North of England Club has notified members that the new fuels introduced to the bunker market to deal with the 0.50% sulphur cap from January 1st 2020 had been diverse in nature, but that one particular characteristic that could prove troublesome was if the vessel was carrying a heat-sensitive cargo.
North Loss Prevention Executive Alvin Forster said that early experience had shown that most VLSFOs were blended products, which meant that they could can vary quite remarkably. Some had very low viscosity, similar to distillate fuels, while others closely resembled traditional high-viscosity heavy fuel oil products. Most were somewhere in between.
The molecular structure of these new fuels has an impact on its storage and use. Some of the new VLSFOs are paraffinic in nature; previously used heavy fuels were asphaltenic. Paraffinic VLSFOs are more prone to waxing.
North noted that, if wax began to form in the ship’s fuel storage tanks it would be very difficult to pump. Transfer pump filters and pipelines were likely to become choked. If the wax formation was extensive, the vessel’s tank heating systems might struggle to re-liquefy the fuel. Manual digging of wax out from the tank might then be required, which would be a costly and time-consuming exercise.
The key, therefore, was to keep the fuel at a temperature above at which wax starts to form.
Before VLSFOs hit the market, determining the cold flow properties of marine fuels was straightforward. Distillate fuels, such as marine gas oil (MGO), were tested to give its Cloud Point (CP) and Cold Filter Plugging Point (CFPP) temperatures, whereas residual fuels (e.g. RMG 380) were tested for Pour Point (PP).
However, the CP and CFPP only apply to distillates because they are clear in appearance. These tests do not work on opaque fuels such as VLSFOs. This leaves only the PP test.
However, because of its paraffinic content, 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 (WAT). 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. This test cannot be carried out on board and must be specifically requested to the testing laboratory.
However, North said that, 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 was a risk that the temperature of the fuel in tanks located adjacent to cargo holds could damage a heat-sensitive cargo.
For example, according to BMT’s Cargo Handbook, a bulk cargo of raw sugar would be at risk of caking at temperatures as low as 25°C. If loaded into a hold which was above a hot double-bottom fuel tank, there was a real risk of cargo damage.
A ship’s chief engineer cannot specify the cold-flow characteristics of a VLSFO when ordering bunkers. The definition of a VLSFO is wide, so it was very much a case of ‘you get what you’re given’.
North said that it was therefore essential that the shipboard engineers find out quickly the characteristics of the fuel soon after bunkering. This would allow them to store and handle the fuel at the right temperature and know the risks if the vessel subsequently loads a heat-sensitive cargo.
An analysis report that shows a fuel having a high paraffinic content might trigger the crew to transfer the fuel to other tanks that are not adjacent to cargo holds. But avoid commingling – paraffinic fuels can be very prone to incompatibility when mixed with other stems.
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 failure. Good levels of maintenance can help prevent this.
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 fuel storage tank temperature records are kept.