Abstract:
The invention relates to a safety joint ( 10; 100; 200 ) for a riser consisting of two telescoping parts that are arranged to break in the event of a predetermined axial tension load. The joint also comprises means ( 21, 41; 121, 141; 221, 241 ) for closing the passage in the joint in the event of fracture where the means are activated by means of the forces generated during the separation.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a joint device, especially a safety joint for a riser extending between a vessel and a subsea installation, which joint comprises two telescoping parts, each defining a fluid channel and interconnected by means that are arranged to be broken at a predetermined axial load. 
     The invention also comprises a method. 
     Operations in subsea wells are normally conducted by establishing a closed column connecting the well with a vessel on the surface, thus providing safe access to the well. A column of this kind is usually called a riser or riser system and includes not only the actual pipe but also several other devices which in addition to the actual pipe are necessary for safe access to the well. All operations down in the well are conducted through the riser, which forms a barrier between well fluids and the surrounding seawater. Work is carried out on a “live” well, i.e. the well is open all the way up to the vessel with well fluids at a pressure corresponding to the formation pressure. The riser therefore must be designed to be able to withstand high well pressure. Otherwise an uncontrolled blow-out may cause the riser to be filled with gas from the well, with the result that the pressure inside the riser sinks to almost zero. 
     The riser system normally comprises a lower riser package LRP with a number of valves for closing down the well, thereby functionally corresponding to a blow-out preventer (BOP). There are also provided an emergency quick disconnect package (EQDP) and a stress joint. At the upper end of the riser, i.e. in the vessel, there is usually provided a surface BOP. In addition the riser may be equipped with a bending member and buoyancy elements, together with any other devices required for operations on a subsea well. 
     When operations have to be performed on wells located at great depths, a vessel is employed which is kept in the correct position by means of propellers and/or thrusters. Such vessels are called dynamically positioned (DP) vessels. These vessels are highly dependent on all systems working satisfactorily and normal practice requires them to be equipped with several systems as security against the vessel drifting out of position. 
     During operations from a dynamically positioned vessel, situations may arise where it is necessary to leave the position above the well quickly. This may be controlled, such as when a warning of deteriorating weather conditions makes it necessary to evacuate the position, or uncontrolled, where some of the systems fail and the vessel begins to drift out of position. Such a situation may also occur in the event of sudden bad weather, but particularly in situations where the vessel&#39;s systems are not capable of keeping the vessel in the correct position above the well. The 
     consequences of such a situation may be that the heave compensation system touches the bottom or that the riser assumes an unacceptable angle resulting in loading that exceeds the riser&#39;s design load. 
     Such situations can result in fracture of the riser. In situations of this kind it is important to control the fracture, i.e. to ensure that it occurs at a location where the well&#39;s barriers remain intact. 
     Fracture of the riser may result in damage to the vessel and constitute a risk to personnel as well as causing environmental damage, i.e. spillage of hydrocarbons, hydraulic fluid or the like. This may occur on account of the energy in the tensioned riser and the content of the riser. A complicating factor will be present if the riser has an internal pressure with an unstabilised fluid or a mixture of gas and fluid. The fluid that then flows out of the lower end of the riser will give rise to an upwardly-directed force that attempts to push the riser up in the rig towards the heave compensation, thereby making the situation more unstable. The most extreme consequence is that the riser may be pushed upwards with such force that serious damage is done to the equipment in the vessel and it may even be wrecked. A situation of this kind may also lead to loss of human life. 
     It is previously known to equip pipes with safety joints that are broken if the pipe is subjected to tension exceeding a predetermined value. This comprises shear pins that are broken when there is tension in the pipe. The position of the fracture can thereby be controlled and it can be located in an area that results in the least possible damage to equipment. However, since the riser has a high internal pressure, dangerous situations may still arise as mentioned above. 
     It is also previously known to arrange valves in a safety joint in such a way that the fluid channel close when the joint are broken. In WO 2004/055316 are shown and described a working string extending through a riser and down in a well. The working string is arranged to be broken at a predetermined upwardly directed load and comprise valves for closing fluid channels over and under the separation joint respectively. The valves are held open by means of hydraulic pressure but are equipped with pull-back springs which actuate the valve to closing when the hydraulic pressure disappears. By rupture the hydraulic supply pipe will also break in such a way that the pressure in the pipe drops. The disadvantage with this embodiment is that it must be arranged hydraulic connection lines from surface and that a hydraulic pressure must be maintained during the work operations. If this pressure for some reason should be lost, e.g. by failure in the system, the valves will close unnecessary and could cause disturbance or even dangerous situations. 
     One other example of the prior art is WO 01/86110 which concern a device for disconnecting an upper part of a riser from a lower part which are connected with, or cemented, in a pipe at seabed. The procedure and mechanism for separating the two parts from each other is relative complicated and result in some operational steps which have to be done in correct sequence. The most important moment here is that in order to disconnect the two parts, the tension in the upper part of the riser must be reduced first (or even be compressed (see col. 30, first section)) in order to release the locking mechanisms and afterwards it is put under tension again to pull the upper part of the riser up from the lower part. Thus one has to undertake an active action in order to release the locking mechanism before the disconnection. The disadvantage with this is that in a emergency situation, where the platform drifting off, there can be a situation where it will be impossible to reduce the tension in the riser, because there will simply not be time nor room to carry out a procedure like this. It is self-evident that in a case like this the tension load will increase, and if there are no weakening devices present the riser will actually break. 
     SUMMARY OF THE INVENTION 
     The invention attempts to redress this problem by providing a riser joint with the features specified in claim  1 . 
     The means advantageously comprise shear elements for cutting a pipe, wire or cable located in the passage. 
     The said means are activated by means of the force to which the riser is subjected during the separation, where a transmission arranged in one part of the joint is employed to activate the closing means in the other part. 
     The invention will now be described in greater detail with reference to the attached drawings, in which: 
    
    
     
         FIG. 1  is a partial sectional view of a joint according to a first embodiment of the invention, 
         FIG. 2  is a partial sectional view of a joint according to a second embodiment of the invention, 
         FIG. 3  is a partial sectional view of a joint according to a third embodiment of the invention, 
         FIG. 4  illustrates a detail of the mechanism in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a joint  10  that forms a part of a pipe system. It may be in the form of an insert part with coupling ends  12 ,  14  for connecting to a riser, especially a working riser. The joint has an axis  16  that is aligned with the axis of the rest of the pipe. 
     The joint comprises an upper telescopic part  20  which defines a fluid channel  13  and has a flange  12  at one end. A lower telescopic part  40  defines a fluid channel  15  and has a flange  14  at one end. The lower part has a portion  17  with an extended diameter of the fluid channel  15  for receiving a pipe socket  22  that forms a part of the upper telescopic part. Sealing elements (not shown) are mounted between the lower part  40  and the pipe socket  22 . The pipe socket  22  is releasably secured in the lower part  40  by means that are arranged to be broken or severed when subjected to a predetermined force. For example, the means may be in the form of shear pins  32 ,  34 . 
     A floating piston is also advantageously provided for pressure balancing when the joint is broken, as described in more detail in our simultaneous application no. 20043933, filed on 10, Sep. 2004. 
     A bending limiter is also advantageously provided in order to reduce the stresses on the riser during normal use, which is well known in the art. It may, for example, be a rubber sleeve which is also described in greater detail in the above-mentioned previously filed application. 
     When the riser is subjected to tension exceeding a predetermined value, the shear pins  32  and  34  will break, thus enabling the upper part  20  to be separated from the lower part  40 . The shear pins are arranged to be broken in the event of an upwardly directed tension. 
     In each of the upper and lower telescopic parts  20 ,  40 , valves  21  and  41  respectively are mounted in the form of oppositely directed rams, which, when the pistons are moved towards each other, are arranged to close the passage  15 . Since the valves are identical, only the lower valve  41  will be described in greater detail as it will be understood that the upper valve  21  contains corresponding parts. As illustrated in  FIG. 1 , the valve  41  may be machined in one piece with the telescopic part  40 , with the result that it is in the form of two projecting valve housings  42 ,  44 . Alternatively, the valve housings may be separate parts that are attached to the telescopic part  40  by means of bolts or the like. The valve housings  42 ,  44  are also identical and arranged symmetrically about the joint&#39;s centre axis  16 . Thus the description of the parts in the valve housing  42  that follows will apply to corresponding parts in the valve housing  44 . 
     The valve housing  42  has a through-going bore  43  whose axis  44 A intersects the axis  16  of the main passage. The bore  43  is closed by an end plate  45  which, by means of bolts  46 , is attached to the valve housing  42 . The end plate  45  has a bore  48 A which is of smaller diameter than, but axially aligned with the bore  43 . A piston  47  with a front part  50  is movably mounted in the main bore  43 . The piston is connected with an actuator rod  48 . The actuator rod extends through the bore  48 A in the end plate  45  and is attached to a crosshead  49 . The crosshead  49  comprises two threaded bores  51 ,  52  for receiving driving rods  53 ,  54 . 
     In  FIG. 4  the valve&#39;s piston  47  is illustrated in greater detail. The front part  50  comprises a first bore  71  in which is mounted a seal  72 . The opposite piston  73  has a corresponding seal  74 , with the result that when the pistons are moved towards each other, the passage  15  is closed. Furthermore, in the front part  50  there is provided a knife  75  which is inserted in a slot  76  in the opposite piston  73 . A slot  77  is also provided in the front part  50  for receiving a knife  78  in the opposite piston  73 . The knives are used to sever an object located in the fluid channel  15  when the valves are closed. 
     Each driving rod  53 ,  54  extend from the crosshead  49  over to a crosshead  55  belonging to the second valve housing  44 . The driving rod  53  has threaded ends  56 ,  57 , with the result that one end has a right-hand thread while the other end has a left-hand thread. The driving rod  53  also comprises a cogwheel  58 . A driving belt  59  has teeth over at least a part of its length and is passed over the cogwheel  58  so that the driving belt&#39;s teeth are engaged with the cogwheel&#39;s teeth. At its upper end the driving belt is attached to a securing device  30  on the upper telescopic part  20  while its lower end is loosened and rolled up as illustrated in  FIG. 1 . 
     As also illustrated in  FIG. 1 , three additional driving belts are provided whereof only two,  61  and  62  are illustrated. The driving belts  59  and  62  are attached in the upper telescopic part and mounted on each side of the joint. The driving belt  62  is connected with a cogwheel mounted on the driving rod  54  in the same way as that described for the driving rod  53 . 
     The driving belt  61  and its non-illustrated corresponding belt on the opposite side are secured at their lower ends in the lower telescopic part  40  at  60  and, in the same way as for the valve  41 , are mounted around corresponding cogwheels on driving rods  23  and  24  in the valve  21  in the upper telescopic part  20 . 
     Alternatively, toothed racks may be used instead of belts. 
     When the joint is subjected to tension that causes the shear pins  32 ,  34  to break, the parts  20 ,  40  of the joints will begin to move apart. The driving belts that are fastened in the upper and lower telescopic parts will rotate the cogwheels, thereby causing the driving rods  53 ,  54  and  23 ,  24  to rotate and drive the crossheads inwards. Since the actuator rods are attached to the crossheads, the rams will be moved towards each other for closing the main passage  13 ,  15 . 
       FIG. 2  illustrates a second embodiment of the invention, in which the safety jioint is indicated generally by reference number  100 . The rams are designed in the same way as those in  FIG. 1 , but they are equipped with hydraulically operated actuators. Thus only the valves will be described in the following, since the other parts are identical to the joint illustrated in  FIG. 1 . 
     In the same way as described for  FIG. 1 , each valve  121 ,  141  comprises oppositely directed pistons which, when they are moved towards each other, are arranged to shut off the main passage  13 ,  15 . In this embodiment the valves  121 ,  141  are also identical and in the following only the valve  141  will be described since it will be understood that the valve  121  is of identical design. 
     In the same way as illustrated in  FIG. 1 , the lower telescopic part  140  comprises valve housings  142 ,  144 , which are also identical but inverted relative to each other. The valve housing  142  has a through-going bore  143  in which a piston  147  with a front end  150  is slidably mounted. The bore  143  extends to an actuator cylinder  145  which by means of, e.g. bolts  126  is attached to the valve housing  142 . The cylinder  145  is closed at its other end by a cap  171  which has a bore  146 A that is of smaller diameter than, but axially aligned with the bore  143 . To the piston is attached an actuator rod  146  which in turn is attached to a hydraulic driving piston  149 . A port  155  is provided in the valve housing  142 . In a similar manner a port  151  is provided in the valve housing  144 . 
     A rod  172  may be attached to the driving piston  149 , extending through the bore  146 A to the outside of the valve housing, thus enabling the piston  147  to be moved manually. 
     The joint  100  also comprises means to provide hydraulic power for operating the driving piston  149  and a corresponding driving piston in the valve housing  144 . In the example illustrated in  FIG. 2 , the means comprise a number of piston and cylinder devices arranged symmetrically around the joint  100 , whereof only two,  131 ,  133  are illustrated in  FIG. 2 . The device  131  has a piston  135  that is movable in a cylinder housing  136  of the device  131 . A driving rod  137  extends from the piston  135  beyond the cylinder with its end attached to a flange  125  on the upper telescopic part  120 . A port  138  is provided in the cylinder wall under the piston  135 . 
     In a similar manner, the device  133  comprises a piston  165  with a driving rod  169  movable in a cylinder housing  166  with ports  167  and  168 . 
     The port  138  is connected via a pipe  152  with the port  155  in the valve housing  142 , while the port  167  is connected with the port  151  in the valve housing  144  via a pipe  154 . Ports in the other non-illustrated cylinders are similarly connected with ports in the upper valve  121 . 
     When the upper telescopic part  120  is pulled out of the lower telescopic part  140 , the piston  135  in the piston device  131  will move upwards in the cylinder  136 . This results in a negative pressure under the piston  135  and, via the pipe  152 , a negative pressure in the actuator cylinder  145 . This will cause the driving piston  149  and, thus, the ram  147  to move towards the joint&#39;s centre axis. The same process will occur with the second ram in the valve housing  144  and the valve will thereby close the passage  13 ,  15 . 
     The same process will occur with the upper valve  121 . 
     Alternatively, the driving fluid may be removed on the top of the piston  135  via a port  139  and passed to a port (not shown) on the back of the driving piston  149 , thereby driving the ram  147  towards closing of the valve. 
     In an alternative version the cylinders may be arranged in the reverse manner, i.e. with the retaining flange located on the lower telescopic part  140 . Alternatively, half of the piston devices may have oppositely directed piston rods, where half are attached to the flange  125  and half attached to the lower flange. 
     The rams are advantageously provided with cutting elements to enable them to sever a pipe located in the passage  15 , as illustrated in  FIG. 4 . 
     The piston rod  137  is attached in such a manner that it will be broken when the telescopic parts are completely separated from each other. They may, for example, be provided with shear pins similar to those for the telescopic part  22  or any other type of weakening device. 
       FIG. 3  illustrates a third embodiment of the invention. A joint  200  is illustrated here where ball valves  221 ,  241  are mounted in the upper telescopic part  120  and the lower telescopic part  140  respectively. Each valve has a drive pin (not shown) connected to an arm  224  and  244  respectively. A driving rod  226  is attached at one end to the driving arm  224  and at its lower end is attached to the valve housing of the valve  241  at a fastening point  228 . Similarly, a driving rod  246  is attached at one end to the driving arm  244  and at its other end is attached to the valve housing of the valve  221  at a fastening point  248 . A combined stop and locking mechanism  230  and  250  respectively is provided in order to restrict the movement of the arms and to ensure that the valves are kept locked in their closed positions. 
     When the joint is subjected to tension that causes the shear pins to be broken, the parts will be pulled apart. The rods  226 ,  246  will thereby move the valve arms, thus causing the balls to rotate and the valves to close. At the fastening points  228 ,  248  the rods are provided with means that cause them to be released from the fastening points when the arms  224 ,  244  are rotated to their extreme position. 
     With the invention a solution has been arrived at where at least one fluid channel ( 13  or  15 ) in a riser can be closed if an event occurs that causes the joint to be broken. The valves will ensure that pressure in the riser is kept inside, thereby avoiding dangerous situations. If the event occurs while work is in progress in the well, a coiled tubing, wire or cable located inside the riser can also be severed. 
     Some embodiments for implementation of the invention have been described above, but for a person skilled in the art it will be obvious that several other methods exist for actuating the valves. For example, a key device may be employed which via a transmission mechanism pushes the rams towards each other. It will also be obvious to a skilled person that the hydraulic actuation can also be employed for the ball valves. 
     The joint may be placed at any suitable location, but advantageously near the seabed end and immediately above the emergency disconnect joint. Alternatively, the joint may be a part of the emergency disconnect unit, i.e. the lower part of the joint forms the emergency disconnect unit.