Abstract:
Disclosed is a riser tower structure including a plurality of elongate objects. The riser tower structure is provided with blocks, preferably of buoyant material, along at least part of its length. The blocks provide the riser tower with a substantially circular cross-sectional profile, wherein one or more strakes are provided on the outside of the blocks. The strakes in a main embodiment are helical. Also disclosed is a corresponding method of constructing such a riser tower structure.

Description:
This Application is the U.S. National Phase of International Number PCT/GB2009/051515 filed on Nov. 11, 2009, which claims priority to Great Britain Application Number 0900097.7 filed on Jan. 7, 2009, and U.S. Provisional Application No. 61/114,141 filed on Nov. 13, 2008. 
    
    
     The present invention relates to Hybrid Riser Towers, and in particular to reduction of the problem of vortex induced vibration on Hybrid Riser Tower structures. 
     Hybrid Riser Towers are known and form part of the so-called hybrid riser, having an upper portions (“jumpers”) made of flexible conduit and suitable for deep and ultra-deep water field development. U.S. Pat. No. 6,082,391 (Stolt/Doris) proposes a particular Hybrid Riser Tower (HRT) consisting of an empty central core, supporting a bundle of (usually rigid) riser pipes, some used for oil production some used for injection of water, gas and/or other fluids, some others for oil and gas export. This type of tower has been developed and deployed for example in the Girassol field off Angola. Insulating material in the form of syntactic foam blocks surrounds the central core and the pipes and separates the hot and cold fluid conduits. Further background has been published in paper “Hybrid Riser Tower: from Functional Specification to Cost per Unit Length” by J-F Saint-Marcoux and M Rochereau, DOT XIII Rio de Janeiro, 18 Oct. 2001. New versions of such risers have been proposed in WO 02/053869 A1. The contents of all these documents are incorporated herein by reference, as background to the present disclosure. 
     The phenomenon of Vortex-induced vibrations (VIV) is a known problem for HRTs, and indeed for offshore oil exploration and production risers in general. VIV are motions induced on bodies, such as an installed riser, facing an external flow by periodical irregularities on this flow. As a result of these periodical irregularities and the slowing of the flow around the riser, vortices are formed. These vortices change the pressure distribution along the riser surface and when not formed symmetrically around the riser cause different lift forces to develop on each side of the riser, thus leading to motion transverse to the flow. VIV is an important source of fatigue damage to risers. These elongate slender structures experience both current flow and top-end vessel motions, which give rise to the flow-structure relative motion and cause VIV. The top-end vessel motion causes the riser to oscillate and the corresponding flow profile appears unsteady. 
     It is known to equip individual risers with strakes, or other protrusions, to disrupt the cylindrical profile and reduce VIV. Difficulties arise, however, when addressing the problem of VIV for a bundle of risers such as required for a HRT, as current fabrication does not make allowance for the fitting of said strakes. 
     Consequently, it is an aim of the invention to address some or all of the above mentioned issues. 
     In a first aspect of the invention there is provided a riser tower structure of a type comprising a plurality of elongate objects, said riser tower structure being provided with blocks along at least part of its length, said blocks providing said riser tower with a substantially circular cross-sectional profile, wherein one or more strakes are provided on the outside of said blocks. 
     Said strakes may be helical in shape. More than one strake may be provided on a single block, circumferentially offset from one another. 
     Said blocks may comprise insulation and/or buoyancy modules. They may be formed out of a plurality of parts. In one embodiment said blocks may comprise a plurality of main sections, preferably two, which are attached together around one of said elongate elements, forming a channel therefor. Said main parts may further comprise recesses, around their periphery and along their length for the location of the remaining of said plurality of elongate objects, said blocks further comprising closing pieces to retain said elongate objects when in place. 
     Said blocks (when assembled together if necessary) may be provided with one or more inserts, each for the location therein of said one or more strakes. Said insert may follow the intended footprint of its corresponding strake. 
     Said strakes may be made of the same material than said blocks. 
     One of said elongate objects may comprise a central core. Said plurality of elongate objects may comprise a plurality of conduits arranged around the central core. Additionally, other elongate objects may make up the riser, such as umbilical and control lines. Said riser tower structure may comprise said blocks along the majority of the riser length. Said riser tower may comprise guide frames along its length, to guide the risers. Said blocks may be provided between successive guide frames. 
     In a further aspect of the invention there may be provided a method of constructing a riser tower structure comprising:
         attaching blocks around a central core of a riser tower structure, said blocks being provided with recesses, around their periphery and along their length.   locating conduits and/or other elongate objects in said recesses;   closing said recesses with a closing piece, thus providing said riser tower structure with a substantially circular cross sectional profile along its length; and   attaching at least one strake to the outside of said riser tower structure.       

     Said riser tower structure maybe any one of those described with the first aspect of the invention. 
     Said riser tower is preferably fabricated in sections, each of said sections being fabricated according to the first aspect of the invention and then assembled together. Each section may be greater than 100 meters long, and may lie between 100 meters and 300 meters in length. In a main embodiment they will be between approximately 150 and 200 meters. 
     Said strake may be attached to the blocks. Said method may comprise the providing of an insert for each strake during fabrication of said blocks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: 
         FIG. 1  shows a known type of hybrid riser structure in an offshore oil production system; 
         FIG. 2  shows a riser bundle having buoyancy blocks adapted for the addition of strakes; 
         FIG. 3  shows the riser bundle of  FIG. 2 , with strake ready for attachment; and 
         FIG. 4  shows the riser bundle of  FIG. 2  with two strakes attached. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIG. 1 , the person skilled in the art will recognise a cut-away view of a seabed installation comprising a number of well heads, manifolds and other pipeline equipment  100  to  108 . These are located in an oil field on the seabed  110 . 
     Vertical riser towers are provided at  112  and  114 , for conveying production fluids to the surface, and for conveying lifting gas, injection water and treatment chemicals such as methanol from the surface to the seabed. The foot of each riser,  112 ,  114 , is connected to a number of well heads/injection sites  100  to  108  by horizontal pipelines  116  etc. 
     Further pipelines  118 ,  120  may link to other well sites at a remote part of the seabed. At the sea surface  122 , the top of each riser tower is supported by a buoy  124 ,  126 . These towers are pre-fabricated at shore facilities, towed to their operating location and then installed to the seabed with anchors at the bottom and buoyancy at the top. 
     A floating production unit (FPU)  128  is moored by means not shown, or otherwise held in place at the surface. FPU  128  provides production facilities, storage and accommodation for the fluids from and to the wells  100  to  108 . FPU  128  is connected to the risers by flexible flow lines  132  etc arranged in a catenary configuration, for the transfer of fluids between the FPU and the seabed, via riser towers  112  and  114 . 
     Individual pipelines may be required not only for hydrocarbons produced from the seabed wells, but also for various auxiliary fluids, which assist in the production and/or maintenance of the seabed installation. For the sake of convenience, a number of pipelines carrying either the same or a number of different types of fluid are grouped in “bundles”, and the riser towers  112 , and  114  in this embodiment comprise each one a bundle of conduits for production fluids, lifting gas, water and gas injection, oil and gas export, and treatment chemicals, e.g. methanol. All the component conduits of each bundle are arranged around a central core, and are held in place relative to each other (in the two lateral dimensions, longitudinal movement not being prevented) by guide frames attached to the central core. 
       FIG. 2  shows a part of a riser bundle having buoyancy blocks adapted for the addition of strakes to counteract the issue of vortex induced vibration (VIV). Shown is a central core  200 , which may or may not double as a fluid conduit with riser conduits (or umbilicals etc.)  210  arranged therearound. Buoyancy blocks, formed in two halves  230   a ,  230   b  are assembled (possibly bolted together) around the core pipe  200 , said blocks forming a channel  220  for said core  210 . Said blocks may be specifically designed to be within the outside diameter of the riser tower. 
     Recesses are formed in the periphery of the buoyancy blocks  230   a ,  230   b , each for the locating therein of the individual riser conduits  210  (in this example; in other embodiments, recesses may be shared by more than one conduit or umbilical etc.). Closing gates  240  are provided to form closed channels for each riser conduit  210 , while providing the structure with a largely unbroken cylindrical cross section (these may be bolted, or bonded in place with adhesive, or both). These gates  240  may be made from the same material as the buoyancy blocks  230   a ,  230   b . Both the central core  200  and risers  210  are loose inside their channels, with the buoyancy force imparted onto the central core via guide frames (not shown) located at various points along the riser bundle. 
     A strake insert  250  is provided onto each of said riser buoyancy blocks  230  during their fabrication. A template may have been used to ensure perfect match with the strake to be inserted therein. The two buoyancy block halves  230   a ,  230   b  should be correctly paired together during installation, which ensures continuity of the strake insert  250 . 
       FIG. 3  shows the assembled riser bundle of  FIG. 2  with strake  300  shown, ready to be attached. In an embodiment, the strake is made from the same material as the buoyancy blocks  230   a ,  230   b , and is specifically matched to a particular pair of buoyancy blocks  230   a ,  230   b . Said strake should be substantially continuous and allow no, or minimal water passage between it and the buoyancy block. 
       FIG. 4  shows the assembled riser bundle with two strakes  300  attached, circumferentially offset from one another, one of said strakes in place in said insert  250 . Of course, the other strake  300  will have its own corresponding insert  250 . The strakes may be bolted in place, and/or bonded with adhesive. The inserts  250  may therefore be provided with threads to receive the bolts. 
     There are a number of advantages of this arrangement over the fabrication of steel strakes attached onto the guiding frames. These would be both heavier and less efficient, as there would be a gap between the strake and buoyancy block. Furthermore, the above embodiment allows for efficient fabrication and assembly. Essentially the foam blocks (with inserts for the strakes), closing gates and strakes can be fabricated at the same time from the same material. Each set of the above should be identified to go together and not be mixed. 
     Consequently the installation of the strakes can become a standard procedure, to take place once the buoyancy blocks and closing gates have been assembled to the riser bundle. 
     The above embodiments are for illustration only and other embodiments and variations are possible and envisaged without departing from the spirit and scope of the invention. For example, the riser arrangements depicted are simply for illustration and may be varied, and in particular the number of strakes may be varied. Strakes do not necessarily have to be helical in shape.