Marine pipeline installation system for laying an offshore pipeline and/or installing a subsea riser, said system comprising: - a storage reel for storage of the pipeline to be installed, - a launch tower which is adapted for launching the pipeline in a firing line into the sea, - a pipeline guide mounted at an elevated position of said launch tower and providing an arctuate guide surface in a vertical plane for guiding said pipeline from the storage reel into the firing line, - one or more tensioners (30,35) mounted on the launch tower below said pipeline guide, - a flexible pipeline end fitting transfer mechanism adapted to transfer an end fitting on a flexible pipeline over the pipeline guide, which transfer mechanism comprises: - a transfer arm which is rotatable about a horizontal transfer arm axis and relative to the pipeline guide, which arm has a leading side and a trailing side when seen in direction of rotation over the pipeline guide towards the firing line, wherein the transfer arm is provided, radially outward from the pipeline guide surface and at the trailing side, with a first retainer for an end fitting adapter, which first retainer is adapted to retain, at the trailing side of the transfer arm, a first end fitting on a flexible pipeline that is provided with an end fitting adapter, wherein the transfer arm is provided with a first sheave at a position radially outward from said first retainer, said first sheave being adapted and arranged to guide a winch driven cable attached via the end fitting adapter to the first end fitting such that, with the transfer arm in a first end fitting pick up orientation thereof, the first end fitting is lifted and the end fitting adapter reaches said first retainer allowing for engagement with said first retainer whilst suspended from said cable, wherein the transfer arm, with the first end fitting retained by said first retainer, is rotatable into a first end fitting release orientation thereof so that the first end fitting passes over and about the arctuate guide surface towards the firing line, characterized in that the transfer arm is provided, radially outward from the pipeline guide surface and at the leading side, with a second retainer for an end fitting adapter, which second retainer is adapted to retain, at the leading side of the transfer arm, a second end fitting on a flexible pipeline, wherein the transfer arm is rotatable into a second end fitting pick up orientation, wherein the end fitting adapter on the second end fitting on a pipeline curving over the arctuate guide surface is engageable with said second retainer, and wherein the transfer arm is rotatable into a second end fitting release orientation thereof so that the second end fitting passes over and about the arctuate guide surface towards the firing line, wherein the transfer arm is provided with a second sheave at a position radially outward from said second retainer, said second sheave being adapted and arranged to guide a winch driven cable attached via the end fitting adapter to the second end fitting such that, with the transfer arm in a second end fitting release position, the second end fitting can be suspended from said cable allowing disengagement of the end fitting adapter from the second retainer and lowering of the second end fitting, wherein said first and second sheaves (75,76) are offset from one another in direction of rotation of the transfer arm, with the first sheave at the trailing side and the second sheave at the leading side of the transfer arm, preferably said first and second sheave being arranged in a common vertical plane. - a storage reel for storage of the pipeline to be installed, - a launch tower which is adapted for launching the pipeline in a firing line into the sea, - a pipeline guide mounted at an elevated position of said launch tower and providing an arctuate guide surface in a vertical plane for guiding said pipeline from the storage reel into the firing line, - one or more tensioners (30,35) mounted on the launch tower below said pipeline guide, - a flexible pipeline end fitting transfer mechanism adapted to transfer an end fitting on a flexible pipeline over the pipeline guide, which transfer mechanism comprises: - a transfer arm which is rotatable about a horizontal transfer arm axis and relative to the pipeline guide, which arm has a leading side and a trailing side when seen in direction of rotation over the pipeline guide towards the firing line, - a transfer arm which is rotatable about a horizontal transfer arm axis and relative to the pipeline guide, which arm has a leading side and a trailing side when seen in direction of rotation over the pipeline guide towards the firing line,