A new marine oil production riser system for use in deepwater applications is disclosed. An efficient means for accommodating movements of the host facility, while maintaining riser top tension within the limits for long-term riser performance. Long riser stroke lengths can be accommodated without requiring complex interfacing with the topsides. The riser assembly comprises: a generally extendable substantially non-vertical section having an upper end adapted to be in flow communication with a generally vertical marine riser carried by a facility floating on the surface of a body of water, and having a lower end adapted to be in flow communication with a fluid source on the seafloor; and tensioning means, mechanically connecting the upper end of the marine riser with the lower end of the marine riser, for biasing said ends towards each other. The tensioning means comprises: a cylinder having one end open to sea pressure, having an opposite end sealed from sea pressure, and connected to one end of the marine riser; a piston within the cylinder disposed for movement within the cylinder; and a piston rod passing through the opposite end of the cylinder and having one end connected to the other end of the marine riser.