Cryogenic ultrasonic flow measurement—an enabling technology for LNG production, storage and transportation

Abstract

Liquefied natural gas (LNG) is becoming an increasingly important energy commodity with the declines in conventional oil reserves. The economics of LNG depend on the use of cost effective production, storage and transportation systems, which can involve use of shared facilities. For example, shared storage and berthing facilities can result in massive cost savings for LNG producers. The potential cost savings associated with shared storage or transportation of LNG can only be realised if each partner can be assured that they are getting their fair share of the final revenue, i.e. reliable systems for production allocation and custody transfer are required. Previously, tank level based volumetric measurement has been the standard in LNG measurement; however, such systems are dependent on the very infrastructure that can be eliminated in shared transportation and storage systems. What is needed for shared facilities is a reliable method of in-line LNG metering. Up until recently, in-line metering of LNG had not been used for allocation or custody transfer owing to a lack of qualified technologies capable of accurate measurement. The two major challenges are to produce a metering technology capable of reliable operation at cryogenic temperatures and to devise a traceable calibration method appropriate for the conditions of operation. This paper describes the development of reliable ultrasonic meters designed for high accuracy measurement of LNG and addresses the issues that could arise as a result of the differences between calibration and field conditions. Data is presented demonstrating the performance and robustness of transducers specially designed for cryogenic operation. Details are provided of a method for transferring laboratory calibration using water to LNG application conditions with minimal uncertainty. The paper includes a case study related to the use of these meters for allocation into shared storage at the Qatar Gas common facilities project. Prior to selection of the meters for this application the project partners undertook an evaluation of the ultrasonic technology at the Kenai LNG plant in Alaska alongside Coriolis flow metering technology. The aims of this field qualification testing are described and results presented.