Hydro-pneumatic tensioner with stiffness altering secondary accumulator

A riser tensioning system having a riser extending between subsea well equipment and a floating vessel having an operational platform engaged with the riser, a tensioner unit comprising:

• a barrel having a port on one end;
• a first piston slidingly carried in the barrel and having an opening in fluid communication with the port;
• a piston rod extending from the barrel and having a bore in fluid communication with the opening in the piston;
• a second piston sealingly and slidably carried in the bore of the piston rod;
• a primary accumulator in fluid communication with the port and having pressurized fluid therein that communicates with first sides of the first and second piston;
• and wherein:
• the bore of the piston rod and a second side of the second piston define a secondary accumulator containing a pressurized fluid;
• the tensioner unit is positioned between the riser and the platform;
• a first section of the tensioner unit is connected to the riser;
• and
• a second section of the tensioner unit is connected to the platform.

A tensioner unit, comprising:

• a barrel including a bore;
• a first piston slidingly carried in the bore of the barrel and having an opening in fluid communication with a pressurized fluid;
• a piston rod extending from the barrel and having a bore in fluid communication with the opening in the piston;
• a secondary accumulator housing having a bore;
• a fluid separator sealingly engaging the bore of the secondary accumulator housing and having a first and a second side to separate a plurality of volumes of fluid;
• a primary accumulator in fluid communication with the bore of the secondary accumulator housing through an opening in at least one of the piston rod and barrel and having pressurized fluid therein that communicates with the first side of the first piston and first side of the fluid separator;
• and
• the bore of the secondary accumulator housing and a second side of the fluid separator defining a secondary accumulator containing a pressurized fluid within.

A tensioner unit, comprising:

• a barrel having a port on one end and in communication with a pressurized fluid;
• a tubular housing surrounding the barrel and having a bore, an opening, and an aperture on one end;
• a piston having a first and second side, slidingly carried in the bore of the barrel, and having a piston rod extending from the barrel on the second side of the piston through the aperture;
• a secondary accumulator housing including a bore, an opening in the bore in fluid communication with the opening in the bore of the tubular housing, and a fluid separator sealingly engaging the bore of the secondary accumulator housing and having a first and a second side to separate a plurality of volumes of fluid;
• a primary accumulator in fluid communication with the port of the barrel and the opening in the bore of the tubular housing, and having pressurized fluid therein that communicates with the first side of the piston and the first side of the fluid separator;
• and
• the bore of the secondary accumulator housing and the second side of the fluid separator defining a secondary accumulator containing a pressurized fluid.

A tensioner unit, comprising:

• a barrel having a bore, a fluid communication opening, an aperture on one end, and having a volume of pressurized fluid therein;
• a first piston having a first and a second side, slidingly carried in the bore of the barrel and having a piston rod that extends from the piston through the fluid communication opening of the barrel;
• a primary accumulator housing having a bore, a plurality of fluid communication openings, and in fluid communication with the fluid communication opening of the barrel;
• a secondary accumulator housing having a bore, a fluid communication opening, and having a fluid separator sealingly engaging the bore of the secondary accumulator housing, the fluid separator having a first and a second side to separate fluid in the secondary accumulator housing, the secondary accumulator housing further being in fluid communication with the primary accumulator housing through the fluid communication opening of the secondary accumulator housing and one of the fluid communication openings of the primary accumulator housing;
• a primary accumulator in fluid communication with the fluid communication opening of the barrel and the fluid communication opening of the secondary accumulator housing and having pressurized fluid that communicates with the first side of the first piston and first side of the fluid separator;
• and
• the bore of the secondary accumulator housing and the second side of the fluid separator defining a secondary accumulator containing a pressurized fluid.

A tensioner unit, comprising:

• a barrel including a bore and an aperture on one end and having a pressurized fluid contained within and forming at least part of a primary accumulator;
• a piston in communication with a pressurized fluid and slidingly carried in the bore of the barrel and having a piston rod that extends from the piston through the aperture;
• a secondary accumulator housing having a bore and a pressurized fluid having a preselected pressure contained within;
• and
• a valve arrangement between the first and second accumulators that allows pressurized fluid to flow from the primary accumulator to the secondary accumulator when the pressure in the primary accumulator is greater than in the secondary accumulator, and that allows pressurized fluid to flow from the secondary accumulator to the primary accumulator when the pressure in the secondary accumulator is greater than the pressure in the first accumulator and also greater than the preselected pressure, and which blocks pressurized fluid from flowing from the secondary accumulator to the primary accumulator when the pressure in the secondary accumulator drops to the preselected pressure.

A method of maintaining a selected range of tension on a riser extending between subsea well equipment and a floating vessel, comprising the steps of:

• providing a barrel having a bore and an aperture on one end, the barrel forming at least part of a primary accumulator;
• mounting a piston slidably in the barrel, the piston having a piston rod extending from the barrel through the aperture;
• providing a secondary accumulator housing having a bore and forming at least part of a secondary accumulator;
• mounting a fluid separator between the primary and secondary accumulators;
• connecting either the piston rod or the barrel to the vessel and the other to the riser;
• applying fluid pressure from the primary accumulator to a first side of the first piston and a first side of the fluid separator;
• applying fluid pressure into the secondary accumulator housing on a second side of the fluid separator until pressure on both sides of the fluid separator equal;
• applying tension to the riser by the force due to pressure of fluid in the primary accumulator, urging the piston rod to extend;
• if vessel moves closer to the subsea equipment, allowing the piston rod to extend farther and allowing the fluid separator to functionally isolate the fluid volume in the secondary accumulator from the fluid volume in the primary accumulator to maintain tension range;
• and
• if vessel moves farther from the subsea equipment, allowing the piston rod to retract and allowing the fluid separator to functionally combine the fluid volume in the secondary accumulator with the fluid volume in the primary accumulator to maintain tension range.

A tensioner unit, comprising:

• a barrel including a bore;
• a piston slidingly carried in the bore of the barrel and having a piston rod extending from the barrel, the barrel and the piston defining a primary accumulator containing a pressurized fluid;
• a secondary accumulator containing a pressurized fluid at a preselected pressure;
• a communication path between the primary and secondary accumulators for flow of the pressurized fluids between the primary and secondary accumulators;
• a control device in the communication path that allows flow from the primary accumulator into the secondary accumulator if the pressure in the primary accumulator exceeds the preselected pressure in the secondary accumulator;
• the control device allowing flow from the secondary accumulator into the primary accumulator if the pressure in the secondary accumulator is greater than the preselected pressure and greater than the pressure in the primary accumulator;
• and
• the control device blocking flow from the secondary accumulator into the primary accumulator when the fluid in the secondary accumulator is at the preselected pressure.

A method of maintaining a selected range of tension on a riser extending between subsea well equipment and a floating vessel, comprising:

• connecting one end of a tensioner to the riser and the another end to the floating vessel, the tensioner having a piston slidably carried in a barrel, defining a primary accumulator;
• providing a secondary accumulator in fluid communication with the primary accumulator;
• pressurizing the primary accumulator with a compressible fluid to a nominal pressure to apply tension to the riser;
• pressurizing the secondary accumulator with a compressible fluid to a preselected pressure greater than the nominal pressure;
• causing the tensioner to retract and extend due to movement of the vessel relative to the riser, the retraction and extension causing the pressure in the primary accumulator to increase and decrease from the nominal pressure;
• preventing any change in the preselected pressure in the secondary accumulator during the retraction and extension as long as the pressure in the primary accumulator remains below the preselected pressure;
• if the vessel movement causes the pressure in the primary accumulator to exceed the preselected pressure in the secondary accumulator, flowing fluid from the primary accumulator into the secondary accumulator, causing the pressure in the secondary accumulator to increase from the preselected pressure, then
• when the vessel movement begins to cause the pressure in the primary accumulator to decrease, returning fluid from the secondary accumulator to the primary accumulator until the preselected pressure in the secondary accumulator is again reached.