a pipe or tube of varying cross ...

s in the cargo tanks onto the walls of the tank. Due to the stickiness of the crude oil, it clings to the tank walls, and such oil adds to the cargo remaining on board. By washing the tanks with crude oil, the amount of cargo remaining on board is significantly reduced, and with the high value of oil, the financial savings are significant, both for the charterer and the ship-owner. If the cargo remaining on board is deemed as 'liquid and pumpable', then the charterers can claim from the owner for any cargo loss for normally between 0.3% up to 0.5%. It replaced the ''load-on-top'' and ''seawater washing'' systems, both of which involved discharging oil-contaminated water into the sea.

MARPOL 73/78 The International Convention for the Prevention of Pollution from Ships, 1973 as modified by the Protocol of 1978, or "MARPOL 73/78" (short for "marine pollution") is one of the most important international marine environmental conventions. It ...

made crude oil washing equipment mandatory for oil tankers of 20,000 tons or greater deadweight. Although crude oil washing is most notable for actual tankers, the current chairman for Hashimoto Technical Service, Hashimoto Akiyoshi, applied the method in washing refinery plant oil tanks in Japan. Hashimoto is currently using this method in the

Kyushu is the third-largest island of Japan's Japanese archipelago, four main islands and the most southerly of the four largest islands (i.e. excluding Okinawa Island, Okinawa and the other Ryukyu Islands, Ryukyu (''Nansei'') Ryukyu Islands, Islands ...

, Chugoku, and Tohoku regions in Japan.494.html

Seawater washing

Originally, oil tankers used one set of tanks for cargo, and about one third of the tanks were for the

water ballast Ballast is weight placed low in ships to lower their centre of gravity, which increases stability (more technically, to provide a righting moment to resist any heeling moment on the hull). Insufficiently ballasted boats tend to tip or heel exce ...

on their empty trips. High-pressure jets of hot seawater were used to clean the tanks, and the mixture of seawater and residue called ''slops'' was discharged into the sea, as well as the oil-contaminated ballast water. The 1954 OILPOL Convention attempted to reduce the harm by prohibiting such discharges within of most land and of certain sensitive areas.

Load-on-top

The discharges from seawater washing were still considered a problem, so during the 1960s, the load-on-top approach began to be adopted. The mixture of cleaning water and residue was pumped into a ''slop tank'', which allowed them to separate by their different densities into oil and water during the journey. The water portion was then discharged, leaving only crude oil in the slop tank. The crude oil was then pumped into the main tanks and the new cargo loaded on top of it, recovering as much as 800 tons of oil which was formerly discarded.

History

Even with the load-on-top process, there is still some oil in the discharged water from the slop tank. Starting in the 1970s, equipment capable of using crude oil itself for washing began to replace the water-based washing, leading to the modern-day technique of crude oil washing. The process reduces the remaining discharge of oil-contaminated water and increases the amount of cargo discharged, providing a further benefit to the cargo owner. Crude oil washing equipment became mandatory for new tankers of 20,000 tons or more deadweight with the 1978 Protocol to the 1973 MARPOL Convention. Revised specifications for the equipment were introduced in 1999. Modern tankers also use segregated

ballast tanks A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide hydrostatic stability for a vessel, to reduce or control buoyancy, as in a submarine, to correct trim or list ...

, which remove the problem of discharge of oily ballast water.

External links

International Maritime Organization description of Crude Oil Washing
Archived fro
the original

"Scanjet Crude Oil Washing Machine"
Archived fro
the original

Seawater washing

Load-on-top

History

External links

See also

References

Further reading