Quantification of Fatigue-Influencing Factors in Critical Tubular Joints of Steel Jacket Offshore Structures in Gulf of Guinea Shallow Waters

Abstract

This study quantifies the key fatigue-influencing factors: hot-spot stresses, Stress Concentration Factors (SCFs), and fatigue damage ratios for six critical tubular joints (J1–J6) of a four-leg steel jacket platform operating in Gulf of Guinea shallow waters. Long-term metocean data were processed through global finite-element analysis and rainflow counting to extract nominal and hot-spot stress ranges under sea states ranging from m to 7.5 m. Parametric SCFs were verified and refined against detailed three-dimensional shell-element models of the most critical joints. Cumulative fatigue damage was evaluated using the Palmgren-Miner rule with the DNV D curve, and a probabilistic assessment based on Monte Carlo simulation was performed to account for uncertainties in metocean parameters, SCFs and S–N curve coefficients. Results show that the lower-bay wave-facing K-joint (J5) is the governing detail (FEA-refined SCF = 3.42, deterministic. , reliability index ). The splash-zone K-joint (J2) ranks second. Moderate sea states ( –4.5 m) contribute 49–56 % of total fatigue damage. The study establishes a reliability-based criticality ranking that provides operators with a transparent basis for prioritising inspection and life-extension interventions on Gulf of Guinea jackets. This is the first assessment that combines FEA-refined SCFs with a full probabilistic treatment of tropical metocean uncertainties for a GoG jacket structure.