Abstract:

For offshore oil and gas jacket platforms with detected structural cracks, a methodology for structural integrity evaluation and maintenance cycle strategy formulation was developed based on engineering critical assessment (ECA) techniques. A case study was conducted on a specific jacket platform. The Morison equation was used for hydrodynamic analysis of the target platform to estimate the ultimate load of joints prone to failure. Hotspot stress assessment was performed on these joints using the finite element analysis method and linear extrapolation. Crack propagation behavior at critical joints was simulated using Paris' law, and the stress intensity factor at the crack tip was determined. Cracks were assessed using failure assessment diagrams (FAD), and the critical crack sizes were provided. Based on the relationship between critical crack depth and crack propagation, a reference maintenance cycle was proposed. The results showed that the joints connecting horizontal braces and risers of the jacket structure were prone to fatigue damage. Failure assessment indicated that failure in the crack depth direction was primarily dominated by collapse, while failure in the crack length direction may involve both collapse and fracture. For the analyzed platform, it was recommended to consider a critical crack depth of 5.3 mm and a critical crack half-width of 9.8 mm. If cracks ranging from 0.5 to 2.0 mm were detected, maintenance was recommended within 13.2 to 5.2 h. This methodology can be extended to similar offshore oil and gas platforms with detected cracks in adjacent sea areas.

Key words: jacket platform, hotspot stress, crack propagation, engineering critical assessment (ECA), critical crack