Patent Publication Number: US-2011048728-A1

Title: Riser support system

Description:
The present invention relates to a device for supporting a riser from a floating vessel comprising a drill floor with an opening through which a riser extends suspended in a gimbal. 
     A riser may be regarded as an extension of a well extending from the seabed up to a floating vessel. The riser is assembled by successive individual pipes being connected on the vessel. As new pipes have to be connected the assembled pipe string is suspended from the vessel. A collar or wedges are normally employed which are mounted in the rotary table on the drill floor and keep the already mounted riser string in position while a new pipe is retrieved from a pipe store and connected to the pipe string. The riser string is then raised slightly to enable the wedges to be released and then lowered, whereupon the wedges again secure the riser when so desired. This continues until the riser has reached its full length and can be connected to the equipment which is fixed on the seabed (Christmas tree, wellhead or BOP). During this entire process the riser&#39;s weight is taken up in the rotary table in the vessel. When the assembly process is completed, however, the riser is attached to a tension compensator which keeps the riser under tension. 
     Another case where the weight of the riser is suspended in the rotary table is where the lower part of the riser has to be disconnected from the seabed installation, for example a Christmas tree on top of a well in an emergency, where such a situation is usually precipitated by bad weather conditions. In the lower end of the riser there is provided an EQDP—an Emergency Quick Disconnect Package—which is activated in such situations, and the riser is left suspended in the water free from the floating vessel or platform. 
     In these cases the weight of the riser suspended in the rotary table can be substantial. Since the search for hydrocarbons is being carried out in progressively deeper water with the result that the risers are getting longer, the weight is becoming greater. The suspension of the riser can be avoided by pulling the riser up on to the vessel again, i.e. dismantling the riser. In the case of long as well as short risers it is neither desirable nor practical to have to pull up the riser, and it is therefore a wish to be able to disconnect the riser only when the weather is so bad that for safety reasons it is not advisable for it to be connected to the well. Furthermore it is desirable for the riser to be able to be suspended in a drill floor for larger operational areas than previously, thereby avoiding pulling up the riser. 
     Risers for use in completion and intervention (so-called “workover” operations) are used for conducting operations in a well under pressure. They are therefore very much heavier than marine risers (which can be relatively thin-walled since they are not exposed to internal pressure). The vessel at the surface is exposed to wave movements resulting in heaving, pitching and rolling. A riser string suspended in the drill floor will therefore be subject to bending moments on account of the dynamic movements of the vessel. Moreover, flow conditions round the riser, for example, will also influence it. The bending moments created in the riser will be dependent on the weight of the riser suspended in the drill floor on the vessel. When it is disconnected as a result of a storm warning, the bending moments can become extremely large on account of the vessel&#39;s movements and the fact that the riser is permitted to move in the water. 
     In GB patent application no. 2 336 382 a device for suspending risers is disclosed and described comprising two spherical surfaces which can move relative to each other. One spherical surface is inserted in the rotary table in the drill floor while the other spherical surface is mounted on a spool piece which can be connected to the riser. The object thereof is to reduce bending stresses when the riser is suspended from the vessel. In such an arrangement the function will be dependent on friction between the two spherical surfaces. With an increase in weight, i.e. longer risers, the friction that has to be overcome in order to obtain movement between the parts will also increase. 
     In U.S. Pat. No. 4,359,095 a slightly different type of support device is disclosed and described. This consists of a bellows of elastic material. Such a device will take up some movements, but in the description it is indicated that the device is mainly intended for use in connection with tension leg platforms (TLP&#39;s) which are not exposed to wave motion to the same extent as a free-floating vessel. 
     Thus there is a need for a device for suspending risers from a vessel in such a manner that substantial bending moments are avoided or at least the bending moments on the riser are reduced. This allows an increase in the permitted “weather window” where the riser can be disconnected and suspended instead of having to be pulled up. Such a device is also desirable in order to increase the operating window when assembling the riser, thereby avoiding having to stop work. 
     These objects are achieved with a device as defined in the following claims. 
     The device according to the invention relates to a suspension device for at least one riser, where the riser extends from a floating vessel containing a drill floor with an opening through which the riser extends. The vessel may be a floating platform, a ship, a tension leg platform or other installation which is floating on the water and is thereby influenced by movement in the water. The drill floor may be arranged in the centre or at one end of a ship or platform or in connection with an outrigger. When we refer to the drill floor, in this application it should be understood to mean a deck with an opening through which the riser extends. This will normally be referred to as a drill floor but in cases where there are several decks it is also conceivable that one of these, regardless of whether this is the drill floor or not, is allocated a device according to the invention for suspending the riser. It is also conceivable for the device to be mounted on a deck in connection with the tension compensating system. 
     According to the invention the device comprises a gimbal containing two frame parts, where one frame part comprises devices for securing to a riser and the second frame part comprises devices for securing to a vessel, where the frame parts are arranged at a distance apart from each other and an intermediate, flexible, annular element is mounted between the frame parts. 
     In this context securing should be understood to mean that in a suspended state of the riser the frame part is so arranged that it substantially follows the movements of the element to which it is secured. It will not necessarily mean that they are fixed and secure relative to each other, but that they cooperate. 
     In an embodiment the intermediate, flexible and annular element may form a complete ring, while in another embodiment it may be designed so that it can be divided in at least one point, thereby facilitating installation and dismantling. The element may be provided with a substantially uniform cross section on the internal cavity viewed in a direction along the circumference of the element. Alternatively, the element may be provided with a varying cross section in a direction along a circumference, thereby forming at regular or irregular intervals chambers, which have a larger cross section than the connections between the chambers. It is also conceivable that control means, for example in the form of valves, may be inserted in the connections between the chambers. One of these valves, for example, may be of such a type that it can be dismantled in order to provide an opening in the annular element for mounting and dismantling thereof. 
     The annular element may be filled at least partly with a fluid, and in a variant partly with an incompressible liquid. The annular element may comprise devices for controlling the fluid pressure internally in the annular element. The annular element may be composed of a flexible material which is stiffened with reinforcing material. The annular element may also have a substantially uniform wall thickness. The annular element may also have a substantially uniform elasticity round the circumference in a cross section. Alternatively, the annular element may be provided with at least one portion with less elasticity, which portions will abut against a surface of the first and second frame parts respectively. In such a case the remaining portions of a cross section of the annular element will be free to distort, thereby taking up relative movement between the two frame parts. According to an aspect the device may also comprise devices for deactivating the freedom of relative movement between the two frame parts. A possible variant of such a deactivating device may comprise a locking device which locks the two frame parts together, thereby preventing relative movement between the parts. Another possible variant of the devices will be to have arrangements which enable the fluid inside the flexible element to be withdrawn, thereby preventing the possibility of achieving relative movement between the frame parts. In such a case these arrangements must also comprise devices for refilling the annular element. 
     The device according to the invention has little or no rotational rigidity and will thereby be able to function in cases where the vessel is facing into the wind. The device according to the invention will reduce or eliminate bending moments arising on a level with the drill floor. With the reduction in the bending moments, the risk of fatigue fracture will also be reduced. The device makes it possible to increase the “weather window” for operations, i.e. maximum permitted wave height and other conditions. It also makes it possible to allow the riser to remain suspended during a storm where otherwise it would have been necessary to pull the riser all the way up on to the vessel. 
    
    
     
       The invention will now be described in greater detail with reference to the accompanying drawings, in which 
         FIG. 1  is a principle drawing illustrating the situation during assembly (“running”) of the riser, 
         FIG. 2  is a principle drawing illustrating the situation after an emergency disconnection, 
         FIG. 3  illustrates a section through a gimbal according to the invention, 
         FIG. 4  illustrates a section through four alternative configurations for a gimbal, 
         FIG. 5  illustrates a section in the circumferential direction of a first possible version of a gimbal, and 
         FIG. 6  illustrates a section in the circumferential direction through a second possible version of a gimbal. 
     
    
    
       FIG. 1  illustrates a vessel  1  floating on a body of water  2 . The vessel  1  comprises pontoons  3  and is preferably dynamically positioned but may alternatively be anchored by means of chains or wires in the normal known manner. The vessel comprises a deck  5  and a turret  4 . 
     The diagram in  FIG. 1  illustrates a situation where a riser  10  during assembly is suspended from a securing device  12 . The riser comprises a stress joint  14 , an emergency quick disconnect package (EQDP)  16  and a pressure safety element  18 , for example a BOP. The riser is arranged to be able to be connected to a Christmas tree  20  ( FIG. 2 ) which is connected to a well  21  extending down to a hydrocarbon-producing zone (not shown). 
       FIG. 2  illustrates the situation where the riser is disconnected from the Christmas tree  20  and is suspended with its full weight from the drill floor  5  by means of a securing device (wedges or collars)  12 . In this embodiment the riser  10  also comprises an upper stress joint  15  at the attachment to the drill floor  5 . 
       FIG. 3  illustrates a device according to a preferred embodiment of the invention, where in this case the device may be called a gimbal. The drill floor  5  has a countersink  22  defining an opening  24 . The countersink  22  is normally arranged for locating a rotary table (not shown). The gimbal consists of a housing comprising a lower frame part  26  and an upper frame part  28 . Between these is mounted a tubular ring  30 . The ring  30  is preferably made of an elastic material such as a rubber material or other elastomer and comprises means (not shown) for supplying a fluid. The ring may be reinforced by means of carbon-, glass-, or Kevlar fibre, metals or the like in order to be able to withstand high internal pressure. The upper frame part  28  comprises an inwardly (towards the centre of the opening) facing flange  29 . The flange  29  serves as a support for a collar  32 . In the normal known manner the collar  32  has a central opening  33  through which a riser  10  can pass. The opening in the collar, however, has a size which is smaller than the shoulder forming a part of the connection constituting an end of the riser (the pin/socket end). A riser&#39;s end shoulder can thereby rest against the collar and be supported thereby relative to the drill floor. The riser  10  is thereby suspended and the drill floor  5  absorbs the forces of the riser  10 , but due to the flexible ring  30  the relative movement between the upper frame part  28  and the lower frame part  26  is permitted and thereby also between the riser  10  and the drill floor  5 . 
     The device according to the invention, the gimbal, may include locking means thereby enabling the gimbal to be locked, i.e. the riser  10  is locked to the drill floor  5  thereby achieving transfer of forces between the drill floor and the riser and they are also kept at rest relative to each other. This may be necessary, for example, in cases where a job has to be done in connection with the top of the riser  10 . The locking means may be locking pins as illustrated in  FIG. 3  which can be moved between an open position (right side of  FIG. 3 ) and a locked position (left side of  FIG. 3 ). The gimbal can thereby be locked in an inactive position. 
     During use the ring  10  is filled at least partly with a fluid, preferably an incompressible fluid such as oil, but it may also be filled with gas. When a riser  10  is suspended from the drill floor  5  on the rig or the vessel  1  and supported by a device according to the invention, such as the gimbal described, any laterally directed wave movements will result in a deformation of the ring on the side with the highest load but this will be restricted on account of the closed system. In addition the fact that the upper frame part is “floating” on the lower frame part will enable the vessel to rotate relative to the riser or vice versa. An increase in weight will only result in an increase in pressure inside the ring. The amount of fluid, i.e. the pressure inside the ring is regulated relative to the weight it has to bear. 
       FIG. 4  illustrates a section through four alternative embodiments. The section is taken in a direction which during use will normally be a substantially vertical direction. In the top alternative in the figure a ring  30  is shown comprising an outer membrane  301  with an inner fluid  302  provided between an upper frame part  28  and a lower frame part  26  where the device also has a centre axis  6  extending through the opening  24  in the gimbal according to the invention. The size of the frame parts  26 ,  28  is such that the ring  30  is located inside an external imaginary box composed of the frame parts. In the alternative under this first variant, an alternative is shown where the ring  30  is composed of membrane parts  301 ′ which are secured to the upper and lower frame parts  26 ,  28 , with the result that all these parts together form an external boundary for the fluid  302  located inside the ring  20 . The third alternative has many similarities with the first alternative, but in this case the size of the membrane  301  is such that it extends outside the imaginary box formed by the upper and lower frame parts  26 ,  28 . In the last alternative the upper and lower frame parts  28 ,  26  are in the form of angled elements, as in the alternative illustrated in  FIG. 3 , but where a movement element  35  is also inserted to ensure good movement between the frame parts  28 ,  26 . The upper frame part  28  will be free to rotate relative to the lower frame part  26 . The fluid in the ring  30  will normally be an incompressible fluid such as water or an oil. The actual gimbal may be mounted on the riser, placed in the rotary table on the drill floor or above the rotary table. 
       FIGS. 5 and 6  illustrate a section through a possible embodiment of a gimbal, in these figures taken during normal use in a horizontal direction. It can be seen that the ring  30  in  FIG. 6  is mounted on the lower frame part  26 , where the ring  30  is made of a membrane  301  which is filled at least partly with a fluid  302 . The membrane  301  is thereby in the form of a doughnut. In  FIG. 5  the ring  30  comprises a plurality of smaller membrane elements  301 ′ arranged round the opening  24 , five membrane elements  301 ′ being shown. These membrane elements  301 ′ are filled at least partly with a fluid  302 ′. The fluid  302 ′ in the different membrane elements  301 ′ is connected with connecting lines  303 . As illustrated, valve elements  304  may be provided in these connecting lines in order to regulate the fluid flow between the different membrane elements. 
     The device may be made so compact that it can be placed inside the rotary table in the same way as an ordinary bushing. Alternatively, it may be placed on the drill floor above the rotary table.