A marine pipeline installation system for laying an offshore pipeline and/or installing a subsea riser includes a pipeline launch tower, a pipeline guide provided at an elevated position for guiding said pipeline to the pipeline launch trajectory along said tower, one or more tensioners, an abandonment and recovery (A&R) system including at least one A&R cable and associated A&R winch and a sheave arrangement with one or more sheaves supported by the pipeline launch tower at an upper position thereof. An auxiliary trolley is provided which is adapted to support the weight of the launched pipeline and which is movable along a rail in the pipeline launch trajectory when the one or more tensioner frames are in the retracted non-operable position, from a position above the uppermost tensioner to a position below the lowermost tensioner, the rail being supported by the pipeline launch tower.