Patent Publication Number: US-7591316-B2

Title: Production system

Description:
This application claims the benefit of United Kingdom Application 0518430.4, filed Sep. 9, 2005, the entirety of which is incorporated herein by reference. 
   BACKGROUND 
   a. Field of the Invention 
   The present invention relates to risers for use in the extraction of hydrocarbons and in particular to risers that are used to extract oil or gas from offshore and deepwater fields. 
   b. Related Art 
   Risers are high pressure dynamic tubular structures used in the extraction of oil and gas from offshore fields. They extend from the seabed to the surface production vessel and are used to transport oil, gas and injection fluids. 
   In deep water (for example a depth of greater than 1000 meters) there is often a limited number of feasible riser solutions for a particular field development. This is due to the many design, operational, commercial and contractual constraints. This limitation is particularly evident on developments in ultra deep water (a depth of typically between 1500 and 3000 meters) which typically require a large number of risers, utilise dynamic production vessels such as turret and spread moored Floating Production, Storage and Offloading (FPSO) vessels and are often located in an environment that has significant wave, current and wind loading. For these applications there is a demand for improved riser technology and system configurations to assist future developments. 
     FIG. 1  shows a schematic depiction of a Single Line Offset Riser (SLOR™), which is recognised as a field proven deepwater riser arrangement that has been successfully deployed on two West African projects. The SLOR comprises a near-vertical steel pipe section  2  which is tensioned by a near-surface buoyancy module  3 . The connection to the production vessel  1  is made via a compliant, flexible pipe catenary section  4 . At the seabed the vertical tension is reacted by a foundation (not shown) that can be either a driven pile, suction pile or gravity base structure. 
   It is anticipated that the SLOR arrangement will be used on future worldwide deepwater developments. However, the potential for structural clashing between adjacent SLORs requires a large separation to be maintained.  FIG. 1  shows schematically that although the vessel  1  may be capable of receiving a significant number of risers it is necessary to provide a separation between the two SLORs shown in  FIG. 1 . In addition, clearance must be maintained with mooring lines and thus the scope of application of the SLORs is greatly limited to developments in which only a small number of risers is required. This can be a serious limitation on large deepwater projects where 20-30 risers is a typical requirement. 
   It is known to use near surface buoys to support a plurality of catenary risers, which connect to a respective plurality of flexible catenaries that provide a connection to a surface vessel. Examples of such arrangements can be found in, for example, U.S. Pat. No. 5,957,074 &amp; U.S. Pat. No. 5,639,187. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention there is provided a production system comprising: a plurality of vertical risers; a plurality of production catenaries; a plurality of buoyancy modules, each of the plurality of buoyancy modules being connected to the upper end of a respective one of the plurality of vertical risers; a support frame comprising a plurality of guide means for receiving each of the plurality of vertical risers, each of the plurality of risers being received within a respective guide means; each of the plurality of vertical risers being connected to a respective lower end of one of the plurality of production catenaries at the support frame; and the upper ends of each of the plurality of production catenaries being connected to a surface vessel. 
   Thus the buoyancy of each vertical riser is provided by the buoyancy module attached to the relevant vertical riser. This is an approach that is not followed in known techniques, such as those described in U.S. Pat. No. 5,957,074 &amp; U.S. Pat. No. 5,639,187, wherein a single buoy provides the buoyancy for all of the catenary risers that are connected to the buoy. In this approach, any movement of the buoy will cause all of the supported catenary risers to move. In the present invention the frame supports and guides the vertical risers to prevent them from clashing or interfering with each other. As each of the vertical risers has its own respective buoyancy module, each of the risers is able to move independently of the frame and the other risers, for example due to thermal expansion or internal pressure. These differences provide significant commercial advantages when it comes to the installation and operation of a plurality of risers. 
   According to a second aspect of the present invention there is provided a method of connecting a vertical production riser to a surface vessel, the method comprising the steps of: a) connecting the vertical production riser to a buoyancy means at the upper end of the vertical production riser, b) supporting the vertical production riser and the buoyancy means within a support frame; c) connecting the production riser to a production catenary at the support frame; and d) connecting the production catenary to a surface vessel. 
   According to a third aspect of the present invention there is provided a method of connecting a plurality of production risers to a surface vessel, the method comprising the steps of: a) positioning a support framework in a position near to a plurality of vertical risers; b) attaching a respective buoyancy module to each of the plurality of vertical risers; c) lifting each of the plurality of vertical risers; d) connecting each of the plurality of vertical risers to the support framework such that the upper end of each of the plurality of vertical risers is secured to the support framework; e) connecting a respective production catenary to each of the plurality of vertical risers at the support framework; and f) connecting each of the plurality of production catenaries to the surface vessel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the following Figures in which: 
       FIG. 1  shows a schematic depiction of a known arrangement in which two SLORs are connected to a surface vessel; 
       FIG. 2  shows a schematic depiction of an arrangement of a plurality of SLORs according to the present invention; 
       FIG. 3  shows a side view of the schematic depiction of an arrangement of a plurality of SLORs according to the present invention shown in  FIG. 2 ; and 
       FIG. 4  shows a schematic depiction of the support frame shown in  FIGS. 2 and 3 . 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows a schematic depiction of an arrangement  100  of a plurality of SLORs according to the present invention and  FIG. 3  shows a side view of the schematic depiction of an arrangement of a plurality of SLORs according to the present invention shown in  FIG. 2 . 
     FIG. 2  shows that the arrangement  100  comprises a surface vessel  10 , a plurality of vertical risers  20   a , . . . ,  20   f  (collectively indicated at  20 ), each of which are connected to the surface vessel  10  by a respective compliant, flexible pipe catenary section  40   a , . . . ,  40   f  (collectively indicated at  40 ). Each of the risers are secured to the seabed with a respective foundation  22   a , . . . ,  22   f  (collectively indicated at  22 ). In place of the single near-surface buoyancy module associated with each of the risers that is shown in  FIG. 1 , the risers are supported by a lightweight support frame  130  which is anchored to seabed foundations by two tethers  140 , which are anchored to tether foundations  145 .  FIG. 3  shows that the riser foundations  22   a , . . . ,  22   f  are laterally offset from the tether foundations  145  so that there is no interference between the risers and the tethers. 
   In use, the support frame  130  is installed before the risers and preferably has sufficient buoyancy that it can free stand, independent of the risers (see below). The frame and its foundations are compact and lightweight so that they can be installed from a small installation vessel such as an anchor handling vessel. The vertical risers  20   a , . . . ,  20   f , are then installed vertically in the usual manner on the out board side of the frame using a conventional installation vessel. 
   After connection of the riser  20   a , . . . ,  20   f  to its respective foundation  22   a , . . . ,  22   f  at the seabed an associated aircan  132   a , . . . ,  132   f  is fully aired-up so that the riser can free stand without support from the surface installation vessel. Subsequently the riser top assembly is laterally deflected to locate into a guide region that is formed within the support frame. Of the six guide regions, five  138   b , . . . ,  138   f  are visible in  FIG. 4 . This can be achieved using a tensioned wire from the installation vessel and assisted by a guidance structure on the frame and visually assisted using an ROV camera. 
     FIG. 4  shows a schematic depiction of the support frame  130  once it has been populated with a plurality of risers  20 . The support frame preferably comprises a number of buoyancy regions  135  that enable the frame to free stand, independent of the risers and/or a surface vessel. Each of the vertical risers,  20   a , . . . ,  20   f  is connected to an associated aircan  132   a , . . . ,  132   f  which is then received within one of the guide regions. After each vertical riser is secured within the support frame then the catenary  40   a , . . . ,  40   f  that links the top of the vertical riser to the production vessel is installed and the vertical riser can be commissioned for production service. 
   In order to facilitate the secure reception of the vertical risers each of the guide regions comprises a funnel. Of the six funnels, five  137   b , . . . ,  137   f  are visible in  FIG. 4 . A swing door clamp assembly is used to secure the riser top assembly in the support frame. Of the six swing door clamp assemblies, five  139   b , . . . ,  139   f  are visible in  FIG. 4 . The swing door clamp preferably comprises half shell Orkot™ type bearings that provide a low friction interface and allow relative movement to occur between the support frame and each individual vertical riser. This movement can occur due to temperature and pressure fluctuations and also due to lateral movement of the support frame due to current and vessel offsets. Once connected into the support frame all of the vertical risers are guided and constrained to displace sympathetically and without the fear of clashing since the support frame maintains a constant separation at the guiding elevation. 
   The support frame size can be designed to suit each particular development but typically facilities for up to 6 vertical risers are provided. In such a case the support frame has a size of approximately 36 m long by 6 m wide. It will be understood that the support frame may accommodate a greater or lesser number of vertical risers and that for support frames accommodating a different number of vertical risers then the support frame may well have a different size. 
   In all other respects the design of the vertical riser and catenary is that of a conventional SLOR. The design of the support frame and the securing means allows the vertical risers to be installed in any order and also accommodates all anticipated movements between the individual vertical risers and the support frame resulting from normal and extreme operating conditions. 
   An additional benefit of the system is that lateral motions at the top of the vertical riser assembly are reduced compared to a conventional SLOR due to the interaction of the tension in each of the individual lines and tethers producing a ‘mooring’ effect. This effect allows the support frame and aircans to be located closer to the water surface than would otherwise be possible with a conventional SLOR, thus simplifying access and installation of the jumper and reducing its required length. Furthermore, the proposed development does not lose the principle technical benefits and cost effectiveness of the SLOR concept: low sensitivity to vessel motions, high fatigue life, pre-installation capability, low vessel payload and pull-in loads and good thermal performance. 
   It will be understood that the preceding references to vertical risers are not intended to act as a geometrical limitation but as defining a functional difference over a catenary riser. In use, a vertical riser will define a vertical or substantially vertical path.