buckling analysis of pipeline under external pressure

The buckling pressure of a uniform pipe under external pressure, can be evaluted by the linear analtical equation:

 I use abaqus to simulate this problem. I tried for different pipe length and boundray conditions, the buckling pressure varies a lot, and even larger than the linear analytical results. Does anyone know how long do I need to build for the model and how to set up the boundray conditions. Many thanks.