Pitting corrosion often occurs due to the presence of various corrosive substances, such as CO_{2} and H_{2}S, in the pipe service environment. As a result of this process, the residual strength of oil pipes is reduced and this can compromise the integrity of the entire pipe string. In the present work, a model is introduced on the basis of the API579 standard to determine the so-called stress concentration coefficient. The model accounts for pitting corrosion shapes such as shallow semi-circles, semi-circles, and deep semi-circles. The relationship between the corrosion pit depth and opening diameter and the residual strength of the oil casing is obtained. The results show that the influence of the pit opening diameter on the stress concentration coefficient is smaller than that of the pit depth. For a constant pit opening diameter, the coefficient increases gradually with increasing the pit depth. The compressive strength and internal pressure strength of the carbon steel oil casing decrease accordingly. When the depth of the corrosion pit is relatively small, the growth of the coefficient is slower; when the depth of the corrosion pit increases to a certain value, the increase in stress concentration coefficient becomes obvious.

With the development of industrial technology, conventional onshore oil and gas fields can no longer meet the energy demand, and exploration and development are gradually growing in harsh environments such as deep formation [_{2} or H_{2}S, which greatly accelerates the corrosion of pipe and leads to the frequent failure of the casing, bringing huge economic losses and security concerns. Therefore, it is of great significance to analyze residual corrosion strength of oil pipes to guide safety in production. Casing residual strength evaluation results from the strength to consider whether the casing to meet the current pressure capacity, whether the need to replace the pipeline in service, to maximize the safety of the pipeline. There are three common corrosion types in oil pipe, local corrosion, uniform corrosion, and pitting corrosion. Corrosion type and form have a very important influence on the residual strength of oil and gas pipes [

Pitting corrosion defects are volumetric corrosion defects, and API579, DNVRP-F101 and ASMEB31G are usually used to evaluate residual strength of pipe with volumetric defects. These standards are clearly defined, but there are some limitations in describing precise failure pressure, and they cannot take account of the nonlinearity of materials, such as elastic-plastic effect of pipelines. Moreover, most of the evaluation methods such as API579 take the pitting corrosion of cube shape with regular length, width, and depth as the research object. Mondalb et al. [

Although the above researchers put forward some residual strength criteria for casing with volume defects, these criteria cannot guide the calculation of residual strength of semi-circular irregular pitting shape. Therefore, it is necessary to evaluate the residual strength of oil casing with semi-circular irregular pitting defects.

The corrosion pit on the inner wall of the carbon steel oil casing is regarded as a two-dimensional circular corrosion pit to solve its stress concentration coefficient. For the stress concentration at the edge of the circular hole on the general infinite plate, elastic mechanics can be used to accurately solve the stress concentration [

When

The quasi-circular corrosion pit whose corrosion depth h is less than the opening radius (

Shallow circular corrosion pits can be analyzed simply as a plate problem, assuming that the stress

The resultant force of _{y}

The solution is as expressed by

Therefore, the stress concentration coefficient at the edge of the shallow semi-circular corrosion pit is:

The

The quasi-circular corrosion pit whose corrosion depth

The balance equation of force in the y-axis direction is:

The solution is as follows:

Therefore, the stress concentration coefficient at the edge of the semi-circular corrosion pit is:

Among them,

The quasi-circular corrosion pit whose corrosion depth ^{2}^{2}

The solution is as follows:

Therefore, the stress concentration coefficient at the edge of a deep semi-circular corrosion pit is:

Among them,

The influence of corrosion on the strength of carbon steel oil bushing is mainly because of the stress concentration at the edge of the corrosion pit, which changes the stress distribution of carbon steel oil bushing at the edge of the corrosion pit, and thus changes the strength of carbon steel oil bushing. Influence of corrosion on strength of carbon steel oil bushing can be evaluated by the stress concentration coefficient. Choi et al. [

API carbon steel oil sleeve internal pressure strength formula is:

The API yield strength collapse formula of carbon steel oil sleeve is:

Choi et al. [_{tg}

The compressive strength of carbon steel oil casing can be modified by using the same method that considers the influence of stress concentration on the compressive strength of carbon steel oil casing [

The formula of compressive strength of corroded carbon steel oil sleeve is:

Steel grade TP140V carbon steel oil casing with

By calculating the distribution of stress concentration coefficient _{tg}

As the diameter of the corrosion pit opening increases, the variation trend of the stress concentration coefficient at the edge of the corrosion pit is shown in

The influence of corrosion pit depth on compressive strength and internal pressure strength of carbon steel oil sleeve is analyzed below. The influence of corrosion pit depth on the compressive strength of the carbon steel oil sleeve is shown in

A steel grade 13Crs-110 pipe with a diameter of _{tg}

The stress concentration coefficient distribution corresponding to different hole depths is calculated as shown in

As the diameter of the corrosion pit opening increases, variation trend of the stress concentration coefficient at the edge of corrosion pit is shown in

The influence of corrosion pit depth on compressive strength and internal pressure strength of oil pipe is analyzed below. The influence of corrosion pit depth on the compressive strength of the carbon steel oil sleeve is shown in

When the opening diameter of the corrosion pit is constant, the stress concentration factor at the edge of the corrosion pit increases with the increase of the depth of the corrosion pit, and the collapse strength and internal pressure strength of the carbon steel oil casing will decrease. When the depth of the corrosion pit is small, the increase of the stress concentration factor is slower; when the depth of the corrosion pit increases to a certain value, the increase of stress concentration factor becomes obvious.

When the opening diameter of the corrosion pit is constant, the stress concentration coefficient of the corrosion pit edge increases gradually with the increase of corrosion pit depth. When the depth of the corrosion pit is small, the increase of the stress concentration coefficient is slower. When the depth of the corrosion pit increases to a certain value, the increase of stress concentration coefficient becomes obvious.

Compared with the corrosion pit opening diameter, the effect of corrosion pit depth on stress concentration factor is more obvious; when the corrosion pit opening diameter is less than the oil pipe wall thickness, the stress concentration factor increases with the increase of the corrosion pit opening diameter. When the diameter of the corrosion pit opening is greater than the thickness of the oil pipe wall, the stress concentration factor will gradually decrease as the diameter of the corrosion pit opening increases.

Radius of circular pit, mm

The radius of the outer boundary of the action area of stress concentration, mm

The correction coefficient of transforming finite plate problem into infinite plate problem, dimensionless

The opening diameter of the circular corrosion pit, mm

Outer diameter of carbon steel oil sleeve, mm

Depth of the corrosion pit, mm

_{1}

_{2}

Intermediate parameters of the derivation process

_{tg}

The stress concentration coefficient at the edge of the corrosion pit, dimensionless

are intermediate parameters in the derivation process

_{b}

Internal pressure strength of carbon steel oil casing, kPa

The internal pressure strength of carbon steel oil casing, kPa

Distance to the center of the semi-circular pit, mm

Thickness of carbon steel oil casing, mm

Poisson’s ratio, dimensionless, is generally 0.31 for steel

Intermediate parameters in the derivation process

is tensile stress, MPa

_{y}

The minimum yield strength of the pipe body, kPa

_{z}

is Axial stress, MPa

Angle used in the derivation of stress concentration coefficient, rad

The project is supported by

The authors declare that they have no conflicts of interest to report regarding the present study.

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