def buckle_thickness(D_o, P_p, sig_y):
    """Return the nominal buckle thickness [t] based on the propagation pressure.
    Considers the worst case maximum external pressure and ignores internal
    pressure - PD8010-2 Equation (G.21).

    :param float D_o: Outside Diameter [m]
    :param float P_p: Propagation pressure [Pa]
    :param float sig_y: Yield strength [Pa]
    """
    return D_o * (P_p / (10.7 * sig_y))**(4 / 9)