For the large D/t>100 pipeline design, as the pipeline'd diameter is 6 metres, and the negative pressure. Does anyone has this kind of expericen to share with us?

The pipe thickness shall be calculated for both internal pressure and external pressure i.e vacuum. For internal pressure, we can use ASME b31.3 for finding out the thickness. For external pressure as formula is not available in Piping code, you can use pressure vessel code , Asme sec VIII div 1, for finding out the thickness for the corresponding L/D and D/T ratios where L is the distance between supports.
Caesar can be used for sustained stress and expansion stresses and piping loads. These large diameter pipes may be subjected to surge pressures due to pump trip and pump start up. The pipe rating shall be checked aginst the maximum pressure surge and also suitable stops and guides shall be provided to take care of pressure surge loads.