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 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 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 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 that is at a same pressure as the fluid pressure in the low pressure chamber. . <EOS>