Petronet, a local company that transports natural gas through underground pipelines across South Africa, had approached MA2K Testing Labs to conduct an investigation on a leakage on the pipeline, which was found during a random leak inspection. The Pipeline operates from Johannesburg to Potchefstroom. Upon inspection a joint was singled out as being a major source of leakage. The affected area was joined using a butt weld and reinforced with a clamp. This section was dissected and inspection revealed that the leakage was due to incomplete fusion in the weld which initiated the formation of hook cracks and cavities in the welded material directly adjacent to the weld. External stresses from the environment may have accelerated the growth of the cavities and cracks yet the clamp increased the strength of the joint aiding in the prevention of any accelerated growths of the defects.
As a short term solution it is advised to replace the damaged section with a new pipe. It must be correctly welded of the same material and as an added precaution to weld an additional sleeve over the newly joined and welded section.
Petronet, a leading company in South Africa which transports natural gas through underground pipelines conducted a random leak inspection survey on one of their pipelines operating from Johannesburg to Potchefstroom, and it was discovered that there was a gas leak at the specific joint. The pipe carries natural gas and it is imperative that the leak should be fixed, due to the volatile nature of gas as well as the loss of resource, in turn profits. The systems comprised of numerous seamless pipes and connected using a butt weld; these joints were reinforced by clamps bolted on.
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MA2K Testing Labs had been approached by Petronet, to inspect the leakage and was employed to identify the true cause of failure and as well as make many suitable recommendations for the repair and prevention of malfunction of the joint. The pipeline was effective since the 20th February 1995; this put the pipeline age in the region of 15 years.
Figure 2.1 (Below) was taken on site
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Figure 1.1: Pipeline on Site
Description
The defective section of the pipe, including the clamp was then marked, cut out and removed from the site for further inspection and analysis. The inspection and analysis was carried out in MA2K Testing Labs in Durban.
Two grade A 106 GR.B SCH40 seamless pipes with outside diameters of 168.3mm and wall thickness of 9mm were butt welded end to end. A butt weld consists of the two pipes heated to a proper fusion temperature then joined by means of force. The joint was further secured by a clamp. The operating internal gas pressure was 2620 kPa (380 psi).
Figure 2.1: Pipe Inspection
Chemical composition of the pipe:
Element
C
Si
Mn
P
S
Cr
Mo
Cu
Ni
V
%
≤ 0.30
≤ 0.10
0.291.06
≤0.035
≤0.035
≤0.40
≤0.15
≤0.40
≤0.40
≤0.08
Possibilities Of Failure
Weld Defects
Penetration is poor at the start of welding
Ensure the grades of pipes are constant to optimize bonding strength
Poor quality welds caused by
Inaccurate timing
Incorrect temperature
Incorrect pressure applied
Failure due to fatigue (welding lowers fatigue strength)
Incomplete Fusion
Fusion did not occur between weld metal and fusion faces.
Incomplete penetration
Weld metal does not extend through the joint thickness
Internal Corrosion
Exposed to H2O and foreign material in the gas, such as CO2, H2S, O2, or chlorides.
Under some operating conditions gases formed may cause corrosion
External Corrosion
Oxygen content (oxidizing salts)
Passing through soils of different mineral content
Contact of different metals, particularly at valves (galvanic corrosion)
Mechanical Damage
Normally consists of gouges and dents.
Generally occurs during extraction or incorrect handling of equipment during assembly.
Fatigue Cracks
Results from cyclic stresses that are below the ultimate tensile stress, or even the yield stress of the material.
Material Defects
Material defects are due to contaminants that combine with the material during the melting of steel, the formation of steel or during the heat treating process.
Faults can happen by incorrect storage or processing.
Material defects can occur in different ways.
Example of various defects mentioned below :
Non-metallic inclusions
bad microstructure
surface corrosion
mechanical damage
surface defect
Weld Cracks
Refers to fracture of weld caused as a result of poor parts fit-up, rapid cooling, and localized stresses exceed the ultimate strength of the metal etc.
Examples of weld cracks :
Arc Cracks
Longitudinal Crack
Transverse Crack
Crater Crack
Throat Crack
Toe Crack
Root Crack
Hot Crack
Cold Crack
Hook Crack
Hydrogen Blistering
When metals absorb hydrogen at internal defects, e.g. Laminations and inclusions, Blisters form and eventually forms cracks.
Inspection
The clamp was removed for further inspection of the weld join. The removal of the clamp was done under supervision by National Energy Regulator of South Africa (NERSA).
The pipe surface was required to be cleaned. An acetone solution was used. Liquid Penetrant examination was performed on the weld area of the pipe. Surface discontinuities were visible on the bottom of the pipe. All examinations and analysis was carried out by MA2K Testing Labs. Through inspection it was deduced that a Butt weld was used to join the pipe line and macrostructure images revealed that there were defects on the welded area. Namely weld cracks, improper penetration and incomplete fusion. Cavities were discovered near/adjacent to the welded section of the pipe material.
Figure 4.1: Inspection Of The Pipe Weld
Surface discontinuities were made visible by the Liquid Penetrant examination. These results further justify the suspicion that there was an improper weld process conducted on the joins of the pipeline and thus the cause of the gas leak. Cavities are visible in the pipe walls. This may be caused by vacancies in the material join in the weld.
Figure 4.2: Analysis Of The Pipe Weld
The extracted sample was investigated by method of etching. A Nital solution was used to etch the pipe material. Nital, a combination of alcohol and nitric acid is used for etching of metals. It is ideal for showing the microstructure of carbon steels. The alcohol used can be either Ethanol, M or methanol. By viewing the etched sample under a Scanning Electron Microscope it was revealed that there was an overlap of material and as a result, it was discovered that there is an irregularity or there is a non-uniform overlap or join of material. This could cause a weakness in the join due to variants in material or state of the material. The non-uniform joint of material gives weak point on the structure of the pipeline and thus resulted in a leak.
Arrows on figure 5.3 clearly indicate all the weld areas, the heat affected zones and the pipe material.
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Figure 4.3: Etched Material
Results
Before the pipe was sectioned for detailed examination, a radiographic examination was performed on the premises of MA2K Testing Labs. The following image was revealed on the radiographic film. The radiograph picture shows the microstructure of the material and identifies the weld overlap with the welded material. There are clear indications of variant material or states which cause weakness in the join when added stress is applied. There are cut out root cavities visible in the radiograph which could be the cause of leakage in the pipeline.
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Figure 5.1: Radiograph Of Material
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Figure 5.2: Radiograph Of Material
In figure 6.3 below: The longitudinal cross section of the pipe showed the weld structure inside the pipeline. This revealed defects in the weld structure and cavities in the wall of the pipeline and a clear indication of the crack and fracture lines that could cause a leak in the pipeline. These defects occurred in the joining of the pipe and together with possible external stresses that act on it due to the environment, no corrosion had occurred and pipe was designed to withstand pressure of the gas within the pipe.
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Figure 5.3: Part Section: Inside Of The Pipe
Longitudinal samples were taken from the extracted piece. Cuts were made across the weld line, to define the differences in the material used in the weld and the welded material.
The two cross sections through the wall thickness of the pipe was polished and etched with Nital. The one macroscopic image (Figure 6.4) revealed a clear leakage path through the wall thickness of the pipe. This shows that there was a weakness in the join between the weld material and the welded material. Further fracture and strain caused the weld to split at the join between the weld material and the welded material. Figure 6.5 shows that there was a lack of side wall fusion in the pipe and that there was another weld bead on the left, this had been made to try to fill the vacancies produced from the lack of fusion.
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Figure 5.4: Part Section: Pipe Wall
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Figure 5.5: Source Of Leakage
Conclusions
After thorough investigation of the defective area of the pipe it was found that the leakage was due to improper weld process and lack of fusion, lack of fusion causes cavities. Cavities form cracks over a period of time. Due to this hook cracks were formed and leakage had occurred. Therefore, the components used contain no post installation defects as there was no internal or external corrosion to the material. The design of the material is suited to the environment as it was able to withstand cyclic loading, fatigue stresses and no signs of hydrogen blistering was present on the material. Mechanical damage may have aided or accelerated the growth of the crack size. The clamp made detecting the source of the leak more difficult and may have helped in the prevention or retarded the rate in which the cracks had grown. The condition of the pipe or clamp base steel compositions or microstructures was as expected for the working life of the system. No softening was found associated with the weld and pipe Heat Affected Zone.
Recommendation
Short Term Solution
As a short term solution it is advised to replace the damaged section with a new pipe. It must be correctly welded of the same material and as an added precaution to weld an additional sleeve over the newly joined and welded section. This may suffice for the next 10 years
Long Term Solution
As a long term solution it is advised to replace the entire pipeline with a new combination of materials as per Petroleum Bulk Storage Regulations, the requirement is that all new underground pipelines be as follows:
Pipes must be either constructed of a non-corrodible material such as fiberglass reinforced plastic, nylon or engineered thermo-plastic, or metal such as steel with a cathodic protection system designed to protect it for 30 years.
Pipes may be in single or double-walled construction
Access ports must be installed to permit tightness testing;
Installation must be in accordance with recognized engineering practices
Pipes and joints must be tightness tested before being covered and placed in.
To reduce downtime and loss of profits, the new pipeline may be installed next to the existing pipeline and until completion will be ineffective. Thereafter the new pipeline will replace the old with minimal downtime. This is another advantage of laying down a new pipeline.
Prevention of Lack of Fusion
Surface should be machined/ground smooth to avoid minute discontinuities.
Ensure welding current is sufficient.
Weld arcs should not be longer than the pipe diameter
The "V" angle should not exceed 7 degrees
The strip width should be appropriate and consistent for the diameter of the pipe
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