Measuring sulphur onboard can help operators make better-informed decisions on regulatory compliance
Shipowners and managers, as well as charterers and operating staff, may have done their due diligence in ensuring incoming bunkers are sulphur compliant, but it may be the case that the bunker delivery note is wrong, either due to error or deliberate deception by unscrupulous bunker suppliers. Furthermore, low-sulphur oils could be blended with high sulphur to ensure that the bunker is not too expensive, but at the same time compliant. Such blends may well push the limits of compliance.
Since measurement science is statistical in nature and includes specifying confidence levels in percentage terms, onboard measurements of sulphur levels in incoming bunkers will provide extra assurance to shipowners, according to Parker Kittiwake principal chemist David Atkinson.
Mr Atkinson presented a paper titled “The science of compliance” at the Cimac Congress 19 held in Vancouver in June this year. The paper makes a case for using portable instruments in shipping.
Sulphur concentration measurements are made using elemental analysis techniques, like inductively-coupled plasma optical emission (ICP) or X-ray fluorescence (XRF) spectroscopy, he explained in the paper. In XRF spectroscopy the technique irradiates samples with high energy electromagnetic radiation and then detects X-rays fluoresced as a result. The wavelengths and intensities of this fluorescence are an indicator of sulphur content. Large XRF instruments are the norm in labs, Mr Atkinson said, but portable ones are available for measurement onboard.
He explained the procedure for calibrating instruments and using them to measure sulphur concentrations. ISO 8754 describes a standard method for calibration of instruments that operating engineers can use for portable instruments.
ISO 8217 is a fuel specification often invoked by fuel supply contracts such as bunker, whereas IMO Marpol Annex VI is a convention for the prevention of air pollution by ships. Both documents stipulate the use of XRF sulphur determination by ISO 8754 (see Section 1.1.2), but differ in their interpretation regarding limits.
Annex L of ISO 8217 indicates that disputes over whether a fuel meets a specification or not should be resolved with reference to ISO 4259. This standard indicates that a fuel buyer should only consider a sample to be non-compliant, with 95% certainty, if a single test result is greater than the specified limit, plus a certain value that factors in the parameters of the instrument. Similarly, a fuel can be considered compliant only if it is below a certain value that is a sum of the limit as well as an instrument parametric value.
Users onboard can evaluate if an incoming bunker sample is compliant or not as per Annex L of ISO 8217. But the problem, said Mr Atkinson, is that Marpol Annex VI produces a pass or fail rather than a 95% certainty.
The paper recalls a recent study on the measurement of bunkers on four barges supplying to merchant ships. Referring to the 0.1% limit in ECA, Mr Atkinson said any supplier in possession of a single sulphur concentration measurement greater than 0.088% cannot be 95% confident that the fuel is compliant. In such a case, an additional measurement, likely made onboard, could be of benefit to both buyer and supplier.
If the measurements are performed twice and the sulphur content is measured to be 0.09% both times, then the confidence level on the fuel being compliant increases and the fuel can pass. An additional measurement can firm up a ‘go’ or ‘no-go’ decision.