Article by Harun Rashid, VPS Senior Technical Manager.

Stability has long been a highly unpredictable character of marine fuels, which is extremely sensitive to a fuel’s formulation and its storage and handling conditions. For these reasons, fuel buyers have no direct control on fuel stability, unlike other physical parameters, other than relying upon a supplier’s due diligence regarding the fuel formulation and the ship’s engineer’s professionalism in managing the fuel on board.

Asphaltenes within residual-based fuels can precipitate as sediment/sludge whenever a fuel’s available solvency to retain asphaltenes in suspension drops below the fuel’s stability reserve. The ISO 8217:2012/17 standard has a sediment limit of a maximum 0.10% m/m to protect buyers’ interest where an elevated sediment value can be indicative of an unstable fuel. Today’s fuels are no longer produced from straight run refining processes, but undergo a series of complex processes, for example, vacuum distillation, vis-breaking, catalytic cracking, in order to extract the lighter end components as much as possible, leaving the bottom end components with reduced reserve stability. The solvency can further diminish with unregulated blending in order to meet the Sulphur compliance levels within MARPOL Annex VI & statutory regulations.

During 2023, VPS handled numerous sludge formation cases which led to severe filter & separator blockages. The majority of these cases involved VLSFO fuels and for a sizable number, the sediment content at the ship’s manifold was actually below the 0.10%m/m specification limit. In one case, a ship received a VLSFO in an Asian port with sediment content 0.07%m/m and reported severe sludge formation soon after the fuel was put into use, with sludge formation continuing throughout the use of the fuel.

The sludge formation was so severe that the engineers had to clean the purifier bowl assembly every 4 hours, compared to a usual cleaning interval of 250 hours. At one point, the engineers discontinued using the fuel, as the vessel ran out of spare parts. Analysis of system samples later confirmed that the sludge formation was indeed due to unstable fuel and not from on-board mixing.

While the number of sludge formation cases clearly singles out VLSFO as more unstable and challenging to manage on-board, compared to HSFOs, another challenge is to prevent wax formation without exposing fuel to excessive heat and accelerated fuel ageing. With VLSFOs being  more parafinnic in nature, a good number may require heating beyond their pour point to prevent wax formation.

VPS 2023 data shows, at least 15.8% of VLSFO samples had a wax apperance temp (WAT) above 40C, including 1.6% above 50C. These samples will precipitate wax if storage temps are not kept above the WAT, compared to the lower storage temperature requirements based on their tested pour points. Wax formation is a reversible process, so theoritically, it should not cause any operational issue as long as fuel remains stable, but unfortunately, fuel stability is not guanteed. Moreover, if wax is allowed to form, wax and aspheltenes (sludge) can co-mingle together as a result, the wax disappearnce temperature may increase significantly and in the worst case, the wax may no longer disappear. Taking a middle approach and strking a balance could be one option where the fuel is not severly overheated, nor the wax allowed to form freely during storage and only increase temperatures during transfer if needed. VLSFO with such high WAT/WDT will need close attention and care during storage, transfer, treatment and use. Prolonged storage should also be avoided for such fuels.  

Sludge formation on the other hand is an irreversible process, meaning it is not possible to convert the precipitated sludge back into the fuel. As there is no quick fix, efficient handling of fuel treatment plants becomes so critical otherwise, severe clogging of filters and separators can lead to fuel starvation to the engine and a loss of propulsion placing greater risk to the safety of the crew, ship, and the environment.

Avoiding a catastrophic situation requires visiting the entire fuel management chain and adhering to best practices. It starts with buying fuel from trusted suppliers, followed by verifying the fuel quality in one of the VPS, ISO17025 accredited laboratories.

VPS offers  an Additional Protection Service (APS) package which includes a stability assessment of the fuel.

VPS additional protection service (APS) package

Parameters such as Separability Number (Reserve Stability Number) and Total Sediment Existent (TSE) together with Total Sediment Potential (TSP) results provides an excellent indication of fuel stability while GC/MS screening and follow up testing ensures the fuel is not contaminated with harmful chemical species at levels considered “unfit for use”. Testing Wax appearance & disappearance temperatures assist in maintaining optimal heating to prevent wax formation whilst also avoiding overheating and fuel ageing.

If a vessel uses stability additives, then its performance evaluation should not only be in assessing the improvement of the reserve stability number, but to conduct a broader assessment across fleet including assessing sediment reduction. Crew training is key, as poor handling of fuel by incompetent crew can also trigger a sludge formation situation.

A deeper understanding of the fuel’s quality can be achieved, coupled with a further reduction of any associated risk, by going beyond ISO 8217 and using the APS test scope. This service will ultimately save time, money and company reputation.

For further information and support in the fuel management of VLSFOs please contact: marketing@vpsveritas.com