Riser with retrievable internal services

Abstract

Systems are described for raising production fluid from a source ( 1 ) on the seabed comprising a riser ( 4 ) having a first, lower, end for connection or connected to the source; a top end support for supporting the riser at or in the vicinity of the sea surface; and an operating device ( 11 ) mounted inside the riser ( 4 ) for displacement within the riser so that the pump is accessible to an operator for replacement or repair. The operating device ( 11 ) may be displaced on a pipe ( 12 ) which extends within the riser ( 4 ) and to a lower end of which the device ( 11 ) is attached. The device ( 11 ) may be inter alia an electric pump, a hydraulic pump, a gas injector, a heater or a cleaning device.

1 . A system for raising production fluid from a source on the seabed, comprising:—
 a riser having an internal passageway for conveying said production fluid and having a first, lower, end for connection to or  
 a top end support for supporting the riser at its second end at or in the vicinity of the sea surface; and  
 an operating device mounted inside the riser for displacement within the riser between a first, operating, position in the riser remote from its second end, and a second, access, position, at the second end of the riser, so that the device is accessible for replacement or repair, the operating device being a pump or a gas injector.  
 
     
     
         2 . A system according to  claim 1  and further comprising means for displacing the device in the riser between the operating and access positions.  
     
     
         3 . A system according to  claim 2 , wherein the displacing means includes a pipe which extends within the riser and to a lower end of which the device is attached.  
     
     
         4 . A system according to  claim 3 , wherein the device is an electric pump and an electric power supply cable for the pump passes through the pipe.  
     
     
         5 . A system according to  claim 3 , wherein said pipe is connected to supply fluid from the upper end of the riser down to the operating device.  
     
     
         6 . A system according to  claim 5 , wherein a further pipe is provided within the riser and is connected to function as a return pipe for conveying fluid from the operating device to the upper end of the riser.  
     
     
         7 . A system according to  claim 6 , wherein the supply and return pipes form a nested pipe arrangement.  
     
     
         8 . A system according to  claim 5 , wherein the device is a hydraulic pump and the pipe is arranged to convey hydraulic fluid down to the pump, which is arranged to discharge the hydraulic fluid into the production fluid passing up the riser.  
     
     
         9 . A system according to  claim 6  or  7 , wherein the device is a hydraulic pump, the supply pipe being arranged for delivering hydraulic fluid down to the pump and the return pipe being arranged for conveying hydraulic fluid from the pump back up the riser.  
     
     
         10 . A system according to any one of  claims 5  to  9 , wherein the operating device is a hydraulic pump and means are provided for delivering heated fluid through the supply pipe for heating production fluid in the riser.  
     
     
         11 . A system according to any one of  claims 3  to  9 , wherein there is associated with said device a heater for heating said production fluid in the riser.  
     
     
         12 . A system according to  claim 10 , wherein a heater is provided in the region of the hydraulic pump, the heater being arranged to be supplied by said heated fluid delivered through said pipe.  
     
     
         13 . A system according to any one of  claims 3  to  12 , wherein the displacing means includes a pipe dispensing and retrieving apparatus on the top end support or an attendant service vessel, such apparatus comprising a rotatable pipe storage device on which the or each pipe is wound, a pipe straightener and drive means for the storage device, selectively operable for straightening a length of pipe and driving it downwardly into the riser to lower the device to said first position and to wind in the or each pipe to raise the device to said second position.  
     
     
         14 . A system according to any preceding claim, wherein a pig introducing device is provided for introducing a pig into the riser at a position below the operating device when in said second position.  
     
     
         15 . A system according to any preceding claim, further comprising a locating device which is mounted on the operating device, and is selectively operable for (i) engaging with the inner surface of the riser and (ii) disengaging therefrom so that the operating device can be repositioned in the riser.  
     
     
         16 . A system according to  claim 15 , wherein the operating device is a pump and the locating device is a sealing device which is operable for both engaging and sealing with the inner surface of the riser so that the pump can pump production fluid in the riser from a low pressure side of the sealing device to a high pressure side.  
     
     
         17 . A system according to  claim 16 , wherein the sealing device comprises a packer mounted on the pump and an inflatable sealing element operable for forming sealing contact with the inner surface of the riser.  
     
     
         18 . A system according to any preceding claim, wherein in addition to said passageway for carrying the production fluid, a second passageway is provided in the riser for connection to a low pressure region in the vicinity of the upper end of the riser and means are provided for expelling production fluid from the first passageway to said low pressure region, so as to reduce the pressure in the first passageway to a lower value than the existing under interrupted on shutdown conditions, thereby inhibiting formation of solid hydrates in the first passageway.  
     
     
         19 . A system according to  claim 18 , wherein the production fluid expelling means comprises a one-way valve providing fluid communication from the first passageway to the second passageway, a source of pressure gas operable for introducing gas under pressure to the first passageway to expel production fluid therefrom through the one-way valve, and means for venting the gas pressure in the first passageway to said region of lower pressure.  
     
     
         20 . A system according to  claim 18 , wherein the production fluid expelling means is said pump.  
     
     
         21 . A system according to any one of  claims 4  to  10 , further comprising a cyclone separator mounted on the pipe for positioning within the riser and having inlet means for imparting swirl to production fluid entering the separator from the riser to effect separation of the fluid into a liquid-rich underflow and a gas-rich overflow, the operating device being a pump, said pump being arranged to receive the separator underflow and pump it up to the top of the riser through said pipe.  
     
     
         22 . A system according to  claim 8  or  9  or any dependent claim thereof, wherein said hydraulic pump comprises a pump section, turbine section and a packer section, the packer section being articulated relative to the pump section.  
     
     
         23 . A system according to  claim 22 , wherein the articulation comprises a universal drive coupling or a flexible coupling.  
     
     
         24 . A system according to  claims 3  to  13 , further comprising a traction device on the pipe operable for applying traction to the pipe to drive the operating device down inside the riser.  
     
     
         25 . A system according to  claim 24 , wherein the traction device is selectively operable from the remote end of the pipe for applying traction to the pipe in either direction for lowering or raising the operating device.  
     
     
         26 . A system according to  claim 24  or  claim 25  as dependent on any of  claims 4  to  10 , wherein the operating device is a pump and a sealing device is mounted on the pump and is arranged to provide a sliding seal with the inner surface of the riser, the pump being arranged to pump between a low pressure and a high pressure side of the sealing device so as to generate a traction force for driving the down-riser pump longitudinally within the riser, the pump and sealing device together constituting said traction device.  
     
     
         27 . A system according to any one of  claims 3  to  13 , comprising a motor mounted on a lower end of the pipe and arranged to be powered electrically or hydraulically from an upper end of the riser, and a rotary cleaning device.  
     
     
         28 . A system according to  claim 27 , as dependent on any of  claims 4  to  10 , wherein a sealing device on the motor is arranged to form a sliding seal with the inner surface of the riser.  
     
     
         29 . A system according to  claim 28 , as dependent on any of  claims 4  to  10  wherein said pump is arranged to provide differential pressure between a low pressure side and a high pressure side of the sealing device.  
     
     
         30 . A system according to  claim 28  or  29 , wherein the rotary cleaning device is arranged to generate a differential pressure between one side and the other side thereof when it is rotating.  
     
     
         31 . A system according to any one of  claims 27  to  30 , wherein the cleaning device comprises at least one of a rotary cutter and a rotary brush.  
     
     
         32 . A system according to any one of  claims 27  to  31 , wherein the motor is an electric motor which is arranged to be powered by an electrical cable passing through said pipe.  
     
     
         33 . A system according to any one of the preceding claims, wherein said riser has a lazy-S, steep wave or steep-S configuration.  
     
     
         34 . A system according to any one of  claims 1  to  32 , wherein said riser has a substantially vertical complaint section, leading from the sea bed to the surface, a substantially horizontal section on the sea bed, and a bend section connecting the substantially vertical and horizontal riser sections.  
     
     
         35 . A system according to any one of  claims 3  to  11 , wherein said device comprises a gas injector for introducing pressure gas into the riser when in said first position.  
     
     
         36 . A method of installing an operating device in a riser connecting a source of production fluid on the seabed to a top end support supporting the riser at or in the vicinity of the sea surface, the operating device being a pump or a gas injector, the method comprising:—
 (a) introducing the device into the riser at the upper end thereof; and  
 (b) driving the device downwardly into the riser to a desired operating position in the riser remote from its end at the top end support.  
 
     
     
         37 . A method according to  claim 36 , wherein the device is attached to the lower end of a rigid pipe and the pipe is driven downwardly into the riser to displace the device to its desired operating position.  
     
     
         38 . A method according to  claim 36  or  claim 37  comprising the further steps of:—
 (c) driving the pipe upwardly to raise the device within the riser to the top end support;  
 (d) removing the device from the riser;  
 (e) disconnecting the device from the pipe, for maintenance or replacement, and  
 (f) repeating steps (a) and (b) with the maintained or replaced device.  
 
     
     
         39 . A method according to  claim 37 , comprising the further steps of:—
 (c) driving the pipe upwardly to raise the device within the riser to the top end support;  
 (d) removing the device from the riser;  
 (e) disconnecting the device from the pipe;  
 (f) removing an end section of the pipe or attaching a new section of pipe to the existing pipe in order to define a new length of pipe;  
 (g) attaching the device to the end of the new length of pipe; and  
 (h) driving the device down the riser to its new operative position in the riser.  
 
     
     
         40 . A method according to any one of  claims 36  to  39 , further comprising at least partially expelling the production fluid from the riser interior, so as to reduce the pressure acting there, in order to inhibit formation of solid hydrates in the production fluid flowing from the source under interrupted flow or shut down conditions.  
     
     
         41 . A method according to  claim 40 , wherein said operating device is a pump and said pump is used for at least partially expelling the production fluid.  
     
     
         42 . A method according to  claim 40 , wherein gas is introduced under pressure into the riser to at least partially expel the production fluid and the gas pressure acting in the riser is reduced to a value lower than its initial value while preventing the expelled production fluid from returning to the space occupied by the gas.  
     
     
         43 . A method according to  claim 42 , wherein a hydrate formation inhibitor is introduced into the riser along with the gas under pressure.