The Fracture Resistance Approach in Order to Prevent Brittle Failure of Offshore Structures Under Arctic Environments

Abstract

This paper is concerned with the challenges related to steel design under Arctic conditions where both loading and temperature have been discussed in relation to material requirements. Today there is a lack of rules and standards for selecting steel materials for bulk engineering for a lower design temperature than −10°C (NORSOK N-004 [1] allows down to −14°C). Both ISO 19902 Steel Structures [2] and NORSOK N-004 Design of steel structures make reference to EN10225 “Weldable structural steels for fixed offshore structures technical delivery conditions [5]” where steel materials are Charpy tested at a lowest test temperature of −40°C and proven for a design of −10°C. Hence, one major challenge for designers are to specify adequate toughness requirements at an early stage of the design process for low temperature applications. Both NORSOK N-004[1] and ISO 19902[2] provide requirements to load combinations that need to be fulfilled, however the relationship between various load types and temperature is not mentioned in any of these standards. Thus, in the design stage the material needs to demonstrate adequate toughness where loading and temperature are treated independently. For the offshore industry, the main question is the balance between materials requirements and cost-effective solutions, and how to address this within an overall design perspective in order to avoid brittle failure. This paper discusses some of these challenges with the aim of starting a focused process leading up to a clear interpretation of the implications of overall design philosophies, necessary in order to define consistent materials requirements ensuring that brittle fracture is not going to represent a significant threat to the structural integrity. The material recommendations provided in the paper are based on the latest research results from the Arctic Materials project (2008–2017) managed by SINTEF and supported by the industry.

Relation between the Fatigue and Fracture Ductile-Brittle Transition in S500 Welded Steel Joints

“…Typically, the highest ductile-brittle transition temperature is found in the coarsegrained and intercritically reheated coarse-grained heat-affected zones [6,19,28,30]. This zone is, however, fairly small, particularly in thin-plated structures with few weld layers, such as in ships.…”

“…Low temperatures cause challenging conditions for the structural integrity of steel structures in Arctic regions [1]. In order to ensure safety of these structures against brittle failure, the structural design has to account for the static and dynamic structural responses that are often more severe than for other engineering structures [2][3][4][5][6]. In particular, the combination of high ice-related loads and low temperatures are difficult to account for in design [7][8][9][10][11][12][13][14][15][16][17][18].…”

Relation between the Fatigue and Fracture Ductile-Brittle Transition in S500 Welded Steel Joints

“…Typically, the highest ductile-brittle transition temperature is found in the coarsegrained and intercritically reheated coarse-grained heat-affected zones [6,19,29,31]. This zone is, however, fairly small, particularly in thin-plated structures with few weld layers, such as in ships.…”

“…Low temperatures cause challenging conditions for the structural integrity of steel structures in Arctic regions [1]. In order to ensure the safety of these structures against brittle failure, the structural design has to account for the static and dynamic structural responses that are often more severe than for other engineering structures [2][3][4][5][6]. In particular, the combination of high ice-related loads and low temperatures is difficult to account for in design [7][8][9][10][11][12][13][14][15][16][17][18].…”