8/16/2019 Integrity Management of Pipe

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Integrity Management of ERW Pipe

How is ERW pipe made? 

In the production of ERW (Electric Resistance Welded) pipe, strip is

 formed into pipe and simultaneously heated and forced together 

through a pair of pressure rolls. This process squeezes the material to

the OD and ID, which is trimmed off. After forming, the bondline is

heat treated by induction heating to temper back any hard

microstructures produced during the welding process.

The types of ERW pipe relate to the heat source used in their

manufacture.

LF – low frequency direct current

HF – high f requency direct current

HFI – high frequency inductance

Many of the problems associated with ERW pipes have occurred in pipe

manufactured pre-1970 using the low frequency (LF) method.

What are the problems? 

Cold Welds

Cold welds are caused by a Lack of Fusion (LoF) between the parent

plates due to inadequate heating and/or pressure during the welding

process. The number of cold welds has reduced since the introduction

of HF and HFI.

Stitching

Stitching is unique to LF welded seams. Non-uniform heating results in

a variation of properties occurring at regular intervals resulting in a

series of black oxide patches on the surface where no bond exists.

Plate Misalignment

Misalignment occurs when the plate edges are not aligned as they are

offered up for welding resulting in a step in the surface which can

initiate further defects.

Heat Treatment Misalignment

The bond line can become twisted as it moves through the rolls and

the subsequent heat treatment becomes aligned in the wrong place.

Hook Cracks

Hook cracks result from metal separation around non-metallic inclusions

in the parent plate. As the material is rolled these inclusions are elongated

parallel to the rolling direction. During welding the edges of the plate

are forced upwards and become oriented perpendicular to the hoop

stress. In this orientation they reduce the pressure carrying capacity of 

the pipe.

What about in service? 

Preferential Corrosion

Preferential corrosion along the ERW line is particularly found in high

sulphur materials that have been not been heat treated. The weld line

in these materials is more susceptible to corrosion due to the

microstructure and the presence of non-metallic inclusions. The weld

area will therefore corrode at a higher rate than the surrounding material.

The result is the formation of a V-shaped groove along the weld line in a

region that tends to have inferior properties to the parent pipe material.

Fatigue

If the pipeline is subjected to a cyclic loading regime, then it is possible

that fatigue cracks could initiate at the pre-existing manufacturing

defects and grow during

the service life of the pipeline.

Approach

Integrity Management of ERW Pipelines

 Assuming the pipeline has a hydrostatic test (either during

construction or sometime during operation) an integrity

strategy can be developed.

 Assess the number of failures that have occurred on the

line. Quantify the defects (length/depth) that would have

survived the last hydrostatic test.

The defects which are common in ERW pipe can be sites for 

the initiation of in-service cracks. Consequently a fatigue

assessment based on actual pressure cyclic data can be

conducted to determine the extent of any growth.

Conduct a fatigue analysis to evaluate when the defects are

expected to reach the dimensions to cause a failure during

normal operating conditions. Then consider utilizing the

Circumferential MFL (TranScan) to characterize the defect

population on the pipeline. Crack opening limitation of 0.1mm.

Utilize the results or reassess fatigue life and TranScan data

to develop a rehabilitation strategy for the pipeline. The

approach is refined to optimize integrity over the

operational life of the pipeline.

If a hydrostatic test was never conducted on the line, and

if it can be independently validated that there are no cold

welds, then the approach described above is applicable.

ERW Integrity Management Example

Background 

• A company in Africa operates a 600 km refined products pipeline.

• The pipe is ERW Grade X52 manufactured in 1964 using the HF method.

• The pipeline has experienced failures from hook cracks, lack of fusion defects, preferential

corrosion and 3rd party damage.

• A Circumferential MFLinspection was used to detect seam weld

defects and dents in the pipeline.

Approach

• Materials testing and metallurgical examination of a pipe sample confirmed the

presence of preferential corrosion and determined that the ERW seam was brittle.

• Therefore an assessment method suitable for the assessment of crack-like defects in low

toughness material was selected.

• The critical seam weld defects were identified, allowing a repair strategy to be

developed.

• In addition, all of the other defects detected by the tool could also be assessed, e.g.repairs, dents, general metal loss, ferrous metal objects.

Fatigue of ERW Pipeline

• One of the consequences of having defects

in ERW pipe is the growth of those defects

under cyclic loading.• Defects can extend as a result of fatigue to

a size that will fail at the operating pressure.

• Given information about the pressure

cycling on the pipeline, fatigue studies can

be undertaken to calculate the fatigue life

of the pipeline.

• As the TranScan tool provides information

on actual defect population, a more

accurate fatigue study can be conducted

 following an inspection.

Hook Crack 

Lack of Fusion/Cold Welds

Stitching

Un-heat treatedMicrostructure

Brittle Weld LineMicrostructure

PreferentialCorrosion Defect

Detection of Seam Weld Cracks

TranScan Inspection Tool

TranScan Signal  Dig Result 

The ERW Manufacturing Process 

Problem Defined

Benefits of an ERW Pipe Integrity Management Program

Crack Excavation Site   Mechanical Damage 

   P   R   E   S   S   U   R   E ,

   B   A   R

READINGS

Line Pressureagainst Time 

FATIGUE

HOOK CRACK

Depth vs. Length

written by Julia Race