Offtake Metering Errors

Stephen Skipp

Asset Manager

(Transmission and Storage)

Metering Systems

• Metering Systems must:-• “Be installed, operated and maintained to the standard of a

Reasonable and Prudent Operator” (UNC – OAD section D, paragraph 2.1.2)

• “Be capable of reading and set to read without systematic bias and within the Permitted Range”

• “Unless otherwise provided in the relevant Supplemental Agreement, the applicable industry standards and best practice recommendations set out in Annex D-2” (UNC – OAD section D, paragraph 2.2.1)

Metering Systems

• UNC – OAD Section D, Annex D-1 describes permitted uncertainty levels :-• Instantaneous Volume Flow Rate +/- 1.0%• CV +/- 0.1MJ/m3 (across range 35 – 44MJ/m3)• Pressure +/- 0.5bar (across range (0 – 85barg)• Temperature +/- 1.00C (across range (0 – 400C)

• Unless superseded by a Supplemental Agreement• Instantaneous Volume Flow Rate

• +/- 2.0% (between 30% and 100% of maximum flow rate)• +/- 3.5% (between 10% and 30% of maximum flow rate)

• CV +/- 0.14MJ/m3

• Pressure +/- 0.4% (across range (0 – 80barg)• Temperature +/- 0.2% (across range (-10 – 400C)

Supplemental Agreements

• Supplemental Agreement were agreed between National Grid Gas and the Distribution Networks at network sale

• Equipment complying with the general terms of the Supplemental Agreement is referred to as “Requisite Metering”.

• The following sites have Supplemental Agreements• 6 sites in Scotland have volume flow and energy measurement• 8 sites in Scotland have volume flow measurement with attributed CV• 11 sites in Southern have volume flow and energy measurement

• 1 site in Scotland complies with Annex D-1

Liaison with National Grid Gas UKT

• Periodic review of emergency procedures (UNC – OAD section C, paragraph

2.3.6)

• National Grid Gas UKT, as the upstream party are entitled to:-• Inspect measurement equipment (UNC – OAD section D, paragraph 2.4.1)

• Witness any validation (UNC – OAD section D, paragraph 3.1.4)

• Request exceptional validation (UNC – OAD section D, paragraph 3.3.1)

• Receive copies of records relating to installation, maintenance, testing, operation, calibration, setting and validation of measurement equipment (UNC – OAD section D, paragraph 7.2.1)

• Provision of maintenance plans (UNC – OAD section G, annex G-2)

• NGG UKT continue to determine daily CVs for each charging area for (SGN) (UNC – OAD section F, paragraph 4)

Liaison with National Grid Gas UKT

Regular meetings• Objectives:

• “To review offtake metering issues and ensure compliance with section ‘D’ of the Unified Network Code”

• “To promote best practice with the aim of preventing or minimising meter errors”

• Where errors are identified “To ensure that the appropriate processes are applied to rectify and account for the error”

• Frequency – approximately bi-monthly

• Agenda:• Routine validations• Validation witnessing• Meter Errors

• Meter technology• Meter records• System Operation alarms

Identification of Meter Error

• Pro-active• Annual Validation

• Routine validation “no less frequently than once every 12 months” (UNC – OAD section D, paragraph 3.2.1)

• Meter Audit• Programme agreed with Ofgem• Normally one per LDZ per annum

• Ofgem Gas Examiner CV / Metering Inspection• Undertaken by Ofgem gas Examiner in conjunction with CV inspection• Aim to complete one audit of every site per annum commencing April 2007

• Re-active• Fault report from System Operation• Report from Operative during site visit• Exceptional Validation requested by NGG

Meter Errors

‘Faults’ found since creation of SGN (June 2005):-Site Dates Cause Error Time to

produce MER

Farningham July 1999 to June 2005 Incorrect LGCF 2.07% under 15 months

Braishfield A June 2004 to July 2005 Orifice plate contamination 0.5% under 7 months

Winkfield So Pre 2000 to April 2007 OMNI not configured to adjust downstream temperature to upstream conditions

0.059% over 1 month

Bathgate Dec’ber 2005 to June 2007 Incorrect ‘K’ factor 0.04% under 1 month

MERs within SGN Geography (based on date found)

0

1

2

3

4

2000 2001 2002 2003 2004 2005 2006 2007

No. MERs (> 0.1%)

No. MERs (< 0.1%)

No. MERs (not quantifiable)

Examples of potential meter errors

• Orifice plate / turbine meter condition• Error assessed on extent of contamination

• Liquid contamination in impulse lines• Error difficult to assess, action to prevent occurrence

• ‘K’ Factor error (turbine meters)• Flow computer configuration error• DWT local gravity correction factor not applied correctly

Analysis of meter errors

• Where incorrect data has been applied (e.g. LGCF)• Identify reason for fault• Identify duration of fault• Initial calculation to identify likely scale of error• Develop rules for control of missing, spurious data• Extract 4 minute data for Inlet Pressure, Differential Pressures (orifice

plate), Temperature, Volume Flow and Gas Composition from archive

NB. Up to 360 packets of data per day• Manipulate data into spreadsheet format• Duplicate spreadsheets for corrected parameters• Embed proven gas flow routines (e.g. GasVLe) into spreadsheets• Run routines for original and corrected parameters• Audit and review output

MeterValidation

0 days

SystematicBias?

+ 14 days

MER

ValidationReport

+ 14 days

UpstreamParty Disputes

MER?

+ 30 days

Dispute resolutionthrough

consultation

ExpertDetermination

Yes

No

Disputeresolved?

No

Yes

Yes

Meter Audit

Gas examinerInspection

SystemOperations report

Existing Meter Error (UNC) Process

SGNNGG UKT

642 / 643Process

0 days

Total energy >50 GWh?

Costs calculatedand passsed to

Billing Operations

Adjustment withinRbD charges at

next invoice

No

End

15 days noticeprovided to allRbD shippers

Yes

Shippersrespond with

concerns

O & Trespond

+ 5 days + 5 days+ 5 days

Shippersrespond

Concernspersist?

Billing OperationsForum

Yes

No

Upstream partypresent case for

reconciliation

Shippers presentconcerns

Reconciliationunanimously

rejected?

Vote for or againstreconciliation

No

Expertdetermination

Expert confirmsUpstream Party's

assessment is correct

Expert confirms UpstreamParty's assessment is incorrect,

but error was present

Expert confirmsno Meter Errorwas present

End

Expert assessment forrecalculation of total

energy and costs

Yes

Existing Meter Error (642 / 643) Process

UNCProcess

Error exceeds0.1%?

End No Yes

MeterValidation

0 days

SystematicBias?

+ 14 days

Draft MER

ValidationReport

Yes

+ 14 days + 30 days

EndNo

Meter Audit

Gas examinerInspection

SystemOperations report

As required

SGNwebsite

issue log

Offtake name,LDZ affected,Description of error,Date error detected,method of detection,Estimated duration of error,Estimated significance,Status of investigation.

+ 14 days

websiteupdate

log

+ 14 days

websiteupdate

log

websiteupdate

log

Expertselected from

agreed list

Expertdetermination

Expert confirmsUpstream Party's

assessment is correct

Expert confirms UpstreamParty's assessment is incorrect,

but error was present

Expert confirmsno Meter Errorwas present

End

Expert recalulatestotal energy and

costs

Yes

Final MER

websiteupdate

log

MER status (draft / final),Error as percentage,Volume of gas (GWh)

Revised Meter Error (UNC) process

642 / 643Process

continues aspreviously

agreedSGNNGG UKT

Error exceeds0.1%?

Yes

Summary of proposed changes

• Extend period permissible for production of MER• Provide updates on progress of meter error calculations

• Initial notification after 14 days• Subsequent updates every 14 days• Information via SGN or third party website

• Obligation for Expert Determination of MER• Where reconciliation required• Expert selected from agreed list• Report from Expert Determination to be circulated to relevant parties

• No change to current 642 / 643 process

Offtake Metering Errors

Stephen Skipp

Asset Manager

(Transmission and Storage)