The spherical shaped tanks/separators represent a common type of equipment in the Oil & Gas industry. The spherical shape is preferred due to the robustness that the geometry offers, in addition to the large fluid surface area that maximizes the gravity separation process. However, these convoluted shapes introduce difficulties in routine inspection and integrity assurance. This paper studies a fitness for service (FFS) assessment of complex damage mechanisms which include localized bottom bulge formation, local shell buckling and underside soil corrosion.

This research analyzes a case study of atmospheric Horton Spheroid separation tank with 20,000 barrel capacity, 20 m diameter and 15 m height. The analysis includes a design calculation review of the spheroid via Finite Element Analysis (FEA), API 620 (API 620 1962-2014) standard review from 1962 to 2014 and simulation of the steam desludging maintenance procedure. 3D nonlinear Finite Element model (FEM) was introduced to analyze the spheroid fitness for service under several bulgings and localized buckling. The FEA results were compared to inspection results to validate and confirm the bulging locations, bulging sizing and spheroid integrity.

The FEA model results showed that the vacuumed spheroid is subjected to inward bulge formation at the crown upper shell and lower bottom plates. Due to the non-uniform curvature of the spheroid, the bulge locations are difficult to be predicted, however, the FEA results were able to detect precisely the location and geometry of bulges. The plastic strain failure criterion was used to assess the fitness for service of the spheroid for continuous operation with the presence of those deformations. The FEA model results showed excellent agreement with internal inspection results and findings. The FEA model results were used to conduct semi-qualitative risk assessment on other identical spheroids; an endorsement extension was granted based on the semi-qualitative assessment.

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