Douglas Raitt, Regional Advisory Services Manager at Lloyd's Register looks at why it is critical for ship owners to order bunker fuel based on the suitability of its use, rather than the compliance of its sulphur content.
It’s now over seven months since IMO2020 came into force and it seems everything has gone smoothly. Certainly, LR’s tests show almost universal compliance – with just four per cent of very low sulphur fuel oils (VLSFO), and less than two per cent of gas oils, being off-specification. But being on-spec doesn’t always mean fuel is always on-point for use. In fact, in some instances, while marine fuel may be supplied to specification, it may not always be fit for purpose when it comes over the rail.
Cold flow properties are an important consideration when it comes to ensuring the safe and efficient use of marine fuels on board vessels. A lot of VLSFOs are highly paraffinic, with poor cold flow properties. This greatly impacts thermal heating management onboard ships. If not controlled effectively, fuel storage temperatures may accelerate fuel instability and impact shelf life and its fitness for purpose.
Storage and wax appearance
The introduction of VLSFO’s poses a set of unique challenges for marine fuel management onboard ships. VLSFOs tend to be more paraffinic in nature than the majority of high sulphur fuel oils they replaced and feature increased wax content and generally lower viscosities. This limits stability and makes them more sensitive to thermal control demands – both heating and cooling.
With these challenges in mind, FOBAS identified fuel treatment during purification as the most critical area in conditioning the fuel. During purification the gravitational forces are multiplied, making them more likely to separate any denser crystallised waxes remaining – which can lead to excessive sludge deposition.
To better predict this sludge deposition, and to ensure the fuel temperature can resolve the waxes back into the fuel, FOBAS developed an in-house performance test method for lower viscosity, lower injection temperature fuels.
The Sediment and Wax Precipitation Point (SWPP) test provides ship owners with the optimum purifier throughput temperature to prevent wax deposition during fuel treatment operation.
In very simple terms, while SWPP is broadly similar to the cold filter plugging point (CFPP) for distillate fuels, CFPP is suitable for detecting the point at which wax crystals become problematic in clear and bright fuels. In contrast, the SWPP can be used on darker opaque oils to detect the point at which combined sediment and wax crystals no longer become problematic.
The key point is that SWPP is a performance-based test to identify the critical temperatures to be maintained during purification, and indicates the temperature at which the wax will disappear into a liquid state.
Other wax measurement tests are offered on the market. The Wax Appearance Temperature (WAT) which is equivalent to Cloud Point (CP) for distillates and is used for dark opaque fuel, while the Wax Disappearance Temperature (WDT) is the temperature required to re-liquify the crystals. Both these are addressed by our combined SWPP performance test. WAT and WDT temperatures are higher than the pour point, which predicts pumpability of fuel.
In essence, although good to know, the practical significance of WAT and WDT in the management of marine fuels is limited. It should be noted that if WAT or WDT temperatures, are maintained for extended storage periods in the bunker tanks to keep all wax crystals in solution, this may lead to fuel overheating. In view of the thermal sensitivity of VLSFO’s, this could then potentially accelerate instability. Therefore, wax crystals in fuel are better addressed in the higher temperature fuel conditioning unit (FCU), with fuel flow pumpability maintained by keeping fuels at least 10°C above the pour point only.