Failure Analysis of RTP for Natural Gas Transportation in Changqing Oilfield

Failure Analysis of RTP for Natural Gas Transportation in Changqing Oilfield Xuehua Cai 1, Dongtao Qi2, Nan Ding3, Jiajia Fu4, Milin Yan51 Engineer, Research Institute of Tubular Goods of CNPC, NO.32 Dianzier Road, Xi’an Shannxi, 710065, caixh@2 NO.32 Dianzier Road , Xi'an, Shannxi, CHN3 NO.32 Dianzier Road , Xi'an, Shannxi, CHN4 NO.32 Dianzier Road , Xi'an, Shannxi, CHN5 NO.32 Dianzier Road , Xi'an, Shannxi, CHNABSTRACTReinforced thermoplastic pipes (RTPs) are being increasing used in China National Petroleum Corporation(CNPC), owing to their good corrosion resistance and easily installation technique. Several thousand kilometers of RTPs has been deployed in natural gas transporting system in Changqing Oilfield since 2007. After one year serving of PET fiber reinforced polyethylene pipe, several failures happened. Therefore, it is necessary to study the effects of environmental factors, including temperature and installation damages, on the strength of PET fiber and RTPs. The primary failure modes of RTP pipe are analyzed and the residual strength tests of RTP pipes, which are cut from the pipes near the failure spot, are carried out. The result shows that the strength of RTP pipe decreases with increase of temperature from 0℃ to 65℃. The damages of the protective layer (the outer layer) have little influence on the strength of pipe, while those damages of reinforced fiber layer have much influence. Finally, some suggestions are given for future application of RTPs in Oilfield(Jerry,2000).INTRODUCTIONReinforced thermoplastic pipes(RTPs) is a kind of product with flexible, light weight, low thermal conductivity, good corrosion resistance, long-length (thousands of feet) and easily installation technique, which make it much different to traditional steel pipes used in oil and gas transportation. The concept for designing RTPs first came out in late 1960’s, the process including continuous manufacture of a tubular product with multilayer structure as a liner inside and wrapped with fiber as reinforced layer and one outside protecting layer. Moreover, besides various advantages of RTPs to steel pipe, the composite pipe can be designed as required which is particularly technique advantage to traditional pipes, it was 1980’s when the newly designed pipes turned to be feasibility for application. From 2000, RTPs began widespread application to oil and gas field in CNPC. Strong corrosion resistant behavior both to the flowing oil or gas and the outside environment, fewer connections and fittings, and ease of installations made RTPs popular in oil and gas field for flow line application.Reinforce thermoplastic pipes first deployed in Changqing oilfield for gas transporting system since 2007 and rapidly developed to several thousand kilometers. During one year serving of PET fiber reinforced pipe for natural gas transportation in the oilfield, the gas flow lines had failures frequently, which made the gas producer disturbed and resulted to reduce the productivity of gas(William et al., 2001). Therefore, few study on fiber reinforce thermoplasticpipes had operated, less knowledge about the failure modes of the newly special pipes wasavailable. It’s a necessity to analyze the factors which effect the pipe’s serving ability.Figure 1. sketch of structure about RTP and example application in sandhills(from indipipe 2006 conference & exhibition. indonesis, 2006: 1-11)FAILURE ANALYSISRtp pipe description. As pipe producer providing, the information about the pipes used inChangqing gas field as followed: PET fiber reinforced thermoplastic pipe used polyethylene pipe as inner pipe, PET fiber as the reinforced layer wrapped outside the liner pipe, and the outerlayer used polyethylene for environmental protect. It was 100m long of single with outerdimension of 68mm, 9mm thickness(protective layer for 1mm, reinforced layer for 3mm and5mm for inner pipe) and normal inner dimension was 50mm. Best service temperature should be from -50℃ to 80℃, the yield work strength was 6.4MPa.Short term burst test of normal pipes. The short term burst strength of RTPs was conducted to determine the actual response of pipe specimens according to different environmentaltemperatures to simulate the actual condition of serving pipes in Changqing Oilfield.After analyzing of the results from short term burst test of RTPs at four temperature conditions, the values of the four different picked environmental temperatures showed the strength of RTPs dropping followed the increasing of atmosphere temperature levels. Moreover, it’s obviousphenomenon that low temperature of environment influent pipe’s quality little to which at room temperature, however, the high temperature working condition made the strength of pipedropped dramatically.Table 1. Short Term Burst Test of RtpsOuter protective layer liner Fiber Reinforced layerEnvironmental Temperature 0℃ 25℃ 65℃Value of Short term burst pressure 29 MPa 27 MPa 17 MPaFiber tensile strength test. As the results received from short term burst test of pipes, the environmental temperature had significant influence on the quality of pipe, which indicated that it was necessary to discover the performance of fiber in the specified environment. Four environmental temperatures chose to simulate the actual condition fibers faced. The samples were conducted in the same procedure except the temperature surrounding them. The results achieved from the tensile strength test of PET fiber correlated with burst strength of pipes for varied conditions. The figures showed a decreasing trend. The quality of PET fiber decreased obviously at high temperature(65℃) while fibers had good character at -20℃, 0℃ and 25℃. Table 2. Short Term Burst Test of Rtps Scratch Failure SimulationScratch styles Outerlayerscratch Little fiberbreakageMuchfiberbreakageLiner pipedefectValue of Short term burstpressure29 MPa 12.5 MPa 1.5 MPa 28 MPa SIMULATION OF FAILURE MODES ABOUT PIPESDuring pipes operating at fieldwork, scratches happened sometimes by worker or by environment. The project simulated several scratched failure modes to achieve information about how scratches affected and learned which breakage style should pay attention to. Firstly, scratch in outer protective layer was common existing example when tools knocked the pipes during installation of careless mistake or rubbing pipes on rough wall of digging channel. Secondly, the experiments simulated deeper cuts caused by stiff tools occasionally, which broke fibers through the outer layer to different extents of breakages. Thirdly, the behavior of liner pipe with defects occurred by flowing media’s brushing pipe or the defects made by liner production process as for pulling section, if extruding not controlled properly minor pores might hidden in the product. All the three possibilities of failure modes carried on short term burst tests to discuss what kind of scratches would result to pipe failure(Tom et al., 2002).The experiments indicated that outer protective layer had less contribution to pressure ability of RTPs. Moreover, small defects in liner pipe would not make obvious changes on pressure ability of RTPs. PET fibers as the reinforced layer mastered RTPs performance toughly, more fibers cut resulted to less pressure ability remained of product.Figure 2. Simulation of scratch on RTPTable 3 – Tensile Strength Test of Pet FibersEnvironmental Temperature -20℃0℃ 25℃ 65℃81 N 80 N 72 N 59 NValue of tensile strength test ofPET fiberFAILURE SAMPLE FROM FIELDWORKFailure sample from field work. There were three gas leak accident happened in one week at March recently. All of them were served by PET fiber reinforced thermoplastic pipes as natural gas transportation line, serving period of them during 1 to 2 years. The accidents were quite similar, several bursts along the pipelines up to several kilometers long and about 50mm long ruptures appeared.It was necessary to know whether the pressure ability had declined already of leaking pipes. Samples picked out from the part near to ruptures of leak pipes. The outer protective layer had some scratches on, they were not smooth. The pipes bended some places, the pipes did not keep natural status. The result of short term burst test of the samples conducted at room condition was 20MPa. The value was more than 3 times security modulus. Experiments showed the value of pressure did not declined much, the pipe remained the pressure quality. The accident pipe was in good condition from pressure test judging.From the research on fiber character, low environmental temperature influent PET fibers weekly. And low temperature had little changes on RTPs performance. The burst value achieved from leak pipe samples showed the fibers in RTPs remain well and the fiber breakages hypothesis could be abandoned. The analysis of simulating scratches on pipes helped to conclude that some small cuts on outer layer would not result to RTPs failure. Ignoring the small mistake on outer layer during installation for the gas leak accidents in Changqing oilfield was acceptable.The accidents pipes had curled and bending for some extent at the gas field during transportation service. Bending easily occurred stress gathering at the bend point when pressure applied. The bending part was the weakness to the pipe. The phenomena gave us reflection that curled and bending appeared in installation might the answer for the failure accidents.Figure 3. failure appearance at accident gas field locationThe gas field is a place full of sand hills and bottom lands. The lowest temperature in winter reaches to -29℃ and highest temperature in summer is 37℃. The temperature variation is in the carrying capacity of PET fiber reinforced thermoplastic pipes. Though digging channels for RTPs, the lowest environmental condition could reach -15℃. RTPs lied in digging channels and went along sand hills or bottom lands. For some lower spots, some liquid flowing with gas would gather at the spots which might froze at critical winter and make the spots as weak point. The frozen liquid could block the normal transportation work and result to pipe’s failure.CONCLUSIONSReinforced thermoplastic pipes are increasing application in China recently for their good corrosion resistance and easily installation method. Oil and gas industries are eager to seeking failures solution after the application. From the project on failure analysis of RTP for natural gas transportation in Changqing oilfield, it concludes that environmental temperature and installation damage must be considered on RTPs application. During research, the pressure performance of PET reinforced thermoplastic pipe decreases with increasing environmental temperature from 0℃ to 65℃. Furthermore, deflects in outer protective layer and liner pipe do little change for pipe’s strength quality while fiber breakages of installation damages drop down pipe’s behavior dramatically.SUGGESTIONSPET fiber appears weak at relative high temperature, serving limit must be paid attention to. Unlike traditional steel pipe, installation should operate at careful method. We suggest that the producer carry on projects for training specified worker to RTPs installation in order to reduce failures made by human carelessness.REFERENCES" Reinforced Thermoplastic Pipes. " R.DET NORSKE VERITAS Technical Report, NO.2001-1078.Jerry G. Williams (2000) ."Composite Spoolable Pipe Development, Advancements, and Limitations." J. offshore technology conference , NO 12029, 327-342.Tom Zimmerman, Gary Stephen, Alan Glover (2002). "Composite Reinforced Line Pipe(CRLP) for Onshore Gas Pipelines." C.International Pipeline Conference , 467-473.Willam D. Stringfellow, Tomas Walsh, Harris Reynoldsl (2001). "Application of Spoolable Composite Pipe to Oil & Gas Industry Flowlines." J. Southwest Petroleum Short Course, 239-245.。