Session: 02-10-02 Fatigue and Fracture Reliability 2

Submission Number: 181440

Reliability Assessment of Crack Orientation Effects on Structural Integrity of FPSO Cargo Tanks 

The structural integrity of cargo tanks in Floating Production Storage and Offloading units (FPSOs) is influenced by corrosion and sustained tensile loads acting on key structural components such as longitudinal stiffeners, transverse frames, and plating. These components operate under constant stress in highly corrosive environments, where small cracks may develop at stress concentration zones or corrosion-induced imperfections. Nevertheless, the presence of microcracks below detectable or structurally relevant sizes, originating from manufacturing and installation processes and exacerbated by fatigue and corrosion mechanisms, often does not immediately affect structural integrity. However, the progressive growth of these initially insignificant defects under sustained loading can gradually reduce the safety margin over time, emphasizing the need for a reliability-based evaluation of their evolution. Understanding how crack orientation influences crack propagation and time to failure is therefore essential for reliability-based assessment of critical structural details. This work investigates how crack orientation affects crack propagation behavior and reliability of longitudinal stiffeners in FPSO cargo tanks under static loading. Finite element analyses employing the Extended Finite Element Method (XFEM) are conducted to simulate crack initiation and growth without remeshing. Cracks with varying orientations relative to the stiffener axis (0°–90°) are analyzed, and corresponding stress intensity factors, such as K_I, are computed to determine the equivalent stress intensity factor (K_eq) governing propagation. The crack growth criterion, K_I ≥ K_IC, is integrated with a reliability model derived from the crack growth rate relation da/dt within a probabilistic framework that accounts for both epistemic and random uncertainties in material fracture toughness, corrosion rate, residual stresses, and initial crack size. This approach enables the direct computation of the time-variant probability of failure (Pf) and reliability index (β) as functions of crack orientation. The results quantitatively identify the most critical propagation paths and delineate how orientation-dependent stress fields accelerate crack growth, thereby providing a scientifically grounded basis for defining fitness-for-service acceptance criteria and optimizing risk-informed inspection and maintenance schedules for critical structural details, ensuring operational safety while acknowledging the tolerability of controlled sub-critical crack growth within the asset’s design life.

Presenting Author: Edilson Gabriel Veruz Escola Politécnica - University of São Paulo

Presenting Author Biography: Bachelor in Mechanical Engineering from Universidade Federal do Paraná. Master of Engineering from University of São Paulo. Edilson is currently developing PhD. program at University of São Paulo. His research topics are structural integrity of FPSOs hulls and corrosion degradation prediction.

Authors:

Fabrício Terciotti Escola Politécnica - University of São Paulo
Edilson Gabriel Veruz Escola Politécnica - University of São Paulo
Gilberto Francisco Martha De Souza Escola Politécnica - University of São Paulo
Fernando Andrade Rodrigues Leopoldo Américo Miguez de Mello Research and Development Center - Petrobras