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A ship transporting oil across oceans is called an oil tanker or petroleum tanker. There are two types of oil tankers: crude tankers and product tankers. Crude tankers transport unrefined crude oil from the production fields to the refineries, and product tankers transport refined products from refineries to consumers.
Crude tankers are normally larger than product tankers and they typically travel longer distances. The size of an oil tanker - normally measured in terms of deadweight - may range from 1,000 to 500,000 metric tons. Tankers transport approximately 2 billion metric tons of oil every year and a single tanker voyage may take as long as 70 days.
The technology of transporting oil in tankers has evolved along with the oil industry. Originally boats and barges were used to transport oil in wooden barrels. There were several problems with this mode of transportation, including the weight and the cost of the barrel, and frequent leakages. As a consequence of initial failures (see section 1.5), the oil tanker design was changed drastically. Instead of one or two large oil holds, several holds were included. These holds were further divided into ports and sections by a longitudinal bulkhead. Earlier designs may have caused the ship to capsize due to oil sloshing from side to side. This approach of dividing the ship's storage space into smaller tanks (Figure 2.18)42 eliminated sloshing of oil and is used universally today.
A major component of a tanker is the hull or the outer structure. A tanker with a single outer shell between the product and the ocean is known as single hulled ship. A tanker with an extra space between the hull and the storage tanks is known as double-hulled. Oil tankers (over 5,000 tons deadweight) are built with double hulls. A hybrid tanker has double-bottom and double-sided design which combines aspects of both single and double-hull designs.
Another important part of the tanker design is the design of the inert gas system. Hydrocarbon vapors are explosive when mixed with air in certain concentrations. The inert gas system is designed to create an atmosphere inside the tanks in which the hydrocarbon oil vapors cannot burn. As the oil in the tank is pumped out it is filled with inert gas until it is refilled with oil. Inert gas systems deliver air with oxygen concentrations below 5% by volume.
Carbon steel is extensively used to build oil tankers. However, other materials including glass reinforced polymer, thermoplastics, polyethylene and fiberglass materials are also increasingly used for fabricating certain components of tankers.
Corrosion may occur at the bottom of the oil tankers if water and sediments are allowed to accumulate. To control corrosion the water may be periodically drained and the internal surface cleaned - typically after each voyage. In addition, the internal surface of oil tankers may be coated.
The space within the double-hull is filled with seawater to increase weight. Sediment buildup, leading to corrosion, may occur if the water used to fill the tanks is not treated properly. In addition, the surfaces of the tankers are protected by both protective coating and cathodic protection. The corrosion inside the ballast tanks may occur in locations where the coating has cracked and where the CP cannot be effectively applied. Such cracking may occur in areas such as sharp angles, welds, transitions between structural, frames, brackets, toes and similar connections, ends of spans and connections between longitudinal and web frames.
The corrosion rate of double-hulled tankers is higher than that of single hulled tanks. Typically, repair and replacement of steel are performed after 15 years in single hulled tankers but in double-hulled tankers the replacement is performed after 5 years. The higher frequency of replacement is due to the use of thinner high strength steels as well as due to higher heat retention in the double-hulled tankers. In certain parts of the world, the oil is pumped at higher temperatures into the tankers. The heat from the hot oil is quickly disseminated by the ocean water in the single hulled tankers, but the double-hull structure acts as insulator, retaining the heat for a longer period.
The outer surface of the tankers is susceptible to localized corrosion, splash zone corrosion, and fouling. For this reason, the outer surface is protected by coatings. The outer surface of the tankers immersed in ocean water is protected by a combination of coating and CP.
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