Ram style tensioner with fixed conductor and floating frame

1. An offshore facility, having a floating platform, a riser having a lower end secured to subsea equipment and an upper portion extending through an opening in the platform, an improved riser tensioner, comprising:

a frame stationarily mounted to the upper portion of the riser;

a plurality of pistons and cylinders spaced circumferentially around the opening, each piston and cylinder being mounted between the frame and the floating platform, the cylinders being supplied with a pressurized fluid to apply tension to the riser;

a tubular guide member mounted to the floating platform for movement in unison in response to waves and currents, the riser extending through the guide member;

a guide roller support stationarily mounted to and extending downward from the frame around the guide member; and

at least one set of guide rollers mounted to the guide roller support, spaced circumferentially around the guide member and in rolling engagement with the guide member as the guide member moves in unison with the platform.

2. The facility according to claim 1 ,wherein:

the platform has an upper deck and a lower deck; the frame is located above the upper deck; and

the guide member is mounted to the lower deck and extends upward through the upper deck.

3. The facility according to claim 2 , wherein the guide member has an upper end that is at a fixed distance from the upper deck and a lower end that is at a fixed distance below the upper deck.

4. The facility according to claim 2 , wherein the said at least one set of guide rollers comprises an upper set of guide rollers located a fixed distance below the frame, and a set of lower guide rollers located a fixed distance below the upper set of guide rollers.

5. The facility according to claim 1 , wherein the guide member has an upper end that is below the frame while the pistons and cylinders are in minimum stroke position.

6. The facility according to claim 1 , wherein the guide roller support comprises a plurality of braces spaced circumferentially around the guide member, each of the braces extending parallel with an axis of the guide member.

7. The facility according to claim 1 , wherein said at least one set of guide rollers comprises a lower set at a lower end of the roller guide support and an upper set positioned to engage the guide member near an upper end of the guide member while the pistons and cylinders are in a maximum stroke position.

8. A method of tensioning a riser extending from subsea equipment to a floating platform, which includes mounting a frame to the riser, and mounting pistons and cylinders between the platform and the frame, and supplying the cylinders with fluid pressure to apply tension to the riser, the improvement comprising:

(a) mounting a guide member to the platform;

(b) mounting a guide roller support to the frame, and mounting a set of guide rollers to the guide roller support and in rolling engagement with the guide member; and

(c) in response to platform movement toward and away from the subsea equipment due to waves and/or current, moving the guide member in unison with the platform and relative to the guide roller support and set of guide rollers, which remain stationary with the riser.

9. The method according to claim 8 , wherein step (a) further comprises extending the riser through the guide member.