Hello Kuramadenz,
Please confirm whether the pressure you have stated is the surge pressure or the line pressure.
Usually for a main LNG lines a surge analysis will be studied during shut down operation in a specialised software(eg, Pipenet). This will give the rise in pressure in the line during shut down operations. Since the valves are not stoped a sudden the surge pressure will not be that high. Since the line sizes are very high in your case it will be too conservative to consider the absolute pressure difference as the surge pressure.