1. Development

The chemical tanker as a ship type has its origin in the late 1940s and early 1950s. The first chemical tankers were developed to meet the needs of a growing petrochemical industry on the gulf coast of the United States. As the petrochemical industry grew and the demand for its products surged it became impractical to continue to ship chemicals in drums or relatively small tanks. The practice of carrying chemical in deeps tanks of freighters was unsafe for cargoes with hazardous properties. The first dedicated chemical tankers were oil tankers modified to provide a greater number of cargo tanks, additional double bottom ballast tanks, and deepwell cargo pumps.

The first seaborne trade of chemicals on chemical tankers was between the chemical producers on the gulf coast of the United States to the manufacturing coenters along the U.S. east coast. Not long after this trade was established, chemical tanker trading began between the U.S. and Europe. As the trade expanded, the features of the vessels evolve with increase cargo segregations, coated cargo tanks, and cargo heating systems becoming common. These vessels were able to carry a wide range of liquid cargoes including chemicals, petroleum products and vegetable oils and animal fats. In the early 1960s the first purpose-built vessels expanded on the design features that had been implemented on the earlier converted vessels, further increasing the number of cargo tanks, still using double bottom ballast tanks to provide improved containment and cargo protection, and utilizing different tank coating systems and stainless steel in cargo tanks. These vessels also included safety systems and alarms to better accomodate the hazardous nature of the cargoes being transported.

Over the next forty years the chemical transportation industry continued to expand and evolve. The innovations and developments were fueled by the needs of the shippers and their cargo as well as by the desire of the shipowners to operate more efficient vessels. These innovations and technologies were developed by a joint effort of the chemical tanker shipowners and operators, ship designers, equipment manufacturers, shipbuilders and classification societies. The growing body of international regulations on ship design and operations provided additional impetus.

2. Types

Chemical tankers can be categorized in a number of different ways. In common with most vessels, size can be used as a means of classification. Unlike oil and product tankers there is no universally accepted size categorization of chemical tankers: However, modern vessels typically fall into one of the following three categories:

• Inland chemical tankers: 500 to 4000 tDWT, typically in the form of self-propelled barges. Commonly used in the river systems of northwestern Europe to load cargo from larger tankers or coastal terminals and transport the material to inland industrial facilities;
• Coastal chemical tankers: 3000 to 10000 tDWT - small tankers, also referred to as short sea tankers, used to transport chemicals coastwise and to transship cargoes into ports and terminals where larger tankers are unable to call because of any number of restrictions. These tankers may load or discharge cargo from a shore terminal or directly from a larger vessel. These vessels are commonly used in the intra-Europe, intra-southeast Asia, and the north-central-south American markets.
• Deep sea tankers: 10000 to 50000 tDWT. These ocean-going vessels typically have a large number of segregations and have either stainless steel tanks or a mix of stainless steel and coated tanks. These vessels operate on the major trade routes between North and South America, Europe, the Middle East and Asia.

Alternatively, chemical tankers can be grouped by the level of cargo containment designed into the vessel, known as Ship Type (ST).
The IBC code defines three specific ship types, with ST 1 providing the greatest level of containment for the transportation of the most hazardous cargoes. Conversely, ST3 provides minimal containment for carriage of the least hazardous cargoes covered by the IBC code. Chemical tanker designs typically fall into one of three ship type arrangements:

• ST 1/2
• ST 2
• ST 2/3

Commercially the lining of the cargo tanks can be used to categorize chemical tankers. The tanks may be mild steel in construction and covered with a specially formulated tank coating, or they may be constructed or lined with corrosion-resistant stainless steel. The cargo tank lining partially determines which cargoes a vessel can carry. Certain cargoes, because of their inherent properties or aggressive nature, cannot be stowed in tanks lined with certain tank coatings. Similarly, many charterers deman stainless steel cargo tanks out of a concern for cargo quality when it would otherwise not be required.

The term chemical tanker is occasionally used loosely to refer to product tankers or chemical / product tankers that carry an NSL certificate. The NSL certificate allows these vessels to carry those products listed in Chapter 18 of the IBC code and assigned the minimum pollution risk.

3. Cargoes

The design of a chemical tanker may allow the vessel to carry hundreds of different liquid cargoes ranging from petroleum products, to inorganic acids, to fish oil, to specialty chemicals. The hundreds of cargoes that can be carried on the modern chemical tanker have properties that can vary greatly from one cargo to the next. For example, phosphoric acid (85%) has a specific gravity of approximately 1.69 while methanol has a specific gravity of approximately 0.79. This wide range is representative of the breadth that any specific characteristic may vary from cargo to cargo.

The cargoes transported by chemical tankers can be categorized in several different ways. The cargoes may be divided into groups based on their chemical composition, such as inorganic and organic chemicals. In CFR 46 Subchapter O, Part 150, the USCG divides all of the chemical cargoes currently being transported on ships in bulk into thirty-six groups and then uses these chemical groups to determine cargo compatibility for stowage purposes.

Alternatively, the chemical cargoes may be divided into the following four groups based on their origin and a specific heavy group:

1. petro-chemical products
2. coal tar products
3. carbohydrate derivates
4. animal and vegetable oils
5. heavy chemicals

Petro-chemical products are those oil products and chemicals that are derived from the refining of crude oil and natural gas.
Coal tar products are derived from the carbonization of coal. The coal tar that results from this process is a source of hydrocarbons used for industrial purposes.
Carbohydrate derivates include molasses and various forms of alcohols. Acohols are used in the manufacturing of various chemicals and products as well as in the beverage industry.
Animal and vegetable oils include oils and fats directly extraced from plants and animals. Cargoes of this nature include fish oil, marine oil, tallow, rapeseed oil, palm oil, and sunflower seed oil. Also falling under the animal and vegetable oil category are various fatty acids and esters of alcohol.
Heavy chemicals are common chemicals that are widely produced for use in a variety of industries. The term heavy results from the high specific gravities of the chemicals. Examples of heavy chemicals include

• sulphuric acid
• caustic soda
• caustic potash
• phosphoric acid
• nitric acid
• sulphur

Cargoes can also be categorized by their intended use or by their stage in the product chain. With regard to regulatory issues, the cargoes can be categorized in terms of the safety hazards associated with their transportation or by the level of marine pollution associated with the cargo. These categorization schemes are found in the International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk (IBC code), and the International Convention for the Prevention of Pollution from Ships, 1973, and Protocol of 1978 (MARPOL 73/78) respectively.

Finally cargoes may be broken into groups based on the requirements for carriage. Examples include easy chemicals, sophisticated chemicals, and clean and dirty products. This grouping is used in the determination of the freight rate for the transportation of a specific cargo. This categorization scheme together with the regulatory categorization is most important to the vessels' operators and thus to the chemical tanker designer.