C.P. data, CIPS & DCVG techniques: another way to predict Corrosion on Gas Pipeline.

Transcription

1 23 rd World GAS Conference, Amsterdam 26 C.P. data, CIPS & DCVG techniques: another way to predict Corrosion on Gas Pipeline. A.Taberkokt ALGERIA 1

2 ABSTRACT One of the most important care of gas operators is how to prevent the presence of corrosion points on their metallic gas pipelines, and therefore repair them before they reach a critical threshold that could lead to an incident. This prevention can be assured by inspecting the pipe wall thickness with an intelligent pig tool. Indeed the evolution of the technology, allows the development of high resolution tools which locate corrosion points ( if they exist ) and restore them in three axial axis dimensions. The gas operators can then identify the critical corrosion points and undertake the appropriate actions to ensure a safe operation of its gas pipeline. Solely, this alternative of inspection, is on one hand costly and on another hand, depends on the technical characteristics of the pipe (diameter, thickness, bends...), without taking into account the time parameter from the call of tender to the data acquisition. In addition to that, the gas operator has to prepare the required conditions before and during the pipe inspection (gas equipments to be replaced, relive of ballast industrial customers in certain cases due to technical reasons, repeating the inspection if some troubles appear in the acquisition data recorder...). Therefore, the gas operators can undertake another alternative which consists of using electrical field survey methods: ON/OFF CIPS combined with DCVG measures over the buried gas pipeline. For which, he has to add historical data of cathodic protection parameters, such as soil resistivities, potential surveys, Off rectifiers time and any other parameters which favor the instability of the steel pipe wall thickness. The analysis of the obtained results leads in preventing the existence of the corrosion points. This alternative is undertaken whatever the gas pipeline characteristics are. The present work shows us an experience done on a part of section of the gas pipeline 2 diameter and 77 Km length gas pipeline (from Relizane town to Sig ) using the two techniques : Inspection by intelligent pig tool and electrical field survey methods. We focus our interest over the first ( 1 ) ten kilometers, cause it s first the section in which our previous study showed us to be the most probable affected part of the gas pipeline, and second because it s the section which supports the high pressure levels (existence of the compressor station ). The results of the both techniques were given; time and costs were evaluated showing the positive and negative sides of the two alternatives. Finally conclusion and recommendations were done for gas operators, who are interested to inspire with this experiment. 2

3 TABLE OF CONTENTS 1. Abstract 2. Introduction 3. Technical characteristics of the gas pipeline 4. Corrosion prediction using CP data and electrical fields surveys. 5. Inspection of the gas pipeline by intelligent tool. 6. Analysis and comparison of the MFL tool and On line surveys results 7. Conclusion. 8. References 9. Annexes 3

4 1. INTRODUCTION On 1977 the Algerian Gas & Electricity company SONELGAZ inherited the 2 Relizane Maghnia high pressure gas pipeline, called 2 Ouest. This pipe made off three sections coated with bitumen product, was strategic for the north west of Algeria, since it permitted the supply of natural gas to the towns situated in the neighboring of gas pipeline. Unfortunately, our company received the gas pipeline in very bad conditions, which required a rehabilitation programs just after putting gas into the pipeline. From these years a particular care was and is still given to the 2 Ouest, by establishing special programs such as inspections by intelligent pig and rehabilitation, to satisfy the required rules of supplying natural gas in a safety condition. From 1979 to 1995 our company has done: - The change of 3 % of the total gas pipelines length, - Reparation of all the severe corrosion points, - Reparation of thousands defects coating. On 1996, because of the possibility to feed a new customers, a power plant situated at 7 Km far the end of the network ( after the extension of the pipeline ), we prepared a rehabilitation study which undertook actions to short, medium and long term. Among these actions, we proposed to change the first 44 Km of the Relizane - Sig section, basing on cathodic protection parameters history, coating condition. In addition, it s the part of the gas pipeline which will support the highest pressures level. The technical committee of our company didn t accepted the proposal of the renovation, in fact, they agreed to undertake this action, only after inspecting the gas pipeline by the intelligent pig tool. So we took the opportunity to investigate, over the first 1 Km of the gas pipeline s first section* by using electrical field measurement methods ( DCVG & CIPS ),combined with the historical CP data parameters to predict where corrosion could take place and compare it with the results of the inspection by intelligent pig. This report throws investigation results, pictures of our undertaken actions, in which time and costs were evaluated showing the positive and negative sides of the two alternatives, a conclusion and recommendations are done in such case, someone could inspire with this experiment. * : the section is the one which support the high pressure levels 2 - TECHNICAL CHARACTERISTICS OF THE GAS PIPELINE: The main characteristics of the gas pipeline were as follow: - Diameter: Ø 2 - Total length: 273,524 Km ( 265 Km initially ) - Thickness: mm - Steel Grade: X 52 - Type of the coating: Km coal tar enamel (new parts) and the rest is petroleum bitumen. 4

5 3 - CORROSION PREDICTION USING CP DATA AND ELECTRICAL FIELD SURVEYS: 3 1 Choice of the area s pipe to investigate Starting from the acceptable basis that, containing external corrosion of buried metallic structures is possible only if two protection techniques are used. The first one is to assure the separation of the structure from the soil by applying an adequate coating. The second is applying cathodic protection ( CP ) to the structure, cause there in no coating which could insulate the structure from the soil at 1 % ( without taking into account that coating defects appear in time ). Our work was then, to delineate the coating defects on the buried pipeline. As we ve seen in the introduction, we focus our work over the Relizane Sig section. After analyzing the historical CP s data, three areas of the gas pipeline knew a level of potentials (to the copper copper sulfate half cell reference Cu/CuSO 4 ), below the criterion protection.(see Fig. 1 ). Three regions presented the same conditions to undertake investigations, we chose the first one cause as we said, it s the part of the gas pipeline which supports the high pressure levels. Potential [ - mv ] Threshold of protection On Potential 95 On Potential 97 On Potential 99 2'' Relizane-Sig On Potential Soil resistivity Resistivity [ Ω.m ] P.K. [ Km ] Area N 1 Area N 2 Area N 3 : New parts of the pipe GRD RELIZANE P.K., Line breaks GRA SIG Fig.1 Synoptic plan of Relizane Sig section and its potential plots P.K. : Kilometer Point. 5

6 3-2 Coating defects research: Principle of the survey methods: Tow survey methods were used simultaneously, the measurement of the lateral voltage drop using two copper copper sulfate half cell ( DCVG ) and the close order potential survey using only one half cell ( CIPS ) ( see Fig.2 ). The measurements were done in ON-OFF ( by connecting a time contactor to the rectifier ), of course, prior to undertake these measurements the pipe s location was done. The measurements were taken each 5 meters ( when necessary it s reduced to locate with accuracy the coating defect s epicenter). When an important defect is located it s marked on a map (experience showed that stakes could easily be stolen). At this time of these investigations we didn t get a GPS equipment, which could gave a big hand to our work. CIPS measurement Test point Half cell Cu/CuSO 4 mv Ω ρ soil resistivity measurement DCVG measurement Pipe detection Fig. 2 Illustration of the combined methods CIPS & DCVG techniques Interpretation of the results: Thousands of data were gathered, to interpret easily the results we plotted (versus the distance covered): - the On Off potential of the pipeline ( P On and P Off ) - the gradient voltage difference VOn VOff ( V ) or IR % (1x V/ P ). It s useful to remind that, when a direct current is applied to a pipeline, a voltage gradient is established on the ground, due to the passage of the current through the soil to the bare steel exposed at the defect. Generally, the larger defect is, the greater the current flow and hence larger voltage gradient. In the same manner we can use the % IR that represents the percentage of the applied potential ( P) which is lost on the fault position ( V), and generally the IR % value increases with the increasing defect size/current requirement. 6

7 Before giving the result of the coating survey it s useful to define the categories of the coating defects: - 1 st category means IR % greater or equal to 1 and less than rd category means IR % greater or equal to nd category is between 1 st and 3 rd ones. The following table synthesizes the surveys results by pipe s sections: Pipe sections Number of defects coating 3 rd category 2 nd category 1 st category P.K.2 TO P.K P.K 1.45 TO P.K P.K 4.4 TO P.K P.K 5.8 TO P.K P.K 6.9 TO P.K P.K 8. TO P.K The first attractive result is that the change of the sections during the 1 st programme of gas pipeline s rehabilitation presents a few number of defects. Once detecting the localisation coating defect, we start analysing them from the point of view of cathodic protection level ( see annexe I), and the soil aggressivity ( for the soil classification see table N 2 ), we can therefore predict the presence of the corrosion at the defect coating. The following table shows an example of how the gathered data are arranged to make easier the analysis: 3 rd category IR % greater or equal to 5 Pipe sections Defects Distance Cathodic Prot. Soil Resistivity Nbr distribution [ Km ] Level [ mv ] Ω.m to P.K 8. TO P.K to to to to to Of course, it s important to recall that we need the historical CP data. For this purpose, we took the last 8 years of the quarterly* ON potential surveys and the monthly* rectifier s measurement reports, in addition to the ON-OFF potential surveys done on and 99 ( see Fig.1 for the years 95,97 and 99 ). Here, it s important to mention that a BANK of PC data is vital for of any existing pipeline. As an example of that, the part of the pipeline situated between P.K 2 and 6 was found protected when we did the surveys, but it was the one which was the most affected: lack of protection for the years 1994 and 1995 (we have reinforced the protection system by a new rectifier at P.K. 6 ) and subjugated to an aggressive soil. * : measurements are done by our regional teams. 7

8 All the 3 rd and 2 nd categories coating defects were well analysed, and we have predicted the possibility of the corrosion presence (with the ration of one corrosion point per one coating defect in which the corrosion process is likely to occur: we don t have tools to predict more than one). The corrosion prediction is given by section of the pipe as shown in the following table: Pipe section Number of the Corrosion points predicted P.K.2 TO P.K 1.45 P.K 1.45 TO P.K 3.3 P.K 4.4 TO P.K P.K 5.8 TO P.K P.K 6.9 TO P.K P.K 8. TO P.K The results given in the above table were obtained, after analysing point per point of the second and third categories coating defects. As an example, the following table shows how the upper table is obtained. Pipe sections Corrosion prediction Distance Soil Resistivity Nbr distribution [ Km ] Ω.m 9 8 to to to P.K 8. to P.K to to to Cost of the survey operation: The DCVG and CIPS surveys were done by our own company team. 4 people (among them two experts), undertook the investigations which spent 8 days ( in site ). The total cost of the operation is 4 K$. 4 INSPECTION OF THE GAS PIPELINE BY THE INTELLIGENT TOOL. 4-1 Principle of the inspection The pipe inspection was done by the Magnetic flux leakage process, using a high resolution equipment with the range of accuracy for depth of defect up to ± 1%. The corrosion points were given in three axial axis dimensions. Many information were given concerning the corrosion point or the pipeline its self. ( see annexe). Fig. 2 shows how corrosion point is visualized in 3 D, by "zoom plot". The Red zones designate corrosion defects. Fig.2 Corrosion points 4-2 Time evaluation: from the call of tender to final results More than one year from the tender of call to the reception of the inspection results ( the time include the tender launching, analysing the technical offers, asking the tenderer for details and additional information, commercial offer, preparation of the conditions to permit the inspection of the gas pipeline.). 8

9 4-3 Inspection results The following table gives the corrosion number per sections of the pipeline. Pipe section Number of the Corrosion points P.K.2 TO P.K P.K 1.45 TO P.K P.K 3.3 TO P.K 4.4 TO P.K P.K 5.8 TO P.K P.K 6.9 TO P.K P.K 8. TO P.K The following graph gives the distribution of the corrosion points for the 1 first kilometres : thickness loss versus distance: 9 8 % T h ic k n e s s lo ss Corrosion Points Distance [ Km ] Fig.3. Corrosion distribution: % thickness loss versus distance 4-4 Cost of the inspection The total cost of the inspection was K$ 9

10 5 - ANALYSIS AND COMPARISON OF THE MFL TOOL AND ON LINE SURVEYS RESULTS : The following table synthesizes the corrosion points obtained by MFL tool inspection with the ones predicted by on line surveys methods: Pipe section Number of the Corrosion points Inspection s results Predicted by online surveys N 1 P.K.2 TO P.K N 2 P.K 1.45 TO P.K N 3 P.K. 3.3 TO N 4 P.K 4.4 TO P.K N 5 P.K 5.8 TO P.K N 6 P.K 6.9 TO P.K N 7 P.K 8. TO P.K At this step, if we had a GPS equipment the comparison of the two results would be very easy and accurate, because the measure of distance covered by the technician could never be equal to the one done by the intelligent pig. The P.K of the pipe s special points and the test points helped us in some cases to correct our results. As the work we ve done for the corrosion prediction, we compare point per point the results obtained and listed in the above table. As an example, the following table gives the way, of how we did the comparison for the pipe s section N 7 : Pipe sections Corrosion points Distance Nbr distribution Observations MFL Surveys MFL Surveys [ Km ] 9 8 to 8.13 Corros. at PK 7.8 and to P.K 8. to P.K to to to 8.98 Difference of 48 m to 9.5 Difference of 28m Difference of 19m If we neglect the number of the corrosion points, the first attractive result is, our prediction failed for the first sections N 1 and 2 (we predicted zero co rrosion point). In fact for the first sections, 15 corrosion points over the 17 (found), are located where we had found a coating defect of second category. We also missed to predict corrosion on these parts of the pipe: - P.K 3.3 to 4.4 : 48 corrosion points. 4 points with respectively metal loss of 25, 25, 22, 17 and 75 at the section P.K The rest are all less than 17 %. - P.K to 9.84: 24 corrosion points with metal loss, less or equal to 13% of the thickness and 2 of respectively 22 and 17 %. For the rest of our predictions, results showed that if we hadn t got approximately the same P.K. of the corrosion locations, we re at least far from of 5 to 7 meters. 1

11 Just after getting the MFL tool results, our company started the reparation of the most severe corrosion points located at P.K , this operation confirmed us that our result : coating defect location and the corrosion prediction. The graphs given in the annexe show the distribution of the corrosion and coating defect versus the distance of the pipe, and give an overview what we ve just explained. (they are better to evaluate rapidly the obtained results ) 6 CONCLUSION. There is no doubt that the best way to detect corrosion in pipeline is the inspection by intelligent tool, because the gas operator will know with accuracy the real condition of the pipe wall thickness and hence ensure a safe operation of its gas pipeline. On other hand if the gas operator has a non-piggable pipelines or he wants to save money the on line surveys ( GVDC CIPS techniques ) gives good results, providing, to get first of all, a large BANK of PC data, and an experiment technicians. 7 - REFERENCES 1. DR. J.M. Leeds A critical review of the techniques used to delineate pipeline coating defects as a precursor to refurbishment. 2. DR. J.M. Leeds The DC voltage gradient method for accurate delineation of coating defects on buried pipelines UK Corrosion W.G.Von Baeckmane (1973) Measurement and importance of the Off potential in applying cathodic protection to pipeline 12 th WGC NICE W.G.Von Baeckmane Taschenbuch fur den kathodischen korrsionsschutz. 5. C.Corradetti, E.Bini and D.Gentile Results of IN SITU coating integrity surveys as routine control on new gas lines impact on gas line quality, design aspects and organization of the work. 11

12 8 - ANNEXES 12

13 3 25 Potential On Off Sample N 1 Potential ON Potential Off Threshold protection Potential [ mv ] ,2,3,4,5,6,7,8,9 1 1,1 1,2 1,3 1,4 Distance [ Km ] IR % IR % IR % 2 1,2,3,4,5,6,7,8,9 1 1,1 1,2 1,3 1, Distance [ Km ] Corrosion points 3 % Corrosion depth Corrosion points,2,3,4,5,6,7,8,9 1 1,1 1,2 1,3 1,4 distance [ Km ] 13

14 Potential On-Off Sample N 2 Potential [ -mv ] Potential On Potential Off Threshold protection 4,425 4,525 4,625 4,725 4,825 4,925 5,25 5,125 5,225 distance [ Km ] 12 1 VOn-VOff VOn-VOff Voltage Gradient [ - mv ] , ,525 4,625 4,725 4,825 4,925 5,25 5,125 5, Distance [ Km ] % Corrosion depth Corrosion points Corrosion points 4,425 4,525 4,625 4,725 4,825 4,925 5,25 5,125 5,225 distance [ Km ] 14

15 Potentila On-Off Sample N Potential On Potentiel Off Threshold protection 1 Potential [ -mv ] ,2 8,4 8,6 8,8 9 9,2 Distance [ Km ] IR % 25 2 IR % 15 IR % ,2 8,4 8,6 8,8 9 9,2-5 Distance [ Km ] Corrosion point % Corrosion depth Corrosion point 8, 8,2 8,4 8,6 8,8 9, 9,2 Distance [ Km ] 15

16 Contrôle De Canalisation Algeria 2" VECTRA MFL Inspection ( 157 ) Sonelgaz_Relazine to Sig (76.883km) Informations D'Ellipsoïde: WGS84 Date of Inspection: 25 September 1999 Méridien Central : 3 E Nombre De Zone: 31 N BJ Process and Pipeline Services Ltd. BJ Chainag e (m) Position Horloge Épais. Paroi (mm) Long. (mm) Larg. (mm) Prof. Max (%) Loc. 4, : EXT. 4, : EXT. 4, : EXT. 4, : EXT. 4, : EXT. 4, : EXT. 4, : EXT. 4, : EXT. 8, : EXT. 8, : EXT. 8, : EXT. 8, : EXT. 8, : EXT. 8, : EXT. Longitude / Latitude (DMS) N: ' " E: 32 ' " N: ' " E: 32 ' " N: ' " E: 32 ' " N: ' " E: 32 ' " N: ' " E: 32 ' " N: ' " E: 32 ' " N: ' " E: 32 ' " N: ' " E: 32 ' " N: ' 6.74 " E: 29 ' " N: ' 6.74 " E: 29 ' " N: ' 6.74 " E: 29 ' 2.18 " N: ' 6.74 " E: 29 ' 2.18 " N: ' 6.74 " E: 29 ' " N: ' 6.74 " E: 29 ' " Sample of a part of the inspection s results 16