Abstract:
Guide arrangement for marine risers, in particular for offshore oil and gas operations. The guide arrangement has at least one guide structure ( 21, 22 ) for a length of riser, a frame assembly ( 10, 10 A-D) for supporting said guide structure, anchor means ( 11, 12 ) at the seabed, tether means ( 13, 14 ) connecting said frame assembly to said anchor means, and a buoyancy element ( 1 ) for keeping said guide structure ( 21, 22 ) at a desired level in the sea during operation. The guide structure ( 21, 22 ) is pivotably supported by said frame assembly ( 10, 10 A-D) with a pivot axis ( 21 P,  22 P) being substantially horizontal.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of priority from Norwegian Patent Application No. 2008 3370, filed on Aug. 1, 2008, the entirety of which is incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    This invention relates to a guide arrangement for marine risers, in particular for offshore oil and gas operations. 
         [0004]    2. Description of Related Art 
         [0005]    Risers are often used for connecting offshore floating platforms or vessels with subsea installations. Such risers may be of various kinds, for example electric cables, fluid pipes, umbilicals or other forms of combined risers being of a flexible character. Typically, these risers are provided at intermediate portions of their length with buoyancy elements so as to obtain a favourable total curve or trajectory of the risers through the water. 
         [0006]    In some cases where dynamic conditions have to be taken into consideration and there is limited space available around the floating platform or vessel concerned, there is a requirement for stabilizing or anchoring the risers so as to reduce or avoid sideways motions thereof. Such motions may be caused by sea currents or waves as well as other influences acting on the risers and/or platform/vessel. 
         [0007]    In a known solution (Subsea Arch System by CRP Group Limited, Lancashire, England) to the above problem there is provided
       at least one guide structure for a length of riser,   a frame assembly for supporting said guide structure,   anchor means at the seabed,   tether means connecting said frame assembly to said anchor means, and   a buoyancy element for keeping said guide structure at a desired level in the sea during operation.       
 
         [0013]    More specifically, the known solution involves the use of a fixed guide structure in the form of an arch having an upward or “convex” curvature when installed for stabilizing one or more risers. However, the combination of a light and flexible riser, such as an umbilical or the like, with large dynamic movements as explained above, will require very large bending stiffeners at the entrance and exit of the arch. This involves highly increased costs. 
       OBJECTS AND SUMMARY 
       [0014]    Substantial improvements in relation to the above are provided according to this invention, by having the guide structure pivotably supported by said frame assembly with a pivot axis being substantially horizontal. 
         [0015]    Advantages obtained with this new guide arrangement are primarily a reduction in bending stress and strain imposed on the riser or umbilical, elimination of very large bend stiffeners, lighter steel structure work and easy installation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Further explanations of the invention follows below with reference to an embodiment of the invention as illustrated in the drawings, of which: 
           [0017]      FIG. 1  is a system overview of a typical riser installation with a “lazy wave” configuration between a surface vessel and the seabed, 
           [0018]      FIG. 2  in elevation shows more in detail the cooperating parts of the guide arrangement included in  FIG. 1 , and 
           [0019]      FIG. 3  shows a plan view according to line II-II in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    In the typical situation of  FIG. 1  a vessel  90  on sea surface  100  is connected through a riser  30  to a subsea installation (not shown). At a middle portion of riser  30  there are provided buoyancy elements  33  so as to obtain a desired configuration of the riser as a whole. Thus, as extended from vessel  90  the riser  30  will have an upward inclination before the buoyancy element portion  33 . At this intermediate portion of the riser there may be a need for some stabilization or anchoring of the riser  30 , in particular against movements in a lateral direction related to the general plane followed by riser configuration  30 . 
         [0021]    Thus, a guide arrangement according to the invention providing for such anchoring, is shown in  FIG. 1  with bottom anchor blocks  11  and  12  at seabed  200 , with tethers  13 ,  14  connected to a frame assembly  10  supporting a guide structure  21 , whereby a buoyancy element  1  serves to keep the guide arrangement at a desired level in the sea. As will be seen better from  FIG. 3 , the guide structure  21  may freely and pivotably adjust itself to the inclined configuration or portion of riser  30  passing through guide structure  21 . 
         [0022]    Referring now to  FIG. 2  as well as  FIG. 3 , the embodiment shown therein comprises two guide structures  21  and  22  provided at respective side parts  15  and  16  of frame  10 , as shown in particular in  FIG. 3 . Thus, this embodiment is useful in the case of two more or less parallel risers  30 , which is a situation being quite frequent in actual practice. 
         [0023]    In this case frame  10  has a rectangular main shape and is adapted to have a substantial horizontal orientation in the sea. For this purpose the frame  10  is suspended by brace members  10 A,  10 B,  10 C and  10 D extending at an inclination from respective attachment points at the frame  10  upwards to a common, central suspension point  3  above the horizontal frame  10 . Between buoyancy tether  2  and the top of braces  10 A-D there is provided a swivel  3 S with a vertical axis of rotation. Guide structure  21  is pivotably supported by frame  10  at a pivot axis  21 P whereas guide structure  22  in a corresponding manner is pivotably supported about axis  22 P. Axis  21 P and axis  22 P are both substantially horizontal, so as to make possible an inclined position of guide structures  21  and  22 , for example as shown in  FIG. 1 . Such angular movement of the guide structures is individual, allowing for different angles of inclination of the two guides. For such movements it is an advantage that the pivot axes  21 P,  22 P are located at a middle portion of the length of each guide structure  21 ,  22 , preferably at a midpoint thereof. Accordingly there will be a kind of balanced arrangement of these guide structures. 
         [0024]    In order that tethers  13  and  14  shall not prevent the movements of guides  21 ,  22  they should be attached to frame  10  in a central region along side members  15  and  16 , preferably adjacent to axes  21 P and  22 P, respectively. For increased stability there may also be provided a third (or further) tether(s) with a bottom anchor at a point displaced from the line between anchors  11  an  12 . 
         [0025]    Another feature of significance is also seen from  FIG. 3 , namely that the length of each guide structure  21 ,  22  is so large as to make the ends of these structures project outside the frame  10 . These ends of the guides are provided with relatively short bend stiffeners  25 - 28 , respectively. Thus, such bending stiffeners have a length being just a small fraction of the length of the guide structures. On the other hand the length of each guide structure  21 ,  22  is many times the diameter of the riser  30 . This will provide for a secure angular movement as desired, when in operation the guide structures are under the influence of risers running through them. 
         [0026]    It is preferred according to the invention to let the guide structures  21  and  22  have a basic pipe shape with an essentially rectilinear configuration. Moreover, for the required fixation of risers  30  through guide structures  21  and  22  against longitudinal displacement, clamps  41  and  42  are provided at a middle portion of each guide structure. Such clamps may be of more or less conventional types and are not shown in detail in the drawing. 
         [0027]    Mounting of the guide structures  21 ,  22  onto a riser  30  can be done at a laying vessel when deploying the riser at the offshore installation site. In the case of pipe-shaped guides (see  FIGS. 2 and 3 ) the riser may be threaded through its guide or the guide may be split longitudinally for placing the riser first in one half pipe whereupon the other half is mounted so as to form a complete, closed guide containing a length of the riser. Then the assembly is deployed into the sea and afterwards each guide is connected to the frame pivot. The arrangement of frame  10 , anchors  11 ,  12 , tether  2  and buoyancy element  1  can be installed before or after deployment of the riser  30  with the guides mounted thereto. 
         [0028]    It will be understood that many modifications are possible, deviating from the exemplary embodiment shown in  FIGS. 1-3 . The riser configuration may be different and the present guide arrangement could be located at other portions of the riser configuration than illustrated in  FIG. 1 . Anchors  11  and  12  could be of any other type of anchor than the gravitation blocks shown, for example pile anchors. There may also be modifications of guide arrangements with only one guide structure, and in such case the frame assembly may be much simpler than illustrated in the drawing.