Terrain-Specific Environmental Loading and Stress-Range Histories for Fatigue Assessment of Gulf of Guinea Steel Jacket Platforms

Abstract

Accurate characterization of site-specific environmental loading is essential for realistic fatigue assessment of fixed steel jackets. This paper presents a terrain-specific methodology for transforming long-term Gulf of Guinea (GoG) metocean data into directional stress-range histories suitable for tubular-joint fatigue analysis. Ten-year wave, current, and wind records from the Okpokonam field were discretized into eight sea states (Hs = 1.0–7.5 m). This study applied a modified Morison equation incorporating a terrain influence factor, mild-slope wave transformation, and marine-growth amplification to a three-dimensional space-frame model. Rainflow counting of the resulting member stress time histories produced long-term stress-range spectra for six critical joints. Results show that moderate sea states (Hs 2.0–4.5 m) contribute 49%–56% of total fatigue damage, while severe and extreme states contribute 35%–46%. The lower-bay wave-facing K-joint (J5) experiences the highest hot-spot stress range of 68 MPa under the design sea state. The derived stress histories provide the necessary input for subsequent S–N and fracture-mechanics assessments and for system-reliability calculations. The derived occurrence-weighted stress-range histograms constitute a reusable data product that supplies the necessary loading input for the companion fatigue-criticality assessment of the same platform and for subsequent integrity-management studies of Gulf of Guinea jackets.