A conduit device for a petroleum well intervention string includes a first end connected to an intervention string drum unit with a drum and a second end connected to an injector head of a wellhead assembly. The conduit device, at any point between said first end and said second end, has a radius of curvature larger than or equal to a given smallest allowable radius of curvature for the intervention string. The conduit device forms a substantially incompressible fixed length path for the portion of said intervention string at any time present between the first end and the second end. The conduit device constitutes a tubular path for the intervention string.