The present invention relates to a marine pipeline installation system for laying an offshore pipeline and/or installing a subsea riser, said system being adapted to carry out the reel lay method and the J-lay method. The system comprises a vessel with a moonpool, a pipeline launch tower arranged on the vessel, a pipeline guide at an elevated position on said tower for guiding said pipeline to a pipeline launch trajectory along said tower, and one or more tensioners. The support assembly of at least one tensioner frame 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. The system further comprises a J-Lay installation.