論文igu2009docswgcfinal00412

1、ELECTROMAGNETIC GROUND CONDUCTIVITY SURVEY TECHNIQUE IN PIPELINE INTEGRITYPablo Federico Cosentino 1, Sergio Nstor Rio 11. Transportadora de Gas del Sur S.A.Keywords: 1. Soil Resistivity; 2. External Corrosion Direct Assessment ECDA; 3. Stress Corrosion Cracking SCC.1IntroductionTransportadora de Ga

2、s del Sur (TGS) S. A. is the leading gas transportation company in Argentina. TGS operates the one of the longest and oldest pipeline systems in Latin America (8500 Km of pipeline,579.090 HP compression power, 74 MMm3/d of contracted capacity). It performs a variety of tasks within its integrity pla

3、n in order to operate its pipeline system at the highest level of reliability, optimizing human and material resources and reducing environmental, personal and business impacts.In order to mitigate the deterioration process of buried pipeline infrastructure, the pipeline industry has widely adopted

4、the use of protective coating systems and the application of cathodic protection (CP). As the pipeline infrastructure continues to age, it becomes a challenge to maintain both CP effectiveness and coating integrity.External Corrosion and Stress Corrosion Cracking (SCC) are two forms of pipeline dete

5、rioration that depend on CP performance, coating integrity and the environment in which the pipeline is buried. If CP or the coating system fails, external corrosion or SCC will probably occur at certain locations along the pipeline. Soil characteristics could be used to predict areas prone to exter

6、nal corrosion or SCC.The External Corrosion Direct Assessment (ECDA) is a structured process that is intended to improve safety by assessing and reducing the impact of external corrosion on pipeline integrity (3). Pipeline operators have traditionally managed several tools to control external corros

7、ion, the ECDA integrates all the external corrosion control technology under one coordinated program.TGS has adopted the ECDA process to asses the integrity of un-piggable pipelines. Design and construction data, operational history, corrosion control history and soil characteristics are used in the

8、 pre- assessment step to define the viability of applying ECDA and if is so, to define specific ECDA regions.TGS highlights:8500 km of natural gas pipelines.579.090 HP compression power. 74 MMm3 of contracted capacity. 8 pipeline maintenance facilities.24”, 30” and 36” diameter trunk lines. Average

9、age of the pipeline system 30 years. Coating type:o Asphalt: 5,640 kmo Tape: 1,230 kmo Three Layer Coating: 1,630 kmFigure 1 TGS transportation system and highlights12DevelopmentSoil characteristics could be used to predict areas prone to external corrosion and SCC. Detailed soil characterization is

10、 the most accurate way to identify those areas. Soil profile, chemical composition, geological classification surveys are conducted to gather the data, but it is an intensive task that requires time and resources.For example TGS - High-pH SCC model study consisted of the following tasks:Provision an

11、d digital processing of high-resolution satellite images. High resolution IKONOSimages (1 meter definition)tereoscopic format are acquired and processed in order toanalyse them in 3D. The processing included the following steps:o o ooGround control point measuring using GPS technology. Creation of a

12、 digital elevation model.Image orthorectification.Creation of a digital mosaic.Detailed scale analysis of researched areas. This analysis is based on IKONOS images, the digital land model and the slope map, combined with a detailed review of previous research (including soil research and geological

13、studies, etc.).Field work for the classification and detailed description of soils in areas under research. This research is carried out through test pits, well studies and natural cracks in the ground.A study of physical land characteristics is carried out, which includes:Relief and micro-relief Na

14、tural drainage Superficial drainageFloodingSurface floraSurface lithology Human influenceThe edaphic profile is also studied, including:Horizon description TextureStructure Permeability Effective depth Porosity StoninessRock outcrop proportionOrganic material content ColorSoil-pH reactionCalcium car

15、bonate content Internal drainageConsistencySoiltability and flood riskPhysical-chemical “itu” parameters are also studied, in relation to the following:oHydrogen potential (pH)Specific conductivityoAnalytical lab description. Included mineral composition and crystalline substance description of rese

16、arched soils/sediments, as well as chemical-analytical description in the aqueous phase.In order to optimise this work, and extend the knowledge of the soil environment along the pipeline system right of way (ROW) in the first phase of the data gathering process, several variables were identified an

17、d studied with the aim to assess the variable significance to be used later as a filter to pin-point smaller areas for further studies.TGS identified the soil parameters that can be related to deterioration mechanisms, studied the influence in the degradation process and collected information about

18、the measuring tools available to gather the data in the field.2Soil resistivity is the capacity to conduct electric current, a parameter that is widely used for soil corrosivity characterization, it is easy to measure with conventional methods and it can be used to filter segments of pipeline right

19、of way (ROW) for further data collection.Soil resistivity is a variable that can be related to High pH SCC and the external corrosion mechanism and plays a very important role:CP current distribution depends on soil resistivity variation along the ROW and coating faults location.The TGS SCC predicti

20、ve model seeks for low resistivity areas, no more than 3 km apart from the Impressed Current Cathodic Protection Units (ICCPU) with high current output during long periods of time:Higher CP current density at coating faults leads to cathodic disbondment.High pH microenvironment formation and shieldi

21、ng of CP current at coating faults.ooDue to the high conductivity values of low resistivity soils, the electrolytic current in the soil produces higher corrosion rates in the exposed metal surface at coating faults.Rapid changesoil resistivity can be related to changes in the soil composition thatpr

22、oduce corrosion cells due to differential aeration in the surrounding soil along the pipeline ROW (clay sand interface).ECDA process seeks multiple indications gathered with indirectpection methods, tolocate coating faults with anufficient cathodic protection level in a corrosive environment.The mat

23、hematical definition of resistivity can be interpreted as the electrical resistance between opposite faces of a unit cube of material; the reciprocal of conductivity. Resistivity is used in preference to conductivity as an expression of the electrical character of soils (and waters) since it is expr

24、essed in whole numbers. When a metallic structure is immersed in a conductive medium, the ability of the medium to carry current will influence the magnitude of galvanic currents and cathodic protection currents.ALResistivity r = R Ohm x cmVIElectrical resistance R =Ohm1rSConductivity s = cm Figure

25、2. Mathematical relationship between resistivity and conductivityResistivity of the soil is a function of soil matrix and the interstitial fluid (moisture) that can conduct electric current. Since interstitial fluids have much higher conductivity than the mineral phase by itself, their presence and

26、the degree of mineralization can significantly reduce resistivity of a soil. Thus, typical soil resistivity measurements represent a composite measure of the moisture content of a soil and dissolved electrolytes in the soil water. Clay minerals, due to the particularities of their crystal structure,

27、 have a relatively high conductivity; so their presence will reduce the resistivity of a sample.Table 1 describes the relationship between soil resistivity and degree of corrosivity defined on a relative scale.3Resistivity (ohm-cm)Degree of Corrosivity 10,000Progressively Less CorrosiveSmall electro

28、des are buried in four small holes in the earth, all at depth b and spaced (in a straight line) at intervals a. A test current I is passed between the two outer electrodes and the potential V between the two inner electrodes is measured with a potentiometer or high-impedance voltmeter. The current i

29、s generated with a AC source to avoid the error due to the electrode polarization with direct current DC.The relationship between the soil resistivity of the sample, the electrical parameters and dimensions of the pin-arrangement is shown in the equation (2). 4pRar =2a2a1 +R = VI-a 2 + 4b 2a 2+ b2(2

30、)Figure 4. 4 pin-method connections. Where:R : calculated resistance . a: electrode separation cm. b: electrode depth cm.r: soil resistivity cm.If b 0.1 a, we can assume b = 0The equation (2) becomes r = 2pRa NovaprobeThistrument uses a probe that is providedwith a measuring sensor in thetipcapable

31、ofmeasuring the resistivity and pH.This kind oftruments is calibrated using solutions of known resistivity and thetrument directlydisplays the recorded value of soil resistivity in the surroundings of the probe tip.Figure 5. Soil probe measuring the resistivity over the pipeline ROW. Electromagnetic

32、 ground conductivityThe electromagnetic ground conductivity (EMGC) survey technique has been used for decades in geological applications. The application of this method to measure the resistivity along the ROW of pipelines has grown in the last decade. The method uses a transmitter coil Tx to produc

33、e a magnetic field, the5magnetic field induces an eddy current in the soil, this electrical current generates a magnetic field that can be measured by a receiver coil Rx (Figure 6).Figure 6. EM ground conductivity principleThe quadrature (out of-phase) component of the secondary magnetic field from

34、these loops is linearly related to subsurface conductivity in units of millisiemens/meter, this is valid under certain constrathat are incorporated into the design of the EMtruments.According to equation 3 the relationship between the intensity of the magnetic fields is lineal and the ratio depends

35、on the soil conductivity when the inter-coil spacing and the frequency has been set for the exploration at a depth of interest.Hi m w s s 2s 0p(3)4HWhere:Hs : secondary magnetic field at the receiver coil. Hp: primary magnetic field at the receiver coil.s: soil conductivity. : 2pff: frequency.s: int

36、ercoil spacing.0: permeability of free space.The conductivity is the mathematical inverse of electrical resistivity as the equation 4 shows:1ro =(4)s: soil conductivity. : soil resistivity.The linearity is valid in the range of 100 Ohm x cm to 1.000.000 Ohm x cm, at lower values of resistivity the r

37、elationship is not longer valid as it is shown in figure 7.The in-phase component can be used to detect both ferrous and nonferrous metal. These two measurements provide a means of assessing changes in geology and the possible presence of buried pipelines or other utilities that may influence resist

38、ivity measurements.These surveys are non-invasive, above ground, electronic surveys, which measure the ability of soil to conduct electrical currents. These surveys have an advantage over the Wenner 4 Pin method o far as data is electronically collected and recorded continuously, rather than at a se

39、ries of discrete points.Thetruments typically survey to depths of 3.0 meters and approximately 1800 data points can becollected per hour. The data logger records the conductivity readings in units of millisiemens/meter (mSm).6Figure 7. Plot of indicated conductivity versus true conductivity.Comparat

40、ive of Measuring MethodsIn the following table we compare the advantages and limitations of each method:7Soil resistivity measurement methodologyAdvantagesand limitations.Soil Box4-pin (Wenner)NovaprobeElectromagnetic (EM)Electricalcontact problems.None.Very sensitive to pin-contact issues in highly

41、 resistive soiltop layer.Depend on moisture content.None.AC Interference.None.Depends on trument AC rejection.Depends on trument AC rejection.Sensible in low-end performancetruments.Explorationat different depths.Soil sample of each layer at different depths.Excellent.GoodFixed depth in truments wit

42、hfixed frequency and intercoil spacing.Dailyaverage survey production.Depends on sampling distance.Depends on sampling distance.Depends on sampling distance.10 km with 2 meter sampling distance.Soil resistivity profile resolution.High resolution cost prohibitive due to exploration wells.High resolut

43、ion increases the cost dramatically.High resolution increases the cost dramatically.Excellent.Surveyinrocky soils.Soil sample must be prepared,itu measurements can be difficult to obtain.Depend on the rocky content of the top layer.Not possible.Contact-free measurement not affected by rocky soils.Pe

44、rsonneltraining and qualificationTechnician with general knowledge.Technicianwith general knowledgeTechnician with general knowledgeTechnician with general knowledgeGPS coupling and Data integration with GISNotruments.Notruments.Measured value not coupled with GPS coordinates.Measured value and GPS

45、coordinates data logging function.trument cost1,000 U$1,000 U$2,000 U$20,000 U$ or more depends ontrument complexityThe methods used to gather resistivity data were compared, the advantages and disadvantages of each method were pointed out. The results of this comparative chart show that EM technolo

46、gy could be the most suitable methodology to survey long segments of ROW, economically, gathering one resistivity value at regular intervals at walking pace.This tool was selected and tested to ensure that the resistivity data gathered with this method was accurate. The preliminary comparisons with

47、the available data of four pin method resistivity values was successful -see figure 8-. Therefore, field survey was planned and executed.Figure 8. EM resistivity values compared with the four pin method.4EMGC Survey Methodology Field WorkThe survey crew consisted in three technicians with a DGPS uni

48、t with submeter accuracy and a EMequipment capable of gathering soil conductivity data up to three meters depth. The crew surveyed up to 15 kilometers per day with a daily average of 10 km Figure 9.The equipment experienced AC rejection problems in ROWs shared with overhead high voltage power lines,

49、 the measuring methodology used in this case was the 4 pin method.The position of pipeline markers and pipeline features must be recorded on the data logs to make itpossible to match results from the conductivity/resistivity surveys with data from other indirect methodologies, using the axial positi

50、on as the common denominator.Data post processingpectionThe collected data was checked for error daily and then sent to the GIS specialist for further processing.Post processing included converting the conductivity values into resistivity values, data validation with resistivity values of prior field works, projecting the data in the correct datum, and control of GPS locations with the existing data layers of facilities, over the line surveys, etc.Figure 9. EM conductivity survey crew in Patagonia desert.8The ECDA implementation uses the E