Patent ID: 7191837

Claim:
An offshore assembly for performing operations on an offshore well, comprising: a floating vessel; an interface device for mounting to a wellhead assembly of an offshore well; a lifting apparatus for lifting the interface device over the wellhead assembly and supporting the interface device while the interface device is in enaagement with the wellhead assembly, the lifting apparatus having a cable with a terminal end extending therefrom, the lifting apparatus being positioned on the floating vessel for movement therewith; a motion compensator connected between the interface device and the terminal end of the cable, the motion compensator being moveable between an expanded position and a contracted position in order to compensate for movement of the floating vessel and the lifting apparatus responsive to the movement of the water; wherein the motion compensator comprises: a first frame assembly comprising a first plurality of rods extending substantially parallel to each other, a first end plate fixedly connected to an end portion of each of the first plurality of rods, and a first medial plate fixedly connected to the opposite end portion of each of the first plurality of rods; a second frame assembly comprising a second plurality of rods extending substantially parallel to each other, a second end plate fixedly connected to an end portion of each of the second plurality of rods, and a second medial plate fixedly connected to the opposite end portion of each of the second plurality of rods, the second frame assembly overlapping the first frame assembly such that the first medial plate is positioned between the second end plate and the second medial plate and the second medial plate is positioned between the first medial plate and the first end plate, the first and second frame assemblies being movable relative to each other between the expanded and the contracted positions; and a piston and chamber connected between the medial plates of the first and second frame assemblies for reducing changes in tension in the cable responsive to the movement of the floating vessel and the lifting apparatus relative to the wellhead assembly, the piston and chamber contracting when the first and second frame assemblies move toward the expanded position.