A riser tensioner, comprising: a cylinder barrel adapted to be connected to a riser; a rod reciprocally carried within the cylinder barrel the rod extending out an upper end of the cylinder barrel, the rod having an upper end adapted to be connected to a floating platform; a flange on a first end of the cylinder barrel, the flange having an opening through which the rod sealingly extends; a piston on an interior end of the rod that slides and seals against an inner surface of the cylinder barrel; a low pressure chamber in the cylinder barrel on a lower side of the piston and containing a low pressure fluid; a high pressure communication passage extending from a sealed annulus through the flange; wherein the sealed annulus is located between the rod and the cylinder barrel, and extends from an upper side of the piston to the flange and contains a high pressure fluid from the high pressure communication passage at a pressure higher than the low pressure fluid to urge the piston downward relative to the cylinder barrel, thereby retracting the riser tensioner and exerting an upward force on the riser; a high pressure seal to seal the piston against an inner surface of the cylinder barrel; a high pressure lubricant conduit, extending from the upper end of the rod to supply liquid lubricant to the sealed annulus; a high pressure seal lubricant conduit extending within the rod from the upper end of the rod to a point on a high pressure side of the seal; a high pressure seal lubricant within the high pressure seal lubrication conduit at a pressure that is the same as the fluid pressure in the annulus; a low pressure seal lubricant conduit (67, 69) extending from an upper end of the rod to a point on a low pressure side of the seal; and a low pressure seal lubricant within the low pressure lubricant conduit (67, 69) that is at a same pressure as the fluid pressure in the low pressure chamber.. a cylinder barrel adapted to be connected to a riser; a rod reciprocally carried within the cylinder barrel the rod extending out an upper end of the cylinder barrel, the rod having an upper end adapted to be connected to a floating platform; a flange on a first end of the cylinder barrel, the flange having an opening through which the rod sealingly extends; a piston on an interior end of the rod that slides and seals against an inner surface of the cylinder barrel; a low pressure chamber in the cylinder barrel on a lower side of the piston and containing a low pressure fluid; a high pressure communication passage extending from a sealed annulus through the flange; wherein the sealed annulus is located between the rod and the cylinder barrel, and extends from an upper side of the piston to the flange and contains a high pressure fluid from the high pressure communication passage at a pressure higher than the low pressure fluid to urge the piston downward relative to the cylinder barrel, thereby retracting the riser tensioner and exerting an upward force on the riser; a high pressure seal to seal the piston against an inner surface of the cylinder barrel; a high pressure lubricant conduit, extending from the upper end of the rod to supply liquid lubricant to the sealed annulus; a high pressure seal lubricant conduit extending within the rod from the upper end of the rod to a point on a high pressure side of the seal; a high pressure seal lubricant within the high pressure seal lubrication conduit at a pressure that is the same as the fluid pressure in the annulus; a low pressure seal lubricant conduit (67, 69) extending from an upper end of the rod to a point on a low pressure side of the seal; and a low pressure seal lubricant within the low pressure lubricant conduit (67, 69) that is at a same pressure as the fluid pressure in the low pressure chamber..