Abstract:
A device is described for maintaining a predominately constant pressure on, and flow of drilling fluid in, a drill string ( 100 ) where drilling fluid is supplied via a circulation system for drilling fluid. The device comprises a mainly elongated, internally hollow body ( 10 ) arranged to surround the drill string where the hollow body ( 10 ) comprises at least an upper pressure chamber ( 40   a ) and a lower pressure chamber ( 40   b ) connected with respective inlet and/or outlet ( 50   a,    50   b ) for drilling fluid from or to the drilling fluid circulation system, as said pressure chambers ( 40   a,    40   b ) are able to be closed and separated by an intermediate valve ( 70 ) arranged for circulation of drilling fluid into or out of the drill string ( 100 ) during coupling up or disconnecting of a new length of drill string ( 100 ). A method to maintain the mainly constant pressure of drilling fluid in a drill string is also described.

Description:
This application is a national stage application that claims priority under 35 USC §§365 and 371 to PCT/NO2008/000228 filed Jun. 20, 2008, which claims priority to NO 20073161, filed Jun. 21, 2007. 
     The present invention relates to a device and method for maintaining a mainly constant pressure on, and flow of drilling fluid in, a drill string, where drilling fluid is supplied via a drilling fluid circulation system, as described in the introduction of the respective independent claims. 
     The drilling industry worldwide experiences many problems during drilling, poor hole stability, formation fracturing and undesirable inflow of formation fluid. When one drills, a drilling fluid (mud) is utilised with a specific gravity which normally lies above the expected pressure from the formation, to hinder inflow of formation fluid, and that a well control situation arises. The pressure of the drilling fluid which acts on the formation must, at the same time, be lower than the pressure which leads to the formation fracturing, something which can lead to the drilling fluid disappearing into the formation and that a well control situation arises. The pressure margin (difference) between inflow of formation fluid and fracturing of the formation can be called a drill window. The pressure on the formation consists of components other than just the weight of the drilling fluid. When one pumps the drilling fluid down and out through the drill string a friction pressure arises in addition, and also that the drilling fluid on the return side contains cuttings normally with a higher density than the drilling fluid. This results in that when one pumps drilling fluid through the drill string the pressure against the formation then increases and when one stops the pressure then drops. The sum of the pressure which the formation is subjected to is called the equivalent circulation density ECD. Changes in ECD usually occur when one stops and starts pumping of drilling sludge through the drill string during coupling or disconnection of a new length of drill string. 
     The present invention has as an object to ensure a most constant ECD during the drilling operation by enabling the circulation of the drilling fluid even during coupling and disconnection of a new length of drill string. This will lead to a more predictable and stabile ECD, something that again will enable drilling of formations which today are difficult, and to some extent, impossible. 
     From prior art, amongst others, WO 02/36928 A1 shall be mentioned, which concerns a device and method for maintaining predominantly constant pressure on, and a flow of drilling fluid in, a drill string during coupling and disconnection of a new length of drill string. U.S. Pat. No. 6,315,051 B1 shall also be referred to, which concerns a method for constant circulation during drilling. 
     The above mentioned objects are achieved with a device which is described herein. According to the invention a device to maintain predominantly constant pressure and flow of drilling fluid in a drill string is provided, where drilling fluid is added via a drilling fluid circulation system, as said device is arranged on a drill floor and comprises: a predominately elongated, internally hollow body arranged to surround the drill string, in which the hollow body comprises, at least, an upper pressure chamber and a lower pressure chamber connected with respective inlets and/or outlets for drilling fluid, from or to the drilling fluid circulation system, as said pressure chambers are able to be closed and separated by an intermediate valve arranged for circulation of drilling fluid into or out of the drill string during coupling or disconnection of a new length of drill string. The upper pressure chamber comprises a seal arranged to surround and seal against the drill string, and a locking anchor for securing the drill string, and the lower pressure chamber comprises a seal arranged to surround and seal against the drill string, and a locking anchor for securing the drill string. The invention is characterised in that an upper part of the elongated body comprises an upper entering cone for receiving the drill string, and that a lower part of the elongated body comprises a landing element with a lower entering cone. 
     The above mentioned objects are also achieved with a method as described by the following steps: to arrange a predominantly elongated internally hollow body about the drill string, where said body comprises a pressure chamber with an upper and a lower pressure chamber separated by an intermediate valve; to separate the drill string, such that the separated pipe coupling of the drill string is inside the hollow body, and that an upper and lower part of the drill string is locked and sealed in the upper and lower chambers, respectively, at the same time as drilling fluid is added internally in the body such that the drilling fluid with a pressure corresponding to the pressure in the drill string is added in the body: to close between the upper and lower chambers with the help of the intermediate valve such that the pressure is maintained in the lower chamber and to balance the pressure in the upper chamber to that of the surroundings; to release and pull up the upper part of the drill string to collect a new length of drill string with a number of drill pipes; to insert the new length of drill string into the upper chamber, whereupon this is locked and sealed in the upper chamber; to supply drilling fluid into the upper chamber to obtain a corresponding pressure to that of the lower chamber, whereupon the intermediate valve between the upper and the lower chamber is opened; and to connect together the upper and the lower part of the drill string, whereupon the drill string is released and the body is made unpressurised. The method is characterised in that said body is lifted up along the drill string such that the pipe connection is made accessible when there is a need to insert a new length of drill string, and to drive in a roughneck and, at least, partially break up the connection whereupon the roughneck is driven away from the drill string and the body is lowered down over the connection which now has a soft-break status still with drilling fluid under pressure and circulation inside. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic view of an illustrative embodiment of a device for maintaining constant pressure on, and flow drill fluid, in a drill string. 
         FIG. 2  is a schematic view of an illustrative embodiment of a drill string. 
         FIG. 3  is a schematic view of an illustrative embodiment of a device in a first configuration. 
         FIG. 4  is a schematic view of an illustrative embodiment of a device in a second configuration. 
         FIG. 5  is a schematic view of an illustrative embodiment of a device in a third configuration. 
         FIG. 6  is a schematic view of an illustrative embodiment of a device in a fourth configuration. 
         FIG. 7  is a schematic view of an illustrative embodiment of a device in a fifth configuration. 
         FIG. 8  is a schematic view of an illustrative embodiment of a device in a sixth configuration. 
         FIG. 9  is a schematic view of an illustrative embodiment of a device in a seventh configuration. 
         FIG. 10  is a schematic view of an illustrative embodiment of a device in an eighth configuration. 
         FIG. 11  is a schematic view of an illustrative embodiment of a device in a ninth configuration. 
         FIG. 12  is a schematic view of an illustrative embodiment of a device in a tenth configuration. 
         FIG. 13  is a schematic view of an illustrative embodiment of a device in an eleventh configuration. 
     
    
    
     DETAILED DESCRIPTION 
     The invention comprises, as shown, a common pressure container  60  in which several components are localised. The components can be threaded, flanged or machined such that they can be put together to create a common pressure container function  60 . 
     An entering cone  80   a  can be found uppermost. The function of the entering cone  80   a  is to guide the drill string into the invention. An upper seal  20   a  can be found below the entering cone  80   a . The seal has a composition which enables it to make a seal around a chosen drill string  100  including the variable diameter which the drill string represents. The seal allows for movement by the drill string  100  both axially and rotationally, at the same time as it seals against the working pressure which is defined in advance. 
     An upper locking anchor  30   a  is arranged below the upper seal  20   a . The locking anchor is arranged so that when it is not connected up (deactivated), it allows a drill string  100  to freely move through. When the locking anchor is connected up (activated) the bottom of the drill string (the pin end)  120  is hindered from passing because of the increased diameter of the pipe connection  110 . The locking anchor is qualified to withstand the forces of separation that can arise in the pressure container during normal operation. 
     An upper pressure chamber  40   a  is placed between the upper locking anchor  30   a  and an intermediate valve  70  in the body  10 . An inlet  50   a  for injection or return of drilling fluid is arranged in the side of the upper pressure chamber. When the valve is open the upper pressure chamber  40   a  is in direct hydraulic connection with the lower pressure chamber  40   b.    
     The valve  70  is arranged between the upper  40   a  and the lower  40   b  pressure chamber. The make-up of the valve is such that when it is open it allows the drill string  100 , including the pipe connection  110 , to freely pass through. When it is closed, the valve is qualified to withstand the working pressure that has been defined in advance and thus to isolate the upper  40   a  and the lower  40   b  pressure chambers both hydraulically and mechanically. 
     A lower pressure chamber  40   b  is situated between the valve  70  and the lower locking anchor  30   b . An inlet  50   b  for injection or return of drilling fluid is arranged at the side of the lower pressure chamber. When the valve is open the upper pressure chamber  40   a  is in direct hydraulic connection with the lower pressure chamber  40   b.    
     A lower locking anchor  30   b  is situated below the lower pressure chamber  40   b . The locking anchor is arranged so that when it is not connected up (deactivated) it allows a drill string  100  to freely move through. When the locking anchor is connected up (activated) the top (box end)  130  of the drill string is hindered from passing through because of the increased diameter of the pipe coupling  110 . The locking anchor is qualified to withstand the forces of separation that can arise in the pressure container during normal operation. 
     A lower seal  20   b  is situated below the lower locking anchor  30   b . The composition of the seal is such that it is able to seal around a chosen drill string  100  including the variable diameter which the drill string represents. The seal permits movement of the drill string both axially and rotationally, at the same time as it seals against the working pressure which has been defined in advance. 
     A landing element with an entering cone  80  is at the bottom of the invention. The landing element is arranged so that it can take up the forces that can arise when one puts the weight of the present device with a drill string that runs through down onto the drill floor. In addition the entering cone contributes to ensure that the couplings on the drill string are led into the invention. 
     The present device can be arranged on drill floors both ashore on floating rigs or platforms. The invention will represent an additional function to the standard functions on a drill floor. In addition it is dependent on established and adjoining systems functioning normally. Typical systems are, for example; iron roughnecks, tongs, mud systems, topdrive systems, handling systems and the like. These are well known by a person skilled in the arts and will not be explained in more detail. 
     The device will normally be dependent on its own systems for control, monitoring and operation. These will not be described in this application. 
     The seals that are used in the device can be of different shape, principles of operation and embodiment. There are different systems for sealing around drill strings on the market today, and also some are under development. Some seals are arranged in a ball/gliding bearing solution such that the whole of the seal rotates with the drill string whilst other seals have a fixed securing mechanism where the seal is held static even if the drill string rotates. There are also variations where several sealing elements are put together to achieve a common sealing function. In addition there are sealing solutions with injection of friction reducing liquid over or directly into the sealing surface and/or between the seals. Some seal solutions are based on the principle of forming a pressure gradient over a set of seals. There are also seals that can be opened and closed against the drill string (annular preventer, pipe ram). All these different seals or combinations of these are described with the common denotation of seal in this application. 
     Bore pipes are used as a common denotation for all types of bore pipes that are used within drilling in oil wells, water wells and gas carrying wells. This includes so-called snubbing operations. The bore pipes can be standard or custom made, with or without special lubrication for threads or seals (o-rings, etc.). 
     Description of Method in Use. 
       FIG. 3  shows the device after it has been fitted around the drill string  100 . Then the seals  20   a ,  20   b  lie against the drill string without being exposed to pressure, something which results in limited wear on the seals. The valve  70  and the locking anchors  30   a ,  30   b  are in open position such that the drill string can freely pass through the body  10 . The drill personnel can carry out drilling operations as normal without taking special care for the invention. During drilling, the drilling fluid is pumped through the drill string. 
       FIG. 4  shows that the body  10  is lifted up along the drill pipe so that the pipe coupling  110  becomes accessible. This occurs when one has drilled so far down that there is a need to insert a new length of drill pipe. A roughneck  90  can then be driven in and break up the coupling  110 . The breaking up shall initially only be carried out with a power/movement that leads to the coupling maintaining its ability to retain pressure at the same time as the power which is later required to open the coupling can be supplied from the topdrive of the rig. This method to break a coupling is called soft-break. 
     When the roughneck  90  has carried out a soft-break it is driven away from the drill string. The body  10  can now be lowered down over the coupling which now has a soft-break status, still with drilling fluid under pressure and circulating inside. 
       FIG. 5  shows that the body is localised over the pipe coupling  110 , the locking anchors  30   a ,  30   b  are activated and the seals  20   a ,  20   b  are functioning. The coupling  110  on the drill pipe is now opened up with the help of the topdrive and the parts  120 ,  130  are separated from each other. The drilling fluid still circulates through the drill string  100  via the pressure chamber  60 . In this phase a pressure from the drilling fluid is established at the same time in the lower inlet of drilling fluid  50   b . The pressure is identical with the pressure in the drill string. The upper inlet for drilling fluid  50   a  is closed during this operation. 
       FIG. 6  shows that the upper end  120  of the drill string is pulled up over the valve  70  and is placed against the upper locking anchor  30   a . Pumping of drilling fluid is thereafter gradually transferred from the drill string to the lower inlet  50   b  for drilling fluid until it is only pumped in via the lower inlet  50   b . The formation has so far not been able to register any pressure variation in the drilling fluid. 
       FIG. 7  shows that after all injection of drilling fluid is transferred to the lower inlet  50   b  and no drilling fluid is pumped through the part  120  of the drill string, which is situated in the upper locking anchor  30   a , the valve  70  can close. The two pressure chambers  40   a ,  40   b  are now hydraulically and mechanically separated. The pressure and the fluid that are in the upper pressure chamber and the drill string can now be bled off and be emptied out via the upper outlet  50   a.    
       FIG. 8  shows that after the upper pressure chamber  40   a  and the drill stem have become unpressurised, the upper locking anchor  30   a  can be opened and the drill string is pulled out to collect a new length of drill pipe. Circulation to the part  30  of the drill string which is in the well now takes place completely via injection in the lower inlet  50   b.    
       FIG. 9  shows that when the new drill string is collected, it is led into the body  10  from the top and down through the upper seal  20   a  and the upper locking anchor  30   a  which is then closed (activated). Thereafter the new drill string and the upper pressure chamber  40   a  is filled with drilling fluid and pressurised to the same pressure as the pressure of the drilling fluid in the lower pressure chamber  40   b . The pressure is then equalised across the valve  70 . 
       FIG. 10  shows that when the pressure is equalised across the valve  70 , this can be opened. Circulation of drilling fluid now takes place in parallel both via the drill string and via the lower inlet  50   b.    
       FIG. 11  shows that the upper part  120  of the drill string is led down toward the lower part  130 . The circulation via the lower inlet  50   b  is gradually stopped until all circulation takes place via the upper part  120  of the drill string. 
       FIG. 12  shows that the drill string  100  is coupled together in that the topdrive (not shown) spins the upper part  120  of the drill string into the lower part  130 . The coupling is made so that it withstands the pressure that is on the inside without leaking (soft make up). After this has been carried out the pressure chambers  40   a ,  40   b  are de-pressurised and the device appears without pressure against the seals  20   a ,  20   b.    
       FIG. 13  shows that after the invention has been made un-pressurised, it is lifted up along the drill string  100  to make room for the iron roughneck  90 . This is brought forward and applies a predetermined connecting force (moment). The drilling can now continue as normal until the next coupling shall be carried out. 
     At pulling/removal of drill pipes, the sequence is repeated in the opposite order.