The term ‘tanker’ often brings to mind an image of the great supertankers plying the seas, carrying bulk and refined oil. These behemoths can have capacities exceeding 400,000 deadweight tonnes (dwt). Chemical tankers are smaller, typically 5,000–40,000 dwt, but are no less important to the world economy. Because chemical cargoes can be extremely corrosive, many of these ships are built with molybdenumcontaining stainless steel tanks for longevity, safety, and flexibility.
Duplex stainless steels, in particular, are increasingly specified for the demanding conditions that the vessels must withstand.
Rules and classification of chemical tankers
All ships are subject to numerous stringent regulations regarding safe navigation, air pollution, and the discharge of liquids and solids. Ships that travel in international waters are subject to the International Maritime Organization (IMO) regulation known as the ‘International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk’, or IBC Code. This code establishes design standards based on the hazards presented by ships’ cargoes.
Because chemical cargoes present many kinds of safety and pollution risks, construction and operation requirements for chemical tankers are much more stringent than those for oil tankers and commodity carriers. Tank design and fabrication are complex.
The IBC Code includes provisions for cargo pumping and monitoring; tank coating, cleaning and venting; vapour detection and fire protection. It also promulgates specific requirements for tank heating, cross-compatibility, corrosion control, and cargo density. Chemical tankers are arguably the most technically sophisticated of all large cargo ships.
Chemical tankers can be categorized according to size, products carried, and markets served. Parcel tankers are relatively large vessels with multiple separate tanks carrying highgrade chemicals, often in stainless steel tanks; product tankers are also large, but they carry less difficult cargoes, frequently in coated-steel tanks; and specialized tankers are small to medium size, carry a limited number of chemicals in a dedicated trade and use either coated-steel or stainless steel tanks, depending on their cargoes.
The ship
Chemical tankers may be distinguished by an array of pipes that run above the ship’s deck. Liquid chemicals are pumped via these pipes and their associated valves to load and unload the various tanks. For example, the large chemical tanker, the Odfjell SE Bow Star, built in 2004 at Poland’s Szczecin New Shipyard, has thirty-four square and rectangular 2205 duplex stainless steel cargo tanks below deck and six cylindrical tanks mounted on the deck. The vessel is 183 meters long with a beam of about 32 meters and a capacity of 39,832 dwt. The construction of the tanks required 3,000 tonnes of duplex stainless steel (some 90 tonnes of molybdenum) and they provide 52,106 cubic meters of storage volume.
The cargo tank
Design and construction:
Inboard tanks contain hatches, ladders, heating systems, piping and drains. Tank walls are typically 20–25 millimetres thick and are corrugated to increase tank stiffness. This enhances the ship’s structural rigidity and saves weight compared to straight-walled tanks. The corrugations are about 1 meter wide and 1 meter deep. Tank fabricators must employ precision welding techniques to ensure weld integrity, and careful post-fabrication cleaning to ensure the quality and corrosion resistance of the stainless-steel surface. A typical size vessel requires about 1,500 tonnes of stainless steel.
Materials:
Early chemical tankers used Type 304L stainless steel, but today a grade with 2.0–3.5% molybdenum is standard. Type 316 austenitic stainless steel and its variants are sometimes specified, but the duplex stainless steel grade 2205 is now the most widely used tank material.
This grade is popular because it is stronger and more corrosion-resistant than Type 316 stainless steel. The alloy’s higher strength requires less steel and therefore reduces the ship’s weight. Its superior corrosion resistance, due in large part to the addition of 3% molybdenum, allows it to carry a wider variety of aggressive liquids. The coefficient of thermal expansion of 2205 is closer to that of carbon steel than Type 304 or 316, making it more thermally compatible with surrounding carbon steel structures.
