The effect of ovality length on imperfect sandwich pipes is investigated using the finite element method in the scenario of local buckling under external pressure. First, the finite element model of the imperfect sandwich pipelines is established in ANSYS and is validated by comparing the results from numerical simulation with those from experiments. Then, the effect of ovality features on the collapse strength of the sandwich pipes is studied. At last, based on the calculation results from 1200 cases, a prediction equation is proposed to represent the relationship between collapse strength and ovality length of imperfect sandwich pipes. Good agreement is achieved between the proposed equation and the calculation results, leading to the conclusion that the proposed simplified model can be an efficient tool in the evaluation of the local collapse strength of subsea sandwich pipes under external pressure.

This paper focuses on the buckle propagation pressure of a pipe-in-pipe system under uniform external pressure. After the validation of the finite element model used in this research, several effects were studied. At first, the effects of the initial imperfection of the inner pipe, including the ovality and eccentricity, were checked out. Second, the influence of the scantling of the inner and outer pipes was tested. It was found that the dominant factor which affects the buckle propagation mode is changeable. Then, the boundary value of the above factor was investigated by varying the ratio of the outer and inner diameters. Finally, after a summary of the calculation results, an empirical formula was proposed to illustrate the relationship between the buckle propagation pressure and the geometrical parameters of the pipe-in-pipe system. Utilizing this formula, the buckle propagation pressure of the pipe-in-pipe system can be estimated rapidly with good accuracy.