A method and system of subsea intervention comprises lowering one or more assemblies of intervention equipment into the sea. Underwater marine units (such as remote operated vehicles or small submarines) may be employed to connect the assemblies to each other and to the subsea wellhead equipment. The subsea wellhead equipment includes a carrier line spool (e.g., coiled tubing spool, wireline spool, slickline spool) and equipment to inject a carrier line from the carrier line spool into the subsea well. The carrier line spool can be located underwater, such as on the sea floor or positioned above the subsea wellhead equipment. The carrier line spool can also be located on a sea vessel. Also, to switch tools, a carousel system having multiple chambers containing different types of tools can be used.