Keywords

1 Introduction

Oil and gas transmission stations are important links in the process of oil and gas transmission, playing the roles of pressurization, receiving, processing, truncation, and metering. [1, 2] Different from long-distance pipelines and gathering pipelines, there are many pipe fittings and equipment in the station and valve chamber, large number of flammable and explosive substances are stored in the station, most of the facilities operate in high-pressure environment. Due to the operation of equipment in some stations, there are adverse factors affecting the service life of station facilities such as vibration. Once the station facilities fail, it may make a great impact on the safety of personnel and pipelines [3,4,5,6].

The author collected 25 station facility failure cases in China in recent years, and statistically analyzed the types and causes of failure facilities. The failure facilities in the station involve pipe, circumferential weld, longitudinal weld, tee, flange, bolt, elbow and valve.

The failure causes include fatigue failure, corrosion failure, failure caused by poor on-site welding quality, failure caused by defects in the manufacturing of pipe fittings and facilities, and failure caused by overload, fire and irresistible external force.

According to the classification of failure facilities, there are 1 failure of safety valve, 3 failures of flange, 5 failures of pipe body, 7 failures of circumferential weld, 1 failures of casing, 2 failures of fillet weld, 1 failure of bolt, 2 failures of tee, 2 failures of elbow and 1 failure of longitudinal weld; According to the statistics of failure causes, there are 3 failures caused by corrosion, 1 failure caused by improper operation, 8 failures caused by unqualified on-site welding quality, 2 failures caused by by fatigue, 2 failures caused by fire and external forces, and 9 failures caused by manufacturing defects. The statistical results of failure facility types and failure causes are shown in Figs. 1 and 2.

Fig. 1.
figure 1

statistical results of types of failed facilities.

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Fig. 2.
figure 2

statistical results of failure causes.

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According to the statistical results of failure facilities in the station, the failure cases of pipe body, girth weld and pipe fittings (flange, tee and elbow) account for 76% of the total failure cases. For 5 failures cases of pipe body, there are 1 failure caused by fatigue (connecting pipe of pressure gauge), 2 corrosion failures and 2 failures caused by external force and fire factors; Among the 7 circumferential weld failure cases, 1 circumferential weld corrosion failure and 1 fatigue failure were caused by insulation joint failure, and the remaining 5 were caused by excessive defects in on-site welding; According to the statistics, the failure causes of the seven pipe fittings are the quality defects of the pipe fittings, which caused the failure in the service process.

2 Typical Failure Case Analysis

2.1 Failure Caused by Fatigue

In 2016, the nipple connecting the compressor outlet pipeline and the pressure gauge of a natural gas processing plant broke and failed. The failed nipple operated for one year and two months. The design pressure of the pipeline was 4 MPa and the actual operating pressure was 2.8–3.5 MPa. The failed connecting nipple was made of 16Mn steel forge piece, with an outer diameter of about 22.2 mm and a wall thickness of about 4 mm. Failure location and failure nipple morphology are shown in Fig. 3.

The morphology and fracture analysis of the failed connecting nipple showed that the connecting nipple of the pressure gauge breaks along the circumferential direction from the root of the last thread engaged place. By observing the fracture morphology, it was found that the fatigue shell line can be clearly observed on the fracture. Most areas of the fracture are in slope shape without obvious plastic deformation. The color of the fracture was dark gray. The section near the crack source was relatively flat and had the characteristics of fatigue fracture. Under the scanning electron microscope, it was found that there was obvious surface damage on the surface of the failed thread, which was mainly manifested in the adhesive wear on the thread surface and thread deformation, which was caused by improper thread screwing operation. According to the field vibration test results, the horizontal vibration speed of the pressure gauge connecting pipeline was as high as 43.8 mm/s. Therefore, it can be judged that the failure mode of the failed nipple was vibration fatigue. Improper screw operation was the main reason for the adhesion and deformation of the thread surface and the source of fatigue fracture.

Fig. 3.
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Failure location and failure nipple morphology

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2.2 Failure Caused by Corrosion

In 2016, the press-leading pipe of a block valve in a station was corroded and perforated. The outer diameter of the impulse pipe was 6.35 mm, the wall thickness was 0.87 mm, the material was 316 stainless steel, and the working pressure range was 7.5 Mpa–8.8 mpa. It has been put into operation for 10 years. The corrosion morphologys of press-leading pipe are shown in Fig. 4.

Through the observation of the appearance of the press-leading pipe, it can be found that the parts with corrosion perforation are the gap between the clamp and the press-leading pipe, and there were no similar corrosion pits on the outer surface of other parts. The morphology of the corrosion pits was similar to that of pitting corrosion. Through the energy dispersive spectroscopy analysis of the corrosion products in the corrosion pits, it was found that the corrosion products contain elements such as C, O, Fe, s, Cl, Si, K, Na, Cr, Ni, Ca and al. The service site of the press-leading pipe was near the sea, and the salt content in the atmosphere was high, which usually contains Cl−, K+, Na+ and other particles. According to literature statistics, the amount of sea salt particles deposited in coastal areas is 50 times that in inland areas. On the other hand, soap water as usually used for the detection of impulse pipe, and soap is the general term of fatty acid metal salt, and the general formula is RCOOM, RCOO is fatty acid radical and M is metal ion. The metals in daily used soap are mainly alkali metals such as sodium or potassium. In addition, ordinary soap has bactericidal effect after adding some sulfur. [7] Therefore, it can be inferred that the Cl element in the corrosion products was caused by the long-term precipitation of sea salt particles, while the K and Na elements are mainly caused by the long-term precipitation of sea salt particles and soapy water. The S element may come from SO2, H2S and other pollutants in soapy water and industrial atmosphere.

According to the location of corrosion (in the gap) and the analysis results of corrosion products, it can be inferred that the dust particles containing corrosive substances in the humid atmosphere are deposited in the gap between the clamp and connecting nut and the press-leading pipe for a long time (the press-leading pipe has been in service for nearly 10 years), resulting in the continuous increase of its concentration, the slow volatilization of water at the gap, and the formation of electrolyte corrosion environment in the gap, This leaded to crevice corrosion.

Fig. 4.
figure 4

corrosion under the clamp.

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2.3 Failure Case Analysis Caused by Welding Defects

In 2013, the weld between the flange and elbow on the filter separator of a compressor station had overproofed defects. Through the service applicability evaluation, it was judged that the use risk was high under the normal operating pressure, and the defective pipe section was replaced. The pipeline on one side of the circumferential weld was made of L415MB straight seam submerged arc welded pipe with the specification of 762 × 25 mm. The grade of flange on the other side was class 900 (pn150), and the type is neck butt welding flange. The defect location is shown in Fig. 5.

Through nondestructive testing and anatomical analysis of circumferential weld defects, it was found that the root incomplete fusion defect with length of 103 mm, the crack occurred along the incomplete fusion defect and extends to the center of the weld, there are non-metallic inclusions in the cracks. The energy dispersive spectroscopy analysis of non-metallic inclusions shows that the non-metallic inclusions are mainly CaCO3 and SiO2, it was the impurity of welding stick introduced during welding. Since the welding of flange and straight pipe was of unequal wall thickness, poor root welding and forming form root incomplete fusion defects, resulting in stress concentration, and the position expansion of inclusions brought in during welding, causing potential safety hazards. The main causes of crack generation and propagation are the lax control of process procedures and improper operation of welding personnel (such as incomplete weld bead or falling welding slag). The metallographic appearance of circumferential weld with cracks is shown in Fig. 6.

Fig. 5.
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The defect location

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Fig. 6.
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metallographic appearance of circumferential weld with cracks.

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2.4 Failure Caused by Quality Defects

In April 2020, a pipeline company found cracking and leaking of a temperature transmitter sleeving in the natural gas station. The sleeving was made of bar stock, with an outer diameter of 30.5 mm, an inner diameter of 17.5 mm, pipe body length of 30 mm, and the form of inner thread was NPT \(1{/}2^{{\prime}{\prime}}\). The material was 304 stainless steel, the working environment temperature was −9–40 ℃, the design pressure was 10 MPa, and the actual operating pressure was 4.1–8.8 MPpa. It has been in service for three years.

The thread morphology of the inner wall of sleeving was observed, it was found that there was slight Mechanical damage marks at the crack position on the outer surface near the port. The mechanical damage marks at the crack location of sleeving end face is shown in Fig. 7. The processing morphology of casing internal thread does not meet the requirements of NPT thread, the thread processing near the port position is relatively standardized. The more inward, the thread height gradually decreases. The non-standard thread processing leads to the extrusion of the thread near the port when the metering skid was screwed in, and the thread close to the inner position was not closely combined, resulting in large compressive stress on the outer surface of the port position. The longitudinal profile of thread is shown in Fig. 8. Under the scanning electron microscope analysis, it was found that the crack fracture was “rock candy block"feature, the fracture mode is intergranular fracture. The intergranular fracture characteristics is shown in Fig. 9. Metallographic macroanalysis showed that the casing material has obvious rolling streamline, which is in the rolling state, not the solid solution state required by the normal supply state, and there are obvious “dot” or “dot connected into linear” precipitates on the austenite grain boundary. Energy dispersive spectroscopy analysis shows that the content of C and Cr precipitates on the grain boundary is significantly higher, and the precipitates on the grain boundary are carbides containing CR.

The raw material of sleeving was not treated with solid solution or the solid solution annealing incomplete, made a large number of Cr carbides were accumulated at the grain boundary of the material, resulting in low anti intergranular corrosion ability and low grain boundary strength. Under the combined action of the corrosion envirmont, the non-standard threading and the stress concentration caused by mechanical damage, stress corrosion cracking occurred along the grain boundary of sleeving leads to failure.

Fig. 7.
figure 7

Mechanical damage marks at the crack location of sleeving end face

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Fig. 8.
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Longitudinal profile of thread

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Fig. 9.
figure 9

Intergranular fracture characteristics, with secondary cracks

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3 Failure Cause Summary

Summarize the collected 25 failure cases of facilities in oil and gas station, and the failure reasons are shown in Table 1.

Table 1. Failure reasons of 25 failure cases.
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4 Suggestions for Preventing the Failure of Facilities in the Station

  1. (1)

    For the instruments and meters connected with the pipeline near the compressor in the station, because the instruments and meters are generally connected with the pipeline by small-diameter pipes, the structure of head (instrument) and neck (connecting pipe) is often thin, which is easy to produce fatigue failure in the case of compressor vibration. It is recommended to conduct vibration reduction treatment for the outlet pipeline of the compressor; Higher strength connecting nipples are used to improve the fatigue strength of the nipples; During thread processing, the transition at the root of the thread should be smooth to reduce the stress concentration at the root of the thread; Prevent thread damage and deformation during thread assembly.

  2. (2)

    For the pipeline welds in the station, due to the large number of pipes and fittings with different specifications in the station, and the large number of circumferential welds and fillet welds with different wall thickness, the root welding position is easy to form defects such as incomplete penetration and incomplete fusion at the root, resulting in stress concentration and cracks at the root of the weld toe; If the position of the fusion line of fillet weld cover welding is not smoothly transferred, it is easy to produce stress concentration and cracks. It is suggested that during the pipeline welding construction in the station, the forming quality of circumferential weld root welding / backing welding and fillet weld cover welding should be strictly controlled to ensure the smooth transition between the weld and the base metal.

  3. (3)

    For the position of clamp and connecting nut, due to the existence of gap, it is easy to produce gap corrosion. Sealant can be used to block the gap to prevent electrolyte from penetrating into the gap and causing corrosion. After leak detection at such positions with soapy water, the residual liquid should be cleaned in time to avoid crevice corrosion [8].

  4. (4)

    For pipe fittings with relatively high possibility of manufacturing defects such as flanges, elbows and tees, the purchased pipe fittings shall be subject to quality inspection in strict accordance with the standards before construction.

  5. (5)

    For stainless steel pipe fittings, before construction, the supplier shall be required to provide the external surface quality and material list certificate of the tee. At the same time, the production process of stainless steel pipe fittings shall be required, and the supplier shall issue corresponding certification materials to ensure that the stainless steel pipe fittings meet the relevant performance requirements.