A jacket structure is designed to support a platform required for drilling and production facilities. The jacket is subjected to complex, multi-directional loading during installation. Overturning moments from wind, current and wave loads are skewed from overturning moments from pile and jacket weight eccentricity. The mudmat geometry can be simple with symmetrical mudmats at the four corners of the jacket base or can be complex due to full rectangular area with an off-center, open, rectangular bay for conductor installation. Mudmat geometry, loads during pile installation and soil conditions combined to produce a challenging overturning stability problem. Equivalent area methods of API RP 2GEO and ISO 19901-4 may not predict the low overturning resistance, and a typical righting moment analysis may not capture the soil-structure interaction. To address the geometry and complex loading, a plasticity analysis of overturning stability was performed and is described herein.
Partial safety factors as recommended by API RP 2GEO, ISO 19901-4 were used to assess stability so that overturning from the jacket dead weight could be treated separately from the wind, current and wave loading. Since the partial safety factors are lower for the stabilizing forces compared to the forces causing overturning moments, the resulting safety factors can be lower. Moment and force equilibrium were imposed, and the minimum overturning safety factor was found. Although the vector sum of the factored loads was oriented away from a principal axis of the mudmat, upper bound plasticity methods were used to investigate kinematically admissible failure mechanisms. The method of analysis easily accounts for irregular foundation geometry and complex, multidirectional loading with varying degrees of uncertainty. The method fills a gap in API RP 2GEO and can be implemented in a simple spreadsheet.
A case study is presented to demonstrate the safety factor variation using API RP 2GEO method and the proposed failure method with varying eccentricity in the gravity loads and overturning moments due to wind, current and wave loads.