—Above-ground crude oil storage tanks (ASTs) in Jonglei State, South Sudan, are founded on soft Sudd wetland clays exhibiting undrained shear strengths as low as 4–8 kPa and consolidation settlements exceeding 300 mm under standard loading. This paper presents a comprehensive finite element analysis (FEA) study of AST foundation performance on these challenging soils, evaluating eight foundation configurations across 36 parametric cases using the Mohr-Coulomb constitutive model implemented in PLAXIS 2D (axisymmetric and plane-strain formulations). Geotechnical characterisation from five boreholes and 120 in-situ vane shear tests established a five-layer soil profile for the Pibor–Bor corridor, with primary consolidation coefficients cv of 0.40–2.50 × 10⁻³ m²/day. FEA results demonstrate that unreinforced shallow foundations and ring-rafts shallower than 1.0 m consistently fail the API 650 bearing capacity criterion (FoS ≥ 2.5) for tank diameters of 30 m and above under design contact pressures of 120 kPa. A 1.2-m-thick reinforced concrete ring-raft achieves FoS = 2.61 and maximum differential settlement of 31 mm, satisfying API 650 shell distortion limits. For tank diameters exceeding 40 m or sites with Su < 10 kPa, piled raft foundations (FoS = 4.20) or stone column ground improvement (FoS = 3.05) are mandated. A parametric Factor of Safety surface as a function of undrained shear strength and raft thickness provides a design nomograph for direct engineering application. Seasonal flood inundation reduces effective Su by 22–35% and lowers FoS by 0.4–0.8 units, necessitating flood protection measures as a prerequisite to any foundation design on the Jonglei wetland plain.