REAL LIFE ULTRASONIC FLOWMETER VERIFICATION FOR UPSTREAM CUSTODY TRANSFER METERING OF NATURAL GAS

Ultrasonic gas flowmeters (USM) employing multiple paths and transit time technology are becoming established in the upstream oil and gas industry for custody transfer flow measurement. This is mainly due to the advantages of wider flow range capability, and lower pressure loss over that of conventional differential pressure type flow meters such as orifice plates. It is now more than a decade since ultrasonic meters were first applied to high accuracy gas flow metering, and standards and guidance documents have developed much in this time [1, 2]. However in the area of on-going measurement verification procedures, there is still minimal standardisation. Current standards provide guidance but leave ongoing verification procedures to the parties involved. Industry regulators such as government departments, pipeline operators etc require that flow measurements are verified on an ongoing basis to the required uncertainty levels. This paper describes a custody transfer USM metering station (measurement uncertainty of +/- 1%) installed on the Petrojarl Banff an FPSO (Floating Production, Storage and Offload vessel). It describes the specifically developed verification techniques, and reviews verification data collected over 4 years of operation. The verification procedures applied have been successful in satisfying the pipeline operator of the suitability of the ultrasonic metering station. It is proposed that the procedures described could form the basis for industry standard USM verification.