Fracture Toughness And Microstructural Studies Of Submerged-Arc Welds For Offshore Structures
Authors: Kamath M.S. , Abson D.J. , Garland J.G. 
Published: October 2008

Information on the effect of welding procedure, consumable type and plate thickness on the fracture toughness, or crack opening displacement (COD), properties of submerged arc welds is relatively sparse. This has been highlighted recently through problems faced by fabricators in meeting as-welded COD requirements for North Sea offshore platforms.

In this investigation, the fracture toughness and microstructure of a series of 50 mm thick C-Mn submerged arc welds have been studied. COD results are presented for welds deposited with wires ranging from conventional 1.5% manganese types up to nickel, molybdenum, and molybdenum-titanium-boron alloyed compositions. The paper describes the manner in which an as-welded COD requirement of 0.25 mm at -10°C was successfully met through appropriate changes in compositional and procedural variables. Finally, the differences in measured fracture toughness in the test series are considered on the basis of variations in weld metal microstructure and composition.

INTRODUCTION

A significant feature of fabrication specifications for the latest generation of North Sea production platforms has been the inclusion of fracture toughness requirements for welds located in highly stressed regions of the platform. On the basis of fitness for purpose design, the use of such fracture toughness criteria has enabled reasonable estimates to be made the acceptability or otherwise of individual welds found to contain defects during non-destructive inspection. One fracture mechanics criterion which has gained considerable acceptance within the offshore industry is the Crack Opening Displacement (COD) approach1. In this context, two major oil companies now operating in the North Sea require their fabricators to attain 0.25 mm of COD at -10°C for welds within the splash zone of a production platform.

Notwithstanding this trend towards increasingly stringent construction specifications for offshore platforms, both oil companies and fabricators have been forced by cost and delivery constraints to examine the possibility of using production routes with a minimal post weld heat treatment requirement. Considerable attention has-focused, therefore, upon the feasibility of attaining the minimum specified fracture toughness in the as-welded condition despite the problems highlighted by early work on the first generation of North Sea platforms2. Further encouragement to the consideration of the as-welded option has been furnished by the extensive research into factors controlling weld metal toughness3–8, motivated largely by the early difficulties. This increase in knowledge has been reflected in both manual and automatic welding consumable development on a variety of fronts embracing manual metal arc electrodes, automatic welding wire and flux8 and, combined with the correct selection of welding procedures, has helped to maximise weld metal toughness.

Despite the basis for optimism provided by such developments, however, first attempts (prior to this study) at achieving consistent as-welded toughness in excess of 0.25 mm COD at -10°C in 50 and 63 mm C-Mn Submerged arc welds were not entirely successful.