The Search for Gas Chromatograph Operational Nirvana Anwar Sutan - i-vigilant
Tropical Nirvana
HOW TO GET THERE? THE TECHNICAL DETAIL
i-vigilant Technologies Based in Aberdeen Founded in 2011 Providing Gas Chromatograph: Conditional Based Monitoring Software Support and Field Service Consultancy Training
Client Base
BS EN ISO 6974-1:2012 Natural gas - Determination of composition and associated uncertainty by gas chromatography Part 1: General guidelines and calculation of composition
Start ISO 6974 A No Step 1 Define Working Range Step 2 Define Analytical Methods Step 3 Select Equip. & Conditions Primary Cal or Evaluation Indirect Comp. Step 5 Relative Response Factors A No Yes Yes Step 4 Type 1 or Type 2 Type 1 Primary Calibration 1. Select Reference Gases 2. Measurement of ref gases 3. Regression Analysis 4. Selection of function Type 2 Performance Evaluation 1. Select Reference Gases 2. Measurement of ref gases 3. Regression Analysis No Routine Operation QA or RC Routine Calibration Finish Yes Step 10 Expanded uncertainty No Quality Assurance Check Quality Assurance Step 7 Sample Analysis Step 8 Calc mole fraction Uncert. Required Yes Step 9 Mole % uncertainty
STEP 1 DEFINE THE WORKING RANGE
Alvheim Working Range This will exist for each individual gases maximum minimum range can be determined this way
ISO 6974: STEP 4 PRIMARY CALIBRATION / PERFORMANCE EVALUATION
Primary Calibration / Performance Evaluation Step 4 Is Primary Calibration or Performance Evaluation Required? Yes Type 1 or Type 2 Type 1 Primary Calibration 1.Select Reference Gases 2.Measurement of ref gases 3.Regression Analysis 4.Selection of function Type 2 Performance Evaluation 1.Select Reference Gases 2.Measurement of ref gases 3.Regression Analysis
Type 1 vs Type 2 Analysis
Case Study 1: Alvheim Norsok I-106 Alvheim: Pipeline Gas Range vs Test Gas Range Test Gas Range
Test Results: Case 1 Test Description Test 1 Test 2 Test 3 Date 08/10/2016 25/03/2017 10/06/2017 Description GC A GC B GC A GC B GC A GC B Result Good Good Pass but Bad Good Good Good Maximum error 0.0232% 0.0239% 0.0946% 0.0280% -0.0413% -0.0346% GC A vs GC B CV discrepancy NA 0.11% 0.006% CV Uncertainty 0.056% 0.056% 0.082% 0.062% 0.062% 0.061% Standard limit 0.30% 0.30% 0.30% 0.30% 0.30% 0.30%
Test Results Test Description (Multiplier modified) (MLC) (DLC) Date 01/08/2017 11/07/2017 27/07/2017 Result Good Good Good Maximum error 0.0079% 0.24% 0.0180% GC A vs GC B CV discrepancy 0.003% 0.013% 0.002% CV Uncertainty 0.058% 0.100% (0.042 MJ/m3) 0.066% Standard limit 0.30% 0.1 MJ/m3 0.30% Comparison to single point 0.009% 0.013% 0.007%
DLC (DUAL LEVEL CALIBRATION)
What DLC Do? Instead of using 7 gases, we just use 2 gases
The impact on Mole Composition Bias is reduced to a negligible level
Test results on CV and Density Bias
Cal Gas 1 Cal Gas 2 Benefit No extra operation cost for operator The spare calibration gas can be used as the test gas for reproducibility tests Linearity check for the range of interest can be done monthly If new constants are required, it can be generated on site using currently used calibration gas and the spare calibration gas Daily use calibration gas Monthly reproducibility test GC
Is Primary Calibration / Performance Evaluation Required? Is Primary Calibration or Performance Evaluation Required? Yes Step 4 Type 1 or Type 2 No? Type 1 Primary Calibration 1.Select Reference Gases 2.Measurement of ref gases 3.Regression Analysis 4.Selection of function If bias is negligible If working range is narrow enough If dual level calibration is implemented If CBM is used to monitor GC daily performance Type 2 Performance Evaluation 1.Select Reference Gases 2.Measurement of ref gases 3.Regression Analysis
STEP 6: ROUTINE CALIBRATION / QUALITY ASSURANCE CHECK
Routine Calibration / Quality Assurance Check Routine Calibration Inject WMS Type 1: Analysis Update Coefficients Type 2 Analysis Update RF (b1) of assumed f(analysis) Quality Assurance Check Inject WMS Determine stability Plot control charts and take appropriate action No Routine Operation QA or RC Yes Routine Calibration Quality Assurance Check Quality Assurance Step 7 Sample Analysis Step 8 Calc mole fraction Finish Step 10 Expanded uncertainty No Uncert. Required Yes Step 9 Mole % uncertainty
New Calibration Regime System has 2 GCs, GC A & GC B. Calibrate GC A every 2 days Monitor GC A uncertainty on calibration reproducibility Set tolerance limit based on individual GC capability GC B is validated by looking at discrepancy against GC A
Case Study: St Fergus Gas Plant PX The use of uncertainty monitoring Daily uncertainty check is done Significant uncertainty increase on 11/7/2017
Case Study: St Fergus Gas Plant PX Discrepancy on CV between GC A and GC B 0.04% 0.11% discrepancy 1/6/2017: Low discrepancy 12/7/2017: High discrepancy, it is time for maintenance on GC A
Conclusion GCs tested are capable of significantly lower uncertainties Example shown 0.11% vs 0.3% stipulated reducing economic risk substantially Existing verification methods can increase risk of unnecessary bias/errors MLC / Exponential RF have minimal benefit in many situations Majority of people never implement MLC Replace Routine Performance Evaluation with Dual Level Calibration, CBM (GCAS with Live CV uncertainty) more practical less cost. UK Sector OGA has issued a no objection letter to DLC method GC maintenance using CBM complies with EN ISO 6974-1:2012 Gas Chromatograph Operational Nirvana achieved.
Acknowledgements Børge Olafsen, Aker BP Frank Tinke, Aker BP Andrew Runcie, PX Jonathan Ritchie, PX Derek Reid, PX Alvheim FPSO 2017 Production Oil: 110,000bpd Gas: 2.2Msm3/d For the supply of data, resources and time
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