Fatigue Life Prediction of Locally Welded Subsea Pipeline Girth Welds Under Nigerian Offshore Cyclic Pressure Loading Conditions

Subsea pipeline girth welds represent critical discontinuities in offshore energy infrastructure and are among the most susceptible sites for fatigue-driven fracture, particularly in the dynamically active shallow and deep-water environments of the Nigerian Continental Shelf. This study presents an integrated fatigue life prediction framework for API 5L X65 girth-welded pipeline sections subjected to cyclic internal pressure fluctuations characteristic of Niger Delta offshore operations. Two complementary approaches are employed and cross-validated: the nominal stress S-N method following IIW recommendations and the DNV-GL FAT class system, and a linear elastic fracture mechanics (LEFM) crack propagation model governed by the Paris-Erdogan law. Nigerian offshore environmental and operational loading parameters, including significant wave heights of 1.5–9.5 m, tidal currents, internal pressure fluctuations of ±1.5 to ±5.5 MPa, and corrosive seawater conditions, were quantified and incorporated as boundary conditions. The influence of welding-induced residual stresses, initial weld toe flaw geometry, weld heat input, and corrosion-fatigue interaction on predicted fatigue life was systematically investigated. Results indicate that uninspected weld toe flaws of depth greater than 1.0 mm can reduce the LEFM-predicted fatigue life by up to 68% relative to an idealised smooth toe, while tensile residual stresses approaching 45% of the yield strength reduce fatigue life by a further 32–41% in deep-water high-pressure scenarios. Post-weld heat treatment and precision weld toe dressing are shown to restore fatigue life to values exceeding the 25-year design target prescribed by DNV-ST-F101. The findings provide actionable guidance for weld procedure qualification, inspection scheduling, and structural integrity management of Nigerian offshore pipelines.