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Chemical tankers routinely carry several different cargoes at once, segregated across many small tanks, and the same tank may carry a different chemical every voyage. The operating regime is set by the IBC Code (International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk), made mandatory under SOLAS Chapter VII and MARPOL Annex II. Getting compatibility, coating choice and cleaning standard wrong can mean a cargo claim, a reactive-chemical incident, or a failed vetting inspection.
| Category | Hazard basis | Associated ship type |
|---|---|---|
| Category X | Present a major hazard to health or the environment — carriage prohibited from discharge to sea of tank washings | Ship Type 1 — most severe containment + location standard |
| Category Y | Present a hazard to health or the environment, or nuisance to other uses of the sea — discharge of residues subject to strict limits | Ship Type 2 |
| Category Z | Present a minor hazard — discharge of residues subject to less stringent limits | Ship Type 3 — least restrictive containment standard |
| Other Substances (OS) | Evaluated and found to present no hazard falling within Category X, Y or Z under MARPOL Annex II | No IBC Code ship-type restriction |
Ship Type 1, 2 and 3 refer to the survivability + tank location standard of the vessel itself — Type 1 must survive the most severe assumed hull damage and carries its most hazardous cargoes in tanks set furthest from the shell and bottom plating; Type 3 has the least restrictive location requirement. A given chemical tanker is certificated to carry only the categories its ship type is designed for, and the IBC Code’s cargo list cross-references each named chemical to its required category and ship type.
The IBC Code includes a chemical compatibility chart used to check whether two cargoes may be carried in adjacent tanks, whether a cargo can be loaded into a tank that last carried a different chemical without a full wall-wash clean, and which cargoes must never be allowed to mix even in small quantities (violently reactive combinations, or cargoes that polymerise, oxidise or generate gas on contact). US Coast Guard compatibility groupings are also widely referenced by chemical tanker operators as a cross-check against the IBC Code chart. Before accepting a cargo, the Chief Officer checks:
Highest chemical resistance and broadest cargo-carrying flexibility of the three options. Enables carriage of aggressive acids and a wide range of organic chemicals without coating breakdown. Higher newbuild cost, which is reflected in higher freight for stainless-tank tonnage.
Lower cost than stainless steel but resistant to a narrower cargo range and subject to coating breakdown or discolouration from certain chemicals over repeated voyages. Coating condition must be tracked tank-by-tank and factored into cargo acceptance decisions.
Common on vegetable-oil and less aggressive cargo tanks. Not suitable for many acids, which attack the zinc. Some cargoes are specifically excluded from zinc-coated tanks in the IBC Code cargo list because of zinc-contamination risk to the cargo itself.
Reactive or oxygen-sensitive chemical cargoes (many of which self-polymerise or oxidise in the presence of air) are carried under a nitrogen pad — a blanket of nitrogen maintained above the cargo surface at a small positive pressure to exclude air and moisture. Nitrogen generators or bottled nitrogen supply the pad, and tank pressure is monitored continuously through the voyage; loss of nitrogen pad on a reactive cargo is treated as an emergency condition requiring immediate remedial action.
MARPOL Annex II requires a mandatory prewash before departure from the discharge port for certain Category X residues (and Category Y/Z residues in some circumstances), with the prewash water discharged to a shore reception facility rather than the sea. Beyond the regulatory prewash, chemical tankers routinely clean tanks to a wall-wash standard for the next cargo: the tank is washed, rinsed and then tested by wiping a defined area of the coating and analysing the wipe for contamination against the next cargo’s purity specification. See tank cleaning for the wider procedure used across tanker types.
Chemical tanker charterers vet tonnage through the Chemical Distribution Institute (CDI) inspection scheme in addition to the tanker-industry SIRE programme. A CDI inspection examines coating condition, cargo-system segregation, cleaning records and crew competence in chemical-cargo handling; findings feed directly into charterers’ acceptance decisions. See SIRE + TMSA for the parallel oil-tanker vetting framework.
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