On 1 January 2016, the IMO implemented a ruling pertaining to the injection of inert gas into cargo tanks of vessels over 20,000 dwt. The goal of the edict was to reduce the risk of explosion and fire, by dampening down the cargo vapours with a blanket of inert gas. However, the exclusion of oxygen in cargo spaces is a well-known risk for those inspecting tanks. Is there another way to verify the pre-loading inspection, without physically entering the tanks?
Inert gas generation is now mandatory on tankers below 20,000 dwt. The gas, usually nitrogen, is pumped into the tanks, where it will force out the potentially explosive vapours from previous cargoes. However, excluding air from the cargo tanks brings its own risks when it comes to certifying the tanks are ready for the next cargo. The atmosphere in the tanks could be a mixture of nitrogen and cargo fumes, requiring double manning and breathing equipment, as per the code on safe work in confined spaces. In a white paper, Responsible Cargo Tank Cleaning – Washing Water Analysis Guy Johnson, managing director of L&I Maritime (UK) Ltd, argues that current pre-loading inspection techniques are redundant, and could be replaced by UV/visible spectrophotometer analysis. An abridged version of the white paper appears below.
Why enter tanks?
Multiple cargo tank entry during tank cleaning operations on board a tanker only occurs to ensure compliance with the pre-loading inspection specifications for the next nominated cargo. Regardless of whether these specifications are for a visual inspection or wall wash, the cargo tanks still have to be entered.
An alternative is to remotely analyse the washing water. This has numerous advantages, including reduced risk, quicker cleaning turnaround times, a reduction in the amount of cleaning fluid used, and less instances of ‘overcleaning’. The washing water technique also reduces the energy requirement and bunker consumption.
Therefore, if a vessel’s washing water analysis is good enough to determine that cargo tanks and lines are clean and free from the last cargo, can the same method not be used for the pre-loading inspection?
Wall wash analysis
The traditional wall wash analysis is considered the strictest pre-loading inspection. Solvent is splashed onto the bulkheads of the cargo tank and then tested to meet a series of pre-determined test specifications. But how reliable is this process and what information does it actually provide?
Random by design
First, and by design, the wall wash inspection is a random process, because it is impossible to write a procedure that exactly and precisely defines which places of a cargo tank should be wall washed; particularly when no two cargo tanks are the same.
Second, the wall wash inspection is highly subjective, as no two inspectors will conduct identical tests. If the wall wash inspection is random and subjective, it follows that it is not reproducible and therefore, it has no legal value in the event of a cargo contamination claim, unless it can be proven that the inspector/surveyor acted negligently.
Furthermore, if only 1% of the internal surface area of the cargo tank is actually being wall washed, it follows that the remaining 99% of the cargo tank will not be tested; this portion of the cargo tank is therefore, by definition, an ‘unknown’. With 99% of the cargo tank untested and potentially un-clean, one has to ask why there are not more off-specification cargoes.
The answer is the dilution effect, which is a quantification of the relationship between surface contamination and how this contamination becomes diluted into the fully loaded volume of cargo. Once this concept is recognised and understood, it becomes apparent that the wall wash inspection has no value nor place in determining whether a cargo tank is suitable for loading.
A dilution factor can be calculated for any cargo tank as follows: a cargo tank has a total internal surface area of 1,000 m2 and a fully loaded volume of 1,000 m3. If it was then possible to wall wash each square metre of this tank with 0.5 litres of methanol, the total volume of the wall wash sample would be 1,000 x 0.5 litres = 500 litres = 0.5 m3 . If this wall wash sample was then diluted into the fully loaded tank, the dilution factor for the cargo tank would be 1000 / 0.5 = 2,000 times.
Clearly, this is an extreme example, but in theory, the cargo could be loaded even though the wall wash sample has 2,000 times the level of allowed contamination. It would appear that passing the wall wash inspection does not guarantee that the next cargo can be loaded successfully.
The most likely source of contamination is the line system through which the cargo is delivered from the loading terminal to the vessel. Neither the shore nor the vessel’s lines can be inspected or wall washed prior to the start of loading, meaning any contaminant that is trapped inside any part of the line system will be flushed into the cargo tank(s) at the start of loading. It is vital to identify that the loaded cargo has become contaminated as quickly as possible to minimise losses, which is why the first-foot sample is so important for satisfactory loading operations.
The owner/operator of the vessel is likely to bear the brunt of any off-specification cargoes unless this can be contested by the first-foot samples.
Washing water analysis
Washing water analysis is the complete opposite of wall wash analysis. It is the identification (by UV spectroscopy) of what is being removed from the cargo tanks/coating during the tank cleaning process, rather than finding out what is left behind after the tank cleaning is complete. When there are no longer any traces of the previous cargo in the washing water, tank cleaning can be stopped, as continued washing will not make the cargo tank any cleaner.
The amount of residue remaining on, or retained in, the surface of the cargo tank after cleaning becomes far less relevant, because all the cargo tank and the cargo lines must be free from previous cargo before the washing water samples are clean. This is also the mechanism that vessels employ to monitor tank cleaning without cargo tank entry. Each step of any tank cleaning procedure needs to be confirmed as complete before the next step can commence. Without using washing water analysis, this confirmation requires at least one crew member to go inside the cargo tank, while a second stands watch in accordance with confined space entry procedures. Using washing water analysis, each step of the cleaning process can be monitored by looking at successive UV traces, which of course does not require any cargo tank entry and takes less than one minute per sample to analyse.
When the graph becomes a ‘flat-line’ or does not change between consecutive samples, this is the trigger that the cleaning step needs to be changed, or that tank cleaning can be stopped. If this trigger can be used to change a tank cleaning step early, there are significant time savings and reduced environmental impact.