- Pipe and other metal components for hazardous liquid and natural gas pipeline systems have been manufactured since the early 1900’s.
- Early steel-making processes did not always remove all of the impurities from the steel that could lead to failures.
- In addition, some early methods for welding the longitudinal seam on some types of pipe produced welds that were susceptible to failure.
- Pipe and components produced using these early steel manufacturing and welding techniques are still in use today.
- Steel-making and pipe manufacturing techniques have improved dramatically over time, with a corresponding dramatic improvement in performance.
- Pipe failures from material and weld defects account for a small percentage of all pipeline failures.
What is a Material Failure and why does it occur?
During the manufacture of steel, impurities can sometimes remain in the molten steel. These impurities can cause an incomplete bonding of the material in the steel plate or solid round steel billet used to produce pipe and other pipeline components. Although early steel-making processes allowed more impurities to remain, newer, improved processes have reduced this potential and vastly improved the quality of the steel used. Still, defects in the steel material can result in failures.
The types of material defects that can exist in the steel used to make pipe and other components include:
- Laminations and inclusions – Laminations and inclusions can occur as a result of oxides or other impurities trapped in the material. As the material cools in the manufacturing process, a small pocket is formed internal to the steel plate or billet. A lamination or inclusion can eventually lead to failure when they are oriented such that they eventually grow to the inner or outer wall of the pipe or pipeline component through pressure cycles.
- Blisters and scabs – These appear as raised spots on the material surface caused by the expansion of trapped gas within the steel. These defects reduce the wall thickness of the pipe or pipeline component, and, if large enough, can reduce the pressure-carrying capacity of the pipe or component.
Defects can also occur due to the manufacturing process used to roll steel plate or convert solid steel billets into pipe. During the manufacturing process, rolled steel plate is moved and shaped by mechanical means to make certain types of pipe. Alternatively, “seamless” pipe (pipe without a longitudinal weld seam) is produced from solid steel billets by passing a mandrel through the billet to produce a hollow steel cylinder.
The types of defects that can occur during the pipe manufacturing process include “hard spots” created by localized quenching (or cooling) of the plate material during the rolling process and, for seamless pipe only, indentations formed by the expanders or mandrels used to make the pipe. Cracking can occur at “hard spots” that eventually grow in size over time, and stress risers can occur at indentations if they are too deep, eventually leading to pipe failure.
And if pipe is not loaded and supported properly prior to being transported long distances, cracks can occur due to a phenomenon called ‘transit fatique”. Transit fatigue occurs when pipe flexes in a certain manner repeatedly over long periods of time during transport, resulting in cracking of the pipe wall. Fortunately, these defects are typically discovered during the hydrostatic pressure testing that occurs prior to the pipeline being placed in service; however, some can remain and grow during pipeline pressure cycles until a failure occurs.
What is a Weld Failure and why does it occur?
Welds are used in the making of the pipe itself, in the joining of pipe and components during construction, and during maintenance and repair of the pipeline system. During the pipe manufacturing process, longitudinal welds join the edges of steel plate to form sections of pipe. Also during manufacture, girth (or circumferential) welds sometimes join sections of pipe – called ”double-jointing” - to speed installation. In addition, many welds have to be performed at the construction site to join together pipe sections and other components to create a pipeline system – these are almost always girth welds. And during maintenance and repair, many other types of welds are also used.
The causes of weld failure that can occur in current manufacturing and construction processes include:
- Pinholes – A pinhole is a small, unwelded area extending through the entire thickness of the weld.
- Toe cracks – A toe crack can occur where the crown of the weld bead intersects the edge of the plate. This defect is seen in pipe joined using the Double Submerged Arc Welding (DSAW) process.
- Off seam weld – A crack can occur because the inside and outside weld of the DSAW process are offset.
- Undercutting – Undercutting exists when there is an inadvertent reduction in the wall thickness in the area of the weld edge.
- Incomplete fusion – This occurs when there is a lack of complete fusion of the weld and the base metal.
- Porosity – This occurs when one or more voids are created in the weld material from shrinkage of the material.
- Slag inclusions – An inclusion exists when non-metallic material becomes trapped in the weld.
The causes of weld failure in older pipe that are typically associated with obsolete pipe manufacturing processes include:
- Burnt pipe edges – A burnt pipe edge sometimes occurred in the manufacture of Lap Welded pipe. In these cases, the edges of the steel plate were heated to too high a temperature and can be susceptible to cracking.
- Incomplete fusion – If the edges of Lap and Flash Welded pipe were not heated to a high enough temperature during manufacture to ensure impurities were forced out of the seam during compression, the edges sometimes did not properly bond together.
- Hook cracks – A hook crack is a lamination that exists in the weld zone that curves, or hooks, towards the inside or outside surface of the pipe. Hook cracks are typically a phenomenon of some types of early Electric Resistance Welding (ERW) processes.
- Cold Welds – A cold weld can sometimes occur where the seam was joined using heat and pressure to form the bond. Either inadequate heat or pressure could result in the plate edges not forming a complete bond.
- Weld metal cracks – A weld metal crack can sometimes form during the manufacturing process when the plate edges are moved before they are cooled completely. This defect occurred in some early seam joining processes where the pipe could not be held securely during seam joining.
What are the risks from Material/Weld Failures?
Material or Weld Failures can result in both public safety and environmental hazards due to the release of petroleum and natural gas products. From 1996 – 2003, there were four injuries attributed to Material/Weld Failures in the natural gas industry.
What is being done to prevent Material/Weld Failures?
Continuous improvements in the manufacture of steel and pipe and in welding processes in general have occurred since pipe was being mass produced for the oil and gas industries in the early 1900’s. The steel produced today has fewer impurities and welding techniques are vastly improved.
Pipe manufacturers pressure test each pipeline section before it is delivered to a construction site. And the welds of new pipeline construction are non-destructively tested to ensure they have no defects. Welders must meet strict qualification and certification requirements, and new pipeline systems or segments must be hydrostatically pressure tested to 1.25 times the maximum allowable operating pressure before the pipeline is placed into service.
Once pipelines are in service, pipeline operators must periodically assess their condition. Assessment methodologies are chosen in order to identify material and weld defects that are suspected. OPS’s Integrity Management regulations require rigorous threat and risk analyses along with periodic integrity assessments to assure that potential material and weld defects are checked and, if needed, repaired.
Material/Weld Failure: What more can be done?
- General Public: Be aware of pipelines located near you. Be observant for signs of pipeline damage, leakage, or security concerns. Report any concerns you have regarding pipeline safety to the pipeline operator immediately.
- Industry : Pipeline manufacturers can continue to improve pipeline manufacturing techniques. Pipeline operators can analyze the risks to their pipelines based on age, manufacturing technique, types of welds, etc. and assess their pipelines for those risks.
- Regulators : OPS and state regulators must continue to pipeline construction to ensure welds are performed properly, welders are properly qualified, non-destructive testing is performed, and hydrostatic testing is properly performed. Regulators need to also ensure pipeline repairs are being performed properly.
Material/Weld Failure: Where can I learn more?
Date of Revision: 07292014