Fracture Toughness Characterisation of Weld Heat-Affected Zones in Offshore Platform Structural Steel Using CTOD Testing

This study characterises the fracture toughness of heat-affected zones in structural steel welds used in Nigerian offshore platform construction, applying crack tip opening displacement (CTOD) testing to assess the resistance to brittle fracture initiation. Offshore platforms operating in Nigerian deepwater are subject to cyclic wave-induced loading at temperatures as low as 4 degrees Celsius at the mudline, and HAZ fracture toughness is a critical fitness-for-service parameter that is sensitive to welding heat input and post-weld treatment. API 2W and S420ML structural steel plates are welded using submerged arc welding at low (1.5 kJ/mm), medium (2.5 kJ/mm), and high (4.0 kJ/mm) heat inputs. Three-point bend CTOD specimens with fatigue pre-cracks located in the coarse-grained HAZ are tested at minus 10 and minus 40 degrees Celsius following BS 7448 Part 2. Fracture appearance transition temperature (FATT) is determined from Charpy impact tests on HAZ notched specimens. Correlations between HAZ grain size, hardness distribution, and CTOD values are established. Findings demonstrate that heat input above 3.0 kJ/mm produces CTOD values below the 0.25 mm minimum required by DNVGL-OS-C101 at minus 40 degrees Celsius, associated with excessive HAZ grain coarsening. The study recommends heat input limits for offshore structural welding procedures. Keywords: fracture toughness, CTOD testing, heat-affected zone, offshore platform, structural steel.