The diverter is an annular preventer with a large piping system underneath. It is utilized to divert the kick from the rig and it can be used when the conductor pipe is set. It is not used if you drill riserless. The large diameter pipe typically has two directions diverting the wellbore fluid out of the rig (see the figure below for more understanding).
The diverter should be used only when the well cannot be shut in because of fear of formation breakdown or lost circulation. Use of the diverter depends on the regulations and operator policies.
The diverter is normally installed on a conductor casing with large diverter pipe pointing to a downwind area. Typically, the selective valves located at each diverter line can be operated separately so the personnel on the rig can divert the flow into the proper direction. It is designed for short periods of high flow rate but it cannot hold a lot of pressure. With high flow rate, the erosion can be happened easily so the bigger of diverter line the better. Additionally, the straight diverter lines are the most preferable.
In the market, there are several models provided by service providers as
Hydril Pressure Control FSP* 28-2000 Diverter
Hydril Pressure Control FS™ 21″ 500-psi Marine Riser Diverter
Reference book: Well Control Books