Analysis of corrosion failure of N80 pipeline underground in offshore oil fields.

An oil
started production in June 2010, with an initial water content of 1.5%. The well is a large slope well with a maximum slope of 74.98o. In October 2015, the pump failed to stop, the water content before the pump stopped 81.7%, after normal maintenance and stable production, by September 2016 water content rose to 91%

. In March 2019, the pipe column was changed, and the original well pipe column 3-1/2" EU trout multiple trout pipe corrosion, and each pipe has about 20 more than 1 to 2 mm perforation, corresponding to a depth of 234 to 369 m. The pipe material of the failure material is N80. In this study, a perforated oil pipe (remembered as A tube) and an un perforated oil pipe (remembered as b tube) were selected as the object of failure analysis, with a view to ascercerging the cause of corrosion perforation.
macro-analysis and wall thickness measurement
1.1 macro-shaped
A tube, B tube tube tube outer wall macro-shaped as shown in Figure 1. It can be observed that the outer wall of the A tube has many floating rust, no obvious oily material attachment, B tube outer wall as a whole attached to a layer of oily material. The macro-shape of the tube end of the A-tube and B-tube is shown in Figure 2, as shown in the figure, the inner wall and end of the male buckle of the A-tube found obvious corrosion pits, the corrosion product is red-brown, sampling for corrosion product analysis, while the outer wall of the B-tube did not find obvious corrosion phenomenon.
Figure 1 A, B tube outer wall macro-shaped map
Figure 2 A, B tube end macro-shaped map
1.2 anatomical analysis
the A oil pipeline along the axis direction of anatomy, further observation found the corrosion of the inner wall of the pipeline, as shown in Figure 3. In addition to corrosion perforated position, the inner wall of the A-pipe also found most corrosion pits, the maximum corrosion pit depth of about 4.9 mm, of which the pipe end wall with a hoop side did not find significant corrosion. The nearest corrosion pit is located approximately 97 mm from the end of the pipe, from which the perforation of the tube body is present in multiple directions, extending axially towards the middle of the tringe, with perforations formed in some locations (in line with the externally observed perforation position).
the internal and external wall shapes of multiple perforated positions, as shown in Figure 4. It can be observed that the corrosion pit area of the inner wall is large, and the center of the corrosion pit extends radially to the outer wall until it is perforated, and the corrosion characteristic of the tringe is internal corrosion.
Figure 3 A inner wall shape of the pipe end on the side of the pipe strap
Figure 4 corrosion perforated position inner wall shape
overall observation of the A oil pipe inner wall corrosion position distribution, as shown in Figure 5, found that the corrosion position is located on both sides of the pipe end, a total of 5 weekly orientations are linearly extended to the middle, where the inner side with the hoop side Wall corrosion pits (including perforated positions) extend as far as 3 m from the end of the pipe, near the middle of the tringe, the last 1 m corrosion pit depth is getting smaller and smaller until it disappears;
Figure 5 A pipe inner wall corrosion location distribution map
Figure 6 A pipe strap hoop side oil pipe inner wall corrosion pit extension
the B oil pipe along the axial anatomy, the end of the pipe on both sides of about 100 mm area did not find obvious internal corrosion phenomenon, as shown in Figure 7. Obvious corrosion pits were found in the central position, and large areas of festering were presented along the axis. The red-brown corrosion product was taken for analysis, and the distribution of corrosion pits also had the characteristics of peri-weekly multi-line distribution, and some orientations of the inner walls of some pipe segments did not see obvious corrosion characteristics, and the overall presentation of local corrosion characteristics, as shown in Figure 8.
Figure 7 B tube without hoops on the side of the pipe end inner wall shape
Figure 8 B pipe in the middle of the inner wall shape
1.3 tube body wall thickness detection
to A, B oil pipe outer wall cleaning, and then along the perimeter to select a representative orientation, using GE DM5E ultrasonic thickness gauge to measure the remaining wall thickness. The results show that the wall thickness of A and B tulle pipes is seriously uneconscionable, such as A pipe, some positions appear perforated, and the same axial position of different directions of thinning is only 0.1 mm, the difference is obvious.
chemical and chemical performance analysis
2.1 Chemical composition analysis
using SPECTRO LABLAVM11 direct reading spectrometer to analyze the chemical composition of the A and B pipeline tube body samples. The results show that the chemical division of the two troughs meets the technical requirements of the order. However, in contrast, A tube, B tube Mn, P, S element content there is a certain difference, should not belong to the same product batch, and B tube central, end of the element content is basically the same, A tube central P, S element content is higher than the end.
2.2 Force Performance Analysis
. . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The results show that the hardness of the two tringes meets the technical requirements of the order.
the extension
the Z600 dual-pillar all-in-one material test machine to stretch the A and B trance test samples. The results show that the stretching performance of the two tringes meets the technical requirements of the order.
impact performance
PSW-750 swing hammer impact test machine for A, B pipeline test samples. The results show that the impact test of the two tringes meets the technical requirements of the order.
2.3 Gold phase analysis
sampling at the non-corrosive end, corrosion center, un corrosive middle, corrosion end (on the other side) of the A-pipe, numbered A1, A2, A3, A4, and sampling at the end, central corrosion and non-corrosive position of the B-pipe, numbered B1, B2, B3, respectively. The gold phase analysis was carried out on the horizontal samples of the A and B tringed tube bodies with the Observer A1m gold phase inverted microscope respectively. The analysis results show that the gold phase tissues of pipeline A and B are refrased, and there is no decarbonization tissue from the internal and external surface tissues of several samples of A2, A3 and B2.
Take A, B tulle tube body vertical sample to observe the ribbon tissue, the study shows that A, B tulle has different levels of ribbon tissue, and the higher the band tissue level of steel, the greater the tendency to corrosion, corrosion more along the direction of ribbon tissue extension.
A the sample taken on the un corroded side of the oil pipe did not find the ribbon tissue, while the sample taken at the end of the corrosion side and the sample taken near the corrosion perforation were found high-level ribbon tissue, and the band tissue level of the sample taken in the middle was low, indicating that the higher the band tissue level, the more serious the corrosion, and the corrosion perforation macro characteristics of the A pipe.
no obvious corrosion occurred at the end of the
B talle, its ribbon level is small, and the band tissue level of the area where corrosion is severe in the middle is high, so the degree of corrosion is also related to the level of ribbon tissue.
corrosion product analysis
3.1 micromorphological and energy spectrum analysis
take the inner wall of the perforated part of the end of the A-pipe, the inner wall of the central pipe segment where no local corrosion occurred, and the corrosion position of the B-pipe respectively The inner wall corrosion product, numbered 1, 2, 3, uses Zeiss EVO 18 scanning electroscope to observe its microscopic appearance and uses the Oxford Energy Spectrometer to analyze the micro-region composition of 3 corrosion products. The results show that the surface of the sample is characterized by particle-like tight accumulation, and the main components of corrosion products are C, O, Al, Si, Mn, Fe and Ni.
3.2 XRD Analysis
The corrosion products numbered 1 , 2 , and 3 are X-ray diffraction (XRD) after treatment with petroleum ether, alcohol dissolved oil, filtration and drying, and the scanning angle is 2 to 3 to 80 degrees, the sampling step width is 0.02, and the wavelength is 1.540 56 nm. Using Search-Match software and combined with the results of EDS, three groups of corrosion products were analyzed.
results show that the main components of the corrosion products of 1 and 2 are the same, both Fe2O3 and FeOOH. Although the XRD map does not match the attribution of C, from EDS data, the C content in both samples is very high, which can be presumed to be the introduction of corrosion environment in the medium. The main components of the corrosion products are FeCO3, FeSO3, FeC, Fe6 (OH) 12CO3, Fe2 (SO4)3.
above, the corrosion products of perforated tubes are mainly iron oxides, the presence of FeCO3 was found in the un perforated tringes, due to the carbon dioxide subpressurized in the environment of 0.003 MPa, according to the DNV promulgated TNB111 stipulates that when the CO2 sub-pressure value is less than 0.021 MPa, there is no CO2 corrosion, so the elimination of CO2 corrosion, where the presence of FeCO3 may be related to HCO3-ions in the fluid medium.
the cause of failure
the pipeline is a normal tringe, corrosion resistance is poor. In the early stages of corrosion, corrosion nuggles appear in some banded tissue sites. With the development of corrosion, the adjacent small corrosion hemp point develops into a corrosion pit in a larger area, and then develops vertically into a deeper corrosion pit. The resulting corrosion product structure is relatively loose, no protection, once there are some points on the metal surface corrosion, corrosion will continue. Due to the presence of corrosion products, an anode region lacking oxidants is formed inside the corrosion product, and an oxidizing-rich cathode zone is formed on the outside, thus forming a thick cell that continues to corrode.
conclusion
(1) the physical and chemical performance test of the pipeline body shows that the material meets the requirements of API Spec 5CT standard;
(2) The corrosion resistance of the trush pipe in the working medium environment is poor, oxygen corrosion occurs, especially under the flushing of high flow, the corrosion rate is further aggravated;
(3) the corrosion characteristics of the trush pipe belong to the internal corrosion form, the wall of the pipe is distributed in strips, and the thick corrosive product layer formed on the inner wall of the
(4) oil pipe forms a thick cell, which further aggravates corrosion.
References (slightly)
Yu Wei, Zhang Guoqing, Chen Wei
(Ocean Petroleum Engineering Co.,
).