An Art and Science of Tank Cleaning On Board Chemical Tankers

Ataul Majid

A chemical tanker as defined in MARPOL Annex II is a ship constructed or adapted for carrying in bulk any liquid product listed in chapter 17 of the “International Code for the Construction and Equipment of Ships carrying Dangerous Chemicals in Bulk (IBC code), as well as industrial chemicals and clean petroleum products. Such ships also often carry other types of sensitive cargo which require a high standard of tank cleaning, such as palm oil, vegetable oils, tallow, caustic soda, methanol, etc.

As the demand and supply complexity for chemicals and products increases, so does the requirement for cleaning cargo tanks. Tank cleaning combined with good seamanship is a special skill to clean a tank properly, several variables must be considered, and these require operators not only to use the best technology but also their experience and knowledge.

The cargo in the parcel chemical tanker can be split into three major categories they are

• Mineral oils: These are typically removed with an emulsifying cleaner containing hydrocarbon solvents and surfactants.
• Animal, fish, and vegetable oils and fats: These are typically removed by acid or alkaline base cleaners about which some confusion is present in the field.
• Chemicals and solvents: These are typically removed by water flushing and rinsing in combination with detergents.

It is always important to know the physical properties and specified hazards of chemical cargo before you make your tank cleaning procedure. The basic properties of the chemicals are to be found in the cargo-specific MSDS (Material Safety Data Sheet). The key points to note are as below

• Cargo density- Whether the cargo is lighter or heavier than water.
• Miscibility- Whether the cargo is easily soluble in water to remove easily through water wash alone
• Melting point- To understand the required wash water temperature above the cargo solidifying temperature.
• Viscosity- Good temperature helps to reduce the viscosity and better cleaning.
• Reactivity hazards- Sensitive to water/air/other chemicals. Need to consider the washing medium depending on the reactivity hazards.
• Self-reactivity hazards- Polymer cargoes are sensitive to heat
• Fire and explosion, static accumulation hazards– Flammable atmosphere and static generation during tank cleaning in non-inert conditions can be dangerous.
• Toxicity hazards- Exposure of crew to toxic vapor can be fatal

From a technical point of view, it is often a time and labor-consuming process because of internal structures, lack of hot water capacity, inadequate pressure of wash water, low number and/or capacity of fixed tank cleaning machines, the ‘shadow areas’ in the cargo tanks, inadequate pumping capacity of cargo pump to continuous removal of wash water from the cargo tank without any accumulation, etc. Additionally, coated cargo piping systems are a source of cargo contamination when the coating is deteriorating. Zinc coating is heavily reacting with an inorganic chemical ex. “Acid” and “Alkali”. Damage to coating is very common with epoxy-coated tanks, Stainless steel or SS clad tanks are the most desirable.

It is, of course, well understood that if the proper cleaning procedure is not followed it may lead to a failure of inspection by the surveyor and cause delays in loading the cargo.

The general tank cleaning phases of the chemical tanker are as below

• Hot/cold(ambient) wash by seawater/freshwater
• Chemical/detergent circulation by IMO approved list of chemicals (annex 10 of Provisional Categorization of Liquid Substances under MARPOL Annex II and IBC Code. Latest MEPC 2/Circ. 27 dated 31^st December 2021. (If required)
• Rinsing with Seawater/freshwater after detergent circulation (If required)
• Rinsing with fresh water to reduce chloride content in the cargo tanks
• Live steaming of cargo tank and line, sometimes conducted to reduce chloride content and remove odor from last cargo
• Drying, gas freeing, and mopping

Most inorganic chemicals can be removed by water only, whereas, there is some chemicals ex. PAPI (Poly-methylene Poly-phenyl Isocyanate) / MDI (Diphenylmethane diisocyanate) etc. which badly react with moisture/ water require to use of chemical additives such MEC (Dichloromethane) /MCB (Chlorobenzene) etc. for pre-cleaning.

Wall Wash test (WWT):

Chemical carriage as a highly specialized operation requires constant monitoring and testing of tank condition before loading, specifications for which Charterers supply data based on the long-accepted Wall Wash Test. The charterers often provide the tank cleaning specification to meet prior loading criteria to load the nominated cargo. Historically, chemical tankers are inspected by third-party surveyors before loading the next nominated cargo, and in many cases that involve tank entry and the taking of Wall Wash Test (WWT) samples to determine cargo tank suitability.

Normally laboratory-grade Methanol is used to collect wall wash samples from a few areas/points of the cargo tank wall/bulkhead. The wall wash sample is then tested in the ship’s laboratory by the crew to determine the presence of HC, Chloride in the cargo tanks. PTT (Potassium permanganate Time Test) is conducted in addition to detect the presence of impurities in alcohols or ketones that reduce potassium permanganate and applies to Methanol, Propanol, Butanol, Methyl Ethyl Ketone, and Methyl Isobutyl Ketone.

The spectrum range for the Hydrocarbon test is 400 nm, APHA Colour is 455 nm, Permanganate Fade Time is 530 nm and Chloride is 420 nm. An example:  APHA Colour – yellow samples absorb in the visible region – around 455 nm. The more intense the yellow color, the more absorption of light. The more absorption of light means a higher concentration of color; the more contaminated the wall wash is.

Wash Water Analysis (WWA)

Washing water analysis is the analysis of samples of washing water for the presence of previous cargo, taken during and throughout tank cleaning operations. It is not a measure of what has been left behind on the cargo tank bulkheads after tank cleaning, rather, it is a measure of what is being removed from the cargo tanks and lines, during tank cleaning.

Reference washing water (either Seawater or Freshwater) samples should be drawn directly from the tank cleaning line before the water enters the cargo tank. A new reference sample should be taken before each cargo tank sample is drawn because the quality of the “clean” washing water can and does change during the tank cleaning process. During the tank cleaning process, samples of washing water should be regularly taken from the manifold area of the vessel, as the water is discharged from each tank to slop or directly overboard. The frequency of sampling should be decided by the vessel, but a good rule of thumb is to sample at the end of each completed washing cycle of the tank cleaning machines, which is typically every 15-30 minutes.

These samples should be immediately analysed using the Spectrometer against the reference sample, noting that all washing water analysis is carried out using the smaller 10mm path length sample cell. During cleaning, it follows that as more of the previous cargo is removed from the cargo tanks, there will be less deviation on the UV graph up to a point where there is either no longer any deviation at all (a “flat-line” graph), or no change between consecutive samples.

At this point it can be determined that the particular cleaning step is no longer effective, meaning it is time to either stop cleaning or change to the next cleaning step.

Human factors that affect the tank cleaning operation

Human behaviour always changes as per the organizational culture and personal beliefs are not a fixed constant. One of the known characteristics of human behaviour is that when work needs to get done, the easiest path to get it completed in time will be chosen. Human beings while working are only taking action. It is only in hindsight that we can recognize them as ‘mistakes’ or ‘errors’.

We need to consider how to re-focus on human factors differently, considering the safety aspect and practices during the tank cleaning operation

• Collaborate: As an industry, we are very globalized and fragmented. Stakeholders need to collaborate rather than confront each other.
• Re-enforce positive behaviour: It is a fact that what gets appreciated gets repeated. It would be motivating to the seafarers if we gave some positive comments rather than only deficiencies or negative feedback.
• Eliminate fear: In the present state of the industry, truthful and honest reporting sometimes leads to commercial losses or punishments. A ship may not be allowed to enter a port or may be subjected to such harsh investigations, that the seafarer or company begins to wonder whether ‘honesty is the best policy.
• Commercial pressure: Tank cleaning, especially on short sea trade vessels are well-known tasks that put pressure on the seafarers. Surely, responsible charterers can co-operate to be reasonable in their expectations.
• Respect the seafarer: Seafarers are highly skilled professionals and deserve to be treated with respect. Port and regulatory authorities must have robust systems to ensure that seafarers are treated well and fairly, during their visits to your port. It will encourage them to be honest and motivated to do their job well.
• Technological/ Regulatory solutions: The industry must look at technological solutions to reduce the workload on seafarers. For example, can we not regulate to have sensors to record the opening and closing of valves, record fluid transfers and vessel positions Can compactors and landing of garbage in every port not be made mandatory? We need to reduce the administrative burden on the seafarer.
• Digitalization: Digitalisation will surely reduce the workload of seafarers. However, it is a ‘chicken and egg situation. It is moving slowly because, as an industry, we have not yet even defined the requirements for sensors. The goal must be simplifying the collection of data and making intelligence out of it. Digitalization is not just converting paper forms to electronic forms. It should eliminate the need for forms.

At the end of the day, owners, managers, charterers, and the regulatory regime have the same goal of safe carriage of cargo with no harm to people, environment, ship, or property. We need to manage our ‘human beings’ well and they will manage safety well! We need to listen to our seafarers and solve their problems through sensible, practical, and sustainable regulations/policies/guidance that should be enforced in an encouraging, collaborative industry and not solely through punishments, criminalization, and blame culture.