Pumps & Systems, February 2009
In the oil and gas industry, custody transfer transactions involve transporting physical substance from one operator to another, including transferring raw and refined petroleum between tanks and tankers, tankers and ships and other transactions. An accurate account of the amount of material transferred is of great value to both the company delivering the material and the eventual recipient. This is especially true in bunker fuel oil delivery since a ship's bunker contributes to the ship's operating cost.
A digital Coriolis meter in use for custody transfer.
The difficulty in measuring bunker fuel oil flow stems principally from air retention in the lines, which is caused by several factors. First, bunker fuel oil is highly viscous, which can cause the pump to draw a heavy vacuum on the suction side. In some circumstances, this can draw air into the pump's gland. Also, fuel oil remaining in the bottom of the tank may draw air as the tank is pumped dry or stripped. In addition, compressed air is introduced into the transfer hose when it is blown clear just prior to disconnection to prevent spillage of bunker fuel oil on the deck. Air entrainment can even be caused by the way in which the barge tank is filled. For example, drop loading and splashing can add air, which becomes entrained. Once air is entrained within the bunker fuel oil, it can take several hours for it to come out of suspension. Temperature and the viscosity of the product are the key variables.
For such reasons, ship owners have resorted to the practice of manually measuring or "dipping" the amount of bunker fuel oil on a fuel barge, but this method does not always result in measurements that all parties agree upon. Two years ago, BP Marine Fuels embarked on a research project to investigate the feasibility of using metering technology to produce irrefutable measurements in the delivery of bunker fuel oil to ships.
BP appointed an independent consultant to evaluate all available technologies. Initial conclusions suggested that ultrasonic meters were both economical and reliable, but the industry view was that Coriolis meters, the alternative metering technology, were unreliable and expensive. Some industry experts felt that the vibration present on bunker fuel oil barges would adversely affect Coriolis meters. However, after detailed discussions with BP's exploration, production, refining and chemical divisions, they suggested, based on their experience, that Coriolis meters were more economical and likely to be more reliable. To arrive at a definitive solution, BP developed a basic statement of requirements, which defined the ideal metering technology in the company's view.
The requirements stated that an accuracy of 0.1 percent was required for single-phase flow, and an accuracy of 0.5 percent was required for two-phase flow. An invitation was then sent to potential partners and vendors to submit details of their metering products that complied with the basic statement of requirements.
Coriolis Technology In Depth
Coriolis technology offers accuracy and reliability in measuring material flow, and is often hailed as among the best flow measurement technologies. Conventional Coriolis meters, however, have had a significant limitation for custody transfer: They have not performed well in measuring two-phase flow conditions, which involve a combination of gas and liquid mass.
Coriolis meters measure flow by analyzing changes in the Coriolis force of a flowing substance. Coriolis force is generated in a mass moving within a rotating frame of reference. That rotation produces an angular, outward acceleration, which is factored with linear velocity to define the Coriolis force. With a fluid mass, the Coriolis force is proportional to the mass flow rate of that fluid.
While custody transfer operations do not typically feature rotating pipes, oscillating a section of pipe is required to exploit the Coriolis principle for flow measurement. Oscillation produces the Coriolis force, which can be sensed and analyzed to determine the rate of flow. To use Coriolis force for measurement, a Coriolis meter has two main components: an oscillating flowtube equipped with sensors and drivers, and an electronic transmitter that controls the oscillations, analyzes the results and transmits the information.
Reliable Coriolis measurement depends on consistent, reliable oscillation, which is determined by four factors: the density of the liquid, the balance of the tubes, the dampening caused by the flow stream itself and the physical isolation of the tubes from the environment.
Compromising even one of these factors will degrade Coriolis meter performance, and two-phase flow compromises every one of them. Consequently, applications involving negligible amounts of entrained gas-even as little as 2 percent volume-have been poor candidates for Coriolis measurement. This has been particularly troubling for those running batch operations, where reliable, highly accurate flow measurement can confer considerable bottom-line advantage, but where it is also necessary to begin with an empty or partially filled flowtube.
Advanced digital Coriolis meters have changed all that. Microprocessors in their transmitters run advanced digital signal processing techniques that provide useful measurements of both mass flow and density, ensuring stable operation in either single-phase or two-phase flow conditions, such as those found in bunker fuel oil transfer.
Applying Coriolis Technology to The Problem
After examining digital Coriolis technology in light of the stated specifications, the BP team concluded that Foxboro digital Coriolis technology from Invensys Process Systems [af1] met their requirements. BP and IPS collaborated on the development of an industry bunker-fuel measurement solution that will be administered by the International Bunkering Industry Association (IBIA).
To house the flowmeter, the manufacturer developed a standardized open skid design. The development team believed that a half 20-foot equivalent unit (TEU) container-sized skid could be easily connected and disconnected from the barge deck. Creating a lock and load system with a barge having a pre-assigned mounting area where the skid would be secured to the deck was essential. The barge flanges needed to connect to the meter lines with simple and quick-mounting arrangements and include two or three plug-in connection points for power and data. The skid had to be simply transported by truck so that the Coriolis meter could be periodically maintained and tested.
It was essential that the integrity of the meter was not compromised, and the system was both visible and secure. If a spot check were to be conducted on the accuracy of the Coriolis meter, the skid would have to be removed from the barge quickly and replaced with a skid carrying a substitute unit. Future skids will comply with the new design, ensuring that the same basic layout is replicated throughout the world.
The marinization phase of the trial ensured that the results obtained in the laboratory could be replicated in the "real world." The Singapore Maritime Port Authority and the BP barge Pride were assigned to run the tests.
To date there have been 50 deliveries, all of which have shown a high level of constancy with barge "soundings." The next step will be for the manufacturer to develop bunker receipt-issuing equipment, which will record data to produce an electronic delivery note-an important development on the road to a fully-automated custody transfer and accounting system.
If bunker fuel oil barge owners adopt it, this Coriolis meter, which would have the port's approval, would be purchased or leased with a licensing arrangement managed by IBIA. Each year an annual fee would be paid to maintain the integrity of the Coriolis meter. Barge owners would recover their capital investment and the integrity management charge through a surcharge on the bunker fuel oil supplied.
BP looks forward to the adoption of the new measurement. "Our efforts and those of our technology partner IPS have now resulted in a bunker fuel oil metering system that can truly claim to provide irrefutable measurement results. We now hand over the baton to IBIA to encourage the rollout of the technology worldwide," said BP Marine Fuels CEO, Graeme Alexander.
Don Gregory, former Director Sustainability and Fuel Technology at BP Marine Fuels, said, "We have taken this initiative to drive forward the use of technology within the bunkering industry to develop, with our partners, a metering system that is both accurate and reliable. However, it has always been BP Marine Fuel's intention to hand over stewardship of the metering technology to IBIA and encourage its widespread adoption for the benefit of all stakeholders in the bunker industry."
In the future, irrefutable measurement data captured by a Coriolis meter can be transferred to a central location where bunker fuel oil expenditure and usage can be monitored centrally. This type of data interchange allows real-time monitoring of bunker fuel oil deliveries from multiple ships worldwide and will therefore be of interest to ship owners, charterers and port authorities. The principal benefit for the barge operator is a faster turnaround, which in turn should improve cash flow and reduce working capital.
For the first time the accurate measurement of all bunker fuel oil delivered to ships can now be guaranteed, much in the same way a gasoline pump measures the delivery of gasoline to a car. The benefits are far reaching, including increased efficiency and improved safety for personnel. It is hoped that the technology will now be adopted throughout the worldwide shipping industry.