Modern heavy steel plates for use in offshore installations - characteristics and production processes

Steels for structural applications for offshore oil and gas as well as offshore wind installations have to fulfil demanding requirements that exceed those for steels used in normal structural steelwork. The applicable steel grades must show an optimum trade-off between sufficient toughness and good strength properties, and at the same time allow easy fabrication, especially concerning weldability. Although sufficient toughness is of importance in order to avoid abrupt failure phenomena such as brittle fracture, good strength properties are necessary to minimize the dead weight of the structure and thus facilitate transport and erection process. The significance of good fabrication properties consist of two facets: good weldability results in easy processing procedures and thus cost-efficiency in fabrication; further, materials with good weldability exhibit a higher resistance to cracking phenomena in the heat affected zone and are therefore more “forgiving” when it comes to non-optimum welding conditions, which may occur during erection in harsh environments.
This steel has to be tailor-made at the steelworks, the aim being to produce it as cleanly as possible with specially adapted alloying concepts. To secure good properties in the half thickness portion of the plates, what is necessary for offshore plates, slabs and ingots have to show an accurate control of the mid thickness segregation. Additionally those plates need higher deformation ratios from slab to plate in order to get a dense structure also in the mid thickness by proper through thickness deformation. These materials are produced with various rolling and heat treatment processes, each chosen depending on the desired grade, thickness and weight of the final plate. The processes in use are normalization of the plates, thermomechanical controlled rolling and the quenching and tempering of the plates.
This article aims to support the designer or fabricator when selecting the steel grade by explaining the different production methods and the resulting properties with respect to the final use.