New tanker generation calls for new rules
A new class notation from DNV GL helps operators of LR2 tankers highlight their ships’ ability to switch between low and high-grade cargoes without risking contamination.
Amid intense competition in the tanker market, ship size and cargo flexibility are key differentiators. As a result, there is a trend among operators to prefer LR2-type tankers which, in contrast to the equally sized Aframax crude oil tankers, have coated tanks. This allows them to carry both “dirty” cargo, such as crude oil or heavy fuel oil, and “clean” cargoes, i.e. petrol (gasoline) or other clean petroleum products.
Typically, cargo owners require carriage of an intermediate cargo, such as diesel oil, for three voyages after carrying crude oil or dirty products before clean products such as petrol can be transported. The intermediate cargo gradually cleans the tanks, pumps and piping for the subsequent clean oil product.
A crucial task: Tank cleaning
An alternative to intermediate cargoes would be to design a ship to enable switching between dirty and clean cargoes on a ballast voyage. This will however require thorough cleaning to remove traces of previous cargo from internal tank surfaces, cargo piping and cargo pumps and avoid contaminating the next product. Tank cleaning is performed by deck-mounted tank-washing machines.
The tanks are washed with seawater during a ballast trip and possibly rinsed with freshwater to remove the salt residue. There are certain designated areas where the washing water cannot be discharged. The discharge must be monitored using oil discharge monitoring equipment (ODME) so the permissible maximum oil content is not exceeded. When the ship arrives at the next loading port, the tanks will be completely clean.
In addition to the arrangement of the tank-cleaning machines, the structural arrangement of the tanks as well as the cargo pump and piping arrangement are important factors to improve tank-cleaning efficiency. A ship’s ability to perform this cleaning routine effectively is key to its competitiveness, since cargo owners insist on shipowners demonstrating their vessels’ ability to minimize cargo contamination.
LR2 (Long-range 2) type tankers, which typically have a 115,000-tonne deadweight (dwt), are more flexible than Aframax vessels and can offer significant advantages in terms of size and profitability. This has prompted DNV GL to develop the new class notation Improved Tank Cleaning (ITC) that is geared exactly towards this ship type, says Olav Tveit, Vice President and Ship Type Expert at DNV GL – Maritime. The new rules quantify the tank-cleaning capability so owners of adequately designed and equipped ships can prove to cargo owners that their LR2 vessels are capable of achieving the required tank cleanliness and cleaning efficiency for the carriage of clean cargoes.
“Most conventional crude oil tankers in the 115,000 dwt range are fitted with horizontal structural elements, such as stringers, as well as vertical deck girders, web frames and large brackets,” says Tveit. “Their horizontal surfaces increase the area where sludge from crude or heavy fuel oil can accumulate, and vertical structures create shadow areas which the tank-cleaning machines cannot reach.”
The ITC class notation for tank cleaning and cargo operations encourages the use of an alternative structural arrangement where corrugated tank walls replace horizontal stringers and brackets, and deep-well pumps are installed in each tank. “While corrugated bulkheads are more expensive, they are much easier to clean because residue will have no horizontal surfaces to settle on,” says Tveit. “Horizontal corrugations have slanted surfaces that cause the cargo and sludge to run off. Furthermore, corrugated, coated tanks offer no obstructions to the tank-washing machines, so they can be cleaned much more easily than conventional ones. This means they can satisfy high cleanliness standards.”
Cargo pumps and piping
“In addition, a traditional tanker is usually equipped with one common pump room with three interconnected pumps for all tanks. These systems involve long cargo lines with large diameters and several interconnections within the pump room, on deck and at the cargo tank bottoms. This complicates stripping of the last remaining cargo from cargo tanks and cargo piping and complicates flushing and cleaning cargo pipes.”
“Using deep-well pumps in each cargo tank instead of a pump room allows shipowners to better strip the cargo tanks and piping from cargo residue and also simplifies the cleaning of the tanks and piping,” says Tveit. “The reason is that all cargo piping is located on deck. In addition, the use of deep-well pumps also enables better segregation when different cargoes are carried on board. Lastly, not having a cargo pump room means that more volume can be allocated for cargo. If owners are willing to make the necessary investments required for the ITC class notation, their vessels will be in a better position than a traditional ship to switch between cargoes rapidly, depending on the qualifiers they choose.”
The ITC class notation specifies that tanks must be fully coated and that the coating must be compatible with any cargo the ship is intended to carry. The cargo tank piping must either be coated or made from stainless steel. There are additional requirements regarding the materials for the heating coils and the tank-washing and pumping systems. In addition, a dedicated oil residue tank must be provided on board. All these specifications are based on DNV GL’s experience in the field, Tveit stresses.
The ITC notation has three mandatory qualifiers. The first qualifier specifies the coverage of the tank-washing machine: 80 per cent of the total cargo tank surfaces must be covered as a minimum. Exceeding this minimum requirement and indicating the actual coverage with the ITC notation will be a competitive advantage, says Tveit. The coverage implies that the tank-washing machines must have a certified throw length. “This means that DNV GL does not consider it sufficient for the water jets from the washing system to simply reach the tank walls at that distance. DNV GL also requires a minimum impact force that translates to a certain cleaning effect of the water jet.”
The second qualifier addresses stripping efficiency, which is indicated by the permissible amount of cargo residue left in a cargo tank and the piping after discharging. The draft version of the new rules proposes a maximum of three cubic metres or 3,000 litres. Ships achieving a lower amount of residue will be at an advantage. Individual deep-well pumps will achieve much better stripping efficiency than a pump room system, stresses Tveit, with values potentially ranging as low as 0.1 cubic metres or 100 litres.
The third qualifier is the combined total area of upward-facing horizontal structures inside a cargo tank, excluding tank bottom and deck, in square metres. The background for this qualifier is that horizontal surfaces represent a potential for the accumulation of sludge and cargo residue. The qualifier indicates the total horizontal surface area per cargo tank on board. A ship with corrugated bulkheads will feature very small surface areas and again be at a significant competitive advantage.
“The ITC class notation will provide owners with a clear competitive advantage by demonstrating that quick and efficient tank cleaning is ensured, so clean and dirty products can be carried interchangeably without risking contamination,” explains Tveit.
This new class notation is primarily intended for new ships since some criteria, including stripping efficiency, are subject to specific test requirements. Nevertheless, it may be granted to existing vessels as well, provided they meet the criteria. The new ITC rules have been published in July 2019, with an effective date of 1 January 2020, says Tveit. “As with all rules, we will gather feedback and experience over time, and fine-tune the requirements where it makes sense,” he adds.
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