Macaw’s Pipeline Defects 9780954429508

Citation preview

MANUFACTURING  CONSTRUCTION  OPERATION  COATING  AND CP  DEFECT  INTERACTION 

-- C1S  :  ,.  ., ' ·ＧＮ. ·Eｾ ｟··LIN ｾ Ｎ .·.. .iE  Ｎ ｾ Ｎ E·F .MAC '  ..'. .  ..:.W.  ' S P·I"P ..  .­ -D-Ｚ -" -r

MACAW.S PIPELINE  DEFECTS 

ACKNOWLEDGEMENTS ｾｄｉｔｩｊｈｬｲＮ

We are grateful to all the colleaguesand  friends in the pipeline industry that have  contributed material and constructive  comments during the preparation of this  book.  In particular we wish  to thank the  following organisations and individuals: 

Neither the publishersor the authors,  editors and contributorsand their  employing organisations, nor any persons  acting on behalf of them a) makesany  warranty, express  or implied,with respect  to the useof any information,apparatus,  method or process described in this  publication or t hat such use may not  infringe privately owned rights or b)  assume any liabilities with respect to the  use of,or for damagesresulting from the  use  of any information,apparatus, method  or process disclosed  in this publication,and  the use of anysuch materials,processesor  methods is solely at the risk of the user. 

UABO 

Editor:   Dr Colin Argent   Technical Ed ito rs:   Dr Kevin Prosser (Metallurgy & Welding)   David Norman (Coatings)  

MACAW Engineering Ltd  GE  PII Pipeline Solutions  KD Efird  J O'Shea  Lithgow Associa tes  David Norman Corrosion Control  TWI ­ TheWelding Institute 

Associate Editors :   Peter Morgan   Roger Weatherhead  

"'00LI5JlER 

­-

Data  contained within this publication is  protected by copyright. All rights reserved.  No part of this publication may be  reproduced, stored in a retrieval system,or  t ransmitted in any form or by any means  electronic, mechanical, photocopying,  recording or otherwise with the prior  permission of the publisher. 

Macaw 's Pipeline Defects   ISBN 0­9544295­0­8   Publi shed by:   Yellow Pencil Marketing Co. Ltd.   IPHouse,London Road, Basingstoke,   Hampshire RG24 7J L UK   [email protected]  

'i) MACAW'S PIPWNE  DEFECTS ｾ

ｾＭ

｟

.•. ｾ Ｍ

Ｍ

Ｍ

Ｍ

Ｍ

Ｍ

Ｍ Ｍ

Ｍ

•  

MACAW'S PIPELINE DEFECTS

INTRODUCTION

Theaim of this book isto illustrate the  range of defectsthat may be encountered  in high pressure steel pipelines and  pipeline coatings.It gives advice on the  probable cause and significance of the  defects and comments on appropriate  remedial actions.This book is not intended  as a complete"Do It Yourself Manual"for  the diagnosis and analysis of defects and  use of this manual will not make the user  an"Instant  Expert': Many of the defects  require analysisand diagnosis by a  qualified expert using specialist analytical  techniques.  The defects included in this book  encompass all aspects of high pressure  steel pipeline manufacture, construction  and operation.  The section on manufacture includes  defects in steel plate, seamless  pipe and  seam welds produced by electrical  resistance welding (ERW) and submerged  arc welding (SAW). Girth welds aredealt 

IV 

with in the section on construction along  with poor construction practicessuch as  lack of cover, electrical shorts, poor pipe  support etc.  The section dealing with operational  defects encompasses internal corrosion,  external corrosion,erosion,fatigue, third  party damage,denting and buckling.  Faults in the external corrosion control  system aredealt with in separate sections.  Coating defects are shown for the common  types of mill applied pipe coating and field  applied girth weld coatings.Common  problems with cathodic protection  schemesare also illust rated.  The final sect ion of the book shows  examplesof the inter­action between fault  conditions to create a new integrity riskto  the pipeline; for example shielding of the  cathodic protection by a disbonded  coating to create t he conditions for stress  corrosion cra cking. 

Each sect ion in this book has been  compiled and edited by experienced  engineers with many years practical  experience in pipeline engineering,  metallurgy, welding, coatings and  corrosion.We believe thisvolume will  provide a useful pictorial reference for  engineers working in the pipeline industry.  We would welcome constructive  comment, suggestions or contributions  that would enhance the practical value of  future editions.  All photographs and information  contained in this document are intended  for guidance only.No comment or  suggestion should be adopted as an  authoritative action without seeking  additional guidance from the relevant  codes, procedures or specialist advisors. No  responsibility can be takenfor any  problems which may ariseas a result of the  use of any information contained within  this document. 

Ｌｾ［

MACAW'S PIPELINE  DEFECTS 

MACAW 'S PfP EU NE DEF ECTS

CONTENTS MANUFACTURI NG DEFEGS Tne first section deals with defects that  occur during steelrnakinq, rolling of the  plate for pipe manufacture and the  manufacturing process  itself. In principle,  all critical defects of this kind should be  eliminated in the proof test carried out in  the  pipe mill. 

OPERATtONAl. DEFEOS The third section of the book deals with  defects  that initiate and grow after the  pipeline has  been commissioned.  These  defects may initiate from a pre­existinq  fault within the pipe or they may initiate  from an areaof the pipe surface that is  freeof any manufacturing or  const ructionfaults. 

The second section considers defects  that  may be introduced into the pipeline  during the construction process.  Critical  defects of this kind should be detected in  the post construction hydrostatic  pressure test. 

Operational faults represent the largest  problem for t he pipeline operator  who  must assess which defects are likely to  develop, t he rate  at  which they will  develop and the consequences for  pipeline safety.  All of these issuesaredealt  with in the  operational risk assessment for the  pipeline and the findings of this risk  assessment  will decide the  frequency  and type of periodic proof tests or  inspections designed  to prevent  operational failures. 

,j MACAW'SPIPELINE DEFECTS 

COATING AND CP DEFECTS The fourth section deals with defects in  the external pipe coating and the  cathodic protection system.  These faults  create the conditions in which external  corrosion can develop. 

DEFECT INTERACTION The final section provides examplesof  how defects  interact  to generate new or  modified risks  to  pipeline integrity. 

MANUFACTURING  DEFECTS 

INTRODUCTION This section dealswith defects generated  in the plate, the pipe or the seam welds up  to the time when the pipe passesthe final  mill test and is declared 'fit for purpose' and  is shipped to the client.  Major manufacturing faults will be  detected by the NDT inspection in the  pipe mill,during the cold expansion of the  pipe (this is applied to most U­and­D  formed,straight seam, submerged arc  welded (SAW)  pipe and some spirally  welded pipe) or by hydrostatic testing of  the individual pipes.  Ifthe pipe was not subject to a high­level  mill test then the critical defectsshould be  detected in the post construction proof  test.  Where there is no record of either type of  test a significance of defect assessment  may be required for mill defects identified  from intelligent pit inspections or nondestructive testing examination. 2

Both the mill tests and the post construction pressure test are designed to locate defects with a significant 'through wall' cornponent.These tests are not optimised to locate defects running parallel to the pipe wall, such as centre line segregations or laps.Defects running parallel to the pipe wall may assume greater significance for structural integrity in specific operating conditions, for example wherethe pipeline is subject to bending loads induced from ground movement or where sour service corrosion mechanisms are active. Minor surface irregularities, such as small laps or rolled in scale, pose no immediate threat to pipeline integrity but can initiate pitting corrosion.Much pipe is shipped by sea, asdeck cargo, or coated at coastal coating plants.In either case these small irregularities can accumulate salt and moisture below the pipe surface. Special cleaning is required to remove all traces of

salt and moisture, including pre-heating, water washing and chemicaltreatment. Surface defects such as laps, slivers and blisters also create 'holidays: or defects in thin film coatings. Manufacturing defects are not normally considered to posea major risk to pipeline integrity if they have been subject to a mill test and a post construction hydrostatic pressure test. Reassessment of manufacturing defects may be required wherethere is evidencefor growth in service,for example by corrosion or fatigue. Manufacturing defects are dealt with in four sections.

r, MACAW'S PIPELINE DEFEers

MANUFACTURING DEFECTS

INTRODUCTION

Section One describes defects in rolled plate including structural features t hat become apparent during standard tests. Plate defects may be carried forward into straight seam, ERW and spirally welded pipe.

Section Two deals with seamless pipe that isformed from a steel billet.The billet is pierced and then progressivelyformed into a pipe.Seamless pipe showsvariable wall thickness along its lengt h due to this pipe forming process.Many of the defects found in seamless pipe originate as defects in the ingot from which the pipe wasformed.

{) MACAW'S PI?EI.INEDEFECTS

Section Three deals with ERW, Electric Resistance Welded,pipe.Early ERW seam weldswere created by passing an electric current through the area of the weld from contacts on eitherside of the fusion line.Control of weld quality was a particular problem with this type of welding.Modern ERW pipe is produced using the HFI, High Frequency Induction, technique and this generally produces seam welds of consistent quality.Most of the features shown in this section relate to weld quality but featureslisted under plate defects may alsobe present.

Section Four deals with straight seam DSAW (double submerged arc welded) pipe and concentrates on seam weld defects.These defects may alsobe found in spirally welded DSAW pipe.In both these typesof pipe the edgesof the plate are crimped before the plate isformed into the pipe shape.Thiscrimping action gives more precise'fit-up'for the sea m welding process. The crimp geometry will be removed in cold expanded pipe but this area may be subject to higher strain during hydrostatic pressure testing. Features listed under plate defects may alsobe present in both straight seam and spirally welded pipe.

MANUFACTURING: PLATE DEFECTS

BLISTERS

A void, close to the pipe surface,enclosed by a thin filamentof steel.

Entrapment of gas released during the solidification of the steel.

Not structurally significant May cause problems with holidaysin thin film coatings.Superficia l blisters may be removed by grinding before applicat ion of the coating. Possible initiation sitefor pitting corrosion if the blister is contaminated with soluble salts during storage or transport.

'; MACAW'S PIPELIN E DEFECTS

MANUFACTU RIN G: PLATE DEFECTS

CENTRE LINE SEGREGATIONS

Concentrations of non-metallic matenal in the mid-wall position Including carbon and manganese rich bands.

During solidification non-metallic material may become concentrated at the centre of the ingot which isthe last area to solidify.

Centre line segregations are generally not structurally significant for pressure retention. They may provide an initiation point for cracking in sourservice and may contribute to weld defects.

e» MACAW'S PIPtLINEDEFECTS

MANUFACTURING: PLATE DEFECTS

CENTRE LINE SPLITS

Cracking aong tbe cenue line of the plate in test pieces.

Line of weaknessin the material caused by aligned inclusions or segregations.

ｾＩ

These features area symptom of the test and are not usually structurally significant.

MACAW'S PIPEUNE DEFK TS

7

MANUFACTURI NG: PLATE DEFECTS

INDENTATIONS

A depression in the pipe surface.When viewed in section the grain structurein the steel follows theoutline of the depression.

Pieces of scrap or foreign bodies that have been rolled into the plate surface.

Commonly thesefeatures are relatively shallow and are too small to be st ructurally signincant. Indentations should be checked with respect to the pipe specification.

8

ｾ Ｇ ｍ

ａｃａ

ｗ

Ｇｓ

PIPELI NEDEFECTS

MANUFAUURING: PLATE DEFEUS

LAMINATIONS

A discontinuity lying parallel to the pipe surface, usually marked by a concentration of non-metallics,

Rolling-out of inclusions, blow holes 0 1 pipes in the parent material.

Large laminations may be structurally significant. Some older pipelines contain pipes with large laminations that have not created a safety hazard in many years service. Surface breaking laminations may contribute to coating faults and may serve as an initiation point for pitt ing corrosion. Superficial laminations may be removed by grinding before application of the coating.

ｾ Ｌ ｍ

ａ ｃａ

ｗ

Ｇｓ

PIPELINEDEFECTS

,9

MANUFACTURING: PLATE DEFECTS

LAPS

A Rap of metal lying flat on the pipe surface,usually with a trapped residue of oxide or scale beneath it.

A metalprotrusion that is folded and rolled into the hot metal surface during rolling ofthe platefrom which the pipe is made.

Generally small laps have little or no structural significance. Laps do create holidays, or defects, in thin film coatings. Soluble saltstrapped in the scale or oxidesbeneath the lap may initiate corrosion pitting. Superflcia l laps may be removed by grinding before application ofthe coating.

;: MACAW'S PIPELINE DEFEO S

MANUFACTURING: PLATE DEFECTS

SEPARATIONS, CHARPY

Cracks, or crack like features that appear in the fractureface of mechanical test pieces.

Separations appear in ( harpy, CrackTip Opening Displacement and Drop Weight TearTest pieces.Their occurrence may be influenced by steel chemistry and plate rolling conditions, particularly heavy deformation at low rolling temperatures. The crack like featuresare related to the steel structure,for example fine bands of martensite.

." MACAW'S PIPELINEDEFEGS

ＭＭＭＭ

ｾＬ

MANUFACTURING: PLATE DEFECTS

SEPARATIONS, CTOD

Cracks, or crack like features t hat appea r in the fracture face of mechanicaltest pieces.

Separationsappearin Charpy. CrackTip Opening Displacement and Drop Weight TearTest pieces.Their occurrence may be influenced by steel chemistry and plate rolling conditions, particularly heavy deformation at low rolling temperatures. The crack like featuresare related to the steel structure,for exampie fine bandsof martensite.

e MACAW'S PIPELINEDEFECTS

MANUFACTURING: PLATE DEFECTS

SEPARATIONS, DWTT

Cracks, or crack like features that appear in thefracture face of mechanical test pieces.

Separations appear in Charpy, Crack Tip Opening Displacement and Drop Weight Tear Test pieces.Their occurrence may be influenced by steel chemistry and plate rolling conditions, particularly heavy deformation at low rolling temperatures. The crack like featuresare related to the steel structure,for example fine bands of martensite.

") MACAW'S F'IPElINEDEfECTS

f i

MANUFACTURING: PLATE DEFECTS

SLIVER

•

A thin. Elongated flap of metal rolled into the pipe surface,often with oxide or scale trapped beneath it.

Rolling out of surfaceasperities, protrusions or solidified splashes of molten metal.

(OMMENF ｾ

- ----ｾＭ

-.ｾ

Small sliversare not structurally significant. Soluble salts contained wit hin scale or oxides beneath the sliver may initiate pitting corrosion.Slivers also create holidays in t hin film coat ings. Superficial slivers may be removed by grinding before application of the coating.

,,) MACAW'S PIPEUNEDEFECTS

-- - - - - - - - - - - - - - - - - _

_-

..__ .... .

MANUFACTU RI NG: SEAMLESS PIPE DEFECTS

COPPER PENETRATION

Crack like features normally at right anglesto the pipe or plate surface.

Copper contamination occurs during the Pilger Mill process, For example following external contamination from defective equipment.Copperon the pipe surface penetrates into grain boundaries and forms a line of weakness that may open up, into crack like features, under st ress.

In extreme cases the cracks can extend through the pipe wall.

ｾＬ

Copper cracking should be detected by the Quality Control and pipe inspection proceduresin the pipe mill.

ｍ

ａ ｃａ

ｗ

Ｇｓ

PIPElINE DEFEGS

ｉ ｾ

MAN UFACTURING : SEAMLESS PIPE DEFECTS

DOUBLE SHELL

A discontinuity in the pipe wall that In extreme cases, splits the pipe wall into two segments.

Commonly associated with an interrupted pour at the ingot stage which allowssurface oxidation to occur within the partiallyformed ingotThe line of oxide remainsasa zone of weakness in the finished ingot and separation occurs along the zone during forging.

Separationsthat run at an angle to the pipe wall may be structurally significant and should be detected by the inspection procedures in the pipe mill. Separationsthat run parallel to the pipe surfacemay not be detected, but may not be structurally significant.

,;. MACAW'S PIPELINEDEFECTS

MANUFACTURING: SEAMLESS PIPE DEFECTS

INGOT DEFECTS

Irregular shaped crack-like defects running into the pipe wall at a high angle to the pipe surface.

Defects in the surfaceof the original ingot that have not been removed during scarfing in the pipe forming process.

Small surface defects are not structurally significa nt but may create defects in thin film coatings.

Ｌｾ

Superficial defects may be removed by grinding before application of the coating.

MACAW'S PIPELI NEDEFECTS

MANUFACTURING: SEAMLESS PIPE DEFECTS

LAMINATIONS

A discontinuity lying parallel to the pipe surface, usually marked by a concentration of non-metallics.

Rolling-out of inclusions,blow holes, pipesor ingot cracks in the parent material.

La rge laminations may be structurally significant. Some older pipelinescontain pipes with large laminations that have not created a safety hazard in many years service.However,such defects could pose a problem if through-thickness stresses are generated in the pipe wall,for example during top tap or repair welding operations. Surface brea king laminations may contribute to coating faults and may serveas an initiation point for pitting corrosion. 18

e: MACAW'S PIPELINE DEFECTS

MANUFACTURING: SEAMLESS PIPE DEFECTS '

LONGITUDINAL SURFACE LAPS

A fiapof metal lying fiat on the pipe surface, usua lly with a trapped residue of oxide or scalebeneath it

A metal protrusion that is folded and deformed into the hot metal surface during the pipe-forming process.

Generally small laps have little or no structural significance.Laps do create holidays,or defects,in thin film coatings. Soluble salts trapped in the scale or oxidesbeneath the lap may act as a corrosion 'hot spot' and initiate pitting. Superficial surface laps may be removed by grinding before applicatio,n of the coating.

tV MACAW'S PIPELINE DEFECTS

19

MANUFAUURING: SEAMLESS PIPE DEFECTS

MANUFACTURING DEFECTS

Irregular shapeddefects in the pipe wall

Solidification defects in the ingot that are carried through into the pipe or piecesof lapped material that have fallen out.

Significant manufacruring defects should be identified by pressure testing in the pipe mill or during the precommissioning pressure test.

C MACAW'S PIPEUNEDEFEG S

MANUFACTURING: SEAMLESS PIPE DEFECTS

SLAG INCLUSIONS

Non-metallic inclusions In the pipe wall.

Ca used when an ingot iscontaminated with slag.ln well controlled pipe production most slag inclusions are removed during the piercing process and when the ends of the ingot are trimmed during the pipe forming process.The shape of t he slag inclusionwill be governed by the forging process.

Slag inclusions are usually too small to be structurally significant. However, surface brea king slag may initiate a corrosion 'hot spot' or a defect in a thi n film pipe coating. Superficial slag inclusions may be removed by grinding before application of the coating.

e MACAW'S PIPE LINE DEF ECTS

,

",-

.1

MANUFACTURING: SEAMLESS PIPE DEF ECTS

SURFACE DEFECTS

Shallow defects that appear at the pipe surface.

Various causes including ingot defects, longitudinal surface laps, slag inclusions, slivers,transversetears and surFace roughness.

Most defects are not structurally significa nt but may create defects in thin film coatings and act as an initiation point for pini ng corrosion.

22

'f, MACAW'S PIPELINEDEFEGS

MANUFACTURING: SEAMLESS PIPE DEFECTS

TRANSVERSE TEARS

Crack like features at the pipe su rface running in the hoop direction.

Cracks in the ingot,for example transverse shrinkage cracks, maybe carried through to the pipe if insufficient material is removedfrom the ingot surface during the scarfing process.

Transverse tears may create defects in thin film coatings. Superficial transverse tears may be removed by grinding before application of the coating.

·0 M ACAW' S ?IPELINEDEFECTS

23

MANUFACfURING: SEAMLESS PIPE DEFECTS

UNEVEN WALL THICKNESS Seamless Pipe

Variation in wall thickness along a pipe, commonly in a spiral pattern.

8

E E

:j

I rn 5 rn

Q)

t: u

-I

-

4

.>c:

ｾ

ｾ

3

j

2

!

Variation in wall thickness may occur during the Pilger process.The pipe is rotated as it progresses through the Pilger mill and this results in the spire] pattern of wall thickness variation.The variation may be -5 - 10% of wall thickness,but in extreme cases the variation may exceed 1O%.

0

0

60

12 0

18 0

240

30 0

360

Location - Degrees

", MACAW'S PIPELINEDEFECTS

MANUF ACTURING: ERW PIPE DEFECTS

BURN MARKS

Local area of transformed,hardened, microstructure.It occurs on either side of the seam weld on the outer surface of the pipe. Burn marks may be associated with lack of fusion in the seam weld.

Poor or intermittent electrical contact between the rollers used to introduce the electrical current to the pipe.The rollers are locatedeither side of the joint line on the outer surface of the pipe.Burn marks arenot found on High Frequency Induction welded pipe.

Good maintenance and operation of the pipemaking equipment in the mill.Repair is not usually required unless the burn marks are extensive when local dressing of the pipe wall,within acceptable ｧ ｲｩ ｮ､ｾ ｧ limits, and subsequent NDT inspection to confirm freedom from cracking should be carried out.

'0 MACAW'S PiPELINEDl:FECTS

•

MANUFACTURING: ERW PIPE DEFECTS

DIVERTED INCLUSIONS

Aligned,non-metallic inclusions in the parent strip are diverted through 90 degrees at the weld seam causing a plane of weakness.

Poor quality, dirty, parent strip.The inclusions become realigned as a consequence of the welding process.

Use good quality, clean, strip. Repair isnot usually required unless the plane of weakness has resulted in a fracture through the pipe wall.When repair is required, replace the pipe or install a repair clamp.

LINEOFWELD ). I

{" MACAW'S PIPELINEDEFECTS

MANUFACTURING: ERW PIPE DEFECTS

HOOK CRACKS

Internal or external cracks within the weld, usually curved in shape.The cracks are axial and at right angles to the pipe surface.

Poor quality, dirty st rip.

Use good quality, clean strip. Replace pipe or install repair clamps or repair shellsas appropriate.

fl MACAW'S PIPEUNEDE, ECTS

MANUFACTURING: ERW PIPE DEFECTS

LACK OF FUSION

Axial,crack like, planar faults at the mid point of the weld.

Incorrect welding parameters,for example low welding current input which leads to insufficient resista nce heating at the joint line. Lack of fusion may be associated with burn markson the outer surface of the pipe.

Ensure good maintenance and operation of the pipemaking equipment in the mill. Replace pipe or install repair clamps or shellsas appropriate.

28

i' , MACAW'S PIPELlNE.DEFECTS

MANUFACTURING: ERW PIPE DEFECTS

MISSED NORMALISING

Lack of grain refined structure at the weld line,leading to poor weld line toughness. Visible on the macro-section as the well defined heat affected zone on either side of the line of the weld.

Misalignment of pipe mill induction heat treatment coil so that on-line post weld treatment misses the weld zone

Ensure correct set up of the weld line tracking equipment in the pipe mill.

ｾ

Missed normalising can lead to failure along the weld line by brittle fracture when the pipeline is subject to severe service conditions. This problem is only detected by pipeline failu re so. replacement is necessary.

MACAW'S PIPELINE DEF[-erS

LINE OF WELD

, I

MANUFACTURING: ERW PIPE DEFECTS

SEAM MISPLACED TRIM

.Axial groove or depression aligned to one side of the seam weld.

Misalignment of the weld trim grinding tool with the result that grinding is offset from the centre line of the weld.

Ensure correct set-up,maintenance and operation of t he equipment in the pipe mill.

LINE OF WELD

01 1

Misplaced trim is unlikely to cont ribute to pipe failure, except in the most extreme operating conditions, so repair is not normally required.

,,>MA,CA,W'S PIPELINE DEFECTS

MANUFACTURING: ERW PIPE DEFECTS

SEAM OVERTRIM

Axial groove or depression on the line of the seam weld.

Excessgrinding of the weld flash.

Ensure correct maintenance, set-up and operation of the pipe making equipment in the mill. Overtrlrn is unlikely to contribute to pipe failure except in the most extreme operating conditions.Repair is not normally necessary.

\'1MACAW'S PIPELINE DEFECTS

LINEOF WELD

tI

MANUFACTURING: ERW PIPE DEFECTS

SEAM UNDERTRIM

"

Linearfeature standing proud of the pipe surface on the line of the seam weld.

Insufficient grinding of the weld flash

Ensure correct set-up, maintenance and operation of the equipment in the pipe mill. Undertrim is unlikely to contribute to pipe failure,except in the most extreme operating conditions,so repair is not normally required.

UNE OF WELD ¢ ｍｾ

Ｎ ｃａ

ｗ

Ｇｓ

PIPEl iNEDEFECTS

MANUFACTURING: SAW/SPIRAL PIPE DEFECTS '

CONCAVE CAP

Concave profile in the weld cap.

Insufficient weld metal caused by a travel speed that is too fast or by too few weld runs.Concave cap is most common at the 3 o'clock and 9 o'clock positions on the pipe.

Use a slow travel speed.Add ast ripper' run to the 3 o'clock and 9 o'clock positions before capping in order to build up the thickness locally. Not a significant defect if the total weld thickness isgreater than the pipe wall thickness.

ｾ ＾

MACAW'S PIPELINEDEFECTS

33

MANUFACTURING: SAW/SPIRA L PIPE DEFECTS

COPPER CRACKING

Cracking in the weld metal,normally close to the weld centre line. Cracks follow the weld solidification structure.

Accidental introduction of copper into the weld pool eitherfrom build up of copperdust from filler wire coating or from melting of electrode contact tube.

Ensure correct maintenance and operation of the seam weld equipment in the pipe mill. Cut out affected pipe and replace.

·r

I I

MANUFACTURING: SAW/ SPIRAL PIPE DEFECTS

LACK OF FUSION, SIDEWALL DE5CRIPlIQN Linear defect at weld sidewall,or in top edge of the weld.

Incorrect positioning of welding head or incorrect welding parameters resulting in lack of fusion.

Ensure weld line tracking equipment is functioning correctly. Ensurecorrect welding proceduresare used. May cause failure during cold expansion in the pipe mill otherwise it mayonly be discovered if the pipe sees severe service conditions Cut out and replace the affected pipe. Alternative repair procedures may be acceptable subject to a full defect assessment.

MACAW'S PIPEliNEDEFEa S

MANUFACTURING: SAW/SPIRAL PIPE DEFECTS

LACK OF INTER PENETRATION

Linear deject between the inside and outsideweld beads. The crack likedefect isaligned through the wall thickness.

Rootface too large or welding conditions incorrect.

I ｾ

Ensure accurate machining of the root face and correct choice of welding conditions. Look for sufficient interpenetration of the external and internal weld beads on macro-sections taken during the Manufacturers Procedure Qualification Test.Checkalso during routine Quality Cont rol testing. May only be discovered if the pipe sees severe service.

e. MACAW'SPiPELINE DEFECTS

MANUFACTURING: SAW/SPIRAL PIPE DEFECTS

MISALIGNMENT

Lack of fusion in the seam weld.

Misalignment of the weld bead resulting in lack of fusion at somepoint in the weld profile.

ZONE OF LACK OF FUSION IN

DSAW SEAMWELD

Misalignment can produce large defects in the seamweld which should be detected in the mill test or precommissioning hydrostatic test.Testing below 105% of the specified minimum yield strength (SMYS) may not reveal all defects.

" MACAW'S PIPELINEDEFECTS

MANUFA CTURING: SAW/SPIRA L PIPE DEFECTS

ROOF TOPPING

Peaking at the SAWseam weld.

Pipe mill defect caused by failure to crimp the plate edges sufficiently before the 'U and 0 ' pipe forming process.

• ,I I

Improve mill practice May be acceptable within specified limits if pipe does not seesevere service. Otherwise reject the pipe

or; MACAW'S PIPELINEDEFEO'S

MANUFACTURING: SAW/SPIRAL PIPE DEFECTS

SLAG INCLUSIONS.

Non-rnerallic inclusions in the pipe wall.

•

Caused when the source steel has a high sulphur content and manganeseor calcium sulphide Inclusions are formed. Their shape dependson the type of inclusion and the plate rolling process.

Slag inclusionsare usuallytoo small to be structurally significant. However, surface breaking Inclusions may initiate corrosion or create a defect in a thin film pipe coating.

" MACAW'S P1PELINE DEFECTS

·f ,

MANUFACTURING: SAW/SPIRAL PIPE DEFECTS

SURFACE LAP ATWELD '

..

Cracking in close proximity to the seam weld and running parallel to it.

Longitudinal ingot cracks that are carried through into t he rolled plate.Such cracks may occur at any point in the pipe plate. Cracksclose to the edge of the plate may open up during the welding process.

Assess the structural Significance of the cracksand repair or reject the pipe as appropriate.

';-r

c; MACAW'SPIPELINEDEFEG S

MANUFACTURING: SAW/SPIRAL PIPE DEFECTS

TOE CRACKS

Cracksat tne junction between the submerged arc bead and the pipe surface.

Causes of toe cracks include: plate surface contamination, poor plate surface quality, roiled in scale, inclusio ns that lead to cracking at the weld toe.Delamination of inclusio ns may occur during weld cooling with subsequent cracking during cold expansion in the pipe mill or during a hydrostatic pressure test.

Use clean, high quality plate. May only be discovered if the pipe sees severe service conditions.

IPEliM: DEFECTS

COATING AND CP: CATHODIC PROTECTION

POTENTIAL PROFILE

CP Potential Profile

A seriesof potential measurements, taken along a pipeline at test posts or in a close interval survey.

-- I

1600 1400 .

The spacing between readings in a close intervaI su rvey may be aslOINas 1m to ensure t hat all areas of ur derprotecuon, however small,are identified.The potential profile on a pipeline typically shows a marked negative spike in close proximity (0 the drain POint, or point of connection to power supply.In contras t, the potential profile usually shows a posmve shift a, coating defects.

ｾＭ

Ｍｮ

';' 1200. >

ｾ

tll

Z

> E III

1000 . 800 600

:;::;

c:

"0

0..

400 200 +--- - - - - -- - - - - - - - - - - - - -_._

_

_-j

0 0

5

10

15

20

25

Dist ance· Km

I.

ON • OFF

··'. 1 ｩＧｉＱ

［ｾｃ

ａ ｗＧｓ

f>IPEJ.. I NE DEFECTS

I

189

COATING AND CP: CATHODIC PROTECTIO N

POTENTIAL RANGE

Severe corrosion Potential comrolled by galvanic effects

Potential r,!1easurernent [Page 188J

Corrosion fisk Natural potential of steel in soil Riskof near neutral pH SCC

Cathodic Dlsbondlng WiigC 158]

Pa rtial protection Some reduction in corrosion rate Riskof high pH SCC Effective cathodic protection in aerobic soil

Effective ca thodic protection In anaerobic soil Increasing overprotection Risk cathodic disbonding of coating Risk of hydrogen cracking in hard spots

or

[PItting Corrosion Page 1041 190

l

COATING AND CP: CATHODIC PROTECTION

SACRI FICIAL ANODE

Zinc or Magnesium alloy sacrificial anode packaged in a low resistance Bentonite fill

Sacrificial anodes are of particular value for small cathodic protection schemes and For cathodic protection in congested areaswhere the low current output avoids interference problems. Zinc anodes may be Ineffectual in sods contaminated with industrial waste, such ascoke.MagneSium anodesare required For effective cathodic protection In these conditions

'. ' MACA W'SPIPEUN[- DEFECTS

191

COATING A ND CP: CATHODI C PROTECTION

STRAY CURRENT

Stray Curren t Interference - 600

- ......__._-

_.

ｾｬ E

ｾ

;:

ｾ

r,

J.

- 70 0

>

.,

- 80 0

ｾ｜

,- 1

- 900

c,

ｮｊｾＱ

)

\ \.f

l",

A,

'I,r

ＭＮＬ

Ｎ

ｾ

ｾ

Aａ

ｾＬ｜

ｾ

='J

.•. _---..-.__ .......

!\ ,0lh' ......

L-J :

I, I

I

ｾｉ

-1 000

r )

.....

Random current flow on a pipeline that may be transient or sustained.

11

- 1100

o

3 00

60 0

900

1200

Time - se cond s

192

1 500

1800

210 0

Sustained stray current imerference may be caused by Interaction with the cathodic pro tection scheme of a nearby pipeline.Transient stray current may be caused by periodic peaks of current flow, for example from electric trains or trams. These transient currents create random spikes (marked by arrows) in a recording of the pipe to soil potential.Telluric acnvny will also create transient fluctuations In the measured pipe potentiaL

..' [V,ACAW'$ PIP l

[DEfECTS

COATING AND CP: CATHODIC PROTECTION

TEST POINT

A cable connection to a buried pipe, that terminates at a test post or box on the surface.The surface connection allows pipe to rail potential measurements to be made.

Accurate pipe to rail potential measurements are needed to maintain the effectiveness of the cathodic protection applied to a pipeline. Routine rnorntonnq may be carried out at test posts only by recordrn g accurate Instant off potentials or by measuring the potential of a test coupon buried at a test post.A periodic close interval potential survey, between test posts may also be required to fully audit the effectiveness of a cathodic protection scheme.

._ MACAW'S r1PELINi'

｛ＩＬ

ｅｆ

..

193

ｾｔｃ

'- - - - -

, -

-

-

-

-

COATII\lG AND CP: CATHODI C PROT ECTION

TRANSFORMER RECTIFIER

Mains or solar powered power supplies for large cathodic protection schemes on cross-cou ntry pipelines and plant sites.

The current output is adjustable 10 provide nnecontrol of the protection field

194

-

'-

-

_._.._--

- -

COATING AND CP: PEr PP COATING DEFECTS

STRESS CRACKING

Irregular cracks peneuating through the polyolefin layer. Cracks are predominantly longitudinal on pipes with a diameter larger than 150 mm, and may be cucurnferential on small diameter pipe.

.

-. _

..

ｾ

.

-

Ｎｾ .

Ｍ y -.

-

A combination of poor stress crack resistan ce in the polyethylene polymer and resid ual stress within the coating from the cooling phase of the coating application process.

Use only polyolefin polymerswith good stress crack resistance. Remove all loose coating If possible blast clean the exposed steel and clean and roughen the coating around the exposed steel. Apply a compatible two-pack, liquid repair system or a circumferential heat shrink repair

ee, IV,ACAW·S PIPI:L1NE DEFECTS

171

COATING AND CP: PE , PP COATING DEFECTS

UV DEGRADATION

Crazing and cracking of the polyethylene

)

)

U\! degradation and embrittlement of a susceptible polyolefin polymer during prolonged storageor in serviceabove ground.

Usc polyolefin polymers with good stress crack resistance. Use carbon black filler to minimise Ultra Violet penetration, Protect the coating from Ultra Violer exposure, Stnp the degraded coating from the pipe Re-preparethe pipe surface and re-coat. i

172 Ｌｾ

Ｂ ＢＧＭ ｾ ｃ ａ ｗ

Ｇｓ

ｐ ｉｲ ｅ ｌ

ｈ ｾ

D€FEc r)

COATING AND CP: LIQUID COATING DEFECTS

DELAMINATION

Loss of adhesion between one layer of coating and the next.

Contaminalion between layers. Fu II cure attained on the first layer before the second layer was applied.

Make sure there is no contamination between layers. Ensure that the manufacturers recommendations are Followed For the timing of over-coating If the delaminated area can be removed prepare the substrate by abrasive blast cleaning and apply another layer of coating. Otherwisecompletely remove all layers of coating and re-coat in the approved mann er,

' ,' MAC;\W'S PIPEUNL' DEFECTS

173

COATING i\ ND CP: LIQU ID ( OAT ING DEFECTS

MIS METERING

Mis-metering is characte rised by blisters in the coating and soft areas of Incomplete cure.

Incorrect mixing ratio, for example due to a pump metering problem. Incomplete mixing of the two components, most usually during hand mixing.

By hand: For pre-measured quantities ensure that all material,from pan' A and B, are used and that the two components aremixed until homogeneous.Bypump: Ensure that pumps,metersand the mixing head arefunctioning correctly. Cut out affected areas backto sound coating.Chamfer the edge of the coating and preparethe surface.Apply a compatible two pack, hquid repair material. 174

COATING AND CP: LIQUID COATING DEFECTS

POOR ADHESION

Coating may be removed from the substrate.

Metal substrate contaminated or not prepared correctly, for example the surface profile may not be rough enough. The 'pot lite' of the matenal was exceeded before application The coating may not have flowed into the depressions In the profile or was applied when the ambient temperature was too high.

Observe good application practice and procedures. Completely remove the affected coating. Re-prepare the substrate and re-coat.

Ｍ Ｌ Ｎｾ

M.4.CAW'S PIPE LINE OCr-ECTS

175

COATING AND CP: LIQUID COATING DEFECTS

SAGGING

Runs and sags visible on the coating surface.

Incorrect metering or application of coating in layers that too thick. Poorapplication technique.

Runs and sags are unsightly but if the coating shows correct adhesion and cure the protection performance may be unimpaired.

176

;-\MACAW'S PIP"E..L1 t - DEFE:CT5

COATING Af\j D CP: TAPE COATING DEFECTS

ADHESION LOSS AT OVERLAP

Spiral pattern of corrosion following the line of overlap between successive turns of tape.

Poorapplication procedures giving insufficient overlap between successive turns of tape.The problem may be aggravated by soil stressing or loading.

Specity the minimum acceptable overlap between successive turns of tape, for example55%.Carry out coating procedure qualification tests and operate an effective quality control policy on site. Remove all of the cold applied laminate tape and prepare the substrate and coating on either side.Apply a compatible two pack, liquid urethane coating or re-wrap with cold applied laminate tape ensuring a minimum overlap of at least 55% between turns.