Unauthorized welding on laundry room equipment triggered a fire aboard the cruise ship Ecstasy off Miami in 1998, injuring 22 people and causing more than $17 million in damages, U.S. safety investigators concluded on Tuesday. The National Transportation Safety Board (NTSB) said that sparks from the welding equipment caused a fire that quickly spread and ignited a large accumulation of lint in the laundry room ventilation ducts. The fire migrated through that system to the aft mooring deck where it fed on dock lines, generating such intense heat that it knocked out an electrical system that helped power the ship's engines. Owned by Carnival Corp.…