Michael+Fox+ +in+Service+Welding+of+Steel+Pipelines

7/27/2019 Michael+Fox+ +in+Service+Welding+of+Steel+Pipelines

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In Service Welding of Steel Pipelines

Presented by Michael Fox



APA Group Presentation 2

What is In Service Welding

Performing a welding process onto a section of pipe which contains a fluid or gas atpressure and or under flowing conditions

Common welding processes used include;

– Manual Metal Arc Welding (Stick Welding)

– Gas Metal Arc Welding (MIG Welding)

– Flux Cored Arc Welding

– Gas Tungsten Arc Welding (TIG Welding)




Why is In Service Welding Performed

Repair of Pipeline Damage Possibly Including;

– Internal / External Corrosion or Erosion

– Mechanical Damage eg; Dent, Gouge

– Cracking

Installation of Branch Connections

To Perform Hot Tap Operations




Types of In Service Welds

Weld Deposition Repair Repair Sleeves

Hot Tap Fittings Branch Connections




What do the Standards Require

AS4645 Gas Distribution Networks

– Part 2 Steel Pipe Systems

• Welding to be performed as per AS2885.2

AS2885 Pipelines - Gas and Liquid Petroleum

– Part 2 Welding

• Section 13 covers welding onto an in-service pipeline

• Defines limits as containing stationary or flowing flammable fluid or internal pressure greater than50kpa

• Requires development and approval of qualified welding procedure

• Development of welding procedure requires worst case simulated testing, destructive and non-destructive testing to meet specified acceptance criteria

• Requires your welders to be qualified to use welding procedures

• Requires a risk assessment to be performed to examine threats to public and operationalpersonnel's safety and the continuity of supply




Role of Welding Procedures & Welder Qualification

Procedure Qualification record (PQR)

– Record of actual welding variables used on simulated test weld and copies of destructive

and non destructive testing reports

– Used to prove that the variables used to make the test weld produce a weld of sufficientstrength and meets allowable limits for defects

Weld Procedure Specification (WPS)

– Developed from the PQR

– A document used to describe to a welder how to setup their machinery and physicallyperform a weld to replicate welds produced in the simulated testing

Welder Qualification Record (WQR)

– Tests an individual welders capability to follow a Weld Procedure Specification andproduce a weld that meets allowable limits for defects




Example WPS




Design of In Service Weld Procedures

Design of In Service weld procedures aims to minimise the risk of

1. Burnthrough – occurs when un-melted area of pipe wall beneath weld pool has insufficient

strength to contain internal pressure

2. Hydrogen Assisted Cold Cracking – where through wall cracks in or adjacent to a weld candevelop after the completion of the weld

Finite Element Thermal Analysis Software Models are available for calculation ofBurnthrough and Hydrogen Assisted Cold Cracking risk

Experimental data and reports are available for guidance in selecting appropriatevalues for variables

Design of weld procedures should always be confirmed by testing and qualification ofthe procedure




Factors Effecting Burnthrough

Pipeline Wall Thickness

– Increased risk with thinner wall thickness due to reduced heat sink capacity and greater

through wall heat affects

– Burnthrough risk extremely remote if wall thickness 6.4mm or greater provided lowhydrogen consumables and normal welding practice is used

Operating Conditions

– Pipeline contents (liquid or gas), flow rate, pressure and fluid temperature all effect heatsink ability

Weld Penetration / Heat Input

– Amount of heat being put into weld by welder

– Heat Input = ( Amps x Volts x 60)

(Travel Speed x 10000




Control of Burnthrough

Primary control of Burnthrough risk is through limiting the heat input

– Achieved through setting preheat, welding current, volts, electrode type & size and travel

speed – Pipeline inside wall temperature calculated using thermal analysis software to predict

likelihood

– Worst case simulated testing can confirm if Burnthrough is possible

Secondary control is adjustment of flow rate

– A higher flow rate will remove heat quicker and reduce the risk of Burnthrough

– A suitable flow rate must be chosen to minimise both Burnthrough and hydrogen crackingrisks

Non destructive testing should be used to ensure pipeline nominal wall thickness isstill remaining

Lowering pipeline pressure can increase risk of Burnthrough due to change in thermalconductivity

Burnthrough can still occur at very low pressure if inside pipe wall temperature issufficiently high




Hydrogen Assisted Cold Cracking




Factors Effecting Hydrogen Assisted

Cold Cracking

Three conditions must be simultaneously present for hydrogen cracking to occur

– Presence of hydrogen in the Weld

• Due to high temperatures in the welding process hydrogen has a high solubility rate

• Hydrogen is trapped in the solidifying weld metal microstructure and can provide easy paths forcracks to develop along

– Presence of a crack susceptible microstructure

• Martensite microstructure is formed on rapid cooling of carbon steel and is very hard and brittle

• Highly dependant on the chemical composition of steels and is affected by carbon content andcarbon equivalent content

– Presence of tensile stress

• Stresses applied to the weld are the driving force for cracks to develop and propagate




Control of Hydrogen Assisted Cold Cracking

Prevention of hydrogen cracking can be achieved by eliminating at least one of thethree conditions

– Perform welding using low hydrogen practices including

• use of low hydrogen electrodes/consumables

• clean working practices removing all forms of grease, oils, solvents etc

• excluding moisture and water from the weld area

– Limit possibility of developing crack susceptible microstructure• Achieved through controlling the weld cooling rate including applying a preheat, selecting a suitable

heat input or applying a post weld heat treatment

• Using steels with a lower carbon content lowering the highest possible hardness

• Using steels with a lower carbon equivalent content lowering the likelihood of developing highhardness areas

– Reduce external applied stresses

• Avoid stress concentrations by applying good sleeve fit up eg: minimise gap between sleeve andpipe

• Apply proper support and backfilling techniques




Practical Implementation of Welding

Procedures

Training of Welders

– The welding process is highly dependant on the skill and dexterity of your welders the

more time you can invest in your welders the better – More training can help welders overcome mental fear factor of in service welding

Supervision

– Is the most critical step once you have a qualified weld procedure is to ensure it isimplemented correctly

– AS2885 says welds should be supervised by an experienced and trained supervisor

Non Destructive Testing (NDT)

– Allow at least 24-48 hrs to perform NDT on in service welds to allow time for hydrogencrack to develop

– Chose NDT method suitable to the type of weld

Qualify procedures to cover ranges

– During the qualification of weld procedures try to test your variables to cover suitableranges to save having to perform additional qualifications in the future

– eg; fitting & pipe sizes / wall thicknesses, travel speeds/heat inputs




Golden Rules of In Service Welding

1. The best way to avoid welding problems is to avoid welding

If you can use a different method which is suitable use it

2. Weld procedures and welders must be qualified and work under good supervision

3. Confirm your actual pipeline operating conditions before welding

4. Burnthrough risk is remote if the pipeline wall thickness is greater than 6.4mm

5. The first line of defence against hydrogen cracking is to limit the amount of hydrogenin the welding process

ie. Use low hydrogen electrodes/consumables

6. Try to develop a smaller number of weld procedures to cover the ranges ofapplications




Delivering Australia’s Energy

www.apa.com.au

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