Hydro-pneumatic tensioner with stiffness altering secondary accumulator

Abstract

A hydro-pneumatic tensioner includes a barrel having an inner bore and a pressurized fluid contained within to form at least part of a primary accumulator having a preset volume of gas at a preselected pressure. A piston having a piston rod extending from an aperture in the barrel is slidably carried in the bore of the barrel and is in communication with the pressurized fluid and positioned to increase the fluid pressure when the piston strokes in the direction of the pressurized fluid. A secondary accumulator also has a preset volume of gas at a preselected pressure. A fluid separator maintains functional separation of the fluid volumes of the primary and secondary accumulators when the primary accumulator pressure is less than a preselected secondary accumulator pressure. The fluid separator allows functional combining of the fluid volumes of the primary and secondary accumulators when the primary accumulator pressure equals or is greater than the preselected secondary accumulator pressure.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] This invention relates to tensioning of seabed-to-vessel marine risers. More particularly, this invention relates to tensioning the marine risers with a plurality of pneumatic or hydraulic cylinders. [0003] 2. Brief Description of the Related Art [0004] A problem presented by offshore hydrocarbon drilling and producing operations conducted from a floating platform is the need to establish a sealed fluid pathway between each borehole or well at the ocean floor and the work deck of the platform at the ocean surface. A riser typically provides this sealed fluid pathway. In drilling operations, the drill string extends through a drilling riser, the drilling riser serving to protect the drill string and to provide a return pathway outside the drill string for drilling fluids. In producing operations, a production riser is used to provide a pathway for the transmission of oil and gas to the work deck. [0005] The riser is ...

AI Technical Summary

Benefits of technology

[0013] In view of the foregoing, an embodiment of the present invention advantageously provides a tensioner unit including a barrel having a bore with pressurized fluid contained within. The barrel forms at least part of a primary accumulator having a preset volume of gas Vg1 at pressure P1. The barrel of the tensioner unit also includes an aperture to allow extension and retraction of a piston rod. The tensioner unit also includes a piston slidably carried in the bore of the barrel. The piston has a piston rod that extends from one side of the piston and through an aperture in the barrel. The piston has one of its sides in communication with the pressurized fluid, and is so positioned to increase the pressure P1 of the primary accumulator when the piston strokes in the direction of the pressurized fluid. The tensioner unit also includes a secondary accumulator housing having a bore which forms at least part of a secondary accumulator having a preset volume of gas Vg2 at pressure P2. The tensioner unit also includes a fluid separator positioned between the primary and secondary accumulators to maintain functional separation of fluid volumes of the primary and secondary accumulators when the primary accumulator pressure P1 is less than the secondary accumulator pressure P2. The fluid separator also allows functional combining of the fluid volumes of the primary and secondary accumulators when the primary accumulator pressure P1 is greater than or equal to the secondary accumulator pressure P2. The tensioner unit is configured so that when the primary accumulator pressure P1 is less than the secondary accumulator pressure P2, the effective total gas volume VgT available to the tensioner to maintain tension on a supported system, such as a riser system, is substantially equivalent to the primary accumulator gas volume Vg1. Correspondingly, when the pressure P1 is greater than or equal to the secondary accumulator pressure P2, the effective total gas volume VgT available to the tensioner to maintain tension on the supported system equals the sum of the primary accumulator gas volume Vg1 plus the secondary accumulator gas volume Vg2. This provides for reduced stiffness and reduced maximum tension applied by the tensioner unit to a supported system when the system is directing a maximum load on the tensioner unit.

Problems solved by technology

A problem presented by offshore hydrocarbon drilling and producing operations conducted from a floating platform is the need to establish a sealed fluid pathway between each borehole or well at the ocean floor and the work deck of the platform at the ocean surface.

If a portion of the riser is permitted to go into compression, it could be damaged by buckling or by bending and colliding with adjacent risers.

However, unlike conventional platforms which are rigidly attached to the subsea floor, TLPs are not designed to resist horizontal forces induced by waves.

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