Uncertainty in Gas Density. Measured and Calculated

Transcription

1 Uncertainty in Gas Density Measured and Calculated Oil and Gas Focus Group December 2012 i-vigilant is a trading name of i-vigilant Limited and i-vigilant Technologies Limited i-vigilant GCAS Limited is registered Conditional in Scotland with Based company number Monitoring SC Registered For Gas office is Chromatographs 56 King Street, Aberdeen, AB24 5AX i-vigilant Technologies Limited is registered in Scotland with company number SC Registered Office is 1 Berry Street, Aberdeen, AB25 1HF

2 Introduction Gas density Continuous direct measurement By calculation using EOS, composition, P, T Use of both provides useful cross check Field allocation On gross energy On a component basis Gas composition required

3 Densitometer Density Vibrating cylinder Excited at it s natural frequency Period is primarily a function of density ρ u = K0 + K1τ + K2τ 2 Temperature Correction ρ t = ρ u 1 + k18 t t c + K19 t t c VOS Correction 1 + K τc ρ A = ρ c t 1 + K τc g 2 2 ρ A = ρ t 1 + K3 ρ t + K4 SG SG γ T c γ T g

4 Densitometer Installation Pressure recovery method Density measured at P2, T3 Densitometer measures density within transducer

5 Densitometer Density Density referred to upstream tapping ρ 1 = ρ A P 1 T 3 z P 2, T 3 P 2 T 1 z P 1, T 1 Full functional relationship used for uncertainty calculation ρ 1 = K0 + K1τ + K2τ k18 t t c + K19 t t c 1 + K 2 τc c P 1 T 3 z P 2, T K 2 P 1 P o T 3 + μ JT P o 1 β 1.9 z P 1, T 1 τc g

6 Densitometer Uncertainty Input Nominal Value Expanded Uncertainty Divisor Relative Standard Uncertainty Relative Sensitivity E(xi)*S(xi) Manufacturers Specification (0.15%) Repeatability (kg/m3) Period (us) Measured Temperature (DegC) Densitometer Temperature (DegC) Upstream Pressure Differential Pressure Joule Thomson Uncertainty in VOSm Uncertainty in VOSc Transducer K - VOS 2.10E Compressibility (T1, P1) Relative standard uncertainty Coverage Factor 2 Expanded Uncertainty % Expanded Uncertainty (kg/m3)

7 Calculated Density Density calculated from composition AGA Report No. 8 Line density calculated from process measurement instruments Composition by Gas Chromatography Utilises gas component thermal conductivity Relationship between conductivity and molecular weight Response compared to known composition Response Factor = Peak Area WRM Mole%

8 Calculated Density Assume Suitable sampling and conditioning adds no additional error Analysis cycle time and lag time are negligible Main Sources of uncertainty Repeatability of the GC Linearity of the GC Quality of the working reference mixture Reproducibility of the GC Calculation Method

9 Calculated Density Component based uncertainty Uncertainty analysis of field GC data Field data uncertainty analysis Uncertainty of the WRM (U wrm ) Uncertainty from GC repeatability data (U repeatability ) Uncertainty from GC reproducibility data (U reproducibility ) U x(i) = U x(i),wrm 2 + U x(i),repeatability 2 + U x(i),reproducibility 2 x(i)= component i within WRM

10 Calculated Density Uncertainty Input Nominal Value Expanded Uncertainty Divisor Relative Standard Uncertainty Relative Sensitivity E(xi)*S(xi) Methane Nitrogen CO Ethane Propane i-butane n-butane neo-pentane i-pentane n-pentane Hexane Pressure Temperature AGA 8 N/A Relative standard uncertainty Coverage Factor Expanded Uncertainty % Expanded Uncertainty (kg/m3)

11 Measured Density Only? Accurate determination of the Pressure and temperature at the densitometer is required In DECC s experience, failure to take account of this factor in the design of densitometer installations is one of the principal causes of significant mismeasurement in North Sea applications DECC Guidance notes for Petroleum Measurement Issue 8 Compositional analysis required in allocation and accounting Gas pipeline allocation is normally performed on a component mass basis The value of natural gas at the point of sale is a function of its calorific value

12 Calculated Density Only? No cross check available Requires an increased degree of scrutiny on critical elements of the sampling and analysis systems VOS potentially a verification tool Common density used in all streams Correlated uncertainties Calculated uncertainty is dependent upon of secondary instrumentation Pressure, Temperature

13 Summary Uncertainty in density from instrument measurement is estimated at ±0.3354% from theoretical analysis Uncertainty in density from compositional analysis is estimated at ±0.3973% from field data analysis A combination of both methods offers potential cross-correlation verification Calculated density can be suitably utilised but requires increased scrutiny and monitoring of the sampling and analyser system Correct installation and operation are critical to the accuracy in each instance

14 Uncertainty in Gas Density Measured and Calculated Thank-you for your time Oil and Gas Focus Group December 2012 i-vigilant is a trading name of i-vigilant Limited and i-vigilant Technologies Limited i-vigilant GCAS Limited is registered Conditional in Scotland with Based company number Monitoring SC Registered For Gas office is Chromatographs 56 King Street, Aberdeen, AB24 5AX i-vigilant Technologies Limited is registered in Scotland with company number SC Registered Office is 1 Berry Street, Aberdeen, AB25 1HF