You need to pay attention to trasnsient effects, particularly the potential frequency and severity of slugging. Calculating this is an art, and you will liekly have to refer the problem to a consultant who has a copy of Pipesim or Olga. Also, you need to understand the differing requirements of B31.3/B31.4/B31.8 and ISO13623, and be aware of which Code applies to each system.

Some pipelines (for instance transfer lines from gathering stations to a processing plant) fall under multiple Code jurisdictions, with the result that an analysis may need to be run twice; once under each Code, and the results for the applicable portions (only) filed in the final report.

You may also need to consider a design case that takes into account release of a lot of vapor, with the effects of isentropic expansion cooling considered in the design temperature. Many times, this requires the use of a Charpy impact tested material for your piping. Occasionally, this will require further precautions, as for instance with high pressure wellhead piping.

Consider the frequency of pigging expected in the piping; this can have an enormous impact on the corrosion life of the system. It's best to use a corrosion engineer familiar with the issues of oil & gas projects.

Consider the flow velocities expected within the system. High velocities can help to accelerate corrosion. Low velocities can create terrible slugging problems. Once again, your two-phase flow consultant will need to provide you with data.

General routing considerations:

Consider doing phase separation of the two-phase flow stream as early as possible to minimize the amount of two-phase piping. Even a partial separation will help enormously.

It is always best if two-phase flow systems slope downward toward the final destination. Even a slight incline can cause major slug formation problems.

If this is not possible, consider creating an artificial high point close to the final destination and installing a slug catcher there, so that transient forces are minimized once you get to the equipment at the final destination. This is a capital expense that will pay for itself many times over during the operating lifetime of the facility due to reduced maintenance.