A Novel Methodology for Fatigue Analysis on Welded Non Tubular Connections on Offshore Platform Decks Using Shell Fatigue Method
Authors: Thomas A. P. , Nekstad O. J. 
Published: October 2023

Offshore structures such as oil and gas rigs and wind turbine foundations typically require additional support in the form of jackets and towers when they are located in waters with shallow to intermediate waves. In addition, similar formations might be found in oceans with a deeper water depth. There are three or more legs and a bracing system that connects the legs. Offshore lattice systems are susceptible to fatigue due to the cyclic environmental stress caused primarily by wave-induced drag and inertia forces and the high number of welded connections between tube elements. Both factors contribute to the production of fatigue. These two factors are contributors to the fact that offshore lattice constructions are sensitive to fatigue. These weld connections, which are frequently referred to as "hot spots" are the portions of the structure that are the most susceptible to fatigue. There is not always a correlation between the weight-efficiency of offshore lattice structures and their construction cost-efficiency. This is typically the case because of the significant expenditures associated with establishing these joints and maintaining them within acceptable safety standards. It is vital to consistently improve the design of the joints' fatigue resistance in order to keep these structures at the top of their level and maintain their competitive edge.

Since materials with greater static strength must now be employed, one of the most important limit state requirements for the successful design of offshore projects is fatigue damage. Throughout the course of history, the aviation industry was the first to make use of fatigue as a design criterion, which was then followed by the nuclear, steel, offshore and marine industries. In the second scenario, the fatigue limit state (FLS) needs to be explicitly stated due to the utilization of higher strength steels throughout the design process. In most cases, the S-N data are obtained via laboratory experiments, which then serve as the basis for the fatigue failure design criteria. In addition to having an acceptable design, inspection, maintenance, and repair (IMR) must be performed throughout the service life of the system to keep the structural safety and the integrity of the system intact.

Utilizing the finite element method for fatigue design and analysis allows for a more precise evaluation of how loads affect the investigated features. This is made possible through the application of discrete element modelling. The efficacy of the finite element model must not be compromised to save time during the modelling process. Simplifying the finite element models may reduce the accuracy of the predicted stress, leading to either an overestimation or an underestimate of the fatigue life. Volume element-based models are regarded as the most accurate representations of the actual object due to their ability to precisely recreate the geometry and accurately depict the stiffness. This is a result of their ability to accurately recreate the rigidity. When utilizing well-constructed finite element models, it is not always apparent how to estimate and extract stresses for a specific fatigue evaluation method. The results of the FEA may be particularly susceptible to the finite element modelling methodology because the stresses collected by advanced procedures are typically positioned in a stress singularity with high strain gradients.