Structural offshore applications require steel plates that have been pre-qualified according to EN, API and NORSOK standards. The aim of pre-qualification is to show the steel’s tolerance to both high and low heat input welding via testing of the heat-affected zone (HAZ). The most critical test is the crack tip opening displacement (CTOD) fracture toughness test of specific regions of the HAZ at -10°C.
Steel concept re-developed
The first step was to improve the base material. OCAS re-developed the steel concept by modifying the chemical composition and optimising the rolling schedule. The resulting 40 and 50 mm plates with fine and homogeneous grain size distribution were CTOD tested − yielding splendid results. With good base material performance of the developed concept demonstrated, an API RP2Z pre-qualification attempt was undertaken for 50 mm thickness (which also qualifies the lower thicknesses). The steel performed very well indeed, meeting all requirements with flying colours. The pre-qualification was witnessed by Lloyd’s Register, who endorsed the testing results. Following API RP2Z, pre-qualification to the other two main offshore standards, EN10225 and NORSOK M-120, was undertaken and meanwhile achieved. This pre-qualified product can be marketed for offshore applications in most regions of the world.
In total, samples were welded with a combined length of 15 m, with the number of passes to weld the full 50 mm thickness varying from to around 20 for the highest heat input to nearly 100 for the lowest. Furthermore, 65 full-thickness CTOD samples were tested and evaluated as well as more than 300 Charpy impact samples. Specific cold cracking tests were performed and extensive base material testing performed to show adherence all the requirements. Moreover, all was done in record time.
Meanwhile OCAS was able to successfully support another mill to pre-qualify its steel plates for offshore applications and they are involved in shipbuilding steel homologations as well.
”In the end, OCAS’ metallurgical expertise solved the issues.”