Two-part telescopic tensioner for risers at a floating installation for oil and gas production
A tensioning device ( 15 ) for a riser ( 5 ) connecting a subsea borehole ( 7 ) with a floating installation ( 1 ) on the surface of the sea ( 11 ), where the tensioning device ( 15 ) is provided with telescoping tubes ( 27, 27 ′) and also several evenly spaced hydraulic cylinders ( 31, 31 ′) arranged in a peripherally encircling manner and mainly in the longitudinal direction of the riser, and where the tension in the riser is exerted through hydraulic pressure in said cylinder ( 31, 31 ′), the tensioning device ( 15 ) consisting of two successive, interconnected telescopic tensioning units ( 23, 25 ), the tensioning units ( 23, 25 ) being designed separately to maintain a prescribed tension in the riser ( 5 ).
1. An arrangement for tensioning a riser, the arrangement comprising:
a riser having a lower end positioned towards a sub-sea borehole and an upper end positioned towards a floating installation on the surface of the sea; and
upper and lower telescopic tensioning units that are integrated into the riser and that are successively interconnected to each other, the upper and lower telescopic tensioning units operating separately to maintain a prescribed tension in the riser;
wherein the upper and lower telescopic tensioning units each comprise a telescopic tube and a plurality of evenly spaced hydraulic cylinders arranged in a peripherally encircling manner around the tube and mainly in the longitudinal direction of the telescopic unit.
2. The tensioning device of claim 1 , wherein each of the upper and lower telescopic tensioning units are separately connected to a hydraulic system to facilitate the separate operation of the upper and lower telescopic tensioning units.
3. The tensioning device of claim 2 , wherein rapid changes in the vertical position of the floating installation relative to the seabed are compensated for by the upper telescopic tensioning unit and slower changes in the vertical position of the floating installation relative to the seabed are compensated for by the lower telescopic tensioning unit.
4. The tensioning device of claim 3 , wherein one of the upper and lower telescopic tensioning units alone maintains the prescribed tension in the riser in a situation wherein the other one of the upper and lower telescopic tensioning units is out of operation.
5. The tensioning device of claim 1 , wherein the lower telescopic tensioning unit has a lower end connected to the riser and an upper end connected to a lower end of the upper telescopic tensioning unit, and wherein the upper telescopic tensioning unit has an upper end connected to at least one of the riser and the floating installation.