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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     Petroleum exploration activities occasionally require specialized drilling techniques to optimise production from certain types of reservoir stratum. One such drilling technique is known as “underbalanced” drilling, which employs singly or a combination of nitrogen, carbon dioxide or other inert gasses, and drilling mud as the primary composite drilling fluid. In this situation, down hole pressure of the composite drilling fluid is monitored within the drill string bore and the well annulus, with the goal of preventing formation fracture due to overly high gas pressures. Another goal of underbalanced drilling is to minimise loss of the composite drilling fluid to the formation, which can be re-circulated until drilling is complete. Clearly, a specialized drilling device is needed to measure the drill string and well bore pressures to make underbalanced drilling possible. 
     BACKGROUND OF THE INVENTION 
     Although there are a variety of devices for measuring downhole drilling fluid pressure, some of the devices require a temporary cessation of drilling operations, which in some cases incur cost and time delays unacceptable to drilling operators in the competitive exploration market. Such a system is described in Canadian Patent 607,352. Other types of systems allow downhole pressure measurement while drilling, generally making use of electronic pressure measurement tools rigidly fixed to the lower portion of the drill string, near the drill bit. While satisfactory for this service, such devices are irrecoverable in the event that this section of the drill string becomes stuck downhole, and consequently abandoned if efforts to free it are unsuccessful. Typically the drill string above the stuck section is disconnected in some fashion and brought to the surface, leaving behind the drill motor, drill bit, pressure measurement tools and the lower section of the drill string. Examples of such systems are described in U.S. Pat. Nos. 4,297,880 and 4,805,449, which are capable of sensing drill bore and annulus pressure, but as mentioned are irrecoverable in the event of drill string abandonment due to their mechanical design. 
     There is a significant need for an electronic downhole system that measures pressure in the drill string bore and the well annulus (the area between the collar OD and the well bore), that is retrievable and re-seatable, and reports pressure measurements to the surface in a timely fashion. Such a system permits drillers to make real-time decisions on how to proceed with the drilling operation based upon this and other information. The value of such a device is greatly enhanced by providing retrieval and reseating capabilities. Retrieval permits the recovery of the device in situations where the drill string becomes stuck and must be abandoned. However, certain situations arise where the tool must be recovered temporarily and then returned to the end of the drill string so that the drilling job may be continued. This is known as re-seating, and offers a level of operational flexibility not observed in the general market for similar devices. 
     SUMMARY OF THE INVENTION 
     The Dynamic Pressure Device (DPD), in accordance with an aspect of this invention measures pressure in the drill string bore and the well annulus (the area between the collar OD and the well bore) and reports the measurement to a transmitter located within the tool string. The transmitter communicates this information to the surface, where drillers make decisions on how to proceed with the drilling operation based upon this and other information. 
     In accordance with another aspect of the invention a drill string section for use in making up a drill string for oil and gas drilling is provided. The drill section carries instrumentation for Measurement While Drilling and Logging While Drilling operations, said instrumentation including a Dynamic Pressure Device for measuring drill string bore pressure of incoming pressurized drilling fluid, and drill string annular pressure of returned pressurized drilling fluid, said instrumentation being retrievable from said drill string when said drill string section is stuck or otherwise abandoned downhole, or otherwise reseatable into said drill string as required when drill string is deemed operational and fit for continued drilling. The drill string section comprises: 
     i) a length of drill string pipe having a bore defined by an inner surface of a pipe wall which has an outer surface, 
     ii) a cylindrical landing sleeve and a support for centering said sleeve in said pipe bore, 
     iii) communicating ports extending through said drill pipe wall from said outer pipe surface to said inner surface and through said support to an inside surface of said landing sleeve, 
     iv) said instrumentation being provided in an elongate cylindrical tool shell, spaced apart seals which engage said inside surface of said landing sleeve and the outer surface of said tool shell, means for locating said communicating ports between said spaced-apart seals, 
     v) said instrumentation in said tool shell having a first terminated passageway in said tool shell between said seals which communicates with a pressure sensor within said shell to sense thereby said drill string annular pressure, 
     vi) said instrumentation having a second terminated passageway in said tool shell in communication with said drill string bore and in communication with a pressure sensor within said shell whereby said drill string bore pressure is sensed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are described with respect to the drawings wherein. 
     FIG. 1 is a schematic of the abandonment of a downhole drill string. 
     FIG. 2 is a section through the drill string of this invention. 
     FIG. 3 is an exploded view of the drill section FIG.  2 . 
     FIGS. 4,  5 ,  6  and  7  show embodiments of the invention where the pressure sensor system and related electronics can be withdrawn from the drill string when it is necessary to abandon the drill string, or alternately re-seated when required. 
     FIG. 8 is an exploded view of an alternative embodiment for the mounting of the pressure measurement system in the drill string. 
     FIG. 9 is a section through the assembly of FIG.  8 . 
     FIG. 10 shows yet another alternative embodiment for the mounting of the pressure measurement device within the drill string. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A representative drilling system is shown in FIG.  1 . The above ground drilling structure  10  has the usual tower  12  with drill string assembly and drive components  14 . The drill string  16  is made up of individual drill string sections  18 , the lower most of which includes a drill bit  20 . For a variety of reasons it may be necessary to abandon the downhole drill string particularly the drill string adjacent the drill bit due to the drill bit becoming stuck or otherwise seized in the formation. The abandonment of the lower most drill string can be costly because of the value of the electronic components in the tool sub-assembly which are used to provide for “measurement while drilling and logging while drilling operations”. In other cases, the tool itself may fail and requires replacement. In these situations, the replaced tool must be lowered down the drill string and re-seated at its original location so that drilling operations may resume. It is understood of course that when the drill bit  20  is abandoned the operator may commence redrilling of the bore and provide for an alternate route around the abandoned drill section as indicated by the dotted lines  22 . A number of contemporary systems provide for retrieval or re-seating of the electronics in circumstances of drill string abandonment or tool replacement, however such systems are not intended or otherwise designed for measuring drill string bore pressure and drill string annular pressure. 
     In accordance with this invention, the system shown in FIG. 2 provides for pressure measurements and at the same time allows retrieval and re-seating of the electronic components from, and into, the downhole drill section. The upper female connector  24  of the drill string section is connected to a male threaded connector  26  of an upper drill string section. Correspondingly the male section  28  is threaded into a female section  30  of the lower drill string  18 . The electronic components for measurement while drilling and logging while drilling are housed in an elongate cylindrical tool shell  32 . 
     The cylindrical tool shell is positioned within a landing sleeve  34 . The cylindrical tool shell may be removed from, or installed into, the landing sleeve in the manner discussed with respect to FIGS. 4 through 7 by grasping a connector stub  36  which is secured to the cylindrical tool shell. A suitable latching mechanism  37  is provided in the drill string to releasably secure the tool shell in the drill string and locate it in the drill string. Although there are a multitude of electrical opponents within the cylindrical tool shell, the specific components of interest in respect to the invention are the devices for measuring drilling fluid pressure in the bore  38  of the tool string and drill string annular pressure in annulus  40 . The annulus  40  is defined between the earth formation  42  and the exterior  44  of the drill string section. Pressure transducer  46  is provided to measure the pressure of a circulating drilling fluid in the drill string bore  38 . A port  48  in the cylindrical tool shell communicates with a passageway  50 , and terminates at the sensor  46 . Drill string annular pressure is measured by pressure transducer  52 . Pressure sensor  52  is in communication with passageway  54 , which in turn communicates through an annular passage formed between the tool barrel and the inside diameter of landing sleeve  34  and ultimately through passageway  56 . Passageway  56  communicates with annular space  40 , noted as the drill string annular region that conveys returned drilling fluid to the surface. 
     Further details of the system are shown in the exploded view of FIG.  3 . The elongate cylindrical tool shell  32  has the respective ports  48  and  58  on the periphery  60  of the shell  32 . Port  48  is in communication with the pressurized fluid within the bore of the drill string section. Port  58  is located between seals generally designated  62  and  64 . The cylindrical tool shell  32  is of a dimension that readily slides through the bore of landing sleeve  34 . Seals  62  and  64  project slightly from the periphery  60  of the tool shell and form an interference fit with the interior surface  63  of the landing sleeve, the resulting seal deformation providing a liquid tight seal with the interior of the landing sleeve. Although in accordance with this embodiment, the seals are provided in the tool shell, it is appreciated that the seals may be provided in the interior surface  62  of the landing sleeve to provide a sealed space when the tool shell is inserted into the landing sleeve. The landing sleeve  34  includes supports in the form of legs  65 , in accordance with this embodiment, to space the outer periphery  66  of the landing sleeve from the interior surface  68  of the drill string section  18 . This allows the drilling fluid to flow through the spaces defined between the periphery of the landing sleeve and the interior of the drill string section. The landing sleeve  34  is secured within the drill string section in accordance with the embodiments to be discussed with respect to FIGS. 4 through 7. In addition the leg  65  includes a port  70  which extends through the leg and the wall section  72  of the landing sleeve. The port  70  is in communication with a port  74  defined within removable plug  76 . The landing sleeve is of course fitted to the drill string section before the drill string section is put into use. When the tool shell is inserted in the landing sleeve, a suitable stop, such as the latch  37 , is provided to locate the seals  62  and  64  on opposite sides of the port  70  so that the pressurized fluid in the annulus outside of the drill string section may flow through ports  74 ,  70  and  58  and through passageway  54  to the pressure transducer  52 . Seals  62  and  64  also prevent the fluid from the drill string bore, which is at a higher pressure than the annulus fluid, from leaking into the annular space  92  formed between the seals, tool barrel  32  and landing sleeve bore. 
     FIGS. 4,  5  and  6  demonstrate the manner in which the cylindrical tool shell may be extracted from the drill string section  18 . On the interior surface  68  of the drill string, cams  78  are mounted on drill string interior to guide insertion of the cylindrical tool shell into the landing sleeve  34  during a seating or re-seating operation. The landing sleeve  34  has its leg portions  65  secured in the drill string wall  82  by way of bolts  84  which are threaded into the respective legs  65  in threaded bores  86 . The pressurized drilling fluid in the drill string bore flows over the tubular sleeve by way of a space defined between the interior  68  and the exterior  66  of the landing sleeve. 
     The extraction tool  80  is shown in FIG. 5 as having clamped onto the connector stub  36 . Extraction device  80  is connected to a wire line or the like  88 . With the extraction device clipped on to the stub  36 , the tool shell  32  may be pulled from the landing sleeve in the manner shown in FIG. 6 where the tubular shell is moving in the direction of arrow  90 . In this manner, the valuable electronic components in the cylindrical tool shell may be recovered before the drill string and drill bit are abandoned. Similarly, the cylindrical tool shell may be re-installed if the drill string and drill bit are restored to service or the electronics require servicing. 
     With reference to FIG. 7 the relative relationship of the exterior bore  74  to the interior bore  58  is shown. The exterior bore  74  extends through the wall  82  of the drill string section. The bore  74  communicates with bore  70  which extends through the leg  65  of the landing sleeve  34 . The bore opens up into the space defined between the exterior surface  60  of the cylindrical tool shell and the interior surface  63  of the landing sleeve. As previously explained there is a slight gap between the cylindrical tool shell and the interior of the landing sleeve to permit insertion and retraction of the cylindrical tool shell. This space is sealed off to each side of the port  70  by seals  62  and  64 . This ensures that all pressurized fluids passing through bores  74  and  70  are contained within the annular space  92 . Port  58  is in communication with the annular space  92  so that any pressurized fluid in space  92  enters port  58  and along passage  54  thereby the pressure of such fluid is sensed by the pressure transducer  52 . In this manner a reliable economical system is provided which permits measurement of drill string annular pressure while at the same time permitting extraction of the cylindrical tool shell. 
     An alternative embodiment for the drill string section is shown in FIG.  8 . The construction of the cylindrical tool shell  32  is essentially the same with the spaced apart seals  62  and  64 . A slight recess  94  is provided for port  58 . The landing sleeve  34  is replaced with an alternative embodiment  96  which is fixed on the interior surface  68  of the drill string section  18  by use of clip rings to be described in more detail with respect to FIG.  9 . The port  70  in the landing sleeve  96  is longitudinally aligned with the port  74  of plug  76  which can be achieved during assembly, however radial orientation of port  70  with respect to port  74  is unimportant. With reference to FIG. 9 the landing sleeve  96  is secured inside the tubular string wall  82  by way of C-clips  98  which engage the faces  93  and  95  of landing sleeve  96 . The C-clips interconnect with groves  100  and  102  in the drill string section wall. This arrangement permits the installation of the C-clips so that they can bear up against the upstream and downstream faces  93  and  95  of landing sleeve  96 . 
     In accordance with this preferred embodiment the cylindrical tool shell  32  has a ledge  104  which defines a stop and which abuts the upstream face  93  of the landing sleeve. Alternatively, the tool shell  32  may be located by other mechanical stops incorporated on the tool similar to other embodiments of the invention. The landing sleeve includes seals  104  to seal the exterior of the landing sleeve within the interior  68  of the drill string section. In addition to or as a replacement for the preferred embodiment showing seals  62  and  64  on the cylindrical tool shell, the landing sleeve may include seals  106  which seal to the exterior  108  of the cylindrical tool shell to ensure a leak tight connection. The port  74  which extends through the wall of the drill string section is aligned longitudinally, but not necessarily radially, with port  70  and leads into annular space  110 . Port  58  leads from pressure transducer  52  and opens into annular space  110 , permitting a reading of drilling fluid annular pressure to be made. Multiple arcuate apertures  112  provide open channels for the flow of drilling fluid along the drill string bore. In a manner discussed with respect to FIG. 2, pressure transducer  46  communicates with port  48  through passageway  50  permitting a pressure measurement of the drill string bore fluid to be made. 
     With reference to FIG. 10, an alternative embodiment for the landing sleeve is shown. In this embodiment the landing sleeve is integral with the drill string section  18  and its wall section  82 . The landing sleeve  114  is machined as part of the drill string bore during the fabrication process. The landing sleeve wall  116  is spaced from the interior wall  118  of the drill string by circumferentially arranged legs  120 . The landing sleeve  114  has arcuate shaped channels  122  which extend through the landing sleeve  114  and provide the necessary flow paths for the pressurized drilling fluid. The landing sleeve  114  has the port  124  extending from the exterior of the drill string section through the wall  82  through the leg  120  and through the wall  116  of the sleeve. The cylindrical tool shell  32  may be constructed in the same manner as that of FIG. 3 so that the seals  62  and  64  are positioned to each side of the port  124 . This provides, as discussed with respect to the prior embodiments, for the usual communication of pressurized drilling fluid on the exterior of the drill section to within the system for measurement by the pressure transducer  52 . 
     Accordingly, various embodiments are provided which demonstrate the effectiveness of a landing sleeve in providing for annular pressure measurements of drilling fluid, and at the same time providing for a retraction or re-seating of the cylindrical tool shell while the drill string is down hole. 
     Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

Summary:
A drill string section for use in making up a drill string for oil and gas drilling carries instrumentation for measurement and logging while drilling. The instrumentation includes a dynamic pressure device for measuring drill string bore pressure of incoming pressurized fluid and drill string annular pressure of returned pressurized fluid. The drill string section comprises a length of drill string pipe having a bore defined by an inner surface of a wall which has an outer surface. The instrumentation is provided in an elongate cylindrical tool shell. The outer surface of the tool shell has spaced apart seals which engage the inside surface of a cylindrical landing sleeve in the pipe bore. The seals are located in the sleeve on either side of communicating port(s) in the drill pipe wall, forming a leak tight annular region that eventually communicates through appropriate ports to a pressure transducer.