Riser and method of installing same

Abstract

A hybrid riser having a lower section ( 6 ) and an upper section ( 10 ), said upper section comprising a flexible pipe, and said lower section comprising a substantially rigid pipe in communication with the flexible pipe, said riser further comprising a buoyancy section ( 16 ) at or in the region of an upper end of said rigid pipe ( 13 ). Said buoyancy section ( 16 ) also comprises an elongate cylindrical buoyancy element, which may be of a coaxial compartmentalised tubular construction having valves such that it may be controllably flooded or evacuated. The hybrid riser may be tethered to a surface vessel or to the seabed. The hybrid riser may be constructed on land, and towed to the vicinity of the installation to which it is to be connected.

1 - 55 . (canceled)  
   
   
       56 . A riser having a lower section and an upper section, said upper section comprising a flexible pipe and said lower section comprising a substantially rigid pipe and forming a catenary in communication with the flexible pipe, characterized in that 
 said riser further comprises an elongate buoyancy section fitted around said rigid pipe at or in the region of an upper end thereof.    
   
   
       57 . A riser according to  claim 56  further comprising a tether.  
   
   
       58 . A riser according to  claim 57  wherein said buoyancy section is tethered by said tether to a vessel on the sea's surface.  
   
   
       59 . A riser according to  claim 57 , wherein said buoyancy section is tethered by said tether to the seabed.  
   
   
       60 . A riser according to any of  claims 57  to  59  wherein a further tether is provided between a point on said rigid pipe above the high bending area thereof, and the seabed.  
   
   
       61 . A riser according to any of  claims 56  to  59  wherein said lower section comprises a plurality of rigid pipes and said upper sections each comprise a corresponding plurality of flexible pipes.  
   
   
       62 . A riser according to  claim 61  wherein said plurality of rigid pipes are arranged around the outside of said buoyancy section.  
   
   
       63 . A riser according to  claim 62  wherein said plurality of rigid pipes are spaced apart evenly about the circumference of said buoyancy section.  
   
   
       64 . A riser according to  claim 62  wherein each of said plurality of rigid pipes is fixed at or near an upper extremity thereof to said buoyancy section.  
   
   
       65 . A riser according to  claim 62 , wherein said buoyancy section is further provided with a plurality of sleeves intended to slidingly receive said plurality of rigid pipes respectively.  
   
   
       66 . A riser according to  claim 61 , further comprising a spacer connected to each of said plurality of rigid pipes, or to each of said plurality of flexible pipes so as to maintain said plurality of rigid pipes or each of said plurality pipes in a fixed position relative to the other rigid pipes or flexible pipes.  
   
   
       67 . A riser according to  claim 66 , wherein said spacer is provided at a point on said rigid riser below a lower extremity of said buoyancy section.  
   
   
       68 . A riser according to  claim 61  wherein each of said at least one flexible pipes is joined to a respective one of at least one rigid pipes at a substantially right angle.  
   
   
       69 . A riser according to  claim 68  wherein the substantially right-angled joins between respective rigid and flexible pipes are spaced apart from one another along the length of said buoyancy section.  
   
   
       70 . A riser according to any of  claims 56  to  59 , wherein said buoyancy section is made of a foam.  
   
   
       71 . A riser according to any of  claims 56  to  59 , wherein said buoyancy section is a tube arranged such that the rigid pipe runs therethrough.  
   
   
       72 . A riser according to any of  claims 56  to  59 , wherein said buoyancy section is made of one or more of: steel, titanium, aluminium and a composite material.  
   
   
       73 . A riser according to any of  claims 56  to  59 , wherein said lower riser section, or parts thereof, is made of one or more of: steel, titanium, aluminium and a composite material.  
   
   
       74 . A riser according to  claim 71 , wherein said tube arranged is arranged coaxially to said rigid pipe.  
   
   
       75 . A riser according to any of  claims 56  to  59 , wherein said buoyancy section further comprises a plurality of bulkheads dividing said buoyancy section into a plurality of closed chambers.  
   
   
       76 . A riser according to  claim 75  wherein at least one valve is provided allowing flow of a fluid from inside said rigid pipe to the interior of a respective at least one of said closed chambers.  
   
   
       77 . A riser according to  claim 75 , wherein at least one valve is provided allowing flow of a fluid from inside a respective at least one of said closed chambers to the exterior of said buoyancy section.  
   
   
       78 . A riser according to any of  claims 56  to  59 , wherein an upper extremity of said rigid pipe is aligned away from the axis of the part of said riser immediately below said upper extremity.  
   
   
       79 . A riser according to any of  claims 56  to  59  wherein said buoyancy section is divided along its length into a second plurality of separate sections, each section having a vent valve communicating with the outside of the buoyancy section, one end of the buoyancy section having an inlet means for the introduction of a high-pressure gas and the buoyancy section comprising a third plurality of open-ended tubes extending between respective pairs of adjacent sections.  
   
   
       80 . A riser according to  claim 79 , wherein the inlet means is at a lower end of the buoyancy section and the vent valves are disposed near correspondingly lower ends of their respective sections, each tube having a correspondingly lower end situated in a first of its pair of adjacent sections and aligned approximately with the vent valve associated with that first section and a correspondingly upper end situated in a second of its pair of adjacent sections and aligned approximately with the vent associated with that second section.  
   
   
       81 . A riser according to  claim 80 , wherein the third plurality is equal to the second plurality, one of the tubes having a lower end which is located in the topmost section of the further pipe, and an upper end which communicates with the outside of the further pipe.  
   
   
       82 . A riser according to  claim 81 , further comprising a control means connected to the vent valves and to the gas inlet and arranged to open each valve in turn starting with the lowest valve, the valve associated with each succeeding section being opened once a water level inside the preceding section has lowered to the point where it reaches the lower end of the tube associated with the relevant succeeding section and preceding section.  
   
   
       83 . A riser according to  claim 79 , wherein the rigid pipes are arranged around the buoyancy section in an approximately circular configuration.  
   
   
       84 . A riser according to  claim 79 , wherein each rigid pipe is provided with its own supplementary buoyancy means.  
   
   
       85 . A riser according to  claim 79 , wherein the buoyancy section is provided with its own supplementary buoyancy means.  
   
   
       86 . A riser according to  claim 85 , wherein the supplementary buoyancy means is constituted by a foam cladding surrounding the respective rigid pipe and/or buoyancy section.  
   
   
       87 . A riser according to any of  claims 56  to  59 , wherein the control means is arranged to, in sequence: 
 (a) close all the valves when the core is in a state of minimum buoyancy;    (b) apply the gas supply to the gas inlet;    (c) open the lowermost vent valve nearest the gas inlet, thereby to allow a liquid medium present in the section nearest the gas inlet to leave that section by way of said valve;    (d) open the next valve up when gas starts to be introduced into the next section up;    (e) repeat step (d) until the desired or a maximum degree of buoyancy has been attained;    (f) stop the gas supply.    
   
   
       88 . A riser according to  claim 87 , wherein the control means is configured so that the valve associated with each succeeding section is opened once a water level inside the preceding section has lowered to the point where it reaches the lower end of the tube associated with the relevant succeeding section and preceding section.  
   
   
       89 . A method of installing a riser, said method comprising; 
 i. constructing a rigid pipe and a buoyancy section on land,    ii. weighting said buoyancy section, such that the buoyancy of the riser is negative,    iii. towing said rigid pipe section and the buoyancy section out to sea to the location where the riser is to be installed by at least a first tug using a first tether,    iv. allowing said rigid pipe and the buoyancy section to land on seabed,    v. connecting the end of the rigid pipe furthest from the buoyancy section at a wellhead or flowline tie-in,    vi. removing said weighting from the buoyancy section, such that a buoyancy force is exerted on the buoyancy section,    vii. allowing the buoyancy section to rise towards the surface of the sea under the guidance of said at least one installation/surface vessel, such that the rigid pipe bends upwards to form a catenary configuration.    
   
   
       90 . The method of  claim 89 , wherein said step of weighing involves at least partially flooding said buoyancy section, and said step of removing said weighting involves expelling fluid from the buoyancy section.  
   
   
       91 . The method of  claim 89  or  90 , wherein one or more temporary buoyancy elements are connected to said rigid riser such that buoyancy is distributed along the length of the riser substantially evenly.  
   
   
       92 . The method of  claim 89  or  90 , wherein the lower end of said rigid pipe is connected to said well head or flowline by means of jumpers or spools.  
   
   
       93 . The method of  claim 92 , wherein a manifold, wellhead, riser base or further subsea structure is connected to the rigid pipe and towed to field together with the riser in one operation.  
   
   
       94 . The method of  claim 89  or  90 , wherein said rigid pipe and the buoyancy section are towed out to sea to the location where the riser is to be installed further using a second tug and a second tether, said second tether being connected to a point along the rigid pipe behind the point to which said first tether is connected.  
   
   
       95 . The method of  claim 89  or  90 , wherein said rigid pipe is pressurized with a gas prior to said step of expelling fluid from the buoyancy unit.  
   
   
       96 . The method of  claim 89  or  90  comprising the further step of a flexible pipe between said installation/surface vessel and the upper end of the rigid pipe.  
   
   
       97 . The method of  claim 89  or  90  comprising the further step of constructing the entire riser structure on land.  
   
   
       98 . The method of any of  claims 79  to  88 , further comprising the step of attaching a tether between said buoyancy unit and said surface/installation vessel or the seabed.  
   
   
       99 . The method of  claim 79  or  89 , further comprising the step of attaching a tether between a point on said rigid pipe above the high bending area thereof, and the seabed.  
   
   
       100 . A method of installing a riser, said method comprising: 
 i. lowering a rigid pipe into the sea by reeling or by lowering successive lengths of rigid pipe section into the sea, each length being connected endwise to the length of pipe section below it;    ii. connecting a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy section extending lengthwise of said further length of rigid pipe section to an upper end of the length of rigid pipe section immediately below it, to form said rigid pipe;    iii. connecting a flexible pipe to an upper end of said rigid pipe;    iv. lowering the flexible pipe;    v. allowing the buoyancy unit to sink;    vi. adjusting the position of the floating vessel such that the rigid pipe assumes the configuration of a catenary in the sea water and;    vii. connecting a lower end of the rigid pipe to a wellhead or flowline.    
   
   
       101 . The method of  claim 100 , wherein said buoyancy unit is allowed to sink before said steps of: 
 lowering a rigid pipe into the sea; and    connecting a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy unit extending lengthwise of said further length of rigid pipe section to an upper end of the length of rigid pipe section immediately below it, to form said rigid pipe.    
   
   
       102 . The method of  claim 100  or  101 , further comprising the step of attaching a tether between a point on said rigid pipe above the high bending area thereof, and the seabed.