Abstract:
An apparatus for venting an annular space between a liner and a pipeline of a subsea riser used for conveying hydrocarbons, the apparatus comprising a permeate recovery device for recovering permeate passing through the liner into the annular space. The permeate recovery device includes a first vent port in a wall of the pipeline at or adjacent a lower region of the pipeline and communicating with a permeate recovery line defining a flow path between the annular space and a permeate collection vessel. A one-way valve is associated with the permeate recovery line for preventing flow from the permeate recovery line into the annular space. A gaseous permeate recovery line connected to a second vent port at an upper region of the pipeline defining a flow of gaseous permeate from the annular space.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a 35 U.S.C. §§371 national phase conversion of PCT/GB2008/000440, filed Feb. 8, 2008, which claims priority of Great Britain Application Nos. 0706745.7, filed Apr. 5, 2007, and 0715347.1, filed Aug. 7, 2007, the disclosure of which is incorporated by reference herein. The PCT International Application was published in the English language. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for venting an annular space between a liner and the pipeline of a riser for conveying hydrocarbons. 
     Grooved liners are a technology used within steel pipelines as a cost efficient alternative to using Corrosion Resistant Alloys (CRA&#39;s) to construct pipelines transporting highly corrosive substances such as hydrocarbons. The system involves inserting a plastic liner, complete with a number of longitudinal external grooves, into the pipeline. The aim of the liner is to protect the carbon steel pipe from the highly corrosive nature of the production fluids. The grooves act as a method of transporting the gases/liquids that inevitably permeate through the liner to a venting location where they are released to atmosphere or stored in a collection vessel. 
     It is desirable to utilise the technology for vertical applications, such as production risers, Steel Catenery Risers (SCR) etc. Such applications involve lining vertical pipelines in exactly the same way as a standard horizontal pipeline. These vertical lines act as a method of transporting the production fluids from the main production transport line to the surface. In some applications, the production fluids are transported down to the seabed. Since some of these vertical lines can be exposed to depths in excess of 1000 m, a method of aiding flow to the surface is usually required. One such method is Gas Lift, in which processed gas is pumped cyclically through the system to push the fluids from the seabed to the surface. 
     In the vertical orientation, the liquid permeate will condense on the inner walls of the pipeline and fall to the base of the riser under gravity, where a column of liquid would quickly develop between the liner and the pipeline. This phenomenon dictates that a reliable venting system is required for the removal of the condensed permeate which collects at the base of the riser. This is necessary to avoid the situation where the liner experiences external overpressure and possible collapse in the event that there is a loss of internal pressure in the bore. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an apparatus for venting an annular space between a liner and a pipeline of a subsea riser for conveying hydrocarbons, said apparatus comprising permeate recovery means for recovering permeate passing through the liner into said annular space. 
     Preferably said permeate recovery means comprises a first vent port provided in a wall of the pipeline at or adjacent a lower region of the pipeline and communicating with a permeate recovery line defining a flow path between said annular space and a permeate collection vessel. Preferably a one-way valve is associated with said permeate recovery line for preventing flow from said permeate recovery line into said annular space. 
     The permeate collection vessel may be mounted on or formed integrally with a suction pile upon which the riser is supported. Alternatively the permeate collection vessel may be formed from one or more pipe sections, the ends of which are closed by flanges or plugs. 
     In one embodiment, said permeate collection vessel includes a first outlet port provided in an upper region of the collection vessel, said first outlet port communicating with a gaseous permeate recovery line, and a second outlet port provided in a lower region of the collection vessel, said second outlet port communicating with a liquid permeate recovery line. One or both of said gaseous and liquid permeate recovery lines may be connected to pump means or other source of vacuum for pumping or drawing said gaseous and/or liquid permeate to the surface and for generating a vacuum in the collecting and/or disposing means. 
     The gaseous collection line and/or the liquid recovery line may be connected to a collection or storage vessel on the surface to permit measurement and/or analysis of the collected permeate. 
     In a second embodiment, a second vent port may be provided in a wall of the pipeline at or adjacent an upper region of the pipeline and communicating with a permeate recovery line for venting gaseous permeate from said annular space, said permeate collection vessel connected to said first vent port collecting primarily liquid permeate therefrom. A non-return valve may be associated with the second vent port and/or the permeate recovery line for preventing the return flow of permeate from said permeate recovery line to said annular space. The gaseous permeate recovery line may be connected to a vacuum source to draw gaseous permeate from said annular space. A storage vessel may be provided for storing said collected permeate. 
     The permeate collection vessel associated with said first vent port may be located on the seabed or an adjacent structure. The permeate collection vessel may be provided with a vacuum pump to aid extraction of liquid permeate from the annular space adjacent said first vent port. The permeate collection vessel may be provided with valve means to permit disconnection and recovery of the permeate collection vessel to enable the vessel to be drained and/or replaced. 
     In a third embodiment, the permeate collection vessel may be associated with a gas lift system for urging production fluids up the riser, whereby liquid permeate may be entrained out of the permeate collection vessel by means of a flow of high speed and/or high pressure gas and subsequently passed into the riser with the production fluids, thereby returning the liquid permeate into the liner and recovering the liquid permeate to the surface with the production fluid. 
     Said high speed gas may be injected into the permeate collection vessel over the surface of collected liquid permeate within the collection vessel, preferably in the direction of a permeate and high speed gas outlet, to entrain said liquid permeate into said high speed gas flow. Alternatively said high speed gas may be injected into a lower region of the condensate recovery vessel below the liquid level to entrain the liquid permeate into said high speed gas flow. 
     A second vent port may be provided in a wall of the pipeline at or adjacent an upper region of the pipeline and communicating with a permeate recovery line connected, preferably to a vacuum pump, for venting gaseous permeate from said annular space, said permeate collection vessel collecting primarily liquid permeate from said first vent port. 
     Alternatively said first vent may be utilised to collect both gaseous and liquid condensate, said gaseous and liquid condensate being entrained into said high speed gas flow of the gas lift system. The flow of high speed gas into the condensate collection vessel may be utilised to create a vacuum in the collection vessel, possible by means of a venturi effect, actively drawing condensate from said annular space. Alternatively a vacuum pump may be used for generating a vacuum in the permeate collection vessel. Such arrangement advantageously avoids the need for any permeate recovery lines extending to the surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of a permeate recovery apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a schematic view of a permeate recovery apparatus according to a second embodiment of the present invention; 
         FIG. 3  is a schematic view of a permeate recovery apparatus according to a modification of the second embodiment of the present invention; 
         FIG. 4  is a schematic view of a permeate recovery apparatus according to a third embodiment of the present invention; 
         FIG. 5  is a sectional view of a permeate recovery vessel of the apparatus of  FIG. 4 ; 
         FIG. 6  is a sectional view of a modified permeate recovery vessel of the apparatus of  FIG. 4 ; 
         FIG. 7  is a schematic view of a permeate recovery apparatus according to a fourth embodiment of the present invention; and 
         FIG. 8  is a schematic view of a storage tank for use with either of the first and second embodiments of the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As shown in the drawings, the riser assembly comprises a suction pile  1  located on the seabed for supporting a lower end of the riser  2 . The riser  2  comprises a rigid steel outer pipeline having an inner liner for protecting the steel pipeline from the corrosive effects of the production fluid (typically crude oil and/or natural gas) conveyed by the riser  2 . 
     A buoyancy module  3  supports an upper end of the riser and the production flow is conveyed from an upper end of the riser  2  to a vessel or surface installation by means of a production jumper  4 . 
     A gas lift system  5  is provided at a lower end of the riser for urging the production fluid up the riser  2 , the production fluid being fed to the lower end of the riser  2  and the gas lift system  5  via one or more production feed lines  6 . High speed gas is supplied from the surface via a feed line  7  connected to a gas lift manifold  8  on the seabed before being injected into a lower end of the riser  2 . 
     In a first embodiment of the present invention, as illustrated in  FIG. 1 , a first vent port  10  is located as low as possible on the vertical riser  2  for providing communication between an annular space between the liner and the inner walls of the riser  2  and a permeate jumper line  11 . The permeate jumper line  11  is connected to a permeate collection tank  12  to collect permeate extracted from said annular space. A non-return valve is provided in the first vent port  10  for preventing return flow of permeate from the permeate jumper line  11  to the annular space. 
     The permeate collection tank  12  may be located on top of the suction pile  1  and may be integrated into the suction pile  1 . Alternatively the permeate collection tank  12  may be located on the seabed separate from the suction pile  1  or may be mounted on other structures of the pipeline assembly. 
     It is envisaged that the permeate collection tank  12  may be formed from a section of pipeline, plugged at each end with blind flanges. The collection tank  12  may be formed from a number of interconnected pipe sections, stacked on top of one another in parallel relationship. 
     A liquid permeate recovery line  13  extends from a lower region of the permeate collection tank  12  to a vacuum pump on the surface to draw liquid permeate from the collection tank  12 . A gaseous permeate recovery line  14  extends from an upper region of the permeate collection tank  12  to a vacuum pump on the surface to draw gaseous permeate from the collection tank and to generate a vacuum within the collection tank  12  to actively draw permeate from said annular space of the riser  2 . 
     On the topside, the liquid permeate can be drained and/or samples taken for monitoring. The gases can be safely disposed of. 
     The first embodiment of the invention allows measurement and analysis of permeated gas/liquid from subsea location as opposed to directly venting to atmosphere and can ensure that the necessary venting process does not damage the environment. 
     A second embodiment of the present invention is illustrated in  FIG. 2 . 
     In the second embodiment, a second vent port  20  is located in an upper region of the riser  2  for extraction of gaseous permeate from the annular space between the liner and the pipeline of the riser  2 . The second vent port  20  is connected to a gaseous permeate jumper  21  leading to a storage tank  22  on the surface. 
     The second vent port  20  for the gas extraction is located at the top of the riser  2  to ensure the gaseous permeate jumper  21  is as short as possible, limiting the possibility of damage during service. The extraction of the gas will be controlled by limited topside equipment such as a vacuum pump. This equipment could also be of lower specification and hence cheaper than the same equipment required for the first embodiment. On the topside the gases can be safely disposed of. 
     As with the first embodiment, a first vent port  7  is located at the base of the riser  2 . This vent port  7  is as low as possible on the riser  2  to maximise efficiency. The first vent port  7  is attached to liquid permeate jumper  11 , complete with anti-return valve, to a liquid permeate collection tank  12 , such tank being of sufficient volume to be capable of containing the volume of permeated fluid for the entire life of the pipeline. The collection tank  12  is fitted with a vacuum pump to aid the extraction of fluids. 
     There are a number of options for the storage facility:
     1. A purpose built storage tank built into the top of the riser suction pile  1 . This would allow the liquid permeate to drain down vertically using gravitational force, aided by the vacuum pump. This is shown in  FIG. 2 .   2. A separate purpose built storage tank secured on the seabed.   3. A section of pipe or multiple sections linked together, both ends fitted with blind flanges. A schematic can be seen in  FIG. 3 .   

     Advantages of the second embodiment:
         Fluids do not need to be pumped to the surface if the storage tank has adequate volume. This removes the need for umbilical lines from the seabed to the surface. This will reduce the risk of damage in service;   Vacuum pump equipment on topside could be of lower specification than if required to vent from seabed;   Feasible for valves to be closed temporarily for change-out of storage tanks for options 2 &amp; 3. This would not affect production.       

     A third embodiment of the present invention is illustrated in  FIG. 4 . 
     Typical vertical lines, such as risers, utilise a Gas Lift system to assist the extraction of production fluids to the surface. The system injects gas at the base of the riser at high pressure with the gas travelling at high velocities. The pressure of the gas lifts the fluids up the riser to the surface. 
     The manifold  8  utilised by the gas lift system  5  on the seabed could be modified to include a small storage tank  30  for the vented liquid permeate. The high pressure, high velocity gas can be routed through the storage tank  30 . The small amount of liquid permeate stored in the tank  30  can then be slowly removed along with the high velocity gas, passing over the liquid permeate collected in the tank  30 , whereby the liquid permeate passes into the riser  2  with the production fluids (see  FIG. 5 ). 
     The second vent port  20  for the gas extraction is located at the top of the riser  2  to ensure the extraction jumper  21  is as short as possible, limiting the possibility of damage during service. The extraction of the gas can be controlled by limited topside equipment such as a vacuum pump. This equipment may be of lower specification and hence cheaper than the same equipment required for the first embodiment. On the topside the gases can be safely disposed of. 
     A variation on this solution may be to inject the high speed gas through the reservoir of permeated liquid within the tank  30 , rather than passing over the top. This would in effect cause a spray which can be introduced into the production flow as described before. A sketch of this variation on the process is shown in  FIG. 6 . 
     Advantages of the third embodiment:
         No umbilicals are required from the seabed to the surface for transport of permeates. This reduces the risk of damage to the venting system in operation;   No external equipment necessary to draw a vacuum in the annulus—the fluid would drip into the container under gravity and hydrostatic load from head of permeated liquid;   Can be incorporated to current standard gas lift/riser designs;   Vacuum pump equipment on topside could be of lower specification than if required to vent from seabed.       

     A fourth embodiment of the present invention is illustrated in  FIG. 7 . 
     In the fourth embodiment, the gas lift system is used to transport the liquid permeate to the surface via the riser  2 . Due to the small amount of liquid permeated, a vacuum may be created in the storage tank  30  that could be used to suck both the liquid and the gaseous permeates from the annular space between the liner and the pipeline of the riser  2 . This system would allow both the gas and the liquid to be reintroduced into the production flow and transported to the surface. This would negate the need for the second vent port  20  at the top of the riser as the gas would be vented from the single base vent port  10 . 
     Advantages of the fourth embodiment:
         No umbilicals are required from the seabed to the surface for transport of permeates. This reduces the risk of damage to the venting system in operation;   No external equipment necessary to draw a vacuum in the annulus—the vacuum would be naturally created by the gas lift flow through the tank;   Can be incorporated to current standard gas lift/riser designs;   No need for topside venting equipment or umbilicals.       

       FIG. 8  illustrates a storage tank  12  for use with either of the first and second embodiments described above. The storage tank is defined by a section of small diameter pipe  100 , identical or similar to the pipe used for the permeate jumper line  11 , the storage pipe  100  being formed into a convoluted or serpentine shape and mounted within a frame  102  mounted on the seabed  104 , whereby the storage pipe  100  can be used to collect permeate from the annular space between the inner walls of the riser  2  and the liner. A first valve  106  is provided at in inlet end of the pipe  100  for controlling communication between the storage pipe  100  and the jumper line  11 . 
     To enable the storage pipe  100  to be drained, a further pipe  110 , of similar diameter and construction to the storage pipe  100 , is arranged in parallel to the storage pipe  100  and connected to an upper end of the storage pipe  100  via a second valve  108 . An inlet end of the further pipe  110  is connected to a source of gas (e.g. air) from an umbilical termination unit and the lower end of the storage pipe  100  is connected to vent or drain pipe of the umbilical termination unit, each via a respective valves  112 , 114 . 
     In normal operation, the second  108  and further  112 , 114  valves are closed and the first valve is open so that permeate from the permeate jumper line is collected in the storage pipe  100 . When it is desired to drain the storage pipe  100 , the first valve  106  is closed, to close communication between the storage pipe  100  and the jumper line  11 , and the second  108  and further  112 , 114  valves are opened whereby air is supplied into the further pipe  110  from the umbilical termination unit via valve  112  and passes into an upper end of the storage pipe  100  via valve  108 , thereby flushing out permeate from the storage pipe  100  into the vent or drain of the umbilical termination unit via valve  114 . Thus a closed circuit is created between the further pipe  110  and the storage pipe  110  for flushing the storage pipe  100 . 
     Various modifications and variations to the described embodiments of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments.