Improved Prewash Procedures for Solidifying or High-Viscosity Substances

Improved Prewash Procedures for Solidifying or High-Viscosity Substances

MEPC77/INF.9 17 September 2021

Findings on improved prewash procedures for solidifying or high-viscosity substances (paraffin waxes)

Submitted by the Netherlands.

This IMO paper relates to a Dutch initiative regarding Paraffin Wax Substances which fall under the new MARPOL Annex II discharge requirements for persistent floaters, which have been found washed upon a number of Dutch beaches in considerable quantities.

The outcome of the Dutch initiative was an improved unloading and washing strategy resulting in hardly any paraffin waxes found on the Dutch coastline since these procedures were adopted.

The findings and recommendations of the Dutch initiative are described below. The pertinent and somewhat surprising sections are in red, with the reasons given for such finding are in purple.

The full text is attached at the end of this page.

Dutch Findings

The Netherlands believes that solidified paraffin waxes would not have been found on Dutch beaches in the amounts mentioned in paragraph 6 if the ship’s crew would carry out adequate stripping procedures at all times. After inspecting several ships calling to the ports of Rotterdam and Moerdijk to unload paraffin waxes, the Netherlands learned that cargo tanks frequently were not stripped. The reason for not stripping was that the ship’s crew didn’t want to risk a clogged stripping line. Freeing the line is a difficult and time-consuming process. The Netherlands also learned that most crew members did not know how to correctly perform a stripping procedure for solidifying cargoes. Furthermore, the Netherlands discovered that the mandatory prewashes that were carried out appeared to be ineffective. The solidified paraffin wax remaining in the cargo tanks couldn’t be removed by using the standard prewash procedure (through cleaning cycle(s) or through using the Q-formula regardless of the K-factor).

In addition, inspections showed that, even when the unloading was done in accordance with MARPOL Annex II, remaining quantities of paraffin wax exceeded the stripping quantities. Inspections of several cargo tanks showed that the “relative” cold ballast water promoted the clotting process of the cargo on the tank walls and bottom. This led to excessive amounts of solidified paraffin wax remaining after the unloading and stripping of the cargo tanks. The Netherlands also noticed large quantities of paraffin wax remaining on tank tops, due to “relative” cold temperatures. Quantities of 6 and up to 12 cubic meters per tank have been found.

Trial of Improved Prewash Procedures

An improved prewash procedure for paraffin waxes of pollution category X and pollution category Y was developed in close cooperation between experienced MARPOL surveyors and a captain of a ship with knowledge of the transport of paraffin waxes and the cleaning of cargo tanks. This resulted in a time-driven washing procedure which was then applied in the ports of Rotterdam and Moerdijk. Essential elements in this improved procedure are heating up the cargo tanks properly and washing the cargo tanks with very hot water for a sufficient amount of time. After cooling down, the tanks were inspected and found sufficiently cleansed.

If the improved prewash is prepared properly and adequately communicated with the crew, the procedure will take the same amount of time as a “standard” prewash procedure. The Netherlands wants to emphasize the fact that, regardless of the amount of solidified cargo remaining and whether the tanks are stripped or not, this prewash procedure will significantly reduce the amount of paraffin wax ending up discharged into the sea.

Improved Prewash Procedure For Paraffin Waxes

To more effectively prewash paraffin waxes of pollution category X and pollution category Y, the following procedure should be carried out:

1/ Cargo must be unloaded at a temperature of at least > 10° C above melting point.

2/ As soon as the tank is empty, hot water shall be entered in the tank until the heating coils are flooded.

3/ The heating coils shall be operated in order to heat up the tanks to melt the wax which is attached to the tank walls.

4/ The tanks and tank walls should be heated with steam for at least 1 to 2 hours.

5/ Subsequently the tanks and tank walls shall be washed with hot water (> 60°C – 80°C) (as normal prewash procedures do not inflict sufficient effect, hot water must be used).

6/ A time factor of at least 45 minutes for washing per tank shall be used instead of the regular K-factor.

7/ Adjacent ballast water tanks should be kept empty (if possible).

8/ Collection tanks must be heated and effluent must be unloaded to Port Reception Facilities or a terminal and subsequently prewashed as well in accordance with the above mentioned procedure.

9/ After unloading the washing water, the tanks should be ventilated to allow for visual inspection.

Knowledgebase Comments

All senior deck officers live in dred of the small-bore superstrip lines becoming blocked, as the Dutch correctly observe can be a difficult process, and it is, or should be, common practice to blow and then blind these lines off prior to loading a high-viscosity product, however that practice should extend to the deck lines only, not to the deepwell pump small-bore lines, which if used as part of a proper stripping procedure will remain open.

A failure to properly strip a cargo tank containing such substances is perhaps more common in vessels fitted with deck-mounted cargo heaters, however when operated by properly experienced crew this failure is mitigated. If necessary, a backflush with hot product from another tank will greatly help towards such stripping.

However, it remains a fact that much depends on the ‘tank sweepers’ who must work in conjunction with the person operating the pump controls. It is therefore imperative that the cargo pump is operated locally, not from the cargo control room, and by a person experienced in such operations; which typically will not be a junior deck officer.

We are all aware of the importance of not having cold water ballast adjacent to and in contact with bulkheads of tanks containing such products, however trim and stability requirements may hinder that aim. However, we cannot stress the importance of avoiding such contact especially in the double-bottoms, and even reducing the ballast water level so that physical contact with the tanktop is avoided will help immeasurably to reduce solidifying of such products.

Hot water must be used for washing, as hot as the tank coatings allow. This used to be a ‘given’, but over the years we have seen this truism diminish. It is time for a return to such practice.

Most chemical tanker operators forbid ‘live steaming’ of cargo tanks following the Stolt Valour incident, but such a practice will, and does, greatly assist in reducing clingage, so we suggest that operators allow an exception in their procedures for such cases.

As an alternative, the senior officers may consider washing the tank bulkheads with the product as part of the discharge procedure. Rather like a crude oil wash procedure. Such a practice used to be common in single-skin Type 3 ships to remove product trapped on the horizontal stringers, and in most cases was conducted by the use of a tank cleaning machine nozzle attached to a 1″ plastic hose, hand-held through a tank cleaning hatch, although a connection to a tank cleaning machine is far more efficient.

Finally, sufficient time is of importance. Do allow the full 45 minutes as the Dutch procedure states, no matter the screams from agents. It will be repaid in spades laten on.


The full text of MEPC77/INF.9 dated 17 September 2021 is able to be downloaded or printed below:

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