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TECHNICAL FIELD 
     The present invention relates to wellbore drilling operations. 
     BACKGROUND 
     Wellbores are drilled in the Earth from the surface to one or more subsurface formations typically by rotating a drillbit against the formation. The drill bit is typically suspended in the borehole by a drill string that extends to the surface. In one example, the drill bit may be rotated by rotating the drill string at the surface. Example of surface rotating systems include a rotary table and a top drive. In another example, the drill bit may be driven by a downhole motor, typically referred to as a “mud motor,” which is typically a component in the drill string, located adjacent to the bit. 
     In a typical drilling system, the drill string defines a flow passage through which drilling fluid, typically referred to as “drilling mud,” is pumped. The mud flows down the drill string to the drill bit, where it exits through jets in the drill bit. The mud then flows up the annulus between the borehole wall and the drill string, carrying drill cuttings to the surface. Through this process, the mud cools the drill bit and cleans the bottom of the borehole from the drill cuttings that are created as the drilling process progresses. 
     The mud is also weighted with the addition of various compounds so that the hydrostatic pressure in the borehole is higher than the formation pressure, thereby preventing a well blowout in the event a pressurized subsurface pocket is encountered by the drill bit. It is noted that some wells are drilled using a technique called under balanced drilling, where the mud pressure does not quite compensate for the formation pressure. 
     Most drilling fluids are a fluid that will gel when the fluid is not pumping. This prevents the drill cuttings from falling back down the hole or from collecting on the low side of a deviated well. If mud flow is stopped, the shear stress in the gel must exceed a certain amount to allow the mud to flow again. 
     SUMMARY 
     In one aspect, the disclosed examples relate to a method for restarting a drilling process that includes applying a surface torque to a drill string in a borehole, detect signals related to one of a torque and a rotational speed experienced at a bottom hole assembly, initiating drilling fluid flow, and lowering a drill bit to a bottom of the borehole. 
     In another aspect, the disclosed examples relate to a method for restarting a drilling process that includes lowering a drill string, detecting signals related to one of a torque and a rotational speed experienced at a bottom hole assembly, initiating a flow of drilling fluid, engage Kelly bushings, and applying a surface torque to a drill string in a borehole. 
     In another aspect, the disclosed examples relate to a method of restarting drilling operations in a wellbore after drilling operations and circulation of a drilling mud have ceased. The method includes providing a wired drill string having a drill bit in the wellbore, downhole sensors positioned in the wellbore and in communication with a controller via the wired drill string, a pumping system to circulate drilling fluid, a rotation system for applying rotation to the drill string and drill bit, a translation system for raising and lowering the drill string relative to the wellbore, obtaining data at the controller obtained from the downhole sensors communicated via the wired drill pipe, operating the rotation system so as to apply torque to the wired drill string, and initiating the pumping system to circulate drilling fluid at a first flow rate upon receiving data at the controller from the downhole sensors indicating that a downhole transient pressure surge has passed. 
     In another aspect, the disclosed examples relate to a method for restarting a drilling process that includes step for generating enough shear stress in a gelatinous drilling fluid located in an annulus to cause the gelatinous drilling fluid in the annulus to flow, step for lowering a drill bit to a bottom of a borehole, and step for generating enough shear stress in a gelatinous drilling fluid located in a drill pipe gelatinous drilling fluid in the drill pipe to flow. 
     The foregoing has outlined some of the features and technical advantages of the present invention in order that a detailed description of an example of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and aspects of the present invention will be best understood with reference to the following detailed description when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  illustrates an example of a wellbore drilling system. 
         FIG. 2  illustrates and example method for restarting drilling. 
         FIG. 3  illustrates another example method for restarting drilling. 
     
    
    
     DETAILED DESCRIPTION 
     Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     As used herein, the terms “up” and “down”; “upper” and “lower”; “uphole” and “downhole”; and other like terms indicating relative positions to a given point or element are utilized to more clearly describe some elements of the embodiments of the invention. Commonly, the terms “up,” “upper,” “uphole,” and other like terms are meant to indicate a position that is closed to the surface along the linear distance of the borehole. It is noted that through the use of directional drilling, a wellbore may not extend straight up and down. Thus, these terms describe relative positions along the wellbore. 
       FIG. 1  provides an example of a wellbore drilling system of the present invention, generally designated by the numeral  10 . A drilling rig  12  includes drawworks  14  to raise, suspend and lower a drillstring  16 . Drillstring  16  includes a number of threadedly coupled sections of drillpipe, shown generally at  18 . The sections of drillpipe  18  may be single joints of drillpipe or stands of made-up joints of drillpipe. In some examples, drill pipe  18  is wired drill pipe, which provides high-speed, two-way communication between surface and downhole systems, independent of the flow of fluid in drillstring  16  or the wellbore. For example, wired drill pipe may have data cables for transmitting various types of electronic signals and couplers, such as inductive couplers at the respective pipe ends, for communicating with the next section of wired drill pipe. Examples of wired drill pipe are disclosed in U.S. Patent Application Publication No. 2006/0225926, which is incorporated herein by reference. 
     A bottom hole assembly (BHA)  20  is located at the bottom end of the drill string  16 . The BHA includes a drill bit  22  to cut through earth formations  24  below the earth&#39;s surface  26 , as well as various sensors, actuators, and other devices that are known in the art. BHA  20  may include various devices such as weighted drillpipe  28 , drill collars  30 , and one or more stabilizers  32  adapted to keep BHA  20  roughly in the center of the wellbore  34  during drilling of wellbore  34 . 
     The drilling system  10  includes one or more sensors  36  for measuring parameters associated with wellbore conditions and the drilling equipment. In various examples, sensors  36  may be located at the surface, various positions along the drill string  16 , and in the BHA  20 . In the example shown in  FIG. 1 , sensors  36   a  represent sensors in the BHA  20 , sensors  36   e  represent sensors located at various positions along the drill string  16 , and sensors  36   b ,  36   c , and  36   d  represent sensors located at or near the surface. In  FIG. 1 , the sensors are shown to illustrate a location of the sensor. Thus, sensor  36   a  is meant to indicate a sensor located in the BHA  20 . Such a sensor may be any type of sensor, and it may relate to more than one sensor. Thus, a description of sensor  36   a  as a temperature sensor is meant to indicate the position of the temperature sensor, and not to exclude a pressure or other sensor from the example. 
     The sensors  36  may include any type of sensor, such as pressure, temperature, accelerometer, magnetometer and strain sensors. In some examples a sensor may include various measurement while drilling (MWD) and logging while drilling (LWD) sensors, as are known in the art. 
     Telemetry for downhole sensors  36   a ,  36   e  may be provided by wired drill pipe to a central processing unit  38 , referred to herein generally as a control system. A wired drill pipe system may provide a high-speed, low-latency communications network between downhole elements and the surface. 
     Drawworks  14  provides a mechanism for lifting, lowering and supporting drillstring  16 . Drawworks system  14  may also include slips and other equipment generally known in the industry but not illustrated in detail. During active drilling drawworks  14  is operated to apply a selected axial force (weight on bit—“WOB”) to the drill bit  22 . Such axial force results from the weight of the drillstring  16 , a large portion of which is suspended by drawworks  14 . The unsuspended portion of the weight of drillstring  16  is transferred to the bit  22  as WOB. 
     Drawworks  14  is also used to lift and lower the drillstring  16  in wellbore  24  for non-drilling operations, such as tripping in or out of the well, and suspending the drill bit  22  off the bottom of the borehole while a new stand of pipe is added. A sensor  36   b  may be functionally connected within drawworks  14  to identify for example the rate of translation of drillstring  16  or the hook load. 
     System  10  may include a surface mechanism for rotating drillstring  16  and thus drill bit  22 , denoted generally herein as rotation system or mechanism  40 . In the illustrated example, rotating mechanism  40  is illustrated as a top drive, or power swivel, but may also be a rotary table with kelly bushing. In other examples, the mechanism for rotating drill bit  22  may be provided in whole or part by a hydraulic motor or other downhole rotating mechanism not shown in detail herein. One or more sensors  36   c  may be in functional connection with the rotation mechanism  40  to provide data such as for example the rotational speed of drillstring  16  and the torque applied to the drill string  16 . Sensors  36   c  may be in functional connection with control unit  38  for communicating the signals from these sensors. The various sensors may allow for determination of rotational speed of drillstring  16  at the surface, the axial load suspended by the drawworks  14 , and the torque applied to the drillstring  16 . 
     System  10  further includes a pumping system, generally denoted by the numeral  42 , for circulating drilling fluid  44  or “mud” during drilling operations. Pumping system  42  may include without limitation a pump  46 , tank  48 , standpipe assembly  50 , and drillstring  16 . While drillstring  16 , including BHA  20  and bit  22 , are rotated, pump  46  circulates mud  44  from tank  48  (or pit) through standpipe assembly  50  to drillstring  16 . Mud  44  flows through the interior of drillstring  16  discharging through drillbit  22  into wellbore  34 . Mud  44  flows back up annulus  52  carrying the drilling cuttings back to tank  48 . 
     Pumping system  42  includes in the illustrated example a sensor  36   d , such as a pressure transducer that generates an electrical signal or other type of signal corresponding to the mud pressure. One or more sensors  36   d  may be positioned so as to determine the mud pressure without limitation at pump  46 , standpipe  50 , and annulus  52 . 
     Control system  38  is in communication with sensors  36  and may be in operational connection with drawworks  14 , rotation system  40  and pumping system  42 . Control system  38  may include circuits for recording signals generated by the various sensors  36  and to control the various drilling systems, such as mud pumping and rotations and translation of drillstring  16 . 
     From time to time it is necessary to terminate or substantially terminate the circulation of mud  44  through the drilling system. This is most frequently done when an additional section of drillpipe  18  is connected to the top end of the drillstring  16  to lengthen the drill string. Typically, when it is necessary to add sections of drill pipe, the rotation and mud flow is stopped, and the drill bit  22  is lifted off of the bottom of the borehole. To restart or initiate drilling operations, the circulation of mud  44  must be started, drillstring  16  must be translated down so that bit  22  is in position to make hole and rotation of bit  22  and typically drillstring  16  will commence. 
     During the transition from stop to conducting drilling operations there can be pressure increases that damage the formation and/or equipment on BHA  20 . For example, translation of the pipe can cause a pressure surge. Additionally, there may be rheological changes in the drilling mud  44  after remaining idle. For example, a typical mud  44  will gel when not flowing, thus requiring that a threshold shear stress be overcome before mud  44  will flow again. In order to limit damage to formation  24  and drillstring  16 , downhole conditions, such as pressure, are monitored; the motion of drillstring  16 , in particular drillbit  22  or BHA  20 , and circulation of mud  44  are also monitored and controlled. 
     An example of a method for starting drilling operations is now provided. For purposes of description the example is described with reference to a top drive rotation system and startup after ceasing drilling operations to make-up a section of drillpipe  18  into drillstring  16 , as shown in  FIG. 2 . 
     Rotation system  40  is initiated, for example by controller  38 , so as to slowly increase the torque applied to drillstring  16 , at step  201 . In this example, the rotary system  40  may be a top drive system that is capable of rotating the drill string before it is lowered. A bottomhole torque sensor  36   a  communicates data via wired drillstring  16  to controller  38 , at step  203 . A torque sensor  36   c  at rotation system  40  communicates the torque applied directly to drillstring  16 . Sensor  36   a  communicates to controller  38  that a torque increase occurs downhole, for example at BHA  20 . Controller  38  maintains rotation mechanism  40  at a set torque until bottomhole motion sensors (for instance either accelerometers or magnetometers)  36   a  indicates that rotational motion has been initiated. Note that surface torque is transmitted downhole at approximately 3000 meters per second in a steel drillstring. When downhole torque sensor  36   a  detects a rise in the torque, the bottomhole torque will continue to increase although the surface torque is maintained at a constant level. If BHA  20  does not move after the transit time of the rotational waves has lapsed, then mechanism  40  may be operated by controller  38  to gradually increase the surface torque. Once motion has been initiated, the surface torque should be reduced to around 60% of the level that was required to initiate motion, due to the lower friction when the drillpipe is rotating. Rotation speed can then be gradually brought up to the desired level. 
     A downhole sensor  36   a  communicates via wired drillstring  16  pressure data to control system  38 . A pressure change will be detected as the gel structure of mud  44  is altered and its viscosity is reduced, at step  205 . Once this initial transient behavior of the downhole pressure has passed, pump  46  or circulation system  42  is started, at step  207 . For example, once the downhole transient pressure has passed, controller  38  initiates pump  46  to circulate mud  44  at a steady low rate. Although the mud in annulus  52  may be liquid, because of the shear stresses induced by rotation of the drillstring, the mud in drillstring  16  may still be gelatinous. A rise in a bottomhole pressure of mud  44  inside of drillstring  16 , communicated by a sensor  36   a  to controller  38 , indicates that all of mud  44  in system  10  is flowing. Controller  38  may then initiate pump  46  to increase flow rate until a surface sensor  36   d  indicates that mud  44  is flowing through annulus  52 . Controller  38  may then operate pump  46  at a specified full flow rate for drilling operations. Changes in annular measurements of temperature are also an indicator of mud motion in the annulus and may used to track how much of the mud column in the annulus is moving. Temperatures measured by sensors  36   a  along the drillstring will rise if mud that has been deeper than the sensor moves past them, and then will reduce as fresher circulating mud reaches them. In order to reach full downhole flow rate as fast as possible, the surface flow rate can be programmed to overshoot the required steady rate and then drop back, without either exceeding surface pressure ratings, or bottomhole flow rate limits. 
     Once a steady flow mud rate is reached, or other desired mud flow rate, then bit  22  may be lowered to the bottom  54  of wellbore  34  by translation system  14 , at step  209 , and drilling may be resumed, at step  211 . Controller  38  controls the rate of translation of drillstring  16  so as to minimize the surge pressure in wellbore  34  and to avoid damaging formation  24 . This is done by making velocity changes smooth, and by timing the motion so that the fundamental resonance of fluid in the annulus is not excited (this requires that the total time taken is not close to half the period of that resonance). 
     In another example method, shown in  FIG. 3 , the rotation system may be a rotary table. Using a rotary table, it may be impossible to begin rotation of the drill string before the drill string is lowered so that the Kelly bushings are engaged. In this example, the method first includes lowering the traveling block until the effects of the motion are observed in the bottom hole weight and motion sensors, at step  301 . Next, the mud pumps may be started, at step  303 . The mud pump start sequence may be initiated in a similar manner to the top-drive case, except that once the fluid near the bit has started flowing, the flow rate must be sufficient to compress the gelled mud in the annulus to the point where the shear stress exceeds the yield stress of the gel. 
     The velocity of the descending drill string and the mud flow must be controlled so that the surge pressure, combined with the hydraulic pressure, does not exceed the desired limits. In one example, the lowering of the drill string and the mud flow rate are controlled by the controller  38 . 
     As the traveling block and the drill string are lowered, the Kelly bushings will approach the rotary table. The descent of the drill string may be slowed, and the Kelly bushings are brought into engagement with the rotary table, at step  305 . Once the Kelly bushings are engaged, the drill string may be rotated, at step  307 , and drill in may continue, at step  309 . 
     In normal drilling, connections are frequent events, and so the response of the system at start up following one connection should be very similar to that at the previous connection. This similarity can be used by the system to modify the automated start-up sequence so as to minimize the total time taken without resulting in undesirable downhole pressures or motions. For instance, if during one start-up sequence the downhole pressure variations are well within the desired limits, the parameters used (eg the plateau flow rates, the rate of increase before the annulus is in motion, or the overshoot flow rate) can be increased until the pressure variations are at the limits, minus a safety margin. 
     The processes may either be entirely automated, partially automated (for instance, the driller still decides when start the pumps or block motion, but does not control the sequence once initiated), or may be in the form of presenting to a human operator the optimal parameters to use and times at which to start operations, or over-ride limits to prevent damage resulting from the human operators actions 
     From the foregoing detailed description of specific embodiments of the invention, it should be apparent that a wellbore drilling system and method that is novel has been disclosed. Although specific examples have been disclosed herein in some detail, this has been done solely for the purposes of describing various features and aspects of the invention, and is not intended to be limiting with respect to the scope of the invention. It is contemplated that various substitutions, alterations, and/or modifications, including but not limited to those implementation variations which may have been suggested herein, may be made to the disclosed examples without departing from the spirit and scope of the invention as defined by the appended claims which follow.

Summary:
A method related to restarting a drilling process includes the steps of applying a surface torque to a drill string in a borehole, detecting signals related to one of a torque and a rotational speed experienced at a bottom hole assembly, initiating drilling fluid flow, and lowering a drill bit to a bottom of the borehole. The surface torque or the drilling fluid flow is maintained or changed based on the signals related to the torque or rotational speed.