What is the regulation for drydock as per SOLAS ?

As per SOLAS CH-I REG -10- a minimum of two inspections of the outside of the ship’s bottom during any 5 years period shall be carried out. The interval between any 2 such inspections shall not exceed 36 months.

 Thus an underwater inspection is mandatory for SOLAS.The inspection may be carried out either in a dry dock or an underwater survey may be carried out as decided by the classification society depending on the vessels condition