1.3 Classification of defects

A defect is any non-compliance with regulated standards. The main reason for the appearance of defects is the deviation of the operating parameter from the standard value, justified by the tolerance.

Defect classes.

The first class includes:

Floated sections of the pipeline (sections of the main gas pipeline that have lost the design position of the axis in water-logged soil with access to the water surface);

Arched emissions (sections of the main gas pipeline that during operation have lost the design position of the axis with access to the daylight surface);

Bulging (pipe sections that bulge as a result of frost heaving of soils, usually when thawed soils containing the pipeline freeze):

a) symmetrical;

b) asymmetrical (in the form of one half-wave sinusoid);

c) “snake” type in the horizontal plane (with two or more half-waves);

Sagging (bare sections of pipe without support on the ground, arising, for example, as a result of karst phenomena or thawing of permafrost soils);

Subsidences (pipe sections that subside when permafrost soils thaw).

The second class includes:

Ovality of the pipe (defects in the geometric shape of the pipeline section, resulting from the transformation of the initial annular section of the pipe into an elliptical one);

Dent (local change in the shape of the pipe surface, not accompanied by wall thinning);

Corrugations (a transverse fold on the surface of a pipe, characterized by a depth that is usually measured against the thickness of the pipe wall).

The third class includes defects in pipe walls of metallurgical origin and those formed during transportation, construction and operation of the main gas pipeline:

Bundles;

Sunsets (discontinuity of metal in the direction of sheet rolling over a considerable length);

Films (delamination of metal of various thicknesses and sizes, elongated in the rolling direction);

Flaws (opened deep oxidized rupture of the metal surface of various shapes);

Liquation (increased content of non-metallic inclusions);

Notch (longitudinal groove formed during pipe rolling).

Defects in pipe walls formed during the transportation of pipes, structures and operations of the main gas pipeline:

Thinning of the pipe wall over a large area;

Local damage to the pipe wall, both single and group;

Linear-extended defects:

a) scratches;

b) bullies.

Causes of pipe defects.

The existing technology of metal rolling, the technology of continuous casting of steel at individual metallurgical plants is one of the reasons for the production of low-quality pipes.

At pipe factories, incoming control of raw materials is imperfect or completely absent - defects in raw materials become defects in pipes.

When cleaning pipelines with pig-cutters, plastic deformation defects occur in local areas of the pipe surface - undercuts.

A defect in a welded joint is a deviation of various kinds from established standards and technical requirements, which reduces the strength and operational reliability of welded joints and can lead to destruction of the entire structure.

Sagging - most often formed when welding vertical surfaces with horizontal seams, as a result of liquid metal flowing onto the edges of the cold base metal. They can be local (in the form of individual frozen drops) or extended along the seam.

Undercuts are depressions formed in the base metal along the edge of the weld.

Burn-through is the penetration of the base or deposited metal with the possible formation of through holes.

Unwelded craters are formed when the arc suddenly breaks at the end of welding.

Assessment of the degree of danger of defects.

The degree of danger of defects should be assessed according to the criteria of static and dynamic stability of product pipelines. According to the criterion of static stability, the danger of classical defects classified as metal loss should be assessed.

According to the criterion of dynamic stability, the danger of defects classified as local stress concentrators in the base metal during repeated static loading of the pipeline with internal pressure should be assessed.

Making a decision about the degree of danger of a defect is based on a conclusion about the nature, location and size, as well as on the ideas of strength physics about the danger of a defect of this kind. In this case, the probability of correct classification of the defect, the accuracy of determining its size and coordinates should be taken into account. If the reliability or accuracy of the results is insufficient, it is necessary to carry out repeated testing, possibly using other methods, for example, radiographic, eddy current.

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The classification of section defects by types and parameters is contained in RD-23.040.00-KTN-011-11.

Pipe geometry defects are defects associated with changes in the shape of the pipe. These include: dent, corrugation, narrowing.

The depth of the corrugation is determined as the sum of the height of the convexity and the depth of the concavity, measured from the generatrix of the pipe.

Pipe wall defects include: loss of metal, reduction in wall thickness, mechanical damage, delamination, delamination with exposure to the surface, delamination in the heat-affected zone, crack, crack-like corrosion-mechanical defect.

Metal losses are divided into combined and single.

Combined metal loss is a group of two or more corrosion defects combined into a single defect if the distance between adjacent defects is less than or equal to the value of four pipe wall thicknesses in the area of ​​the defects.

A single metal loss is one metal loss defect, the distance from which to the nearest metal loss exceeds the value of four pipe wall thicknesses in the area of ​​the defect.

Mechanical damage to the surface of the pipe wall, classified according to GOST 21014 as “risk”, “scratch”, “scuff”, “tear”, “surface dent”, are identified according to VIP data as “risk”.

Defects in a welded joint (seam) are defects in the weld itself or in the heat-affected zone. The types and parameters of defects in welded joints are regulated by relevant regulatory documents. Weld defects include:

Crack, lack of penetration, lack of fusion - defects in the form of discontinuity of metal along the weld, which, according to the VIP data, are identified as “planar type discontinuity” of a transverse, longitudinal, spiral weld;

Pores, slag inclusions, sink marks, undercuts, excess penetration, sagging, scaliness, deviations of weld dimensions from the requirements of regulatory documents, which, according to VIP data, are identified as an “anomaly” of a transverse, longitudinal, spiral weld;

Edge displacement is a discrepancy between the levels of location of the internal and external surfaces of the walls of welded (welded) pipes (for a transverse weld) or sheets (for spiral and longitudinal seams) in stud welded joints, which, according to VIP data, is identified as a “displacement” of transverse, longitudinal, spiral weld;

An oblique joint is a welded butt joint between a pipe and a pipe (with a coil, with a connecting piece), in which the longitudinal axes of the pipes are located at an angle to each other -

The difference in thickness of joined pipes with a wall thickness ratio of more than 1.5 is a defect (with the exception of joints made according to special technical conditions, with a corresponding entry in the welding log as part of the as-built documentation).

A circumferential weld containing one or more defects is a “defective weld.” In databases containing information about defects, “defective welded joints” are subject to recording without indicating the number of defects.

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A– defect on the pipe; b – mechanical treatment of the defect; V– diagram of the welds; 1 – surfacing layer; 2 – filling layers; 3 – contour seam;

4 – facing seam.

Repair by welding is subject to pipes that have separately located single defects, which are considered to be defects that have a distance between them:

ü not less than 300 mm with a maximum defect size less than or equal to 35 mm;

ü not less than 500 mm with a maximum defect size of 35 to 60 mm.

In this case, the number of defects per linear meter of pipe should not exceed two.

The following defects on pipes cannot be repaired by welding:

ü defects whose dimensions exceed the values ​​are given in table. 9.7; corrosion cavities and

ü sinks located on connecting nodes and parts;

ü defects located at a distance of less than 300 mm from longitudinal and circumferential welds;

ü defects that have cracks or visible delamination of the metal, as well as those located on dents. Welding of defects on pipes is carried out by manual arc welding with basic type electrodes.

Before welding, the defect is processed mechanically (with a grinder or cutter) for the purpose (Fig. 9.9.b):

· obtaining a crater shape that ensures a uniform and high-quality position of the metal;

· complete removal of corrosion products and possible surface microcracks.

The areas adjacent to the crater are cleaned to a metallic shine to a width of at least 15 mm with the preliminary removal of remnants of the insulating coating, rust, dirt, and oil stains.

Heating of pipe metal before welding is established according to table. 9.8.

Table 9.8

Preheating of pipe metal

Heating of pipes is carried out, as a rule, by electric or gas heaters. Temperature control is carried out by devices of the TP-1 type.

Welding of pipe defects with a tensile strength from 42 to 55 kgf/mm 2 is carried out using electrodes of the brands UONI 13/55, "Garant", LB52A(I) with a diameter of 2.5-4.0 mm, welding of pipe defects with a tensile strength of 55- 57 kgf/mm 2 – with electrodes of the brands “Schwarz 3K”, VSF-60 or similar.

Surfacing of metal onto a defect includes: the first surfacing layer, filling layers, contour weld, facing seam (Fig. 9.9.c). The first surfacing layer and contour weld are made with electrodes with a diameter of 2.5-3.25 mm, filling and facing - with electrodes with a diameter of 3.0-4.0 mm at the welding modes given in table. 9.9.

Table 9.9

Defect welding mode

When welding defects, the number of guided layers must be at least two (excluding the contour layer of the seam). Welding is performed with beads no more than 20 mm wide with mutual overlap of no less than 3 mm. The contour layer of the seam is made with oscillations perpendicular to the boundary line (Fig. 9.9.d), while the width of this seam , ranges from 8 to 14 mm. The welds are applied tightly with fine flakes (0.5-0.7 mm), which ensures a smooth transition to the base metal of the pipe.

After welding of the defective section of the pipe is completed, the outer surface of the surfacing is processed mechanically, and the surface must be smooth, without visible scaliness, and the reinforcement must be uniform over the entire area. The gain height should be in the range from 0.7 to 1.5 mm (Fig. 9.10) and controlled using an indicator.

Rice. 9.10. Mechanical processing of the deposited area

Thus, welding (welding) of single cavities is carried out.

Group cavities are considered to be a cluster of cavities of at least 10 per 100 cm 2 of the pipe surface.

When the diameter and depth of group cavities significantly weaken the pipe wall, part of the pipe wall with cavities is cut out and a patch is welded in its place flush with the surface of the pipe.

The patch for welding flush with the surface of the pipes is made of oval shape from pipes with a wall thickness equal to the wall thickness of the gas pipeline being repaired, from steel of the same grade or with the same physical and mechanical characteristics. The dimensions of the patches should be no more than 250 mm wide (along the pipe ring), but no more than half the diameter of the pipe, and no more than 350 mm long (along the axis). Minimum patch size: width – 100 mm, length – 150 mm (in the same directions). In all cases, there should be a difference between the length and width of the patch within 50¸100 mm.

The dimensions of welded patches when repairing gas pipelines of various diameters are given in Table. 9.10.

Table 9.10

Dimensions of welded patches

The edges of the welded patches are cut with beveled edges. The hole in the pipe is cut in the shape of the patch, also with beveled edges. The edges of the patch and hole are dulled. To ensure complete penetration, the patches are welded with backing rings (Fig. 9.11.a).

The backing ring is made of sheet steel 3¸4 mm thick and 20¸30 mm wide and is welded to the patch from the inside so that its edges protrude beyond the edges of the patch by 10¸12 mm. The gap between the edges of the patch and the pipe to ensure penetration should be 2¸5 mm. The patch is welded with electrodes type UONI 13/55. For a patch thickness of up to 12 mm, the weld is welded in three layers. The root of the seam is made with electrodes with a diameter of 3 mm, subsequent layers - with electrodes with a diameter of 3¸4 mm. Welding is performed in reverse - with a stepped seam around the entire perimeter of the patch in three or four steps (Fig. 9.11.b). To press the patch to the pipe for the time being, tacks are used with a device installed above the hole, which is a U-shaped bracket with a screw in the middle, equipped with a gripping device. A small bracket is also welded onto the patch for this purpose.

Rice. 9.12. Layout of cuts and patches:

A=IN=WITH=500 mm; D=1500 mm; L=9360 mm

distance between hubs along the perimeter – 200 mm

A pipe with a diameter of 820 and a wall thickness of 9 mm, made of 19G steel and in operation for 27 years, with the following characteristics was selected for research:

ü longitudinal stresses s t= 407 MPa, s vr=555 MPa;

ü transverse stresses s t= 456 MPa, s vr= 557 MPa.

5 identical cuts were made on the outer surface of the pipe: with a length of 200; depth – 3.5 and width 3 mm. The scheme for making cuts is shown in Fig. 9.12.

To get closer to operating conditions, the pipe was subjected to hydraulic tests in a cyclic mode. The cycle range was: Pmin= 2.0 MPa; Pmax= 4.0 MPa.

The analysis of the pipe condition was carried out in stages after exposure to a load of 200 cycles.

Cut No. 5 was certified before the start of testing, sequentially at each stage one cut was welded, a window measuring 150 x 200 mm was cut out and a patch was welded. After 600 cycles, four cuts were welded (Nos. 2,3,4,5) and patches Nos. 6,7,8 were welded. The pipe was then subjected to 5,000 cycles of stress, after which notch #1 was cut out and a patch was welded in its place. The sequence of repairing cuts and welding patches is given in Table. 9.11.

Methods of cap. repairs to restore the MTP wall.

Defects of the MTP wall.

TECHNOLOGY FOR REPLACING A DAMAGED SECTION OF OIL PIPELINE

Types of repair work on the linear part of the MTP.

Repair of a damaged section of a pipeline by replacing it is carried out upon detection (presence):

cracks 50 mm or more long in the weld or base metal of the pipe;

rupture of the annular (assembly) seam;

rupture of the longitudinal (factory) seam and pipe metal;

dents with a depth exceeding 3.5% of the pipe diameter;

scratches with a depth of more than 30% of the wall thickness and a length of 50 mm or more.

Depending on the adopted technology for carrying out work, replacement of a pipe section can be carried out: with a stop in oil pumping through the pipeline for the entire period of restoration work, while the emergency section can be completely or partially cleared of oil; with the installation of a bypass (bypass) line, which requires stopping pumping only for the period of its installation and connection.

After pumping is stopped, the detected emergency section is blocked from the rest of the route by two linear valves. In case of accidents on oil pipelines with a telemechanization system, the pumping units are automatically switched off and the damaged section is localized using linear valves.

See question 22

Pipe wall defects are defects that do not lead to a change in the flow area of ​​the pipe. They are divided into the following groups:

loss of metal (corrosion, erosion, dent in rolled steel, nick, scuff, flaw) - a change in the nominal thickness of the pipe wall, characterized by local thinning as a result of mechanical or corrosion damage or due to manufacturing technology;

risk (scratch) - loss of metal from the pipe wall that occurs as a result of the interaction of the pipe wall with a solid body moving along it;

delamination - discontinuity in the metal of the pipe wall; usually is a rolled out accumulation of non-metallic inclusions;

change in wall thickness - a gradual thinning of the pipe wall formed during the production of pipes or rolled sheets;

crack - a rupture of the base metal of the pipe wall, characterized by a small transverse size;

defect of St. weld (lack of penetration, time, slag inclusion, undercut, weld crack) - a defect in the weld itself. weld or OSH caused by a violation of welding technology.

Based on the degree of impact on the bearing capacity of the oil pipeline, defects are classified into dangerous and non-hazardous.

Dangerous defects include:

geometry defects adjacent to welds or directly on the seams, if their measured depth exceeds 3% of the nominal outer diameter of the pipe;

defects that are dangerous according to the results of static strength calculations (the calculated fracture pressure of the defective pipe is lower than the factory test pressure);

wall defects associated with loss of metal, with the residual thickness of the pipe wall at the level of the technically possible minimum measurement limit of the flaw detector projectile.

Hazardous defects are subject to selective repair in accordance with established hazardous defect repair methods.

Non-hazardous defects include defects for which the calculated fracture pressure of the defective pipe is not lower than the factory test pressure. Operation of the pump in the presence of non-hazardous defects is permitted without restrictions on pumping modes during the inspection period.

According to the criterion of the need for additional defectoscopic inspection (DDT), defects are divided into those requiring DDT and those not requiring DDT.

A number of defects in pipes and welds are repaired without cutting out the defective area. Corrosion ulcers can be welded during the repair of oil pipelines under the pressure of pumped oil up to 3.5 MPa.

Damage to the pipeline wall up to 5% of the pipe thickness (scratches, pits, scuffs, nicks) is eliminated by grinding. In this case, the wall thickness should not be taken beyond the minus tolerance of the pipes.

Corrosion damage with a depth of more than 5% of the pipe wall thickness can be repaired in accordance with the “Instructions for the safe conduct of welding work during the repair of oil and product pipelines under pressure.” If there is continuous corrosion, the oil pipeline is repaired by welding overhead reinforcing elements (patches, couplings).

The technology for welding corrosion damage consists of two stages: preparatory work (surface cleaning) and welding itself. The welding site is cleaned to a metallic shine within a radius of at least two damage diameters (largest linear dimensions). Surface cleaning can be done manually using sandblasting machines. It is possible to use other cleaning methods (for example, chemical) to completely remove corrosion products.

If dents with a depth of up to 3.5% of the diameter of the pipeline are detected, they are allowed to be straightened using non-impact devices.

Damage to the pump in the form of fistulas and cracks up to 50 mm long is repaired without emptying the pumped product by welding patches, clamps, and couplings.

The dimensions of the overlay elements and couplings must cover the defect location by at least 40 mm from its edges. The patch should have an elliptical shape. The length of the coupling without technological rings should be in the range of 150-300 mm. When the coupling length is more than 300 mm, process rings must be used.

23. Cap. repair of defects with cutting out the “coil”.

This scheme can be used for selective repair of oil pipeline sections that have dangerous defects, i.e. violation of the geometry of pipe walls (dents, corrugations) is above permissible limits.

Repairs are carried out by cutting out the defective part of the TP and replacing it with a new one, stopping the pumping. The length of the cut out defective area must be at least 100 mm greater than the defect itself on each side. The minimum permissible length of the “reel” is not less than the diameter of the oil pipeline being repaired.

The work begins with the preparation of working documentation based on in-line flaw detection data.

On-site repair of a defective area begins with opening the defective area and preparatory work for pumping out the oil.

The opening of the defective area and the development of a pit for dismantling and installation work are carried out using a single-bucket excavator. Digging under an oil pipeline can be done simultaneously when opening it with an excavator with a rotary bucket or manually.

Cleaning the exposed section of the oil pipeline from the old insulating coating is carried out with a cleaning device or manually, after which a thorough inspection of the TP is carried out to ensure that there is no product leakage.

Having measured the distance between the treated ends of the oil pipeline, prepare a “reel” from a pre-pressured pipe or the pipe as a whole.

If you have a device for marking the pipe, it is possible to initially prepare a “reel” of a given length, according to the dimensions of which the ends of the oil pipeline are marked and prepared.

The “coil” is joined to the TP using a pipelayer or truck crane, the joint is assembled using external centralizers and the joined ends are fixed using tacks evenly around the perimeter.

Requirements for the qualifications of welders, assembly, welding and quality control of welded joints of oil pipelines remain the same as for the construction of new oil pipelines.

Quality control of welds - visual and radiographic, regardless of the category of TP sections. If the quality of the weld is satisfactory, the technological holes are plugged with metal plugs and scalded after filling the pipeline with oil until it reaches operating mode.

If, when emptying the pipeline, oil was pumped into an earthen pit or rubber-fabric tanks, then it is necessary to pump it into the oil pipeline being repaired before resuming pumping water through it and dismantle the piping circuit of the oil pipeline with the pumping pump unit.

The next significant and complex technological operation is removing air from the oil pipeline.

Cleaning and applying an insulating coating to the oil pipeline of the section being repaired is carried out using appropriate cleaning and insulating devices or manually. This depends on the length of the section, the diameter of the pipes and the type of insulating coating.

The work ends with the reclamation of the fertile soil layer, leveling and cleaning of the surrounding area, restoration of route structures, signs, etc., if they were disturbed during the work.

Repair of defects in the base metal of the pipe (dents, corrugations, corrosion, loss of metal, scuffing, delamination, cracks, etc.) is proposed to be carried out using special equipment. The universal underwater chamber (caisson) is designed to repair damage to underwater passages of oil pipelines in dry conditions under normal pressure using the same repair methods as on the surface.

This camera allows you to repair defective sections of pipes in various ways (installation of welded couplings, installation of composite couplings, inserting coils, grinding, welding, etc.), repairing the insulation of the main gas pipeline and other dry work on pipes with a diameter of up to 1420 mm. Working depth - up to 30 m.

The disassembled camera can be quickly delivered to any area by any means of transport, incl. aviation. The equipment is patented, has a GOST R certificate of conformity and permission for use from Rostechnadzor.

Welders are certified to level I in the NAKS system with permission to work on oil and gas production equipment, taking into account the additional requirements of Transneft AK

Installation of an underwater chamber (caisson) during repair of a main oil or gas pipeline:

Figure 7 Universal underwater chamber (caisson) for gas pipeline repair - an inside view

Figure 8 Classification of pipeline defects (main oil pipeline and gas pipeline)

Oil pipeline defects are divided into defects subject to repair (DSR), from which, according to the degree of danger, defects of priority repair (POR) are distinguished.

A defect subject to repair is each individual non-compliance with regulatory documents: walls, welds, geometric shapes of the pipe, as well as connecting, structural parts and welded elements on the oil pipeline or included in its composition that do not comply with regulatory documents.

A priority repair defect is a defect that limits the operation of an oil pipeline section for a period of 1 year or less and reduces the design bearing capacity of the oil pipeline, as well as a defect for which strength and durability are not determined to be repaired.

Pipe geometry defects

“Dent” is a local decrease in the flow area of ​​a pipe at a length less than 1.5 times the nominal pipe diameter D, without a break in the axis of the oil pipeline, resulting from transverse mechanical impact.

“Corrugation” is a reduction in the flow area of ​​a pipe, accompanied by alternating transverse convexities and concavities of the wall, as a result of loss of stability from transverse bending with a break in the axis of the oil pipeline.

“Narrowing” is a reduction in the flow area of ​​a pipe with a length of 1.5 times the nominal pipe diameter or more, in which the pipe cross-section has a deviation from the circle (Dн-d)/Dн, 2% or more, where Dн is the nominal outer diameter of the pipe, d is the minimum measured outside diameter of the pipe.

Pipe wall defects

“Metal loss” is a local decrease in the thickness of the pipe wall as a result of corrosion damage to the oil pipeline. Metal losses are divided into combined and single. Combined metal loss is a group of two or more corrosion defects combined into a single defect if the distance between adjacent defects is less than or equal to the value of 4 pipe wall thicknesses in the area of ​​the defects. A single metal loss is one metal loss defect, the distance from which to the nearest metal loss exceeds the value of 4 pipe wall thicknesses in the area of ​​the defect.

“Reduction in wall thickness” is a gradual thinning of the wall formed during the manufacturing process of a hot-rolled pipe or a technological defect in the rolled product.

“Delamination” is an internal violation of the continuity of the pipe metal in the longitudinal and transverse directions, separating the metal of the pipe wall into layers of technological origin. “Delamination with access to the surface” (sunset, rolling film) - delamination extending to the outer or inner surface of the pipe. "Delamination in the heat-affected zone" - delamination adjacent to the weld (the distance of the seam transition line to the base metal to the edge of the delamination is less than or equal to the value of 4 pipe wall thicknesses).

“Crack” is a defect in the form of a rupture in the metal wall of an oil pipeline pipe.

“Surface defect” is a rolled defect on the pipe surface (rolled out contamination, rippling, scaliness, overheating of the surface, rolled-in scale, scale pits, indentation pits), which does not bring the pipe wall thickness beyond the maximum dimensions according to GOST 19903-74.

Defects in the welded joint (seam)

Crack, lack of penetration, lack of fusion - defects in the form of discontinuity of metal along the weld. Pores, slag inclusions, sink marks, undercuts, excess penetration, sagging, scaliness, deviations of weld dimensions from the requirements of regulatory documents - “anomalies” of a transverse, longitudinal, spiral weld.

Edge displacement is a discrepancy between the levels of location of the internal and external surfaces of the walls of welded (welded) pipes (for a transverse weld) or sheets (for spiral and longitudinal seams) in butt welded joints.

An oblique joint is a welded butt joint between a pipe and a pipe (with a spool, with a connecting part of a main oil pipeline), in which the longitudinal axes of the pipes are located at an angle to each other. A connection with an angle of 3 degrees or more between the pipe axes to each other is classified as a defect “oblique joint” of the transverse weld.

The procedure for repairing a main pipeline (oil pipeline, gas pipeline). Elimination of defects in a pipeline (oil pipeline, gas pipeline) subject to repair is carried out by selective repair of individual defects in accordance with the methods regulated by this document, and during major repairs with replacement of pipes and replacement of insulation. During a major overhaul involving replacement of gas pipeline insulation, all existing defects in the area that need to be repaired must be repaired, followed by replacement of the insulation. More information about the technology for replacing the insulation of a main pipeline (oil pipeline, gas pipeline)