Heavy Wall Thermomechanically Rolled Heavy Plates For Offshore Structures In Arctic Regions

摘要

The exploitation of many offshore fields, especially in exposed areas like the arctic region, has become economically feasiblernin the last years. For the construction of heavy offshore structures a plate thickness up to four inch is common. In order tornhave the benefit of high strength and toughness together with good weldability, thermomechanically rolled plates arernpreferred.rnDue to the low LAST (lowest anticipated service temperature), special steels had to be developed.rnAlong with a R&D program, there is now the possibility of the production of heavy plates up to 80mm (target 100mm) with arnyield strength of minimum 414MPa which meet the high demands for the use in arctic regions.rnTo fulfill the mechanical requirements, especially impact toughness at -60°C and fracture toughness COD (crack-openingdisplacement)rnat -40°C, a lot of investigations and investments (a new continuous caster, new main drives on the four highrnrolling stand, new accelerated cooling unit, modern analysis concepts, …) were made.rnIn addition to the standard extent of a testing program, plates in 80mm thickness were welded and tested according tornAPIRP2Z. Welding was done with a heat input of 0,8kJ/mm by flux-cored arc welding (FCAW) without preheating and arnmaximum interpass temperature of 100°C. Additionally two submerged arc weldings (SAW) were done. The first with arnminimum preheating and interpass temperature of 250°C and a heat input of 4,5kJ/mm, the second with a heat input ofrn3,0kJ/mm and a preheating temperature of 100°C.rnA low carbon-content and carbon-equivalent together with sophisticated reheating-, rolling- and cooling parameters arernnecessary to meet the high demands. Charpy-impact values in half thickness of the plate up to 250J at -80°C are common onrnthis base metal as well as in the coarse-grained heat affected zone (CGHAZ) and the subcritical heat affected zone (SCHAZ).rnThe COD-values (Crack-Opening-Displacement) in the HAZ meet the requirements of 0,25mm at -40°C.rnFurthermore, tests to determine the susceptibility to cold cracking as bead on plate test, Y-Groove test and CTS-tests werernmade with excellent results.rnPlates with such extraordinary properties have not been available so far. The significantly improved weldability, toughness,rnand fracture-toughness characteristics in combination with a high strength level are a milestone in the field of offshorernstructural steels.