Subsea gas energy and quality measurement using ultrasonic flow meters

International trading of natural gas is based on high-precision, traceable, and accredited fiscal flow metering systems and international pipeline transmission infrastructures. Norwegian O&G industry is today moving towards subsea solutions for fiscal allocation metering, to replace today's topside metering systems. Gas is traded on basis of energy flow rate. Thus, in addition to the volumetric flow rate, the energy and quality parameters (QP) of the gas need to be measured. Traditional methods used topside cannot be used in subsea systems. An industrial solution to this challenge is mandatory and urgent.

The objective of the research project is to develop necessary scientific basis and technology components, as a fundament to - in a follow-up industrial project - realize traceable and accredited gas energy and quality measurement by ultrasonic gas flow meters (USMs) in subsea fiscal metering systems. The approach is based on use of the sound velocity (VOS) measurement already available in relevant USM technologies. However, no method is available today to ensure necessary traceability and accreditation of this VOS measurement.

The work includes development of a measurement method and a laboratory high-pressure VOS measurement cell for gas, as a basis to achieve high-precision calibration of the USM's subsea VOS measurements. Secondly, it includes further development of an already operational VOS-to-QP algorithm to ensure - from measurements available from the USM and the metering system - high-precision and accredited calculations of the energy and quality parameters under subsea gas operational conditions.

The results of the project will serve as a necessary fundament to enable accredited and traceable subsea measurements of the USM's VOS reading, as input to an accredited and traceable VOS-to-QP algorithm, which together with the already accredited USM volumetric flow measurement provides the gas energy flow rate with accredited fiscal accuracy.

In particular, the project addresses development of necessary scientific basis and technology components, as a fundament to realize traceable and accredited gas energy and quality measurement by ultrasonic gas flow meters (USMs) in subsea fiscal measurement of natural gas. This includes:

(1) Development of laboratory high-pressure VOS cell to address scientific research challenges that need to be solved to realize construction of a high-precision VOS cell enabling accredited and traceable calibration of the USM's VOS reading in natural gas;

(2) Development of a full 3D system model for the VOS cell measurement method, to establish necessary and traceable time delay corrections in the measurement method; and

(3) Development of robust Gas Analyzer algorithm for VOS-to-QP calculation for a wide range of gas compositions, with documented accuracy sufficient for fiscal gas flow metering.