Natural gas - Gas metering NTNU 3 October 2007

Transcription

1 Status: Draft Natural gas - Gas metering NTNU 3 October 2007 Endre Jacobsen [email protected] Fiscal metering and field allocation

2 2 Content Introduction Why and where to measure Commonly used measuring principles for gas metering Parties Design and construction Challenges

3 3 Needs and locations for metering in different parts of gas transport- / value- chain of wellstream subsea flowlines from satelite fields platform stream locations test separator gas and liquids fuel gas consumption flare gas export gas to pipeline transportsystem at gas processing terminal in dry gas transportsystem at sales / exit point to gas buyer

4 Allocation of gas and condensate - principle 4

5 5 Economic aspects of metering Value of information Hardware costs Operating costs

6 Metering uncertainty and cost 6

7 Transportsystem for norwegian gas 6600 km pipelines Operator: Gassco 84 x 10 9 Sm3 (2006) 0,5 % error equals 630 x ,50 NOK/Sm3 7

8 8 Metering and analysis in transport system determination of delivered energy (mass x calorific value of gas) to be analysed for: hydrocarbon dewpoint water content CO 2 content H 2 S content - calorific value, wobbe index Operator must ensure that gas specifications given in contract and national requirements for distribution network are met.

9 9 Gassco key metering / allocation aspects Company (shipper) make nominations and sell their own gas Capacity booking at entry / exit Defined tariff regime; tarif costs according to booking Example: delivered quantities (50,0) = nominated quantities (50,0) metered quantities (50,2) Excess metered gas (0,2) are deducted from next day nomination Dry gas system allocation based on energy Fiscal metering / analysis at every entry / exit point

10 10

11 11 Gas metering at offshore platforms Gas to export (pipeline) ref transport (entry point) Allocation metering of tie-in fields (may be dedicated separators) CO 2 tax 0,78 NOK/Sm3 (2005); NOx tax fuel and flare metering systems; Stat C. approx Sm3/d fuel Sm3/d flare (ca 170 mill NOK/år) norwegian sector approx NOK Plant mass balance monitore performance / GOR (gas oil ratio) Gas injection Well testing by using test separator; determine well performance relationship (well-head pressure, multiphase/wet gas flow meter) Process regulation (level control of separators/scrubbers, compressor control, etc) EU MRG Directive

12 12 Monitoring and Reporting of Greenhouse gas emissions (MRG) EU-directive for EU MRG directive effective from 2008; program for monitoring and reporting of CO 2 emmisions to be agreed with norwegian pollution authorities (SFT) Detailed and complex set of requirements putting forward uncertainty for activity-data (t / Sm 3 ) and carbonfactor-data (tco 2 /t gas or tco 2 /Sm 3 gas) on fuel, flare and diesel used and burned on a facility. Requirements varies with type of industry, yearly CO 2 emision quantities, type of source stream, definition of system border, each source relative contribution of total yearly CO 2 emmisions Petroleum Industry have identified severe technical challenges (problems) to implement all requirements;

13 13 LNG fiscal metering; ref Snøhvit LNG storage and transport at very low temperature (liquified gas at C in special designed tanks) Fiscal metering of LNG based on level measurement of ships cargo tank (spherical or rectangular) according industry methods (principle next page) 2 independent level systems (main system: radar) Temperature readings at minimum 5 levels pr. tank Certified tank volume from 3rd party inspection Analysis of product (mainly methane)

14 14

15 Bodies involves in gas metering norwegian sector. Authorities: Oljedirektoratet (NPD) + Justervesenet (Legal measurment) + SFT (Environmental) 15 Set requirements in law/regulations, 2 EU directives MID (Measurement Instrument Directive (liquid metering systems) & MRG (Measurement and Reporting of Greenhouse gases)) Approval role Audit Gassco Govermental owned; operator of gas transport and gas processing systems: Overall responsible, administrate capacity, approve and give access to the transport systems by objective and non-discriminating conditions. System infrastructur are owned by Gassled. Field owner and operators (StatoilHydro, Conocophillips, ExxonMobil, Shell, ENI, Total, BP... ) Vendor- and contractor-industry Test laboratorier (K-lab, IKM-lab, Teknologisk Institutt),

16 16 Components and functions in a metering system Flow element Piping configuration Pressure, temperature, fluid density Sampling and analysis Computer part (calculation, monitoring and reporting) Operation, maintenance and calibration

17 17 Uncertainty in metering Keep traceability - calibrate metering componts against primary/secondary equipment with known uncertainty Document measurement uncertainty within agreed limits (+/- 1% for fiscal gas metering): Establish functional relationship between input and output (need a model) Quantify uncertainty in input and in model; including type of distribution (often normal distribution) Find if any correlated input terms and determine correlation coeff. Determine expanded uncertainty based on given rules for correlated and uncorrelated contribution terms

18 18 Measurement principles for gas metering Ultrasound technology: Compact and new technology; Differential pressure; conventional technology with limited turndown Orifice; ISO 5167, Venturi; ISO 5167, V-Cone; short upstream lenghts ( Turbine meter; conventional Coriolis; new technology, limited capacity, direct mass meter

19 19 Orifice, venturi and V-cone V-cone Measure pressure drop over a restriction caused by fluid flow Each meter has a specific calculation formulaes Overall functional relationship is about the same for all 3 meter types venturi blendeplate

20 Orifice meter ISO

21 21 Orifice Pressure profile

22 22

23 23 Turbine meter Measure actual volumeflow Low friction bearings required. Rotational speed is proportional to gas flow Rotational movement detected by use of magnetic material on rotor, generating a voltage pulse in pick-up located on wall Need calibration against a known reference on laboratory. Sensitive to foreign material

24 24 Ultrasonic flowmeter Measure time for a ultrasound pulse to travel with and against a flowing gas. Direct or wall reflection varies between vendors Gas velocity and speed of sound given as: v = Lp 1 1 a 2cosφ t AB t BA C= Lp t AB t BA High capacity and turndown, can cover 3 to 4 orifice meters of same size More robust against asymmetric/swirl flow compared to orifice and turbine.

25 25

26 26 Algorithm For A Chordal Multipath USM Chord Location V m = R 0.309R 0.309R 0.809R Weight W A B C D

27 Data Calculation Summary 27 Measure transit times Calculate individual chord velocities Weight chord velocities Calculate average flow velocity Calculate average volume flow rate Convert to m 3 /hour 2 L V = 2 X. V Weight A = Weight B = Weight C = Weight D = = 4 i= 1 t t V ( r ) W i t. t i i Q = V i πd. 4 Q (m 3 s -1 ) x 3600 = Actual Volume Flow rate (m 3 /hour) 2

28 28 Five path matrix combination

29 Coriolismeter emersonprocess.com 29

30 30 Orifice + documented in standard ISO generally accepted + low cost + no moving parts + no need for flowcalibration + no temp / pressure limitations + mechanical robust - sensitive for pulsating flow - limited turndown (1:5) - significant pressure drop - sensitive for assymetric/swirl flow profil - can accumulate dirt / liquid - possible damage with high flow - sensitive for particles

31 31 Turbine meter + low uncertainty over a large flow range + generally accepted + medium cost + easy interface signal transmission - need flowcalibration and re-calibration - significant pressure loss - movable parts need lubrication - can be damaged of particles, wet gas and high flow rate

32 32 Ultrasound flow meter + low uncertainty over a large flow range + self diagnosis possible -> indicate if re-calibration is needed + no moving parts + no pressure drop - need normally flowcalibration (cost) - can be affected by acoustic noise from surrounding regulating valves - possible pressure and temperature limitations + high capacity + possible for bi-directional use

33 33 Coriolis meter + massrate as output + water calibration may be valid for gas + not sensitive for flowprofile + bi-directional use - high pressure loss - sensistive for crosstalk and for external vibrations - size limited to 6 10

34 34 Criterias when establishing metering principle Check agreements, authority requirements and standards Determine the specific need (mass? volume? energy?) Evaluate capacity, turndown and expected production profile Review prosess conditions (P, T, liquid drop-out?, high CO 2 content?) Availability and concequences if shut-down or out of service Required uncertainty Cost / benefit analysis on simplified systems Unmanned operation? Remote area? Subsea?

35 35 NPD regulations for fiscal metering Total measurement uncertainty for gas < +/- 1% (mass) includes: instrument, signal loop, calculation and control interval. Total uncertainty to be documented according to recognised ISO method. Include effects from each part of system (pressure, temperature, flow element, density) Develope a maintenance program Ensure updated documentation Internal control Competence (incl. dedicated system responsible) Procedures Audit and review activities by NPD, partners and company internal

36 Specification and requirement areas from NORSOK / ISO standards Procurement of systems Functional, operational and test requirements Specific desing areas (lay-out, etc) Calibration of components 36

37 37 Typical activities from design to start-up Perform cost/benefit evaluation and propose simplification if effective Achieve approval among the different partners Describe and present concept to NPD via PUD application Maturing the metering concept by Engineering contractor / Company (FEED phase) Develope package specification and issue bid document, choose vendor Ensure compliance and quality of delivered equipment (follow-up activities, testing, doc. review, etc) Install, test on site and prepare system operation (procedures, training etc)

38 38

39 39

40 40 KVITEBJØRN GAS METERING 2 x millsm3/d ca 40 ton

41 41 KVITEBJØRN GAS METERING Instrument enclosure Ultrasonic meter FMC MPU 1200

42 42 Main challenges and future work areas develope compact and weight saving concepts reduce maintenance and calibration, remote monitoring (integrated operations), improved self diagnoses ensure robust concepts document traceability and uncertainty in the measurement systems gas quality determination develope simpified and effective subsea systems (wet gas metering)

43 43 Summary of covered areas Why and how to measure natural gas Regulating bodies and requirements Metering principles Regulations and standards Design and construction

44 44 References (Coriolis) (Norwegian Petroleum Directorate) (Transportsystems in North Sea) (NORSOK I-104) (norsk forening for olje og gassmåling) (interactiv course)