Abstract:
A riser system including: at least one riser for extending from infrastructure on a sea bed and each riser having a riser termination; an end support restrained above and relative to the sea bed and having attachment means to couple each riser termination for storage and decouple each riser termination for coupling to a floating vessel; and an intermediate support supporting an intermediate portion of the riser to define a catenary bend between the intermediate support and the riser termination device.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to riser systems for communicating between infrastructure, such as a production well on a sea bed and a floating vessel. 
       BACKGROUND OF THE INVENTION 
       [0002]    Risers are used in offshore oil and gas production for communicating between infrastructure on the sea bed and floating vessels on the surface. 
         [0003]    It is sometimes desirable to disconnect the surface vessel from the riser. For this purpose risers are sometimes provided with a floating buoy that allows the riser termination to float when not in use. Australian patent no 677204 describes such a system. Australian patent no 677204 also describes the use of an intermediate support at an intermediate point along the riser for supporting and elevating the riser at the intermediate point and defining a catenary bend between the intermediate support and the riser termination/buoy. The riser termination and buoy float at the water surface and are allowed to drift about in a relatively large drift circle when disconnected from the floating vessel. 
         [0004]    The upper strata of water in the sea is relatively turbulent. Floating riser termination and buoy combinations are from time to time damaged by this turbulence. 
         [0005]    The addition of the floatation buoy to the riser termination requires specialised equipment which can add significant time and cost to the installation of the riser system. Additionally the use of a floatation buoy connected to the riser requires modification to the standard floating vessels which adds further cost. 
         [0006]    Objects of the present invention include to provide an improved riser system, to provide an improved method of storing disconnected riser terminations, or at least provide alternatives in the market place. 
       SUMMARY OF THE INVENTION 
       [0007]    There is provided a riser system including:
       a riser for extending from infrastructure on a sea bed and having a riser termination;   an end support restrained above and relative to the sea bed and having attachment means to couple the riser termination for storage and decouple the riser termination for coupling to a floating vessel; and   an intermediate support supporting an intermediate portion of the riser to define a catenary bend between the intermediate support and the riser termination.       
 
         [0011]    The riser system may be configured such that the catenary bend portion is supported above and clear of the sea bed when the riser termination is supported by the end support. 
         [0012]    The riser termination is a device at the end of a riser which is used to connect the riser to a vessel floating installation or a platform. The riser termination is generally pulled into a connection conduit on the vessel installation or platform. 
         [0013]    Preferably the end support has multiple attachment means to support multiple risers. A socket, hook or clamp are suitable forms of attachment means. The end support might be a rigid structure extending from the sea bed but is preferably a tethered buoy. The attachment means preferably include lead-in features, such as V-shaped extensions, for leading the riser, or riser connection means such as a line, into the attachment means. 
         [0014]    Preferably the relative depth and spacing of the intermediate support and end support are selected so that contact between the end support and the riser is prevented when the riser termination is coupled to a floating vessel. 
         [0015]    Advantageously the riser system includes a plurality of risers each having a respective intermediate support and riser termination. While more than one end support may be used, it is preferable that the riser and terminations are able to cooperate with a single end support. 
         [0016]    In another aspect of the invention there is provided a method of storing a riser termination for later coupling to a floating vessel, the method including the step of releasably coupling the riser termination to an end support, the end support being above and in a fixed position relative to the sea bed. 
         [0017]    Preferably the end support has multiple attachment means to support multiple risers. A socket, hook or clamp is a suitable form of attachment means. The end support might be a rigid structure extending from the sea bed but is preferably a tethered buoy. The attachment means preferably include lead-in features, such as V-shaped extensions, for leading the riser, or riser connection means such as a line, into the attachment means. 
         [0018]    The method may include supporting an intermediate portion of the riser using an intermediate support to define a catenary bend between the intermediate support and the riser termination. The intermediate support may be a float or floats attached to the riser. Preferably the intermediate support is a tethered buoy. The catenary bend portion may be supported above the sea bed. 
         [0019]    In another aspect of the invention there is provided a method of installing a riser system including the steps of guiding a riser termination to couple with an end support restrained above and in a fixed position relative to a sea bed; extending the riser from the end support to an intermediate support for supporting the riser to define a catenary bend portion between the intermediate support and the end support; and extending the riser from the intermediate support to a low connection point, such as infrastructure on the sea bed. 
         [0020]    Preferably the step of guiding the riser termination to couple with the end support includes extending a line between a top portion of the riser termination and a first vessel and suspending the riser termination in an inverted position, preferably from a second vessel; extending the riser such that the riser termination is lowered and is suspended from the line in an upright position, and manoeuvring the riser via the line to couple the riser to the end support. 
         [0021]    It is preferable to install the riser system without the catenary bend portion contacting the sea bed. For this purpose, the length of line may be sufficiently shorter than a sea bed depth so that the riser termination may be lowered and suspended from the line in the upright position without the riser contacting the sea bed. 
         [0022]    Preferably the end support has multiple attachment means to support multiple risers. A socket, hook or clamp is a suitable form of attachment means. The end support might be a rigid structure extending from the sea bed but is preferably a tethered buoy. The sockets preferably include lead-in features, such as V-shaped extensions, for leading the riser into the socket, hook or clamp. 
         [0023]    The intermediate support could be a float or floats attached to the riser. Preferably the intermediate support is a tethered buoy. 
         [0024]    The various aspects of the invention are complementary and each may incorporate the features of the other aspects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The figures illustrate preferred embodiments of the invention. 
           [0026]      FIG. 1  is a schematic view of a riser system; 
           [0027]      FIG. 2  illustrates the inverted lowering of the riser termination; 
           [0028]      FIG. 3  illustrates an upright riser suspended from a line; 
           [0029]      FIG. 4  illustrates the manoeuvring and coupling operation; 
           [0030]      FIG. 5  illustrates the riser being extended to an intermediate support; 
           [0031]      FIG. 6  illustrates the riser supported by the intermediate support; 
           [0032]      FIG. 7  is a perspective view of the buoy portion of an end support; 
           [0033]      FIG. 8  is a close up perspective view of the mounting sockets on the buoy; 
           [0034]      FIG. 9  is a top view of the sockets on the buoy; 
           [0035]      FIG. 10  is a perspective view of the buoy supporting multiple risers; 
           [0036]      FIG. 11  is an end view of the buoy supporting multiple risers; 
           [0037]      FIG. 12  is a side view of the buoy supporting multiple risers; 
           [0038]      FIG. 13  is a perspective view of the buoy supporting multiple risers; and 
           [0039]      FIG. 14  is a top view of the buoy supporting multiple risers. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0040]      FIG. 1  illustrates a riser system according to an embodiment of the present invention coupled to a Floating Production Storage and Offloading vessel (FPSO)  20 . 
         [0041]    The illustrated embodiment includes two risers  11  and  11 ′. Each riser extends from the sea bed  22  to a riser termination  12 ,  12 ′. The risers  11 ,  11 ′ are supported by intermediate supports  16  and  16 ′. Catenary bend portions  14  and  14 ′ are respectively defined between intermediate supports  16  and  16 ′ and the riser termination  12  and  12 ′. 
         [0042]    “Catenary” is used herein per its usual use in this art i.e. the shape of a chain supported at both ends. It refers to the curved shape assumed by the suspended riser. The term is not limited to the curve assumed by a uniform heavy cord (as in the strict dictionary definition). For example, the net buoyancy of various points along the catenary bend portion  14  could be varied, e.g. by the addition of weights or floats, which would alter the shape. This altered shape is intended to fall within the scope of the term catenary bend as used herein. 
         [0043]    The use of intermediate supports  16 ,  16 ′ and a catenary bend portions  14 ,  14 ′ allows the location of the riser to be more accurately controlled and minimises the maximum tension experienced by the riser. 
         [0044]    The riser  11  is illustrated in the in use position. Riser termination  12  is engaged with the FPSO  20  for communicating with infrastructure (not shown) on the sea floor  22 . Riser  11 ′ is illustrated in the “storage position”. The riser termination  12 ′ is coupled to the end support  18 . 
         [0045]    In this embodiment, the end support  18  and the intermediate supports  16  and  16 ′ are tethered buoys as illustrated. Alternative arrangements are possible. For example, the end support  18  might be a rigid structure extending from the sea floor. Similarly, the intermediate supports might simply be a float or a series of floats positioned along a portion of the risers. 
         [0046]    The tethered supports preferably include a buoy, a tether and ballast. Alternatively, the buoys could be tethered to a structure fixed to the sea bed. In an embodiment, the ballast may be buoyant for manoeuvring into position and then flooded such that it sinks to the sea floor  22 . Alternatively a tethered buoy may be configured such that it may be flooded and sunk to the seabed for later refloating when required. The tether is preferably heavy chain but may be wire or synthetic fibre. It is desirable to provide a redundant tether, which might be a synthetic cord, in case of damage to the chain. 
         [0047]    As illustrated, the intermediate supports are spaced from the end support and are relatively higher than the end support  18 . The relative height and spacing of the intermediate supports  16 ,  16 ′ and the end support  18 , and the length of the riser within the catenary bend portions  14 ,  14 ′ are selected such that the catenary bend portion  14  when in the coupled position (as illustrated) cannot interfere with the end support  18  thereby avoiding damage to the riser  11 . Similarly, the relative height and spacing of the intermediate supports  16 ,  16 ′ and the end support  18  and the length of riser material within the catenary bend portions  14 ,  14 ′ is selected so that when the riser  11 ,  11 ′ is in the storage position, the catenary bend portions  14 ,  14 ′ cannot interfere with the sea bed  22  thereby avoiding damage to the catenary bend portion  14 ′. 
         [0048]      FIG. 2  illustrates the initial stages of installing a riser system in accordance with an embodiment of the invention. As shown, the riser termination  12 ″ is lowered into the water from an installation vessel  26  in an inverted position. Riser termination  12 ″ is suspended by riser  11 ″ such that it can be lowered by extending riser  11 ″ from the installation vessel  26 . A line  24  is attached to an upper portion of the riser termination  12 ″. In this embodiment the line  24  is attachment to the upper portion of the riser termination  12 ″ via a short line  48  and a coupling device  50 . As will be described in more detail below short line  48  extends from the upper portion of the riser termination  12 ″ to the coupling device  50 . The coupling device  50  is in turn connected to the line  24 . Of course, the upper portion of the riser termination  12 ″ is downwardly disposed whilst the riser termination  12 ″ is in the illustrated inverted position. The line  24  is secured to a vessel  28  (but in other embodiments may be secured to a crane or winch aboard the installation vessel). 
         [0049]    As illustrated, the vessels  26  and  28  are positioned proximal to an end support  18 ″ and an intermediate support  16 ″. Both the intermediate support  16 ″ and the end support  18 ″ take the form of tethered buoys in this embodiment. As illustrated, the end support  18 ″ in this embodiment includes a buoy portion  30  and a pair of tethers  32 . The use of a pair of tethers serves to stabilise and orient the buoy portion  30 . 
         [0050]    As riser  11 ″ is extended from the vessel  26 , the riser termination  12 ″ is lowered until it is suspended from the line  24  and thereby turned from the inverted position (illustrated in  FIG. 2 ) to the upright position illustrated in  FIG. 3 . The buoy portion  30  includes sockets  36  for receiving the coupling device  50  and thereby coupling with the end portion  12 ″ of the riser  11 ″. Details of the sockets  36  are discussed in more detail below. 
         [0051]    After uprighting the riser termination  12 ″, the vessel  28  is manoeuvred such that the short line  48  with coupling device  50  connected is captured within socket  36  as illustrated in  FIG. 4 . Line  24  is then reeled out until the coupling device  50  (connected to the short line  48  and in turn connected to the upper portion of the riser termination  12 ″) is engaged with socket  36 . The line  24  is then released from the coupling device  50  connected to the short line  48 . The riser termination  12 ″ is thereby coupled to the end support  18 ″. 
         [0052]    As illustrated in  FIG. 5 , the installation vessel  26  is then manoeuvred toward and over the intermediate support  16 ″ such that the riser  11 ″ is engaged with the intermediate support  16 ″. An engagement feature  38  is fitted to the riser  11 ″ for this purpose. The riser  11 ″ is thereby engaged with the intermediate support  16 ″ such that a catenary bend portion  14 ″ is formed as illustrated in  FIG. 6 . The installation vessel can then continue on to lay the riser to the infrastructure (not shown). 
         [0053]    As illustrated in  FIG. 6 , the catenary bend portion is supported above the sea bed  22 ″ such that damage to the riser  11 ″ is avoided. The riser termination  12 ″ is securely coupled to the end support  18 ″ by the engagement device  50  and short line  48  connected to the riser termination  12 ″ and thereby kept safely away from the turbulent zone in the upper strata of the water. 
         [0054]    The stored riser termination  12 ′, (or  12 ″) is then readily retrieved for attachment to an FPSO  20 . 
         [0055]    Using this method allows the riser system to be installed without the FPSO  20 . This has the significant commercial advantage. Both the FPSO  20  and the installation vessel  26  are expensive long lead time items. Requiring the FPSO to be present during installation of the riser system, as in prior art systems, leads to significant risk of expense which is saved by the preferred embodiments of the present invention. 
         [0056]      FIGS. 7 to 14  illustrate an embodiment of the buoy portion  30 ′ of an end support. The buoy portion  30 ′ includes two buoyant substantially cylindrical tanks having hemispherical end portions. The tanks  44  are in adjacent parallel horizontal juxtaposition and connected by a pair of bracket portions  46 . 
         [0057]    The buoy  30 ′ includes a bank of four (in this particular embodiment) sockets  36 ′ on each outwardly disposed side face. The sockets  36 ′ are arranged to catch the line  24  and couple to the riser  12 ″ as illustrated in  FIGS. 4 and 5 . 
         [0058]    Each socket  36 ′ includes lead-in features  42 . The lead in features include a pair of extending members which, in plan view as in  FIG. 9 , outwardly diverge. As best illustrated in  FIG. 8 , each outwardly extending portion of the lead in features  42  is formed by a loop of bar material lying in a vertical plane. The lead in features  42  each lead into a respective vertical open-ended slot which provides an entrance into a keyhole feature having an upwardly open cavity with an upper lead in feature for receiving a coupling device  50  attached to a line  48 , which in turn supports the riser termination  12 , such that the coupling member is retained within the upwardly open cavity by the weight of the riser  11  and the riser termination  12 . The coupling device  50  is preferably a master link. Another preferred detail (not shown) is a locking device, such as a pin, to further retain the coupling device  50  within the upwardly open cavity. 
         [0059]    The preferred embodiments of the present invention allow the riser termination  12  and its connected riser  11  to be recovered from the end support  18 . This gives considerable commercial advantage in allowing the connection of the riser termination  12  and its connected riser  11  to an FPSO or other surface vessel without the need for a construction vessel. 
         [0060]    The riser termination  12  is decoupled from the end support  18  by connecting a line from a winch or crane at the surface to the coupling device  50 . This winch or crane could be aboard an FPSO or second vessel. By lifting the coupling device  50  upwardly from the upwardly extending cavity the coupling device  50 , connecting line, riser termination  12  and its connected riser  11  can be recovered for connection to an FPSO or other surface vessel. 
         [0061]    The preferred embodiments of the present invention also permit the riser termination  12  and its connected riser  11  to be recoupled with the end support  18  should the FPSO or surface vessel need to be disconnected from the riser. The preferred embodiments of the present invention permit this to be performed, at commercial advantage, without the need for an expensive construction vessel to assist. This would be conducted in a reverse manner to that described previously. A coupling device  50  and short line  48  would be reconnected to the riser termination  12 . A line from a winch or crane aboard an FPSO or second vessel would be connected to the coupling device  50 . The riser termination  12  and its connected riser  11  would be lowered back down to the end support  18 . The short line  48  would be guided into the socket  36  by the lead in features  42  and the coupling device  50  lowered into the upwardly open cavity in the socket  36 . The line from the surface winch or crane would be disconnected from the coupling device  50  leaving the riser termination  12  and its connected riser  11  again supported by the coupling device  50  and short line  48  from the end support  18 . 
         [0062]    It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention. 
         [0063]    It will also be understood that the term “comprises” (or its grammatical variants) as used in this specification is equivalent to the term “includes” and should not be taken as excluding the presence of other elements or features.