Operational Guide · Registers & Logbooks

# Tank Cleaning

From Crude Oil Washing on tankers to chemical washing on chemical tankers: different procedures for different risks, but one common principle — accurate recording of every stage.

tank cleaning COW MARPOL Annex I MARPOL Annex II

## Operational Explanation

Tank cleaning varies significantly depending on ship and cargo type: on tankers, Crude Oil Washing (COW) uses the crude oil itself as the washing agent to reduce oily residues; on chemical tankers, washing is specific to the NLS substance carried, according to the P&A Manual, and is often a prerequisite for the next cargo (to avoid contamination between different products).

In both cases, the main risk is not technical but documentary and procedural: a wash carried out correctly but not adequately recorded, or carried out using a procedure that does not conform to the ship's specific manual, still generates a deficiency during inspection.

## Regulatory Reference

MARPOL Annex I, Regulation 33 (Crude Oil Washing, mandatory on tankers meeting the applicable requirements) and MARPOL Annex II with the ship's specific P&A Manual for NLS tank cleaning.

## Scope of Application

Tankers subject to COW under MARPOL Annex I Regulation 33; chemical tankers for NLS tank cleaning under their own approved P&A Manual.

## Procedure / How to Complete It

1. For tankers: carry out Crude Oil Washing according to the ship's approved COW Manual, checking pressure, number of tank-washing machines and the sequence specified.
2. For chemical tankers: identify the specific washing procedure for the NLS substance just discharged, according to the P&A Manual, before proceeding.
3. Check the compatibility between the previous product and the one to be loaded next, to avoid cross-contamination.
4. Record every stage of the wash (start, end, quantity of residues, destination) in the appropriate record book (Oil Record Book or Cargo Record Book).
5. Where required, verify the tank's actual cleanliness through visual inspection or specific tests before the next loading.

## Practical Example

Example on a chemical tanker: washing of tank no. 2 after discharging methanol, according to the specific procedure in the P&A Manual, with verification of the absence of residues before loading the next product, which is incompatible with residual traces of methanol.

## Real Cases

Contamination between incompatible chemical products, with significant commercial and sometimes safety consequences (unwanted chemical reactions), is frequently traced back to tank cleaning that did not follow the specific procedure required for the sequence of products actually carried.

## Common Mistakes Mistake Library

Mistake | Consequence | How to avoid it
--- | --- | ---
Generic washing procedure applied instead of the specific one in the P&A Manual | Residues not adequately removed, risk of contamination | Always consult the P&A Manual specific to the combination of products involved
COW carried out without respecting the pressure or sequence specified in the COW Manual | Reduced washing effectiveness, oily residues higher than expected | Rigorously follow the parameters of the approved COW Manual
Incomplete or generic recording of the wash | PSC deficiency, difficulty demonstrating compliance in case of a dispute | Record every stage in detail (tank, quantity, destination of residues, time)

## PSC Observations

On tankers, PSCOs check the recording of COW in the Oil Record Book and its consistency with the COW Manual; on chemical tankers, the consistency between the Cargo Record Book, the P&A Manual and the sequence of products carried.

## Operational Tips

- Always consult the specific P&A Manual before every wash, especially with unusual product sequences.
- Do not depart from the pressure and sequence parameters of the COW Manual to save time.
- Record every stage of the wash in full detail, not just the final outcome.

## Checklist

- Specific washing procedure identified (COW Manual or P&A Manual)
- Compatibility between the previous and next product verified
- Washing parameters (pressure, sequence) respected
- Every stage recorded in the appropriate record book
- Actual cleanliness verified before the next loading, where required

## FAQ

Is Crude Oil Washing mandatory on all tankers?
It is mandatory under MARPOL Annex I Regulation 33 for tankers meeting the applicable criteria (typically large tankers in certain configurations), unless equivalent systems are installed.
Must tank cleaning always be recorded separately from loading/discharging?
Yes: tank cleaning is a distinct operation that requires its own record, detailing the quantity of residues produced and their destination (ashore or, within the permitted limits, at sea).
What should be done if two successive chemical products are not compatible even after washing?
The specific compatibility must be checked against the IBC Code and the P&A Manual; if incompatibility persists, a more thorough wash, the use of a different tank, or the involvement of a chemical cargo specialist may be necessary.
🎬 Additional photos, videos and interactive diagrams for this topic will be available in a future version of the platform.

## Related Topics

Cargo Record Book Enclosed Space Entry

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