In the 1990s and early 2000s, intentional illegal discharges and mystery spills were often very hard to trace to a miscreant vessel. When the source of a spill is unknown, it is very difficult to prosecute the spiller. It can also be very difficult to recover the costs of responding and cleaning the spill. With suitable samples from the spill and from suspect ships, chemical analysis can be used to fingerprint the oils and create a match, so identifying the spiller.  Prosecutions can then take place under local laws derived from the International Convention for the Prevention of Pollution from Ships (MARPOL).

Oils undergo chemical and physical changes once spilled into the environment. These changes are called “weathering”. As the oil changes over time its chemical properties also change, and so analytical (chromatographic) profiles for weathered oils can look very different to profiles for fresh oil samples from ships. Oil weathering studies can provide an estimate of how long since the oil was spilled. This can then improve modelling capabilities for response and can be used to estimate a time and location for the spill for legal investigations. 

This research looked at the effects of weathering at sea of various grades of bunker oils. The summary report below was completed by Leeder Consulting.

Research outcomes

A series of marine bunker fuel oils of different viscosities (180, 280 and 380 cSt) were artificially weathered in a laboratory under moderate and extreme wind and sea temperatures. The weathering process was videoed and the physical properties (viscosity and pour point) and mass balance of the remaining oil was determined after 9, 24, 48 and 96 hours of simulated summer and winter conditions. 

Samples were taken of the oil residues for chemical (biomarker) fingerprinting analysis by gas chromatography and mass spectrometry (GC/MS). Results of the GC/MS chemical analysis technique undertaken on the weathered bunker fuel oil residues demonstrated that even after extreme weathering at summer conditions over 96 hours the biomarkers were still intact. This highlights that the specific biomarker used and the ratios calculated are highly resistant to weathering and an excellent marine oil spill source identification tool.

This provides demonstrated proof of the effectiveness of the oil spill fingerprinting techniques used in investigations of oil pollution incidents.

The weathering, mass balances and oil properties tested provided some interesting results:

• Bunker fuels that were rated with equivalent oil specifications on viscosity had quite different weathering rates and residual oil properties.
• Both the pour point and kinematic viscosity of the bunker fuel oils were found to increase with time as the oil weathered.
• The viscosities of the weathered bunker fuels after 96 hours increased on average by two orders of magnitude.
• All bunker fuels tested were found to have lost an average of 30 per cent by volume after 96 hours weathering under all test conditions. Even after the most extreme weathering, approximately 70 per cent of the bunker fuels remained as a solid residue on the water surface.
• Once the pour point of oil exceeded the seawater temperature, generally after nine to twelve hours of weathering, the oil became a solid, and dispersant effectiveness was very low.
• After only nine hours weathering of all the oils tested, even under moderate winter conditions, the pour point of the remaining oil was higher than the sea temperature and became a solid residue on the water surface.

This showed that even when the initial grade and specification of the bunker fuel and weathering conditions experienced at sea are known, it is not possible to predict the physical properties of the weathered residues.  Bunker fuels are a diverse blend of heavy residual oils and lighter viscosity oils, which vary too widely in their physical and chemical properties, to have predictable weathering.

Thus, to assess weathering processes and outcomes for any given bunker fuel, it would be necessary to conduct controlled laboratory weathering.  Caution should also be taken with any oil weathering models in spills involving marine bunker fuel oils that are not fully characterised.