Crude oil washing (COW) is washing out the residue from the oil tanker using the

crude oil Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...

cargo Cargo consists of bulk goods conveyed by water, air, or land. In economics, freight is cargo that is transported at a freight rate for commercial gain. ''Cargo'' was originally a shipload but now covers all types of freight, including trans ...

itself, after the cargo tanks have been emptied. Crude oil is pumped back and preheated in the slop tanks, then sprayed back via 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. A nozzle is often a pipe or tube of varying cross sectional area, a ...

s in the cargo tanks onto the walls of the tank. Due to the sticky nature of the crude oil, the oil clings to the tank walls, and such oil adds to the cargo 'remaining on board' (the ROB). By COWing the tanks, the amount of ROB is significantly reduced, and with the current high cost of oil, the financial savings are significant, both for the Charterer and the Shipowner. If the cargo ROB 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 The International Convention for the Prevention of Pollution from Ships, 1973 as modified by the Protocol of 1978, or "MARPOL 73/78" is one of the most important international marine environmental conventions. MARPOL 73/78, MARPOL is an amalg ...

73/78 made this mandatory equipment for oil tankers of 20,000 tons or greater deadweight. Although COWing is most notable for actual tankers, the current chairman for Hashimoto Technical Service, Hashimoto Akiyoshi, applied this method in washing refinery plant oil tanks in

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

. Hashimoto is currently using this method in the

Kyushu is the third-largest island of Japan's five main islands and the most southerly of the four largest islands ( i.e. excluding Okinawa). In the past, it has been known as , and . The historical regional name referred to Kyushu and its surroun ...

, Chugoku, and Tohouku regions in Japan.http://www1.ocn.ne.jp/~hts.wak/473/494.html __FORCETOC__

Seawater washing

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

ballast Ballast is material that is used to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within a boat, ship, ...

on their empty trips. High pressure, hot,

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...

jets were used to clean the tanks and the mixture of seawater and residue called ''slops'' discharged into the sea, as was 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 particularly sensitive areas.

Load on top

The discharges from seawater washing were still considered a problem and 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'' and allowed 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. This was 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 load on top 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 current technique of ''crude oil washing''. This reduces the remaining deliberate 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

Convention. Revised specifications for the equipment were introduced in 1999. Modern tankers also use segregated ballast tanks and these remove the problem of discharge of oily ballast water.

External links

International Maritime Organization description of Crude Oil WashingScanjet Crude Oil Washing Machine

Sources

{{Reflist2.http://www.hts-g.co.jp/maintenance-eng.html Petroleum production Ocean pollution