Oil and gas procurement has extended to even harsher environments. Arctic application represents the severest challenge for today’s structural steel plates. Lower and lower design temperatures require steels showing improved fracture toughness combined with high strength and high thickness. Dr Frank Hanus and Wolfgang Schütz report.
Because of the rough climate and the dangerous nature of the Sea of Okhotsk (ice, waves with a height of up to 19m, seismic activities, gusts, temperatures down to -32°C even -35°C), Sakhalin II (Fig.1) is considered as a challenging project in the Arctic field, as highlighted in Fig.2. A special steel was developed to satisfy the specified requirements and finally Dillinger Huette GTS supplied more than 10000 tons of this S450-Arctic (thickness range: 15–150mm) specification for the topside frames of the two platforms sketched in Fig.1. The plates were mainly produced by Thermo-Mechanical Control Process (S450M-Arctic); only the extremely heavy plates were produced by quenching and tempering (S450Q-Arctic).
A prerequisite to produce heavy plates which fullfil low temperature toughness requirements is the use of clean slabs. The steel plant of Dillinger Huette is, due to its long history in supplying HIC resistant linepipe steels, equipped to cope with this basic requirement. The extraordinary cleanliness of the slabs is reached by a sophisticated secondary metallurgy with 100 per cent vacuum degassing and the possibility to use the thickest continuously cast slabs produced by the continuous casting machine. A soft reduction in the final stage of solidification of the slab effectively minimises the segregation.
One portion of the Sakhalin plates was produced by Thermo-Mechanical Control Process (TMCP) using such thick continuous cast slabs.
The steel composition is characterised by a very low carbon content, the addition of alloys improving the toughness, pure Nb-microalloying and low Pcm-values of about 0.17percent.
During the TMCP all metallurgically relevant production parameters in the rolling mill are controlled: from the reheating, the deformation of each rolling sequence, the start and final temperatures and the accelerated cooling. The final rolling temperatures for the arctic plates are above the Ar3-temperature in the austenite region. The subsequent intensive accelerated cooling with low final cooling temperatures and high cooling rate ensures a homogeneous extremely fine grained microstructure.
Finally the plates undergo a stress relieving treatment to ensure adequate workability.
The thickest plates up to 150mm were produced, from ingots, via the quenching and tempering process.
The production results show that the plates have excellent toughness (Figs.3 and 4) combined with a stable yield strength level above 450MPa.
During the development of the Offshore Grades for arctic applications the weldability of different chemical compositions was assessed. The achievement of sufficient HAZ toughness was an important parameter for the selection of the steel composition. The impact requirements for the HAZ were 60J average/45J minimum single at -60°C. CTOD at -40°C seemed a higher hurdle to take. In order to obtain in the HAZ a sufficiently high resistance against brittle fracture, the steel was alloyed with more nickel than a conventional offshore steel. Relative to plate thickness ranges, different compositions were necessary for S450M-Arctic plates and for the S450Q-Arctic plates. Commercial aspects had also to be taken into account aiming to keep the alloy costs as low as possible.
An important weldability verification programme was finally carried out on the 90mm thick S450M-Arctic plates and on the 125mm thick S450Q-Arctic plates. Hardenability, cold cracking sensitivity, preheat temperature and toughness of the HAZ were investigated for several welding conditions and processes.
The impact energy at the straight fusion line was tested at -60°C. The average results were above 100J and all tests were safely exceeding the required values. CTOD testing was specified with the fatigue crack positioned at the weld fusion line. Deep notched full thickness specimens producing high constraints at the fatigue crack tip were investigated. The results are shown in Fig.5. Increasing the weld heat input tended to produce lower impact and CTOD values. Due to the higher nickel content, the results of the S450Q-Arctic steel were slightly superior to those of S450M-Arctic steel.
Depending on the CTOD level to be achieved, different limits for the weld energy must therefore be set during fabrication.
With the production of the heavy plates for the Sakhalin II project, a real challenge has been achieved. Pushing the limits of the feasibility, the delivered steel met all the severe requirements in terms of strength and toughness for high thicknesses.
Dr Frank Hanus, Manager of Welding Laboratory, and Wolfgang Schütz, R&D-Group Manager TMCP Plates, are with AG der Dillinger Hüttenwerke, Dillingen, Germany. www.dillinger.de