Over the last couple of decades, tank cleaning has turned from a tradition-based chore into a scientific exercise in energy consumption optimisation
The start of a new decade is always a good time for reflection and it is instructive to compare tank cleaning concerns at the end of 2009 with those at the end of 2019. In 2009, the question was whether a tank cleaning machine should have one nozzle or two. This debate was a legacy from the days of the original Butterworth tank cleaning machines. The industry had been divided on the merits of single- versus double-nozzle tank cleaning machines, and opinion has oscillated between the two. There was a stage during the 1970s and early 1980s, when the twin nozzle was favoured; single nozzle systems then experienced a surge in popularity in the 1990s and 2002s.
Another 2009 concern was how many cleaning machines to fit in a tank and the best way to position them. The standard at the time had been outlined by IMO rules for crude oil tankers. This was not necessarily the standard required for chemical tankers however, with shadow areas resulting. The then Norwegian class society DNV (now DNV GL) noted that its Effective Tank Cleaning (ETC) notation revealed that shadow areas totalling 7% were being thrown up – implying that more than one machine is needed in a tank. For a chemical tanker, the lack of another cleaning machine in the tank had profound implications in terms of downtime and turnaround.
Today, Alfa Laval offers a solution to this problem. Its G-Pass design software goes beyond shadow diagrams to produce a 3D simulation of the tank – showing corrugations, stringer platforms and other internal obstructions from all angles. The aim is to identify the best position, quantity, jet length and jet hit angle of the tank cleaning machines. This ensures an optimised installation that prevents product build-up and reduces fluid and energy use.
Returning to the 2000s, the focus moved to the role of pre-loading inspections. Guy Johnson, now of L&I Maritime, was one of the first in the pages of Tanker Shipping & Trade to discuss the issue of absolute quality for cargo owners and charterers. The problem arose due to the ability of modern equipment to detect minute traces of contamination from any source in the supply chain. As a result, the tanker industry was being forced to achieve and maintain levels of quality which are far higher than they need to be for the safe carriage of goods.
At the time, Mr Johnson noted that it was not unusual for the same cargo to be shipped by different charterers with very different pre-loading inspection requirements, even when the cargoes had the same export specifications. He has since campaigned for charterers to replace the requirement for a wall-wash inspection with that of washing water analysis. The education process is working and has brought about a significant change in procedure, with safety and environmental benefits.
Indeed, environmental concerns are at the centre of current tank cleaning thinking and will increasingly dictate the profile of the tank cleaning industry in the 2020s, as it strives to meet owners’ demands to optimise every facet of tanker operations to reach IMO targets.
From days to hours
The first step in this process involves reducing fuel consumption during tank cleaning operations. Kockumation Group head of sales and marketing Johan Ljungbeck believes the use of digital sensors during tank cleaning can reduce a full-day procedure to just four hours.
“With sensors, operators can monitor tank cleaning by machines, manage temperature and produce digital reports to prove whole tanks are cleaned,” he said at Riviera’s Tanker Shipping & Trade Conference last November.
A typical tank cleaning operation includes nine hours’ use of heated seawater, three hours of washing with fresh water and then 12 hours to refill the tanks with nitrogen gas to inert them ready for the next cargo.
But using sensors and Kockumation’s Washmaster tank cleaning system, the seawater cycle can be reduced to three hours, the fresh-water cycle to just one hour, while the tank itself is already inerted. Plus, the tanks are digitally inspected during the wash cycle.
Washmaster includes software for automated tank cleaning and to ensure full coverage. Mr Ljungbeck said wash cycles can be tracked, while the progress and positioning of cleaning processes can be followed in real time.
Another tank cleaning machine provider, Scanjet, which supplied the tank cleaning equipment for Carl Büttner Holding’s new 38,000 dwt IMO II product and chemical Handysize tanker CB Adriatic, is also working on reducing fuel consumption during the tank cleaning process. Scanjet Marine managing director Niklas Falkmer says: “Tank cleaning is a crucial operation for all tankers that change type of cargo frequently. The operation is both time consuming and costly, as a lot of bunker is being used to heat up the tank cleaning water.”
Scanjet is in the process of optimising emissions output across its range of products, from P/V valves to tank cleaning machines. The company has launched a fully controllable tank cleaning machine that will contribute to less bunker being used and shorter cleaning operations. Scanjet also has a monitoring system called WashTrac which enables the operator to log how a successful cleaning have been made and monitor and record the results from inspections and washwater analyses.
Another step along the optimisation curve involves understanding the impact that the application of the tank coating can have on its resilience to cargoes and repeated washing cycles, and how this influences the ability to reduce temperatures and cleaning cycles.
Safinah’s marine sector manager Alan Walker notes that the curing of coatings is a crucial step that owners need to monitor closely. Hot-air curing could lead to a hot spot at the point the heat is injected into the tank, he explains, while hot-water curing also has drawbacks and can lead to stress and brittle paint films.
Safinah is an independent coating consultancy and is called upon to investigate cargo tank coating failures. Mr Walker notes that in the last two years there has been an increase in such failures. An investigation of the logs and electronic records has shown the heat and humidity during paint application remained within guidelines, the cargoes were loaded in accordance with regulations and the tank cleaning and chemicals used were within manufacturer’s guideline. So why was the coating flaking off after only a few years? Safinah’s inspection of the paint flakes revealed grit on the underside of the flakes.
These application problems can be compounded by in-service abuse of the coating, for example, mandatory water flushing. This needs to be minimised after the carriage of miscible and hydrolysable cargo such as methanol. Mandated ‘over-cleaning’ after methanol is a serious issue, as Mr Walker explains: “Methanol will suck water into the tank coating and cause blisters. It is a basic chemistry, but now it is mandated that (operators) must do (water flushing) after carrying methanol.”
He warns that the flushing should be as short as possible and the tank dried with dry air as soon as possible to prevent the coating blistering.
He adds that in general, operators should keep water temperatures as low as possible and only use the amount specified to clean the last cargo as per regulations. He says that to preserve tank coatings, operators should: “Experiment with lower temperatures and shorter cleaning times. Do not just follow traditional procedures. If you have applied low absorption coatings, you can save time and money of reducing the water temperature.”
The cleaning products used can influence the ability to lower temperatures. Teco Marine, the producer of the MarClean range of tank cleaning products, states that it works closely with tank cleaning stakeholders to develop its products. Feedback is gathered from supercargoes, chief officers, crew and shipowners and combined with its own data to produce a greener series of products.
Also following the green approach to tank cleaning detergents is Navadan of Denmark. Navadan produces the NavaClean range of cleaning products, used across a wide range of product tanker cargoes including veg oils, fish oils and other cargoes and specialist products.
In 2009, Hamburg-based ChemServe took the concept of providing cargo tank cleaning advice one stage further by creating the online Miracle Tank Cleaning Database. On its introduction, the database provided cleaning guidance and advice for more than 1,000 different cargos and around 1,500 different cleaning proposals. The latest version of the database was released in January 2020 and added a further 60 cargoes, including the latest low-sulphur compliant fuels, bringing the total range to well over 10,000 cargoes. Each cargo can be searched by synonym and each has a cleaning recipe provided by tank cleaning experts. The latest version includes an e-learning package on washwater analysis, and steps on reducing confined space entry and increasing the safety of crews and inspectors.
Washwater analysis is used on Waterfront Shipping’s methanol dual-fuel 49,000 dwt methanol carriers Mari Couva and Mari Kokako, via an inline spectrometer. On Odfjell’s Hudong-49 series super-segregator chemical carrier Bow Orion, an innovative high-capacity reversed osmosis system, produced by the Norwegian company Norwater, produces the ship’s fresh water. The Norwater system has a very low energy and carbon footprint and produces sufficient freshwater so that seawater is not required in the tank washing processing.
The use of reverse osmosis installations reduces the need to bunker demineralised water and provides drinking water. Another reverse osmosis demineralised water equipment manufacturer, Hatenboerwater of the Netherlands, explains that producing freshwater from reverse osmosis for tanker flushing saves a significant amount of energy and is yet another step on the ladder towards optimising the tank cleaning process.
Optimising all operations and equipment on a vessel is now a critical step toward reaching greenhouse goals. See and hear more at the Optimised Ship Forum in Singapore on the 1st of April 2020.