Volume 20 Preprint 72
Residual Strength Analysis of Three-phase Separator Containing Corrosion Defects in Service
C.J. Han, R. Xie, Y. Xiao and J. Zhang
Keywords: Three-phase separator; corrosion defects; residual intensity; finite element analysis
Three-phase separator, for separating oil, gas, water three-phase and solid phase, is widely used in oil field development and production process. Since the long-term effects of oil and gas in an aqueous medium, the inside of the three-phase separator usually corrodes, thus, reduce its loading capacity and its service life, and endanger the safety of equipment and oil and gas production system. Based on, the paper set up a simulation model of the three-phase separator containing the problem of corrosion defects, and made model analysis on the residual intensity of the three-phase separator containing corrosion defects. Analysed the impact of position and geometry on the residual intensity of the three-phase separator. The result as follows: plastic deformation of the cylinder corrosion defects region primarily occurs at the axial edges of the defect. As the defect length, width, depth enlarged, the separator failure occurs earlier, among which the depth of defects impact greatest on the failure pressure. Plastic deformation of the corrosion defects head region occurs first in the centre of the defect, and extends to the surrounding. The larger the depth and cross-sectional area of the defect is, the sooner separator failure occurs, wherein the depth of defects impact greatest on the failure pressure. The impact of the head defects overpassed that of the cylinder defects on the residual intensity of the three-phase separator.
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Residual Strength Analysis of ThreeThree-phase Separator
Containing Corrosion Defects in Service
C.J. Han1, R. Xie1, Y. Xiao1, J. Zhang1,2*
1.School of Mechatronic Engineering, Southwest Petroleum University, Chengdu, China
2.Key Laboratory of Energy Engineering Safety and Disaster Mechanics （ Sichuan
University）, Ministry of Education , Chengdu, China.
*J. Zhang, email@example.com
Three-phase separator, for separating oil, gas, water three-phase and solid phase, is widely
used in oil field development and production process. Since the long-term effects of oil and
gas in an aqueous medium, the inside of the three-phase separator usually corrodes, thus,
reduce its loading capacity and its service life, and endanger the safety of equipment and
oil and gas production system. Based on, the paper set up a simulation model of the threephase separator containing the problem of corrosion defects, and made model analysis on
the residual intensity of the three-phase separator containing corrosion defects. Analysed
the impact of position and geometry on the residual intensity of the three-phase separator.
The result as follows: plastic deformation of the cylinder corrosion defects region primarily
occurs at the axial edges of the defect. As the defect length, width, depth enlarged, the
separator failure occurs earlier, among which the depth of defects impact greatest on the
failure pressure. Plastic deformation of the corrosion defects head region occurs first in the
centre of the defect, and extends to the surrounding. The larger the depth and crosssectional area of the defect is, the sooner separator failure occurs, wherein the depth of
defects impact greatest on the failure pressure. The impact of the head defects overpassed
that of the cylinder defects on the residual intensity of the three-phase separator.
Keywords: Three-phase separator; corrosion defects; residual intensity; finite element
The three-phase separator is to rely on the incompatibility and the density difference
among oil, gas and water. It is suitable for the purification of high water content well,
especially oil and gas well containing a large amount of free water, as is shown in Fig.1. In
the process of oil and gas field development and production, three-phase separator is one
of the most important equipment in oil and gas water separation, and it also to ensure that
the output of crude oil and withdrawal of water are up to standard . Therefore safety
performance and service life of oil gas water separator have an important influence on oil
and gas field development, However, under service condition, due to the mixed effects of
oil, gas, water and other material, the internal wall of the three-phase separator face
seriously corrodes and local residual strength decreases significantly, so this has seriously
endangered the safe operation of the separator. The corrosion mainly concentrates in the
water chamber of the floating ball, the bottom of the separator, coalescence plate and the
circumferential weld, internal support, oil pipelines, water pipelines, oil-water interface and
other parts. According to the corrosion factors of the three-phase separator, the
researchers launches a series of research work, they combine the research with the results
of the analysis and put forward many feasible protective measures .but the operation
process to ensure real-time monitoring of residual strength is still relatively small, and the
crevice corrosion , bacterial corrosion and internal structure defects will make local wall
thinner, leading to the insufficient of local strength to some unknown degree. There are
serious security risks in the three-phase separator under service condition.
Fig.1 The structure of three-phase separator
Understand the development situation of corrosion of the three-phase separator to ensure
its safe operation. It often appears to induce accidents, causing serious economic loss and
personnel injury . If focusing on detection and maintenance, some oil and gas fields have
less spare separator configuration the construction, some oil and gas even don’t have
separator as spare one. This may lead to a halt in production for detection, so it is
necessary to predict the corrosion rate and analyze the changes of the residual strength
evaluation of three-phase separator caused by corrosion so as to assess the safe operation
of life expectancy of separator. And the corrosion rate of oil and gas separator can usually
be specific through continuous statistics or indoor safety testing data of simulated
corrosion experiments, this paper mainly discusses the determination of residual strength
of corrosion state of separator.
Building Finite Element Models
Finite element analysis (FEA Finite, Element Analysis) is a method of using mathematical
approximation to simulate the realistic objects of the physical system, including geometry
and loading conditions. It replaces complicated problems with a relatively simple one and
then solve it . Finite element method can get rid of a lot of difficult field operation, using
simulation to replace the actual situation. Therefore, through the establishment of
numerical model of three-phase separator containing corrosion defects, this paper analyses
residual strength of three-phase separator on corroded defects by using finite element
software. Besides, it also studies the rule of the influence that corrosion defect position and
size changes have on the residual strength of the three-phase separator, when the position
and size of corrosion defects change.
The purpose of this paper is to analyze the influence of corrosion defects of three-phase
separator and cylinder head residual strength, so it can be simplified as the actual threephase separator as is shown in Fig.2. The analysis model is based on an three-phase
separator on service. So it can describe the corrosion defect accurately and can ensure
accuracy of calculation result [8-10].
Fig.2 Simplified model of the three - phase separator
The separator is affected by internal pressure, and when the internal pressure is 4MPa, we
can calculate the two-way stress according to the formula (1) and formula (2).
4 × 106 × 2200 × 10−3
4 × 16 × 10 −3
σ ′′ =
4 × 106 × 2200 × 10 −3
2 × 16 × 10 −3
Where, σ ′ is tensile stress, MPa. σ ′′ is tangential stress, MPa. d is three-phase separator
cylinder diameter, mm. P is internal pressure of three-phase separator, MPa. δ is threephase separator thickness, mm.
Fig.3 Finite element analysis model of corrosion defects on cylinder
Fig.4 Finite element analysis model of corrosion defects on head
Under the actual conditions, because the stress received by the corrosion near defect is
bigger, so in the presence of corrosion defects, corrosion defects of three-phase separator
and the area around it damaged first, simulation analysis usually selects the part of cylinder
head of a section or the part containing corrosion defects. Since corrosion of the main
defects in the three-phase separator of is square or round, so according to the symmetry
characteristics of the model, the separator barrel body segment model 1/4 is selected to
analyse . In the inner surface of cylinder head section and the top model of the threephase separator were established the square cylinder in the analysis of the corrosion pits.
When some defect areas are affected, take the 1/4 model of the barrel to fill the corrosion
pit evenly, as is shown in Fig.3. Change the position of corrosion defects to the inner
surface of the top of head position, when head position in the analysis of the defect area is
affected, the entire head and a separator cylinder are to establish analysis model as is
shown in Fig.4.
Applied load process
Apply the displacement constraints in section in the contact head and cylinder. Limit axial
displacement is zero. Apply the frictionless constraint in the head section which is parallel
to the axial direction. And the four inner surface defects are applied 4MPa internal pressure
in the model. The operating pressure presents a linear increase in the 10s from 0MPa to
Allowable stress is the ultimate strength of materials under different conditions and working
conditions, calculated according to formula (3).
For plastic material:
σ = s
Where, [σ ] is the allowable stress, MPa. σ s is the yield strength, MPa. ns is the safety factor,
under static loading, safety factor of plastic material is ns = 1.2 ~ 2.5 .
The allowable stress is the highest limit of the working stress of the component, that is to say
the working stress is not more than the allowable stress. Therefore σ ≤ σ , the strength
condition can be checked according to the strength to confirm the allowable load.
Considering the material, the applied load, the component simplification, the reasonable degree,
the importance of the component in the equipment and the working conditions, and combining
the practical experience, the safety factor ns = 1.45 is selected. The yield stress σ s = 245MPa of
the Q245R material is introduced into (3) and the allowable stress is calculated through formula
(4), and the safety of the in-service three-phase separator with 170MPa is evaluated.
σ = s =
Simulation calculation of defect parameter setting
Table 1 Geometric parameters of corrosion defect on cylinder
Based on the effect the establishment of corrosion defects have on the separator simulation
analysis, the design is shown in Table 1 and table 2. It shows multiple defect geometric
parameters and comparatively analyses the influence of geometric parameters has on the
residual strength of corrosion defects of separator.
Table 2 Geometric parameters of corrosion defect on head
Simulation results analysis
Influence of corrosion defect location
As is shown in Fig.5, when the same corrosion defect is located in the cylinder and the head
position, equivalent stress nephogram of corrosion region under the pressure 4MPa is listed.
It can be seen from Fig.6, the maximum stress and the equivalent stress of the corrosion
defects of the cylinder are not the same as those of the head area. It describes the
relationship between the two positions of the corrosion area of the maximum equivalent
stress with inner pressure in the range of 0-4MPa. From Fig.6, in the early stages of inner
pressure load application, the curves of the maximum equivalent of the two corrosion area
almost overlap, but with the increase of working pressure, maximal equivalent stress of
head defects gradually gets away from the cylinder defects and is greater than that of
cylinder defect area. From the Figure we can see, when the internal pressure reaches about
1.48MPa, the equivalent of two defects stress showed small fluctuations, this is because the
separator material enters the yield limit and causes the phenomenon. In addition, in the
early stage of the increase of internal pressure, the two position defect areas’ maximum
equivalent stress approximately grow linearly, the head of corrosion defect area’ equivalent
pressure is to achieve the maximum yield strength. It is earlier than cylinder corrosion
defects to enter the yield stage. In the middle stage, it fluctuates in a small range. With the
increase of internal pressure, the maximum equivalent pressure of the head of the
corrosion region is to achieve ultimate tensile strength earlier, than the cylinder corrosion
(a) Internal of cylinder defect
(b)Inner surface of cylinder defect
(c) External of head defect
(d) Outer surface of head defect
Fig.5 Equivalent stress cloud diagram of the corrode area at different positions of defect
Fig.6 The relation of the maximum equivalent stress with work pressure
(a) Internal of cylinder defect
(b)Inner surface of cylinder defect
(c) External of head defect
(d) Outer surface of head defect
Fig.7 Plastic strain nephogram at different positions of defect
Fig.7 shows equivalent plastic strain distributions under the operation pressure 4MPa, when
the corrosion defects with the same geometric size are located respectively in the cylinder
and head, the corrosion area of. The two positions’ equivalent stress nephogram are alike
the two positions’ equivalent plastic strain distribution and the maximum equivalent plastic
strain are not the same. The strain distribution of the defect area of cylinder decreases from
the edge to the central axial and the strain distribution of defect region of head decreases
from the defect centre to the periphery ring.
Fig.8 The relation of the maximum strain with work pressure
In addition, as is shown in Fig.8, when internal pressure was in the range of 0-4MPa,
cylinder and head defect areas’ maximum equivalent plastic strain varies with the increase
of working pressure. From Fig.8, the defect area and cylinder head, the positions’
maximum equivalent plastic strain increases with the increase of the working pressure, the
trends are similar. But in the head position of the corrosion defect under the working
pressure1.68MPa, the corrosion area of the maximum equivalent plastic strain increases
slightly after the start. When the work pressure is 3.28MPa, the maximum equivalent plastic
strain similar to the exponential function increases rapidly; while in the corrosion defect
barrel location reaches 2.48MPa under the working pressure, the corrosion area the
maximum equivalent plastic strain begin to increase slightly, the internal pressure increases
to 3.68MPa, the plastic strain similar to the exponential function increases rapidly, and the
corrosion defect area increases later than that of the head position. In addition, the
maximum equivalent of head corrosion defect area’ maximum equivalent plastic strain is
larger than that of the cylinder defect stress. so it shows that the effect of corrosion defects
in head position have bigger effect on the wind sharp plastic deformation, which will more
easily lead to failure of the separator due to the deformation and failure of head position.
Table 3 Failure pressure of different defective position
positions of defect
The allowable stress
Failure pressure (MPa)
When the maximum equivalent stress reaches the corrosion area of the three-phase
separator material allowable stress, three-phase separator is in dangerous working
conditions, there is a high possibility of failure, the working pressure is the failure pressure
at that time. Through the finite element analysis results of the post-processing failure
pressure of the same size square defects respectively in three-phase’ cylinder and head is
calculated，as is shown in Table 3. From the data it can be found that compared with the
cylinder position of corrosion defects, defects in the head position have failure pressure
earlier, that is to say, it is more prone to have failure deformation. In addition, in the
absence of corrosion conditions, separator working pressure is 1MPa, and in the presence
of corrosion defects afterwards, the failure pressures of the cylinder and the head position
are both less than 1MPa, so the corrosion defects will reduce the separator work and
influence the normal operation of the separator.
Influence of corrosion defect location
Fig.9 Influence of geometric parameters about corrosion defect of cylinder
In order to analyse the influence of corrosion defects of different geometrical size have on
the failure pressure of the separator, Fig.9 and Fig.10 compare the failure pressure under
the same conditions in the cylinder and the head position of different corrosion defect
geometric parameters. Fig.9 describes when the corrosion defects are in the cylinder body,
the effect that the defect length, width and the depth have on the failure pressure. Fig.9 (a)
shows that with the increase of corrosion defect length, failure pressure decreases, but the
decrease is very small, the defect length increases from 60mm to 260mm when the failure
pressure drops less than 0.1Mpa, so the impact that cylinder’s corrosion defect’s length has
on the failure pressure is so small. And under the same defect length and depth condition,
influence of cylinder position corrosion defect width has
on the failure pressure as is
shown in Fig.9 (b) ,we know that with the increase in the width of the corrosion, failure
pressure is gradually smaller ; but the corrosion defect’s width changes evenly, and failure
pressure shows even changes.. Therefore, influence the barrel body corrosion defect width
has on the failure pressure doesn’t have linear relationship. Under the same defect length
and width conditions, the effect that corrosion defect’ depth has on failure pressure is
shown in Fig.9 (c) shows that with the corrosion depth increasing, separator failure
pressure gradually decreases, but the decline also decreases gradually. But compared with
the influence of the length and width of corrosion on the failure pressure, the change of the
depth of corrosion has more influence on the separator.
When corrosion defect is located in the head, the influence of defects’ depth and width on
failure pressure of the separator is shown in Fig.10. Fig.10 (a) shows, due to the
assumption that the defect is square (length and width are equal), in the same width, with
the increase of the defect depth, separator failure pressure changes significantly. And it
gradually decreases, the trend is basically linear. And in the same corrosion depth,
influence of defects’ width on the failure pressure is shown in Fig.10 (b). From Fig.10 (b)
can be seen, with the increase of corrosion defects’ width, separator’s failure pressure
changes slightly. It has very little influence on the width of defect failure pressure.
Furthermore, comparing the effect of head position’s corrosion defects’ depth to that of the
width on failure pressure of the separator, effect of defects’ depth on failure pressure is
greater than the width of defects, and it more easily make cyclone separator invalid.
（a）The influence of depth
（b）The influence of width
Fig.10 The influence of geometric parameters about corrosion defect of head.
The geometry of corrosion defect size under the same condition, the corrosion defect in
head position, the maximum equivalent stress of regional corrosion defects in any of the
same internal pressure is greater than that in the cylinder corrosion stress region, and when
the corrosion defect is located in the head position of corrosion, residual strength of the
three-phase separator is more significant so it more easily lead to three-phase separator
When the square corrosion defect is located inside of the cylinder, with corrosion defect
length, width and depth increasing, the corresponding failure pressure of the three-phase
separator of is smaller and smaller, and it is more likely to fail in advance. The influence of
corrosion defect depth on residual strength is greater than the length or width of separator
defect on the residual strength of separator.
When square corrosion defect is located inside of head, with increasing cross section width
and depth, the three-phase separator’s failure pressure is smaller and smaller, and
approximately changes linear. In addition, compared to the impact of corrosion defect
width, the depth of the head defect has more significant influence on the ultimate bearing
capacity of the three-phase separator.
This paper is supported by Project of Key Laboratory of Energy Engineering Safety and
Disaster Mechanics (Sichuan University) (No.EES201604), Ministry of Education, Chengdu
science and technology plan (No.2016-HM01-00306-SF) and Science and Technology
Innovation Talent Engineering Project of Sichuan Province (No.2016115).
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