CN101726456B

CN101726456B - Residual intensity evaluation method of corrosion defect contained steam injection pipeline compensator bent pipe

Info

Publication number: CN101726456B
Application number: CN2008102244414A
Authority: CN
Inventors: 张广利; 罗金恒; 赵新伟; 张华�; 陈志昕; 董保胜
Original assignee: China National Petroleum Corp; CNPC Tubular Goods Research Institute
Current assignee: China National Petroleum Corp; CNPC Tubular Goods Research Institute
Priority date: 2008-10-15
Filing date: 2008-10-15
Publication date: 2012-01-11
Anticipated expiration: 2028-10-15
Also published as: CN101726456A

Abstract

The invention relates to a residual intensity evaluation method of a corrosion defect contained steam injection pipeline compensator bent pipe, which comprises the following steps of: computing the bearing capacity of the bent pipe through a formula plimit=Sigma f/(r Eta+3r2 Alpha Eta 2), wherein plimit in the formula is the ultimate bearing capacity of the bent pipe, Sigma f is the flow stress of a pipe, r is the mean radius of the bent pipe, alpha is the ellipticity of the bent pipe, Eta is an intermediate variable, t in the formula is the wall thickness of a pipeline, d is a defect depth, k is an intermediate variable, L in the formula is a defect length and Dm is the mean diameter of the bent pipe; carrying out full-size hydrostatic bursting test verification by utilizing the compensator bent pipe with corrosion defects, and comparing a measured value with a theoretical value, wherein the theoretical value is smaller, and an error is 24.7 percent. A result indicates that the method has higher engineering utility value and reasonable safe reliability.

Description

Method for evaluating residual strength of bent pipe of steam injection pipeline compensator containing corrosion defects

Technical Field

The invention relates to a residual strength evaluation method for a steam injection pipeline compensator bent pipe containing corrosion defects in heavy oil thermal recovery.

Background

Steam stimulation and flooding are two main methods for heavy oil recovery, and both recovery techniques require continuous injection of high temperature steam into the ground through a surface steam injection pipeline and a downhole steam injection string. Under the operation conditions of alternating high and low temperatures, the steam injection pipeline can generate elongation and shortening deformation. In order to eliminate the stress generated by the telescopic deformation, a compensator bent pipe is usually connected between linear pipe sections in the design and construction process of a steam injection pipeline, so that the axial and transverse thermal deformation of the pipeline is compensated, and the safe operation of the pipeline is ensured.

Due to the high-temperature and high-pressure working environment of the steam injection pipeline (the working temperature is more than 300 ℃ and the working pressure is more than 10 MPa), the concentration of corrosive ions in steam is too high, and the influence of an oil extraction process, the steam injection pipeline is easy to corrode and damage, particularly in the bent pipe part of a steam injection pipeline compensator, the erosion corrosion is more serious, the stress state is very complex, and the method is a weak link of the whole steam injection pipeline network. If the residual strength evaluation is not carried out on the compensator bent pipe containing the corrosion defect in time, once a failure accident occurs, huge economic losses such as shutdown and production halt and even serious accidents such as casualties can be caused.

The research of the residual strength evaluation method of the pipeline containing the corrosion defects is carried out from the beginning of the 70 th 20 th century internationally, and the standards and specifications of ASME B31G (1991), CAN/CSA Z144-M86 (1986), DNV RP F101(1999), API RP579 and the like are formed at present. Two industrial standards SY/T6151-1995 and SY/T6477-2000 are issued in the aspect of the method for evaluating the residual strength of the pipeline containing the corrosion defects in China.

However, the existing standards and specifications mainly focus on the evaluation method of the residual strength of the straight pipe, and do not relate to the evaluation method of the residual strength of the bent pipe. Moreover, the stress state of the bent pipe under the action of internal pressure is substantially different from that of the straight pipe, and the existing standards and specifications are not suitable for evaluating the residual strength of the bent pipe. At present, research work aiming at the bent pipe mainly focuses on analysis of the stress state of the bent pipe, and the evaluation of the safety state of the bent pipe is not reported.

Disclosure of Invention

The invention aims to design a residual strength evaluation method for a steam injection pipeline compensator bent pipe with corrosion defects, solve the problem that the existing residual strength evaluation standard is not suitable for the bent pipe, and fill the blank of the pipeline residual strength evaluation method in the aspect of bent pipe evaluation.

The evaluation steps of the residual strength of the compensator elbow containing the corrosion defects are briefly described as follows:

1. under internal pressure, the stress state of a bent pipe is essentially different from that of a straight pipe because it is a shell limited by double curvature. One unit face in a curved tube bent at any radius was studied, as shown in fig. 1. If the moment acting on this cell is neglected, then according to the Laplace Equation, the following relationship can be written:

N₁/ρ₁+N₂/ρ₂＝p (1)

in the formula N₁And N₂-minimum and maximum force values in the tangential direction of the surface;

ρ₁and ρ₂-minimum and maximum principal radii of curvature;

p is the internal pressure value.

The two major radius of curvature values are calculated for any point on the elbow section at angle θ, as shown in FIG. 2. The first curvature radius is at the center of the cross section of the elbow; and the centre of the second radius of curvature being at the intersection of the normal line under study and the axis of the torus

ρ₁＝r，ρ₂＝r+R/sinθ (2)

R-bending radius of bend axis

r-radius of bend

Substituting the obtained value into expression (1) to obtain

In order to simplify the calculation and with sufficient practical accuracy. We consider that: the longitudinal stress in the bent pipe is the same as the stress in the straight pipe, i.e. N₂Pr/2. Will N₂The value substituted into expression (3) is

To N₁Solving equation (4) to obtain

For stress, we obtain the final expression for calculating the approximate value of the stress in the bent pipe under the action of internal pressure

In this formula pr/t is the value of hoop stress in a straight pipe σ_hAnd expression of

Indicating the change in stress when the bent tube is compared to the straight tube.

2. Influence of rounding of cross section on stress of bent pipe

A large number of bent pipes are bent from straight pipes, in the pipe bending process, besides the change of the wall thickness, certain ovalization can be generated on the cross section, when only internal pressure acts, the cross section tends to be restored to a circular cross section due to pressure, and additional axial force and bending moment are generated in the thickness direction of the cross section of the pipe. In the process of analysis and calculation, the higher-order terms after the series expansion are taken as omitting simplification processing to obtain additional moment caused by ovalization

Wherein ovality

Since the wall thickness is much smaller relative to the bend radius, it is believed that the additional stress is linearly distributed along the tube wall in the radial direction when the R/R is_m>>1, assuming that the additional stresses of the elliptical bent pipe and the straight elliptical pipe are the same, the bending moment causes an additional positive stress on the pipe wall which is linearly distributed along the radial direction

The stress state is as shown in fig. 3 because the major axis of the elliptical cross section tends to be short and the minor axis tends to be long (circularity) due to the internal pressure. The elliptical bend stress can be expressed as

And x is the distance from the stress point of the cross section to the neutral axis.

3. Influence of wall thickness variation on stress distribution of bent pipe

The straight pipe is bent into the bent pipe, and the wall thickness of each point changes due to large plastic flow. The concrete expression is that the outer arc area is thinned and the inner arc area is thickened. Assuming that the wall thickness variation is fully compensated by axial deformation, the wall thickness at any point is inversely proportional to the bending radius at that point, i.e.

Substituting the above wall thickness formula into formula (9)

When R > > R, the above formula is simplified to

Because the hoop stress is more than 1 time larger than the axial stress, the strength of the elbow is mainly determined by the hoop stress. Thus, the equations presented herein are applicable to stress calculations for elbows with ovality. Since the actual bend is not a true elliptical cross-section, which reduces the hoop stress at the camber line, the wall thickness correction increases the camber line stress, and the effect of the wall thickness is approximately considered to cancel the non-elliptical effect. Therefore, under the premise of confirming that the circumferential stress of the outer arc line of the bent pipe is maximum, the maximum stress of the bent pipe can be written as

4. Effect of Corrosion Defect on residual Strength evaluation results

The foregoing teaches a method for calculating hoop stress of a complete bend, which requires adjustment of relevant parameters for bends with corrosion defects. It was found that replacing t in the original formula with a wall thickness t' comprising the length of the etch would be more reasonable. t' is expressed as

Wherein d is rottenDepth of etching defect, D_mThe average diameter of the pipe, L, is the corrosion defect length. When the elbow is applied to actual engineering, the L can be the length of an outer arc of the elbow. Wherein,

k = \sqrt{1 + 0.8 {(\frac{L}{\sqrt{D_{m} t}})}^{2}} .

the circumferential stress sigma ' of the corrosion defect thick bent pipe is obtained by replacing t in the formula (13) with t ' in the formula (14) '_θIs composed of

5. Establishment of bent pipe residual strength evaluation method

Is sigma'_θ＝[σ]^tFor the failure condition of the bent pipe, the estimation formula for obtaining the ultimate bearing capacity of the bent pipe containing the corrosion defect is

p_{lim it}＝[σ]^t/(rη+3r²αη²) (16)

In the formula,

<math> <mrow> <mi>η</mi> <mo>&equiv;</mo> <mfrac> <mrow> <mi>tk</mi> <mo>-</mo> <mi>d</mi> </mrow> <mrow> <mi>tk</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>-</mo> <mi>d</mi> <mo>)</mo> </mrow> </mrow> </mfrac> <mo>;</mo> </mrow></math>

[σ]^tthe allowable stress of the material at the operating temperature of the compensator.

And the formula (16) is a residual strength evaluation formula of the steam injection pipeline compensator bent pipe containing corrosion defects. If the ultimate bearing capacity is greater than or equal to the working pressure, the pipeline is in a safe state; if the ultimate bearing capacity is smaller than the working pressure, the pipeline is in an unsafe state and needs to be depressurized to operate or maintain defects.

The method is based on the static balance theory and according to the Laplace equation, the circumferential stress distribution characteristics of the bent pipe are researched, and the scientific and accurate method for evaluating the residual strength of the bent pipe of the steam injection pipeline compensator containing the corrosion defects is established. In the process of establishing the residual strength evaluation method, the ovalization of the bent pipe generated by the cross section of the pipeline in the straight pipe manufacturing process is considered, the additional bending moment and the additional stress generated by the rounding tendency of the elliptical cross section to the bent pipe under the action of internal pressure are strictly analyzed, and the influence of the wall thickness change in the bent pipe manufacturing process to the stress distribution of the bent pipe is considered; by correcting the wall thickness of the pipeline, the problem of stress concentration caused by corrosion defects is solved, and thus the method for evaluating the residual strength of the compensator bent pipe containing the corrosion defects, which meets the actual engineering, is established. And the full-size hydrostatic bursting test verification is carried out by processing the compensator bent pipe with corrosion defects, and the result shows that the method has higher engineering practical value and reasonable safety and reliability.

Drawings

FIG. 1 is a schematic view of a unit surface of a curved pipe of a rotating thin shell mechanical model of a bent pipe

FIG. 2 is a cross section of a rotary thin shell mechanical model elbow of the elbow

Wherein: n is a radical of₁And N₂-minimum and maximum force values in the tangential direction of the surface; rho₁And ρ₂-minimum and maximum principal radii of curvature;

p is the internal pressure value; r is the bending radius of the axis of the bent pipe; r-bend average radius;

Detailed Description

The present invention will be further described with reference to specific examples.

The method for evaluating the residual strength of the compensator bent pipe containing corrosion defects in service comprises the following steps:

(1) measuring the specification and the corrosion defect geometric dimension of the compensator bent pipe;

(2) measuring the tensile strength and the yield strength of the compensator pipe at the working temperature, namely the stress-strain curve of the material;

(3) establishing a limit state equation of the safe operation of the compensator according to the specification of the compensator, the geometrical parameters of the defects and the mechanical property of the pipe;

(4) and calculating the pressure bearing capacity of the compensator in the current operating environment by using the mechanical properties of the material at different temperatures, and judging the safety state of the compensator.

Example 1

(1) The specification of the compensator elbow and the geometric dimension of the corrosion defect are measured, the tensile strength and the yield strength of the pipe are measured, and the rheological stress of the pipe is calculated. The results are shown in table 1:

TABLE 1 relevant parameters of test tubes

Depth of defect (mm)	Defect Length (mm)	Ovality	Wall thickness (mm)	Average pipe diameter (mm)	Average radius (mm)	Flow stress (MPa)
							3.01	70.40	0.001	10.86	103.14	51.57	410

It should be noted that in calculating the burst pressure, the allowable stress of the material is not used, but rather the rheological stress of the pipe is used. This is because the allowable stress is not an inherent property of the material, and the allowable stress is a reaction of the ultimate strength of the material in different shapes and different working environments. And plastic deformation occurs in the pipe section blasting process, so the final blasting pressure of the bent pipe is calculated by using the rheological stress of the pipe as the strength limit of the material.

(2) Compensator bend bearing capacity calculation

Substituting the data in the table 1 into the following formula to calculate the pressure bearing capacity of the elbow

p_{lim it}＝σ_f/(rη+3r²αη²)

In the formula, p_{lim it}The ultimate pressure-bearing capacity of the elbow;

σ_f-the rheological stress of the pipe;

r-average radius of the bend;

alpha-ovality of the bend;

eta-the intermediate variable,

wherein t is the pipe wall thickness;

d-depth of defect;

k is an intermediate variable which is a variable,

k = \sqrt{1 + 0.8 {(\frac{L}{\sqrt{D_{m} t}})}^{2}};

wherein, L is the defect length;

D_m-average diameter of the bend.

(3) Full-scale hydraulic bursting test

In order to verify the evaluation formula of the residual strength of the compensator bent pipe, the actual burst pressure of the pipeline is determined by adopting a full-size burst test, and is compared with the calculated limit pressure obtained by adopting the formula, and the loading condition in the test process is shown in fig. 3. The results are shown in Table 2.

TABLE 2 burst pressure test values and theoretical values for pipe samples

As can be seen from table 2: the theoretical value of the tube sample burst pressure is smaller than the measured value, which shows that the formula provided by the invention has certain conservatism and is biased to be safe in engineering. The measured value is smaller than the theoretical value by 24.7%, which is acceptable by engineering. Since the actual bend is not a true elliptical cross-section, which reduces the hoop stress at the camber line, the wall thickness correction increases the hoop stress at the camber line, and the effect of the wall thickness is approximately considered to cancel the non-elliptical effect. Therefore, the method for evaluating the residual strength of the steam injection pipeline compensator elbow containing the corrosion defect is accurate and reliable.

Claims

1. A method for evaluating the residual strength of a steam injection pipeline compensator elbow containing corrosion defects is characterized by comprising the following steps:

(1) the specification of the compensator bent pipe comprises the following steps: the wall thickness, the average pipe diameter and the ovality of the compensator bent pipe; measuring the geometric size of the corrosion defect: including defect depth, defect length; measuring the tensile strength and the yield strength of the compensator bent pipe; calculating the rheological stress of the bent pipe of the compensator;

(2) compensator bend bearing capacity calculation

Calculating the pressure-bearing capacity of the elbow