September 2021, Vol. 248, No. 9
Features
Increased Safety for Natural Gas Pipelines
By Andreas Thieme, Dillinger
Local hard zones, also called hard spots, are small areas (only a few tenths of a millimeter in thickness) on the surface of steel plates.
Formed on pipes transporting sour natural gas, local hard zones can cause sudden stress corrosion cracking in the through thickness direction. To detect and eliminate these local hard zones reliably during plate production, Dillinger, Europe's leading heavy plate manufacturer, developed a special eddy current testing system.
This method is now recognized by various oil and gas companies. First, pipes made of D-TECT-branded steel were tested for local hard zones on a large scale.
Because of global warming, the world is being forced to change from the old carbon economy to a generation of energy, which will not impact global warming as much. This is a long process, however, requiring bridge technologies that are already securing a significant reduction in CO2 emissions.
One of these bridging technologies that has a reduced carbon footprint is the use of natural gas. The supply of this less harmful combustible has to be secured, of course, in the most efficient way. There is only one reasonable solution – transportation through pipelines.
Pipelines are transporting natural gas at high pressures through all kinds of areas, e.g., cold arctic tundra, hot deserts or seismically active areas. They also are laid offshore in deep waters or at low temperatures.
Besides mechanical loads, this transported medium can cause corrosion problems. In the worst case, acid sour gas can cause various types of cracking inside the pipe wall or on the surface.
This forces the designer to specify challenging mechanical-technological and corrosion properties at the stage of materials selection. These materials are characterized by special toughness at low temperatures, distinctive crack-stop behavior and special corrosion resistance, which must be adjusted evenly throughout the entire production.
The design of pipelines is accomplished with knowledgeable parties so that life, environment and invested capital are protected for the duration of the operation of the pipeline, which can last several decades. Knowledge can be gained about possible new damage throughout the life of operational pipelines with new projects and technical improvements.
Usually, technological advances and knowledge come in small steps of development. However, when unknown damage mechanisms are causing larger failures, it can force the whole supply chain to develop and introduce new technologies in a short amount of time to solve the issue.
Stress Corrosion
As an example, a fatal failure of a pipeline in Asia occurred in 2014. In the presence of H2S as a promotor for the absorption of hydrogen in the steel, stress on the pipe and susceptible areas on the surface was caused by a severe sour gas attack in new, as yet unknown form.
This sour gas attack led to sulfide stress cracking-type (SSC) cracks through the wall of the pipes, stopping operations. These cracks were assumed to be related to very thin local hard zones on the surface of the pipes.
Since this attack, all manufacturers of plates for natural gas pipelines have become aware of this phenomenon occurring on high-tech steel plates for sour service, produced by thermomechanical rolling and accelerated cooling (TM+ACC).
In the meantime, the local hard zones have been investigated. Findings show that the formation of these zones is generally related to several steps in the production process of this type of plates.
These local hard zones have four characteristics:
- Hardness is well above the hardness level of the base material.
- With a thickness of a few tenths of a millimeter, these hard zones are very thin.
- Occurrence of the local hard zones over the plates is irregular and rare.
- Local hard zones cannot be prevented completely.
Conventional hardness testing methods cannot detect these zones on the plates because they have lateral dimensions of only a few centimeters and are statistically distributed on plates with a rolled length of up to 40 meters.
Sour Service
The oil and gas majors do not allow the use of TM+ACC material for the transport of sour natural gas anymore because of uncertainties after finding the harmful phenomenon of local hard zones.
However, because of its advantages in terms of mechanical-technological properties and its efficiency, it is the first choice in production and processing today.
Local hard zones can’t be avoided completely, so a method to find them on the plates in a large-scale production had to be developed. To achieve this, Dillinger, together with an industrial partner specialized in nondestructive testing (NDT), developed a testing method based on eddy current (EC) that can detect small hardness differences on the plate surface.
Eddy current testing (ECT) as such is a well-known and proven testing method that has been used in steel production to detect cracks on hot wide strip materials.
The ETC system can detect areas that are 20 mm in diameter or larger using physical characteristics of the different structures in comparison to the base metal. These show, within a certain threshold, the hardness increase vs. that of the base.
These indicated areas can be repaired through light grinding. The EC device, currently in operation in both of the Dillinger’s plate mills in Germany and in France, is testing plates inline the production without any interruption of the flow.
Since there are no standards or specifications that deal with the subject so far, it was necessary to approve the testing method in the production of heavy plates.
Some oil and gas majors have participated in the testing and final approval of the method and installation. Together with the end-users, a test program based on the Dillinger ECT procedure was established.
The sensitivity of the system and the probability of detection (POD) were demonstrated. This approach for verifying new test methods is described in DNV.RP-F118 and in mathematical models.
Test plates with different kinds and sizes of artificial and natural local hard zones, covering the known different material structures responsible for hardness variations were driven through the EC device. On the one side, the detectable minimum size of these areas was checked depending on size and thickness of the hard layer.
On the other hand, the repeatability of the test was determined. At the end of the test, it could be proven that the system could find hardened areas in sizes of 15 mm in diameter and larger with the same thickness of 100 μm as the natural ones. Further, it was shown in multiple successive tests that the reproducibility was very high.
Since the artificial hard zones were circular and significantly smaller than those occurring naturally in combination with the high reproducibility, it can be stated that the system can detect the local hard zones in the plate production safely.
Both the POD of the ECT device and test procedure were approved by several oil and gas companies. Through its subsidiary Europipe, 160,000 tons of plate for Qatargas’ NFPS project will now be supplied exclusively by Dillinger from Germany and its identically equipped sister plant in Dunkirk, France.
Author: Andreas Thieme has worked for heavy plate steel producer Dillinger for more than 30 years, both as a research and development engineer and in marketing.
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