Session: 03-02-01 Fatigue Performance and Testing

Submission Number: 180656

Development and Validation of a Local Fatigue Design Concept for Welded Joints in Offshore Structures Subjected to Multi-Axial and Variable Loading 

The fatigue assessment of welded maritime structural components is critical for the safe and economical design of modern offshore structures such as wind turbines, platforms, and navigation aids. Conventional approaches, such as the nominal stress or structural stress concepts, often reach their limits when applied to complex, non-proportional loading, variable amplitudes, and thick-sections, leading to overly conservative and cost-intensive designs. In the SMATRA project, a novel local design concept was developed based on the notch stress approach and fracture mechanics, enabling a realistic consideration of local effects including weld geometry, post-weld treatments, and multi-axial loading. By integrating high-resolution 3D surface scans, automated finite element modeling, and a two-phase life prediction model (crack initiation and crack growth), the prediction accuracy was significantly improved. The accuracy for multi-axial, non-proportional loading increased by 24 to 77% compared to established standards (IIW, Eurocode 3, FKM). Validation was performed on a large number of small-scale specimens and full-scale test on a double-Y-joint, where crack initiation sites were accurately predicted. Furthermore, a practical design environment was developed, reducing the time required for fatigue assessment by 80% and enabling seamless integration into design workflows. Comparative studies demonstrated material savings exceeding 25% in weld volume and reduced plate thicknesses while maintaining the same safety level. The results open significant potential for more efficient, safe, and sustainable design of future offshore structures. SMATRA established the foundation for transferring local assessment methods into industrial standards.

Presenting Author: Moritz Braun German Aerospace Center (DLR) Institute of Maritime Energy Systems

Presenting Author Biography: Moritz Braun, Dr.-Ing. habil., is the head of the department for ship reliability at the German Aerospace Center of Maritime Energy Systems in Geesthacht, Germany. He holds a B.Sc. in mechanical engineering (2012) from University of Rostock, an M.Sc. in Marine Technology (2014) from NTNU Trondheim, and a Doctor of Engineering (2021) and Doctor habilitatus (2025) from Hamburg University of Technology. For his doctoral thesis, he received the Curt Bartsch Award of the German Society for Maritime Technology. His research focuses on the structural integrity and reliability assessment of large engineering structures. He has more than 100 publications in the corresponding fields. He is the delegate of the German Welding Association (DVS) at the International Institute of Welding (IIW) in ‘Commission XV: Design, Analysis, and Fabrication of Welded Structures’, the German representative in the International Ship and Offshore Structures Congress (ISSC) technical committee ‘V.7 Structural Assessment During Operations’, and the deputy chairman of the working group ‘Damages of Ship Structures’ of DVS.

Authors:

Moritz Braun German Aerospace Center (DLR) Institute of Maritime Energy Systems
Sascha Maurice Isay Hamburg University of Technology Institute of Ship Structural Design and Analysis
Shi Song Hamburg University of Technology Institute of Ship Structural Design and Analysis
Jan-Hendrik Grimm Hamburg University of Technology Institute of Ship Structural Design and Analysis
Niklas Michael Bauer Fraunhofer Institute for Structural Durability and System Reliability LBF
Jörg Baumgartner Fraunhofer Institute for Structural Durability and System Reliability LBF
Leon Wilms Ramboll Deutschland GmbH
Dennis Birkner Ramboll Deutschland GmbH
Karsten Schürmann Ramboll Deutschland GmbH
Marc Voßbeck Ramboll Deutschland GmbH