156Where, as a result of any occurrence taking place while a ship is carrying a cargo of persistent oil in bulk, any persistent oil carried by the ship is discharged or escapes then, whether or not the owner incurs a liability under section 153—
(a)he shall not be liable otherwise than under that section for any such damage or cost as is mentioned therein; and
(b)no servant or agent of the owner nor any person performing salvage operations with the agreement of the owner shall be liable for any such damage or cost.