• Stang, Henrik
• Haselbach, Philipp Ulrich
• Kolios, Athanasios
• Waldbjørn, Jacob Paamand
• Katsanos, Evangelos
• Al-Hagri, Ammar

Abstract

Within the realm of offshore engineering, steel jacket structures operate in harsh marine environments. These environments are characterized, for example, by repetitive wind- and wave-induced loads and slamming forces due to extreme waves, while the capacity of structural elements may degrade due to corrosion phenomena. One of the most dominant and well-observed impacts of these adverse conditions is the fatigue-induced crack propagation that commonly takes place at the joints in the so-called "hot spots" of the steel jacket structures. Therefore, it is critical to conduct fatigue lifetime assessment of the joints during both design and operation phases. This is best achieved by performing actual fatigue tests to understand how the fatigue-induced cracks propagate at the joints during their lifetime. However, given the considerable expense, complexity, and vast scale of steel jacket structures, a more practical alternative is to perform fatigue analysis based on finite element (FE) simulations. This necessitates the availability of an FE model representing the offshore structure accurately combined with fracture mechanics theory. In this regard, the development of a meticulous model via the use of solid FE elements favors the stress intensity factors (SIFs), which are calculated at the crack fronts and have a central role in calculating the fatigue-induced crack propagation. Hence, this study investigated how different modeling choices may influence the calculation of the SIFs. Especially, the modeling with different fidelities approaches were studied herein in terms of their influence on the SIFs at the crack fronts. Moreover, this study elaborated on the sensitivity of the SIFs due to the consideration of joints’ flexibility. The findings emphasized the viability of employing multi-fidelity modeling, as this approach demonstrated a favorable compromise between computational efficiency and the precision of SIFs calculations. Conversely, employing a submodelling approach for analyzing a high-fidelity submodel represented by a k-joint resulted in a deficiency in estimating the SIFs with a notable deviation of 46-57%. Moreover, the incorporation of the joints’ flexibility revealed a corresponding rise in the computed SIFs, reaching up to 7%, attributed to the resultant reduction in the overall stiffness of the steel jacket structure. The outcome of this study is anticipated to provide valuable insights for the reliable design and assessment of the fatigue lifetime for steel jacket structures.

Topics

• impedance spectroscopy
• corrosion
• phase
• theory
• simulation
• crack
• steel
• fatigue