A marine pipeline installation system for laying an offshore pipeline and/or installing a subsea riser, the system at least allowing to carry out the reel lay method, wherein the system comprises:one or more storage reels for storage of pipeline, a pipeline launch tower adapted for launching the pipeline in the direction of the sea, a pipeline guide at an elevated position for guiding said pipeline to a pipeline launch trajectory along said tower, one or more tensioners, each tensioner comprising a tensioner frame and multiple tracks supported by said tensioner frame, said tracks being adapted for engaging the pipeline and supporting at least part of the weight of the launched pipeline, the tensioner frame being supported by said tower via an associated support assembly. The support assembly of at least one tensioner frame, preferably of all tensioner frames, is adapted to allow for displacement of the tensioner frame between an active position, wherein the pipeline launch trajectory extends through the tensioner frame between the tracks, so that the tracks can engage on a pipeline, and a retracted and non-operable position.