Nonlinear analysis of a prestressed steel pipeline crossing

Externally prestressed steel tubes are effective solutions to pipeline crossings up to 60 m in span. For the case study described herein it represents an economy of 12% with respect to the other alternatives. Finite element models of localized stress concentrations at intermediate stiffeners and stiffening ring supports are presented. A shell model with stiffeners is studied to avoid the damaging ovalization due to the prestressing force. In order to assess stresses, a rational simplified method is developed. For the load levels considered the stresses are well below the yield limit of the structural steel used in the pipe. Second order effects are of primary concern, therefore nonlinear analyses are studied. Water pressures must be applied as “follower” forces, otherwise a significant error is introduced rendering nonlinear displacement smaller than those of the linear analysis. Two sources of second order effects are identified: axial force due to prestressing and water internal pressure. A simplified rational method is presented to take into account both effects using critical loads as parameters. The amplification factor obtained is very close to the results of the full nonlinear finite element model and is therefore recommended for initial design purposes.