The new fuels introduced to the bunker market are diverse in nature. However, one particular characteristic can prove troublesome if the vessel is carrying heat-sensitive cargo.
Very-low-sulphur fuel oil (VLSFO) is the umbrella term for marine fuel (other than marine gas oil) with a maximum sulphur content of 0.50%. With prices falling, its popularity has increased rapidly since the introduction of the IMO’s global sulphur cap earlier this year.
Early experience shows that most VLSFOs are blended products and can vary quite remarkably. Some are very low in viscosity, similar to distillate fuels, while some closely resemble traditional high-viscosity heavy fuel oil products. Most are somewhere in between.
The molecular structure of these new fuels has an impact on their storage and use. Some of the new VLSFOs are paraffinic in nature whereas the heavy fuels used previously were asphaltenic. The issue with these paraffinic VLSFOs is that they are more prone to waxing.
The problem with wax
If wax begins to form in the ship’s fuel storage tanks, it will be very difficult to pump. Transfer pump filters and pipelines are likely to become choked. If wax formation is extensive, the vessel’s tank heating systems may struggle to re-liquefy the fuel. Manual extraction of wax from the tank might then be required, but this is a costly and time-consuming exercise.
The key, quite simply, is to keep the fuel above the temperature at which wax starts to form.
Finding the right temperature
Before VLSFOs hit the market, determining the cold-flow properties of marine fuels was straightforward. Distillate fuels, such as marine gas oil, were tested to give cloud-point (CP) and cold-filter-plugging-point (CFPP) temperatures, while residual fuels (e.g. RMG 380) were tested for pour point (PP).
However, the CP and CFPP tests only apply to distillates because they are clear in appearance; they do not work on opaque fuels such as VLSFOs. This leaves the PP test, but because of the paraffinic nature of these fuels, there is a risk that wax can still form at temperatures higher than the traditional PP + 10°C ‘rule of thumb’ for heavy fuels.
An alternative means of measuring the cold-flow properties of a VLSFO is the wax appearance test. Developed by fuel analysis experts VPS, this test does not, however, form part of the suite of tests usually carried out on bunkered fuel under ISO 8217. It cannot be conducted on board and must be specifically requested to the testing laboratory.
Warm fuel, damaged cargo
Whatever the test method, the fuel analysis report provided to a vessel could recommend a relatively high fuel-storage and transfer temperature to prevent waxing and solidification.
In such cases, there is a risk that the temperature of the fuel in tanks located adjacent to cargo holds could damage heat-sensitive cargo. For example, according to BMT’s Cargo Handbook, a bulk cargo of raw sugar is at risk of caking at temperatures as low as 25° C. If cargo is loaded into a hold located above a hot double-bottom fuel tank, there is a significant chance of damage occurring.
Good fuel management
Know your fuel:
A ship’s chief engineer is not able to specify the cold-flow characteristics of a VLSFO when ordering bunkers. As the definition of a VLSFO is broad, it is a case of getting what you are given. It is therefore essential that the shipboard engineers identify the characteristics of the fuel soon after bunkering. In this way, they can store and handle the fuel at the right temperature and will be aware of the risks if the vessel subsequently loads a heat-sensitive cargo.
An analysis report revealing that a fuel has a high paraffinic content might prompt the crew to transfer the fuel to other tanks that are not adjacent to cargo holds. However, care should be taken to avoid commingling: paraffinic fuels can be prone to incompatibility when mixed with other stems.
Fuel heating system maintenance:
It can be difficult to accurately control the steam heating of fuel storage tanks. Temperature sensors and steam-control valves work in a harsh environment and can be vulnerable to falling out of calibration, if not failing entirely. Proper maintenance can help to prevent this.
Accurate record keeping:
In the event of a claim or dispute, evidence is essential. A vessel presented with a cargo damage claim will be far better placed to successfully defend it if reliable records are kept on the temperatures of the fuel storage tanks.
Find out more
Know your fuel – simple guidance on new fuels for ships’ engineers: www.nepia.com/publications/2020-shorts-know-your-fuel
Visit the dedicated 2020 Vision web area: www.nepia.com/topics/2020-vision
Alvin Forster, who joined North P&I in 2011, is responsible for Loss Prevention publications. He is involved in all aspects of loss prevention for both North and Sunderland Marine. Forster previously worked as a Marine Engineer Surveyor/Consultant and before that as a Technical Superintendent with a shipping company. A qualified Chief Engineer, his seagoing experience included 12 years at sea as an Engineer Officer on passenger ships. As well as holding Incorporated Engineer status he has attained a Chartered Insurance Institute Advanced Diploma in Insurance (ACII).