Vital difference between COA and consecutive voyage contract is that the former is based on the cargo while the latter is based on the named vessel.
In Voyage Contract, if the ship becomes a total loss, the obligation of the carrier ends as the carrier is no longer available to the charterers. While in COA, if ship owner intends to carry the cargo in his own vessel, even though he had the option of carrying it in any vessel. If that vessel becomes a total loss then the ship owner is liable to charterer and obliged to fulfill its contract.