Crude oil scrubbing cleans oil tanker storage tanks using crude oil as a solvent, dissolving sediment back into the liquid oil for land-based storage. This method is required by law since the late 1970s and reduces pollution and cargo waste. It has replaced the old method of using high-pressure seawater pipes, which resulted in contaminated water being pumped out to sea. The buildup of solid material on the inside walls of storage tanks, known as clingage, can significantly reduce the capacity of storage tanks over time and cause pumping mechanisms to become clogged or malfunction. Crude oil scrubbing has become the only legal method of cleaning large tanker storage tanks due to its environmental benefits.
Crude oil scrubbing is a method of cleaning oil tanker storage tanks using crude oil as a solvent. When crude oil is shipped long distances, layers of sediment separate from the oil and settle on the walls of the storage tank. In the past, this was removed using high pressure seawater pipes, with the resulting contaminated water generally being pumped out to sea. By cleaning the walls of the storage tank with a high-pressure spray of crude oil, the sediment is dissolved back into the liquid oil and can be pumped into land-based storage containers along with the rest of the cargo. This method has the advantage of reducing pollution and cargo waste, and the use of crude oil scrubbing in large tankers has been required by law since the late 1970s.
In its raw state, oil contains various compounds that must be removed in the refining process to create a usable product. Some of these, including waxes, tars and sediments, tend to separate from liquid oil and build up on the inside walls of storage tanks. This buildup of solid material, known as clingage, can significantly reduce the capacity of storage tanks over time and can cause pumping mechanisms to become clogged or malfunction. As a result, at the end of a journey, storage tanks are typically removed.
Until the late 1960s, scale was removed by scrubbing the interior walls of storage tanks using the same high-pressure seawater pumps used to clean ship decks. It took a lot of water to clean the tanks this way, as crude oil is not water soluble. The resulting mix, which could hold up to 800 tons of crude oil on a 20,000-ton tanker, was used as ballast and then pumped into the ocean some distance from shore. Due to growing concerns about the pollution caused by this method, alternatives were sought.
The crude oil scrubbing method came into widespread use in the early 1970s. Pumping mechanisms capable of producing a high-pressure stream of crude oil were devised, which was used to spray the walls without residue. Unlike seawater, crude oil acts as a solvent and dissolves solid petroleum sediment into liquid oil. Instead of an unusable emulsion of water and oil, the result is cargo crude that can be pumped ashore and sold along with the rest of the cargo.
Because of the environmental benefits of washing crude oil, it has become the only legal method of cleaning the storage tanks of large tankers. In 1978, crude oil scrubbing was made mandatory for all new tankers, and those with capacities of 20,000 tons or more must be built with specialized crude oil scrubbing systems. This requirement, together with the creation of segregated ballast tanks that prevent the mixing of ballast water and oil, has greatly reduced the pollution associated with international oil trade.
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