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Tank cleaning is the operation of preparing a cargo tank to receive the next cargo without risk of contamination, chemical reaction, or Cargo Wall Wash quality-test failure. It is a routine operation on product tankers and chemical tankers, and a demanding technical + regulatory + safety activity involving MARPOL Annex II (chemical), MARPOL Annex I (oil), IBC Code chemical compatibility, tank-cleaning-chemical selection, enclosed-space entry (for gauging + wall wash), and slop-handling. Chief Officers on chemical tankers spend a substantial share of their working time on tank-cleaning planning + execution.
Product tankers carry refined petroleum products: gasoline, gasoil, jet fuel, HFO, VLSFO, MGO. Cleaning procedures are governed by MARPOL Annex I. Typical scenarios:
Chemical tankers carrying IBC Code chemicals face a much more demanding regime under MARPOL Annex II + IBC/BCH Code. Every cargo change requires: (a) verified cleaning to the receiver's wall-wash acceptance spec; (b) prewash for MARPOL Annex II Category X cargoes; (c) documentation retained in the Cargo Record Book; (d) certified by the Chief Officer.
| Category | Environmental risk | Discharge rules | Example cargoes |
|---|---|---|---|
| Category X | Major hazard | Prewash + slop to reception. NO overboard discharge. | Acrylonitrile, benzene, hydrogen peroxide, DPPA |
| Category Y | Hazard | Limited overboard slop discharge outside 12 nm + speed > 7 kn | Methanol, ethanol, styrene, xylene, MEG |
| Category Z | Minor hazard | Overboard slop discharge with limits | Sugar solutions, palm oil, edible oils |
| Other Substances (OS) | No hazard | No MARPOL discharge restrictions | Fresh water |
The Miracle Guide, produced by Dr. Verwey Chemicals + updated by cargo experts, is the industry-standard reference for chemical-tanker cleaning. It provides cleaning procedures for hundreds of specific chemical cargoes, including: recommended cleaning temperature; recommended chemical cleaner; wash cycle time; solvent rinses required; wall-wash spec; and compatibility notes. Chief Officers on chemical tankers routinely reference the Miracle Guide when planning tank-cleaning operations. Similar guides: Dr. Chan Chemical Tanker Guide, INTERTANKO Chemical Tank Cleaning Guide.
Wall wash: sampling of the tank wall for residual contamination via a methanol solvent rinse. The sample is analysed for water reactivity, acid, chloride, permanganate value, and total residue. Receivers of high-purity cargoes (methanol, MEG, aromatics) specify wall-wash acceptance criteria that must be met before the vessel is nominated as clean.
OBQ (On-Board Quantity): the residue remaining in the tank after discharge, measured by dip + gauge. Zero OBQ is typically required for high-purity next-cargo loading. Chemical-tanker Chief Officers must document OBQ + wall-wash results in the Cargo Record Book.
Before loading a new cargo, the Chief Officer must verify chemical compatibility with the residual of the previous cargo. The IBC Code Appendix + IMO Compatibility Chart lists reactive combinations that must NOT be allowed together. Notable reactive pairs: acids + bases; oxidisers + hydrocarbons; peroxides + almost anything. Failure to check compatibility has caused several notable casualties (e.g. the Bow Mariner 2004 explosion attributed to incompatible cargo residue reaction).
Wall wash + gauging + inspection requires enclosed-space entry into a recently-emptied cargo tank. The atmosphere may contain HC vapours from the previous cargo, methanol from wall wash, and cleaning-chemical vapours. Full enclosed-space-entry procedures under /reference/enclosed-space-entryapply. Multiple fatalities have occurred from tank-cleaning enclosed-space entry — a routinely-underestimated risk.
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