This study investigates buckling propagation mechanisms of subsea pipe-in-pipe (PIP) systems under hydrostatic pressure. Unlike previous studies which covered propagation pressure of PIPs with carrier pipes of diameter-to-thickness (Do/to) ratio smaller than 26, in this study PIPs with Do/to of 30 and 40 are examined. Experimental tests on 1.6 m long PIP with Do/to 40 and of inner pipe diameter-to-thickness (Di /ti ) ratio of 25 is carried out inside a hyperbaric chamber. The experimental results are used to validate a 3D nonlinear (material and geometry) finite element (FE) model. Using the validated FE model a parametric study is conducted to capture the effect of the parameters affecting the buckling collapse pressure mechanism of the PIP system. Based on the results from the FE study, the collapse propagation modes of the PIPs are identified and empirical formulae for the propagation pressure of the PIP system are proposed.