Investigation of weld cracking of a BOG booster pipeline in an LNG receiving station
Introduction
Liquefied natural gas (LNG) booster pipeline represents an essential piece of equipment in LNG receiving stations as they facilitate the peak regulation of the gas supply. Therefore, it is essential to ensure the safety of boil of gas (BOG) booster pipelines. One welding of a BOG booster pipeline in a LNG receiving station of China cracked and caused to failure of entire LNG booster pipeline, this failed weld used the gas metal arc welding (GTAW) welding method by using TGS-308L welding wire with a diameter of 2 mm. The welding current was 85-100A, the arc voltage was 13–14 V, the welding speed was 7–9 cm/min, and 99.9% argon was used for protection. The crack was located between one straight BOG inlet main pipe and support branch pipe with a line length of 135 mm, as showed in Fig. 1. According to the design document, main pipeline and the supporting branch pipes were made according to the ASTM A240-13 standard type AISI 304L [1]. The outer diameters of the main and supporting branch pipes were 324 mm and 273 mm, respectively, and the wall thickness of main pipeline and branch pipes were 4.57 mm and 4.19 mm, respectively. The design pressure of the BOG booster pipeline were 1.0 MPa, and designed working temperature was under 40 °C. When crack failure occurred, the pipeline operating pressure and temperature were 0.69 MPa and 23.5 °C, respectively.
Many researchers have examined the failures of LNG equipment, pipelines and welds. LaFleur et al. [2] used the failure modes and effects analysis (FMEA) to identify the high risk failure modes of a liquefied natural gas (LNG)/diesel hybrid locomotive. Baek et al. [3] investigated the fatigue crack growth rate and fracture toughness of type 304L stainless steel and weld metal. It was noted that the orientation did not influence the fatigue crack growth rates, and the fatigue crack growth rates and crack tip opening displacement (CTOD) values decreased with a decrease in the test temperature. Bagherifard [4] analyzed the failure of a subsea ball valve used in an oil piping line and noted that the failure occurred due to two concomitant factors, specifically, severe notch effects and incorrect thermal treatment. LaFleur et al. [5] reviewed and compared the significance and status of the engineering critical assessment (ECA) of the embedded flaws in pipeline girth welds and found that the FEM techniques and frameworks could facilitate advancements in stress based ECA. Li et al. [6] established the finite element models of corroded pipelines with different types of defects and examined the interaction among the defects to formulate pipeline operator forum (POF) interaction rules, which could be used to identify whether any interaction among the defects occurs. However, the literature pertaining to a similar failure analysis of BOG booster pipelines in LNG applications is scarce, and the lack of investigation impedes the safe operation of such pipelines and prevention of similar failures.
In order to find the reason for the failure of the BOG booster pipeline in the LNG receiving station, the failed part of the pipeline was intercepted and examined by macroscopic observation, physical and chemical inspection, optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) techniques. Moreover, the microhardness of the weld and nearby pipe materials was tested, and the stress concentration of the pipe and weld caused by compressor vibrations was determined using the finite element simulation method. Finally, the reason for the failure was analyzed, and countermeasures were suggested to avoid the occurrence of similar accidents.
Section snippets
Experimental procedure
The chemical composition of the crack failure pipeline was analyzed using a Baird Spectrovac 2000 direct reading spectrometer and LECO CS-444 IR carbon–sulfur spectrometer according to the ASTM A751-14a standard [7]. Metallographic and fracture samples were cut near the crack position, after being ground and polished with diamond pastes of 6 μm and 1 μm, the microstructure, grain size, inclusion and crack morphology of the main pipe, branch pipe and weld of the failure samples were analyzed
Macroscopic observation
The macroscopic observation of the failure BOG booster pipeline showed that the crack in the failure position of the main pipeline ran through the entire wall thickness in an arc shape, parallel to the welding seam, as shown in Fig. 2. Inside the welded branch pipe, there is a welded small branch pipe with a diameter of approximately 219 mm, which is probably for strengthening the support of the branch pipe. The welding of the two branch pipes on the main pipe was not concentric, and its
Discussion
The previously obtained results indicated that the mechanical properties of the main and branch pipes were in accordance with the requirements of the ASTM a240-13a standard, and thus, the possibility of failure caused by the defects of the main pipe and branch pipe body could be excluded.
The simulation analysis of the pipeline vibration indicated that the occurrence of vibrations likely led to a certain increase in the stress in the welding seam, however, this stress caused by vibration was
Conclusions
The weld cracking failure of BOG booster pipeline in a LNG receiving station was systematically examined, analyzed and simulated by using FEA, and the following conclusions were derived:
- (1)
The mechanical properties of the main and branch pipes were in accordance with the relevant requirements of the ASTM a240-13a standard.
- (2)
Due to the poor machining dimensional accuracy of the welding fit between the main pipe and the supporting branch pipe, multipass welding with a large amount of filler metal in
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge the financial support provided by the National Key R&D Program of China (2016YFC0802101), Major science and technology project of CNPC (2016YFC0803201).
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