Local hard zones, also called hard spots, are small areas on the surface of steel plates only a few tenths of a millimetre in thickness. Formed to pipes transporting sour natural gas, they are held responsible for sudden stress corrosion cracking in the through thickness direction. In order to detect and eliminate these local hard zones reliably already during plate production, Dillinger, Europe’s leading heavy plate manufacturer, developed a special eddy-current testing system. This method has now been recognised by various oil and gas companies. First projects started using pipes made of D-TECT branded steel tested for local hard zones on a large scale.
Big changes in technology with niches for adapted “old” technologies
What we know about global warming, the world is forced to change the generation of energy back from the old carbon economy towards new technologies with less impact on the global warming. But this can’t be done simply by flipping a switch. It’s a long process with a slow conversion from old to new, requiring bridge technologies that are already securing a significant reduction in CO2 emission. One of these bridging technologies with 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: Transport in pipelines.
Pipes for the transport of oil and gas
Although a pipe does not look very spectacular, it has a lot to accomplish. Pipelines are transporting natural gas at high pressures through lovely to inhospitable areas, e.g., cold arctic tundra, hot deserts, or seismically active areas. Also, they are laid offshore in deep waters or at low temperatures. Beside these mechanical loads, also corrosion problems can be caused by the transported medium. In the worst case, it shows acid sour gas characteristics that can be cause various types of cracking inside the pipe wall or on the surface. This forces the designer already in the stage of the material selection to specify challenging mechanical-technological and corrosion properties. These are characterised by special toughness at low temperatures, distinctive crack-stop behaviour, and special corrosion resistance, of course to be adjusted evenly throughout the entire production.
The design of pipelines is done with the best knowledge and belief of all parties involved. Life, environment and invested capital have to be protected for the duration of the operation of the pipeline, which can last several decades. With growing experience over the operational life of pipelines, with more and more projects built and with general technical improvements also new knowledge can be gained about possible new damage. Usually, this knowledge comes 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 time basis, to solve the issue.
Fatal stress corrosion cracking as a challenge
As an example, a fatal failure of a pipeline in 2014 in Asia can be cited here. In presence of H2S as a promotor for the absorption of hydrogen in the steel, stress on the pipe and susceptible areas on the surface, a very severe sour gas attack occurred in new, as yet unknown form. This sour gas attack led to Sulphide- Stress-Cracking-type (SSC) cracks through the wall of the pipes that did not allow continuing operation. These cracks are assumed to be related to very thin local hard zones on the surface of the pipes. Since this case became known, all manufacturers of plates for natural gas pipelines dealt more or less intensively with the phenomenon that is 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 well investigated. It became clear 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:
- Their hardness is well above the hardness level of the base material.
- With a thickness of a few 10th of a millimetre, these hard zones are very thin.
- Their occurrence over the plates is irregular and rare.
- Research showed that they cannot be prevented completely.
Conventional hardness testing methods are not able to detect these zones on the plates, as they have lateral dimensions of only a few centimetres and are statistical distributed on plates with a rolled length of up to 40 metres.
Problem solving for current sour service pipeline projects
Due to the uncertainties after finding the harmful phenomenon of local hard zones, most of the oil and gas majors did not allow the use of TM+ACC material for the transport of sour natural gas any more. However, due to its advantages in terms of mechanical-technological properties and its efficiency, it is today’s first choice both in production and processing. Therefore, every attempt had to be made to regain confidence in this type of material and a technical solution to solve this issue is from general interest for the whole industry.
The solution: Inline inspection for local hard zones
As it was clear, local hard zones can’t be avoided completely, a method, to find them on the plates in a large-scale production, had to be developed. In order to achieve this, Dillinger, together with an industrial partner specialised in NDT, developed a testing method on basis of Eddy Current (EC) that is able to 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 so far for example to detect cracks on hot wide strip materials.
In cooperation between Dillinger, plant engineers and research institutes, the system was adapted in such a way that differences in hardness on the surface were detectable. The system is detecting areas of diameters 20 mm and above by using physical characteristics of the different structures in comparison to the base metal. These show with a certain threshold the hardness increases versus that of the base. These indicated areas can be repaired light grinding. The Eddy Current device, currently in operation in both of the Dillinger plate mills in Germany and in France is testing plates inline the production without any interruption of the flow. It enables Dillinger heavy plate mills to check 100% of the surface of the plates on both sides on the occurrence of these local hard zones. Orders in largest quantities, as they are typical for pipeline projects, are processed. All plates tested on the appearance of local hard zones are marked with the brand name D-TECT (Dillinger – Totally Eddy Current Tested).
Stakeholders are convinced
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. Most important, the testing and the final approval of the method and the installation had to be done together with the end customers like some of the big oil and gas majors. Together with them, a test programme on basis 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 above having the same thickness of 100 μm as the natural ones. Further it was shown in multiple successive tests that the reproducibility was very high. As 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 fulfils the task to detect the local hard zones in the plate production safely.
D-TECT is entering the market
Both the POD of the Eddy Current Test device and test procedure were approved by several oil and gas companies. Combined with the experience for large complex pipeline projects, Dillinger is the first supplier for the upcoming highly complex sour gas projects especially in the Middle East. Through its subsidiary Europipe, 160,000 tonnes of plate for Qatargas NFPS project will now being supplied exclusively by Dillinger from Germany and its identically equipped sister plant in Dunkirk, France.
As a special highlight due to the current time with reduced possibilities to travel, the inspection of the plate production can be done remotely with the Dillinger myE-Service. This system allows following up all relevant production steps directly and inspecting of all the material testing on a web-based platform.