Marine pipeline installation system for laying an offshore pipeline and/or installing a subsea riser, said system at least being adapted to carry out the reel lay method, wherein the system comprises: - one or more storage reels for storage of the pipeline to be installed, - a pipeline launch tower which is adapted for launching the pipeline in a pipeline launch trajectory into the sea, - a pipeline guide provided at an elevated position for guiding said pipeline to the 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 via an associated support assembly, the support assembly of one or more tensioner frames being adapted to allow for displacement of the tensioner frame with respect to the tower between an active position, wherein the pipeline launch trajectory extends through the tensioner frame and between the tracks, so that the tracks can engage on a pipeline, and a retracted non-operable position, - an abandonment and recovery (A&R) system comprising 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 adapted to support the weight of the launched pipeline 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; characterized in that the auxiliary trolley comprises: ∘ an A&R connector allowing the A&R cable to be connected to the auxiliary trolley at the position above the uppermost tensioner and thereby allowing the auxiliary trolley to lower with the launched pipeline suspended therefrom; ∘ an A&R cable guide (50b', 50b") allowing the A&R cable to be guided by the auxiliary trolley and pass along the auxiliary trolley during A&R of the launched pipeline connected to the A&R cable, at least at the position below the lowermost tensioner; ∘ a collar clamp comprising two clamp parts and (50a") which are moveable between an open configuration and a closed configuration in which the clamp is adapted to engage a pipeline, pipe section or accessory in the pipeline launch trajectory; ∘ two trolley parts (50' and 50"), each part comprising an A&R cable guide (50b', 50b") and a clamp part (50a', 50a"), the clamp parts being movable via respective frame parts (50c'; 50c") slidably engaging with a horizontal guide of the trolley. - one or more storage reels for storage of the pipeline to be installed, - a pipeline launch tower which is adapted for launching the pipeline in a pipeline launch trajectory into the sea, - a pipeline guide provided at an elevated position for guiding said pipeline to the 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 via an associated support assembly, the support assembly of one or more tensioner frames being adapted to allow for displacement of the tensioner frame with respect to the tower between an active position, wherein the pipeline launch trajectory extends through the tensioner frame and between the tracks, so that the tracks can engage on a pipeline, and a retracted non-operable position, - an abandonment and recovery (A&R) system comprising 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 adapted to support the weight of the launched pipeline 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; ∘ an A&R connector allowing the A&R cable to be connected to the auxiliary trolley at the position above the uppermost tensioner and thereby allowing the auxiliary trolley to lower with the launched pipeline suspended therefrom; ∘ an A&R cable guide (50b', 50b") allowing the A&R cable to be guided by the auxiliary trolley and pass along the auxiliary trolley during A&R of the launched pipeline connected to the A&R cable, at least at the position below the lowermost tensioner; ∘ a collar clamp comprising two clamp parts and (50a") which are moveable between an open configuration and a closed configuration in which the clamp is adapted to engage a pipeline, pipe section or accessory in the pipeline launch trajectory; ∘ two trolley parts (50' and 50"), each part comprising an A&R cable guide (50b', 50b") and a clamp part (50a', 50a"), the clamp parts being movable via respective frame parts (50c'; 50c") slidably engaging with a horizontal guide of the trolley.