Metering Marine Fuel, The Advanced Instrumentation Research Group.
Metering Marine Fuel
The ability to identify non-ideal operating conditions for an instrument is particularly important when metering a high value product such as fuel. In-built intelligence and the monitoring of additional data channels can not only detect non-ideal conditions but potentially provide corrected measurements and on-line uncertainty assessments where models can be developed. The $150 billion ship fuel (bunkering) industry still uses largely manual methods for determining the transfer of fuel from the bunkering barge to the receiving ship. These techniques are prone to error and potential fraud, with Maersk, the world's largest ship operating company, estimating it is defrauded of 1.5% of its $7 billion annual spend of fuel. Conventional flow metering does not work because of entrapped air in the viscous fuel. However, a new generation of meter is able to detect and correct for this condition. The detection of air can in turn improve the operation of the barge leading to better asset utilisation. Detailed trials and studies have been carried out demonstrating the benefits of the new approach.
In 2006, approximately 350 million tonnes of marine fuel were supplied worldwide. At current prices, this equates to approximately $150 billion of transactions annually. Increasing economic and environmental importance is attached to the measurements taken during the bunkering process (i.e. the physical transfer of ship fuel [1]), in terms of both quantity (usually measured in volume but sold by mass), and of fuel quality (such as viscosity, density and sulphur content). For example, the International Marine Organisation (IMO) is introducing a series of measures to impose significant cuts in the permitted levels of sulphur in marine fuel [2], in order to reduce harmful emissions. For the efficient enforcement of such measures, reliable methods are needed for monitoring both the sulphur content and the quantity of fuel at the point of supply.
There are two distinct classes of marine fuels: distillate grades, often described as gas oils, and the residual fuel grades. “Residual” refers to the crude oil content remaining after the lighter and more valuable components have been removed by the oil refining process. Currently about 90% of marine fuel is residual, however many ships use at least some gas oil to drive auxiliary power systems. Conventionally, residual fuels have been specified simply by viscosity; current standards (e.g. ISO 8217:2005 [3]) provide more thorough specifications for the fuels grades (including density, flash point, pour point and limits on components such as sulphur), but the grades are still labelled according to their nominal viscosities, so that for example the fuel grades RMF-180 and RMG-380 specify maximum kinematic viscosities of 180cSt and 380cSt respectively, measured at the standard temperature of 50ºC. At ambient temperature in high latitude ports, some fuel grades are effectively solid. As most bunker fuel is supplied to vessels via bunker barges, it is thus necessary to ensure that the fuel supply pipework on the barge is kept entirely clear between transfers (usually achieved by “blowing down” with high pressure air at the end of each transfer), to prevent solid fuel blockages. Even at an elevated pumping temperature, the viscosity of the fuel is high, and so air entrainment is a significant challenge to any metering system that might be used for fuel bunkering.
Another regular feature of bunkering operations is “tank stripping”, whereby the last dregs of fuel (together with increasing amounts of air) are pumped out of one storage tank on the bunker barge before the supply is switched to another. A typical modern bunker vessel of perhaps 4,000 tonnes capacity may have 6-10 such fuel storage tanks operating in port/starboard pairs: a high proportion of bunker transfers will thus include a period of tank stripping.
Currently, the most widely used means of measuring the fuel transfer from bunker barge to receiving ship is by manual tank dipping. In this procedure, the level of fuel in each bunker barge tanks is recorded, along with its temperature, prior to the transfer of fuel. This exercise is repeated at the end of the transfer. Barge-specific calibration tables are used to map the recorded dip levels into corresponding volumes, and the difference in volume between the beginning and end of the bunkering operation gives the transferred quantity. The tank calibration tables are normally derived from calculated data and the tank is not calibrated by measurement and filling. Further calculations, based on the density of the fuel (as certified by the supplier) yield the delivered mass. One difficulty of current practice is that regular recalibration of the measurement system, as practiced in most custody transfer applications, is rarely possible. A further difficulty is the measurement error that may be introduced by entrained air.
Dissastifaction with the current state of the industry has been most notably declared by Maersk, the largest shipping company in the world [4]. On an annual basis Maersk buys 13 million metric tonnes of marine fuel and undertakes 12,000 bunkering operations, but claims an average discrepancy of 1.5% between the readings taken by Maersk vessels and those provided by suppliers or barge operators. Until recently, all but the most specialised flow instrumentation was incapable of providing accurate measurement of two-phase flow (e.g. oil/air) mixtures. Indeed Coriolis metering was known to be especially vulnerable.
Developments at the UTC (described in Coriolis Research) have pioneered Coriolis metering technology capable of maintaining flowtube oscillation. Thus although the uncertainty of the measurements are higher than for single-phase fluids, it is becoming accepted that Coriolis meters are capable of generating usable measurements with two-phase flow.
One consequence of the new Coriolis metering technology is that the previous vulnerability to two-phase flow has been transformed into a very high sensitivity to two-phase flow, whereby the damping on the flowtube (as represented by the parameter known as the drive gain) can be used to detect the presence of even small quantities of gas, a feature particularly useful to the bunkering industry.
This is illustrated in figure 1, which shows data from an actual bunker transaction recorded during a trial in Singapore, as discussed later. As the first results using Coriolis metering for bunkering are shown in conferences and publications, the question most frequently raised is “how can you prove that there really is air in the bunker fuel?” The data in figure 1 is used to illustrate how additional channels of data, such as the drive gain, can be used to determine the presence of air.