Horndon SMER 29th November 2011

Overview Background Methodology Results Comparison with other sites Bias and Standard Deviation Changes in Gas Source Results Daily Average Temperature Error Daily Corrections Overall Result

Identification of Error Problem started in Sept 2008. Prior to this there were also intermittent low temperature readings

Correction of Error Nearby sites investigated for temperature correlation were: Matching Green, Luxborough Lane, Shorne, Yelverton, Roudham Heath, Gt Wilbraham and BarkingPS.

Bias and Standard Deviation 1 Temperatures can only be compared during flowing periods. Low DP cut-off will be used to indicate if the site was flowing or not Downstream to upstream temperature correction of 0.5 °C/bar used Bias will exist between temperatures because of differences in site installations and locations. The bias was investigated for a known good period (July 2010 to Dec 2010) Count Bias Standard deviation Luxborough Lane 17088 -0.52 1.64 Matching Green 14670 -1.32 1.11 Shorne 8313 +0.26 1.84 Yelverton 17144 +2.44 1.56 Gt Wilbraham 18016 +2.90 2.01 Roudham Heath 18004 +3.51 1.81

Step Changes in Temperature

Change in gas composition

Change in pressure profile

Bias and Standard Deviation 2 Calculation of bias separated by pressure difference between Horndon and Matching Green. This reduced the standard deviation. Points at which a site started flowing were excluded from the analysis until the temperature stabilised and reflected the real gas temperature. Higher pressure at Horndon than Matching Green (H) Lower pressure at Horndon than Matching Green (M) Initial calculation Bias Std Dev Matching Green +1.72 0.76 +0.83 0.59 +1.32 1.11 Luxborough Lane +1.09 0.63 -0.47 0.83 +0.52 1.65 Shorne +1.15 0.99 -1.23 1.00 -0.26 1.84 Yelverton -1.61 0.97 -3.60 1.02 -2.44 1.56 Roudham Heath -2.58 1.32 -4.91 1.08 -3.51 1.81 Great Wilbraham -1.90 1.55 -4.42 1.22 -2.90 2.01

Correction of Error Yelverton, Gt Wilbraham and Roudham Heath were excluded from the final analysis because of higher bias and standard deviation values than other sites which may be attributed to the distance from Horndon. The average of the (bias corrected) temperatures was used to calculate the temperature error. The temperature error was consistent for long periods from July 2008 to July 2009 and July 2009 to July 2010. Before and after this period the error is taken to be zero with explainable fluctuations around zero.

Daily Average Temperature Error

Intermittent Negative Temperature Differences

Intermittent Negative Temperature Differences Not an Error

Data Processing Rules Where data is erroneous (such as during periods of validation) the erroneous data was corrected before calculation. Where data was missing for up to 30 minutes the last good value was used. Where data was missing for longer than 30 minutes no correction was made, except for essential data (Horndon DP, P, T; Matching Green P) which were interpolated. The temperature error was calculated on a daily basis. Where no suitable flowing data was present the temperature error for the surrounding days was applied (Except in Sept 2008).

15th & 22nd Sept 2008 No daily average temperature due to lack of flowing data and transient temperature error. Detailed analysis of non-flowing data on these dates highlighted errors of 7.4 °C and 22.1 °C respectively.

Correction of Error The flow error was found to be sensitive to pressure fluctuations therefore it was necessary to recalculate the flow rates from the raw data. One set of calculations was made on a four-minutely basis using the corrected temperature and another using the recorded temperature. Daily volumes were calculated for both sets of data, the ratio between them being described as the ‘daily correction factor’. The uncorrected calculated daily volume was compared to the billed daily volume and where the difference was more than 10% a default daily correction factor was applied. This was the average of the factors on surrounding period. The daily correction factor will be applied to the daily volume and energy totals.

5th – 8th July 2010 RBD data missing from 10:06 on 5th July 2010 until 17:03 on 8th July 2010 Assumed default daily correction factor for 5th and 7th July 2010. Only count half of Error on 7th July 2010.

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