Heavy Wall Thermomechanically Rolled Heavy Plates For Offshore Structures In Arctic Regions
Authors: Lengauer Harald , Tischler Guido 
Published: May 2011

The exploitation of many offshore fields, especially in exposed areas like the arctic region, has become economically feasible in the last years. For the construction of heavy offshore structures a plate thickness up to four inch is common. In order to have the benefit of high strength and toughness together with good weldability, thermomechanically rolled plates are preferred.

Due to the low LAST (lowest anticipated service temperature), special steels had to be developed. Along with a R&D program, there is now the possibility of the production of heavy plates up to 80mm (target 100mm) with a yield strength of minimum 414MPa which meet the high demands for the use in arctic regions.

To fulfill the mechanical requirements, especially impact toughness at -60°C and fracture toughness COD (crack-openingdisplacement) at -40°C, a lot of investigations and investments (a new continuous caster, new main drives on the four high rolling stand, new accelerated cooling unit, modern analysis concepts, …) were made.

In addition to the standard extent of a testing program, plates in 80mm thickness were welded and tested according to APIRP2Z. Welding was done with a heat input of 0,8kJ/mm by flux-cored arc welding (FCAW) without preheating and a maximum interpass temperature of 100°C. Additionally two submerged arc weldings (SAW) were done. The first with a minimum preheating and interpass temperature of 250°C and a heat input of 4,5kJ/mm, the second with a heat input of 3,0kJ/mm and a preheating temperature of 100°C.

A low carbon-content and carbon-equivalent together with sophisticated reheating-, rolling- and cooling parameters are necessary to meet the high demands. Charpy-impact values in half thickness of the plate up to 250J at -80°C are common on this base metal as well as in the coarse-grained heat affected zone (CGHAZ) and the subcritical heat affected zone (SCHAZ). The COD-values (Crack-Opening-Displacement) in the HAZ meet the requirements of 0,25mm at -40°C.

Furthermore, tests to determine the susceptibility to cold cracking as bead on plate test, Y-Groove test and CTS-tests were made with excellent results.

Plates with such extraordinary properties have not been available so far. The significantly improved weldability, toughness, and fracture-toughness characteristics in combination with a high strength level are a milestone in the field of offshore structural steels.

Introduction

Fossil combustibles like oil & gas or coal have been in the past decades and will be for at least the next decade the main energy source of the world (World Energy Outlook; 2007). In order to maintain the steady flow of oil and gas the industry is forced into regions of the planet that are not easily accessible like the bottom of the oceans or arctic territory. The materials used under arctic conditions need to have mechanical properties that exceed those commonly used in construction.

The necessary development of the steel industry to fulfill those needs leads to an overall enhanced standard in the engineering of offshore structures. Typically a step to lower transition temperatures, better fracture mechanics and easier welding procedures were the results of these efforts. One of the driving forces that imposed changes in the fabrication of offshore plates was the demand for thermo - mechanically (TM) rolled plates in high thickness to have the combination of strength and toughness with a low carbon equivalent and therefore improved weldability.