PRODUCTION • PROCESSING • HANDLING
Thermomechanical tank technology
Highlighting the advantages of TMCP steel plates for the construction of storage tanks used in the oil & gas industry
wind power, shipbuilding, steel construction and most popularly, for the construction of large diameter oil and gas pipelines based on the API standards. Considering the construction of pressure vessels and storage tanks, there is a contrary situation. In spite of the evident advantages of thermomechanical controlled processing (TMCP) steels, their usage in this sector is rather uncommon. For pressure vessels comprising heads and shells,
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where a new quality heat treatment after processing can become mandatory, or for pressure vessels with high wall thicknesses, which need a considerable post weld heat treatment (PWHT), there are clear arguments against these steels. But regarding the fabrication of large storage tanks for hydrocarbon products and ammonia, the application of thermo- mechanically rolled steels offers considerable technical and economic benefit for the manufacturer as well as the end user. Tis benefit increases if these steels are applied in
sour service conditions, where the risk of hydrogen induced cracking (HIC) or sulphide stress cracking (SSC) is present. As the pressure equipment industry is highly
regulated, it is very important that the use of TMCP steels is permitted by the applying codes. Consequently, Dillinger experts reviewed the
here have been more than four decades of experience in the successful application of thermo-mechanically hot rolled heavy plates in the fields of offshore oil and gas platforms, offshore
globally established construction codes such as ASME BPVC, API 620, EN13445, EN14620 and so on, (Table 1) with the result that all of them allow the application of thermo-mechanically rolled steels up to about 40mm in thickness.
THE BASICS OF TMCP STEELS Steels made by TMCP are not only characterised by high strength and excellent toughness, but also by outstanding weldability. Conventionally it has been impossible to obtain these characteristics simultaneously and one had to be chosen at the expense of the other. With regard to normalised steels, higher strength could only be achieved by increasing the content of alloying elements, which in turn has a negative impact on the weldability. Te situation is different with TMCP steels. Tey achieve their strength properties by a combination of a process-optimised, very lean chemistry (Table 2) and a special rolling technique (Fig. 1), resulting in a unique, fine grained microstructure compared to normalised steels (Fig. 2).
STRENGTH LEVELS AND ALLOYING CONTENTS Te relationship between achievable strength levels and required alloying contents is clearly shown in Fig. 3. Te content of alloying elements is described by the carbon equivalent. It can be seen that it is possible to achieve the same strength level with a considerable reduction in alloying
Table 1. Conformity of TMCP steels with different construction codes
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