NO844026L - PRESSURE PULSE GENERATOR - Google Patents

Description

The present invention relates to pressure pulse generators in general, and in particular to pressure pulse generators such as the "mud siren" type used in the oil industry's MWD operations (measure-'ments-while-drilling) to send measurement information from the borehole to the surface via a mud column in the drill string.

There are many systems for sending data representing one or more measured conditions in a borehole up to the surface during drilling of a wellbore. Such a system, described in US patent 3 309 6 56 (Godbey) uses a pressure pulse generator or modulator down the borehole, and is used to send modulated signals carrying coded data at acoustic frequencies to the surface via the mud flow in the drill string. IN

such a system, it has been found expedient to supply the electrical components in the borehole with electrical power from an independent, mud-driven turbine generator (known as a mud turbine), located downstream of the modulator.

Existing mud siren type modulators are usually

in the form of turbine-like signal generator valves placed in the drill string near the drill bit and exposed to the circulating drilling mud. A typical modulator of this type consists of a fixed stator and a motor-driven, rotatable rotor placed coaxially with each other. The stator and rotor are both formed with a plurality of block-like radial protrusions or tabs located circumferentially around a central hub, such that the openings between adjacent tabs constitute a plurality of openings or ports for the inflowing drilling mud. When the corresponding ports in the stator and rotor are directly in line with each other, they form the largest passage for flow of drilling mud through the modulator. When the rotor blades rotate relative to the stator, the alignment between the respective ports is changed so that the flow of drilling mud is interrupted and generates pressure pulses in the form of acoustic signals. Rotation of the rotor relative to the stator in the circulating mud flow generates a cyclic acoustic signal that travels up the mud column in the drill string to be detected at the borehole surface. By selectively varying the rotor's rotation to generate changes in the signal, modulation can be achieved in the form of a coded pressure pulse that carries information from instruments down in the borehole to the surface for analysis.

The tab design and relative positioning of the stator and rotor elements in conventional modulators is such that it subjects the rotor to dynamic fluid forces due to the mud flow, causing the rotor to seek a "stable lid" position, in which the tabs on the rotor blocking the ports on the stator. There is thus an undesirable tendency for the modulator to settle into a position that blocks the free flow of drilling mud every time the rotor, even temporarily, goes out of service. This increases the likelihood that the modulator will jam, when solid objects carried by the mud flow are forced to pass through restricted modulator runs. Restarting the rotor becomes more difficult because the reduced mud flow affects the turbine generator's generation of rotor power. Prolonged modulator shutdown can block the flow of drilling mud to such an extent that lubrication of the drill bit and other important functions of the drilling mud are so detrimentally affected that the entire drilling operation is jeopardized.

A number of methods have been proposed to solve the problem arising from the tendency of the existing modulators to assume the closed position as described above. One such approach, described in US Patent 3,792,429 (Patton et al) is to use magnetic force to bias the modulator towards an open position and hold it there in the event of rotor failure. Magnetic attraction between a magnet attached to the modulator housing and a co-operating magnetic element based on the rotor shaft develops sufficient torque to overcome the dynamic torque caused by the mud flow. This method has the disadvantage that the tool must be extended to make room for the magnets, and that the introduction of an additional magnetic field down the hole can affect measurements of the earth's magnetic field (used to derive tool orientation).

In commercial MWD operations, the space between the rotor and stator components of the modulator must be narrow to generate satisfactory acoustic signals. This requirement makes the modulator particularly susceptible to jamming or blockage by solid materials present in the mud flow. A system to avoid such stalling, described in US Patent Re29,734 (Manning), includes a control device which is sensitive to conditions which tend to reduce engine speed (such as an increase in the pressure difference across the modulator and an increase in the demand on driver torque) to temporarily separate the rotor and stator to allow rust to be removed from the modulator by the flowing drilling mud. Such a system can be used to remedy the decreasing mud flow experienced with a closed modulator by separating the modulator parts as a result of the increase in pressure difference that results from the modulator taking a closed position.

The present invention provides an improved pressure pulse generator or modulator of the type used to transmit information between points in a wellbore via a fluid stream in a tubing string comprising a housing adapted to be connected in the drill string so that the fluid stream in the drill string will at least partially flow through the house; a stator, permanently mounted inside the housing; a rotor mounted rotatably inside the housing near the stator; the stator and rotor each have a plurality of spaced tabs at openings therebetween, which form a plurality of ports for fluid passage such that rotation of the rotor relative to the stator will shift the alignment of the respective stator and rotor ports from a position that provides greatest fluid flow to a position that gives the smallest fluid flow, so that fluid flow through the housing will be interrupted to generate and send a pressure pulse signal against the flow. The invention is characterized in one of its aspects as a device that is sensitive to the flow of fluid in the drill string to establish dynamic fluid forces that bias the generator in a stable open position.

A pressure pulse generator, structured according to an aspect of

the present invention comprises a fixed stator and a rotatable motor, both mounted inside the housing arranged to be connected in a pipe string so that liquid flowing in the string will at least partially flow through the housing. The rotor is mounted close to the stator and below it in the direction of current. Both the stator and rotor are formed with a plurality of radial projections or tabs, with intermediate openings between adjacent tabs serving to form a plurality of ports or openings for the passage of fluid flowing through the housing. Rotating the rotor relative to the stator will vary the blocking effect

of the rotor protrusions against the current from the stator ports, so that the relative alignment of the respective stator and rotor ports changes between a position that gives the greatest stroke for fluid flow through the housing (open position) and a position that gives the smallest stroke for fluid flow through the housing (closed position). This vein creation interrupts the fluid flow in such a way that it causes the generation and voltage of a pulse signal through the fluid against the flow. The relative position of the stator and rotor, and the special design of their respective lobes, is such that dynamic fluid forces are established as a result of current, but of fluid in the housing, which biases the rotor to such an orientation that it provides the greatest fluid -run through the generator. Should the generator fail or in some other way come out of operation, the fluid forces will force it into the position that provides the minimum blockage to the flow.

In general, the forces are developed from the fluid flow by providing each lobe of the rotor with sides that slope outwards in the direction of flow, and which partially lie below the stator lobes. The inclination of each side of the rotor blades is preferably in the range from about 8° to about 30° in relation to a vertical axis.

In another aspect of the present invention, the rotor tabs are designed in such a way as to cause the rotor to oscillate between an open position and a partially closed position due to the dynamic action of the fluid. This serves to prevent rust from blocking the flow of fluid through the modulator, and provides a periodic movement and signal whose frequency varies with the flow rate. The oscillations take the form of aerodynamic vibrations, created by the fact that the rotor blades at their trailing edges, near the base, are equipped with non-sloping areas of reduced width. The sides of the rotor blades may also have non-sloping areas at the leading edges, near the top of the blades, to produce a sheathing effect on debris passing into the ports and into the space between the stator and the rotor.

The modulator according to the present invention produces an improved signal source with a good ability to avoid obstruction. It is particularly applicable in the oil industry for measurements during drilling, downhole testing and monitoring of completed wells as a signal source for communication from down the borehole to the surface, from the surface down into the hole, and between intermediate points in a well. Other applications include its use as a sound source for underwater seismology surveys, use as a current measuring device, and use as a one-way flow valve.

Construction, operation and advantages of the invention can be better understood by reference to the drawings, where: Fig. 1 is a schematic diagram of a pressure pulse generator according to the present invention, shown connected in a drill string for a typical drilling operation, in its application for communication between a downhole MWD tool and the surface of the well; Fig. 2 is a side view, partly in section, of the generator in fig. 1;

Fig. 3a is a perspective view of the generator in the figures

1 and 2; Fig. 3b is an unfolded end view of the stator and rotor tabs in the generator in fig. 3a; Fig. 4a is a top view of the stator in fig. 3a; Fig. 4b is a section taken along the line 4b-4b in fig. 4a; Fig. 5a is a top view of the rotor in fig. 3a; Fig. 5b is a section taken along the line 5b-5b in fig. 5a; Fig. 5c is a partial end view seen from the line 5c-5c of one of the tabs in the rotor of fig. 5a; Fig. 6 is a schematic perspective view identifying reference symbols which may assist in understanding the relative dimensions; Fig. 7a is a top view of a modified embodiment of the pressure pulse generator of Figs. 1-5c; Fig. 7b is an end view as seen from the line 7b-7b in a set of the stator and rotor tabs of Fig. 7a; Fig. 8 is a sectioned perspective view of a further modification of the generator in Figures 1-5c, where the rotor has a reduced part near the rotor hub; Fig. 9 is a top view of the rotor in fig. 8; Fig. 10a is a "perspective view of a further modification of the generator of Figs. 1-5c, illustrating a construction to cause aerodynamic vibration; and

fig. 10b is an unfolded end view of the stator and rotor tabs of fig. 10a.

In all the drawings, the same reference numbers are used to

identify equal parts.

Fig. 1 in the drawings shows a tubular tool 20 for measurement while drilling (MWD) connected in a tubular drill string 21

with a rotating drill bit 22 connected to the end thereof and arranged for drilling a borehole 23 for a well through various soil formations. As the drill string 21 is rotated by a conventional drilling rig (not shown) on the surface of the borehole 23, a significant volume of a suitable drilling fluid (known as drilling mud) is continuously pumped down through the drill string 21 and discharged from the drill bit 22 to cool the drill bit and to carry away materials removed by the drill bit. The muds are returned to the surface through the annulus between the walls of the borehole 23 and the outside of the drill string 21. The circulating mud stream flowing through the drill string 21 serves as a medium to send pressure pulse signals with information from the MWD tool 20 to the surface, as described in more detail below.

A data signaling unit 24 down in the borehole has transducers mounted on the tool 20 in the form of one or more state-sensitive devices 26 and 27, connected to suitable electrical data coding circuits, such as the coding circuit 28, which sequentially generates coded electrical digital data signals that represent measurements taken with the transducers 26 and 27. The transducers 26 and 27 are selected and arranged as needed for the particular application of measuring such parameters in the borehole as pressure, temperature, and resistivity or conductivity of the drilling mud and adjacent soil formations, as well as measuring other conditions in the borehole similar to those that are currently measured with electric logging instruments.

Electrical power for operating the data signaling unit 24 is provided by a typical rotary axial mud turbine 29 with an impeller 30 which responds to the flow of drilling mud and drives the shaft 31 to generate electrical energy.

The data signaling unit 24 also comprises a modulator 32

which is driven by a motor 35 to selectively interrupt or prevent the flow of drilling mud through the drill string 21 to generate digitally-encoded pressure pulses in the form of acoustic signals. The modulator 32 is selectively driven as a result of the data-encoded

the electrical output signals from the encoder circuit 28 to generate a corresponding encoded acoustic signal. This signal is sent to the surface of the well through the drilling mud that flows in the drilling

the string 21 as a series of pressure pulse signals, which are preferably coded binary representations of measurement data for the drilling parameters in the borehole, as measured by the transducers 26 and 27. When these signals reach the surface, they are detected, decoded and transformed into understandable data by a suitable signal detector 36, such as that shown in US Patent No. 3,309,656; 3,764,968; 3,764,969 and 3,764,970.

The modulator 32 comprises a fixed stator 40 and a rotatable rotor 41 which is driven by the motor 35 in response to signals generated by the encoder circuit 28. Rotation of the rotor 41 is controlled by the computer coded electrical output signals from the encoder circuit 28 to generate a corresponding coded acoustic output signal. This can be achieved by using well-known techniques to vary the direction or speed of the motor 35 or to controllably engage/disengage the rotor 41 from the drive shaft of the motor 35'.

The stator 40 has a plurality of equally spaced, block-like lobes 71 arranged circumferentially around a central hub. The opening between adjacent tabs 71 provides a plurality of ports for passing the incoming drilling mud through the stator which flows more or less parallel to the stator hub axis. The rotor 41 has a similar design to the stator 40, and is placed close to it in the flow direction below the stator to rotate about an axis which is coaxial with the stator's hub axis. As the rotor 41 rotates, its tabs 72 will successively move in and out of positions that shut off the flow of drilling mud through the ports in the stator 40, to generate a pressure pulse signal which is sent against the flow of the circulating drilling mud.

When the rotor 41 is rotated relative to the stator 40 so that it currently presents the greatest obstacle to the flow of circulating drilling mud, the acoustic signal will be of maximum amplitude. As the rotor 41 continues to rotate,

the amplitude of the acoustic signal generated by the modulator 32 will decrease from its maximum to its minimum value as the rotor moves to a position where it presents the least obstacle to the drilling mud flow. Further rotation of the rotor will cause a corresponding increase in signal amplitude when the rotor again approaches the next maximum current limiting position.

Those skilled in the art will understand that rotation of the modulator rotor 41 will generate an acoustic output signal with a cyclic waveform, which alternates successively between positive and negative peaks around a mean pressure level. Continuous rotation of the rotor 41 will generate a typical alternating or cyclic signal of a fixed frequency, which will have a determinable phase relationship with another alternating signal, such as a selected reference signal generated in the signal detector circuit 36. In momentary announcement, sinking , stopping or reversing the rotation of the rotor 41 as a result of the output signal from the encoder circuit 28, the rotor may be selectively shifted to different positions relative to the stator 40 than it would have been if it had continued to rotate without change. This selective shifting causes the phase of the acoustic signal to shift relative to the phase of the reference signal. Such a controlled phase shift of the signal generated by the modulator 32 acts to send measurement information from the borehole, via the drilling mud flow to the surface of the well to be detected by the signal detector circuit 36. Shifting the phase at a given moment means a binary bit "1" ( or "0") and the absence of a phase shift means a binary bit "0" (or "1"). Other signal modulation techniques are usable, and selection of the particular coding, modulation and decoding technique to be used in connection with the modulator 32 is a matter of preference, and a detailed discussion thereof is unnecessary to an understanding of the present invention.

As shown in fig. 2, both the stator 40 and the rotor 41 are mounted inside a tubular housing 42 which is press-fit inside a part of the drill collar 43 by means of the enlarged annular parts 44 and 45 of the housing 42, which are in contact with the inner surface of the drill collar 43. A plurality of "0" rings 46 and 47 provide sealing contact between the collar 43 and the housing 42.

The stator 40 is mounted by means of threaded connections 50 (see also fig. 4b) to one end of a support structure 51, placed above all inside the housing 42 and locked in place with a set screw 56. The space between the end of the threaded part of the stator 40 and an adjacent shoulder of the support structure 51 is filled with a plurality of "0" rings 55. The support structure 51 is held in a spaced relationship with the inner wall of the housing 42 by means of a spacer 52. The spacer 52 is attached to the support structure 51 by means of a plurality of hex bolts 53 (only one of which is shown), and is in turn attached to the housing 42 by a plurality of hex bolts 54 (only one of which is shown). The spacer 52 is provided with a plurality of separated ports to allow the passage of drilling mud in the annular space formed between the support structure 51 and the inner wall of the housing 42.

The rotor 41 is mounted for rotation on a shaft 60 in the motor 35 (Fig. 1) which drives the rotor 41. The rotor 41 has a rotor bushing 59 (Fig. 2) guided near the end of the shaft 60 and forced into contact with a shoulder 61 on the shaft 60 by a bushing 62, which is also guided on the end of the shaft 60. The bushing 62 is forced against the rotor bushing 59 by means of a hex nut 63 which is threaded on the free end of the shaft 60. An inspection port 58 is provided for to allow examination of the stator and rotor tabs 71,72 to measure the rotor/stator separation and to detect wear.

The shaft 60 is supported inside a bearing housing 65 to rotate around a bearing structure 66. The bearing housing 65 is supported at a distance from the inner wall of the housing 4 2 by means of a spacer 67, fixed to the bearing housing by means of the hexagon bolts 68, and is in turn attached to the housing 42 by means of the hexagon bolts 69.

As shown in Figures 2 and 3, drilling mud flows into the top of the housing 42 in the direction shown by the arrows 70 (Fig. 2), through the annular space between the outer wall of the support structure 51 and the inner wall of the housing 42, and flows through the ports in the stator 40 and the rotor 41. The fluid flow continues past the rear spacer 67 and to the drill bit 22 (Fig. 1). The shaft 60 drives the rotor 41 to interrupt the fluid flows passing the ports in the stator 40 to generate a coded acoustic signal which moves against the flow.

According to the present invention, the rotor 41 is placed

in the flow direction below the stator 40, and its tabs 72 are designed to produce dynamic fluid forces due to the flow of drilling mud which drives the rotor 41 to an open position relative to the stator 40 whenever the rotor 41 is not driven by the motor 35. More particularly , the relative geometry and placement of the stator 40 and the rotor 41 establish dynamic fluid biasing of the rotor 41 to an orientation in which the tabs 72 provide the least obstruction to fluid flowing through

the ports in the stator 40.

Figures 3a-5c show features of a first embodiment of the modulator 32 which shows such "stable open" behaviour. Figure 6 identifies dimensions that are important for the understanding of these features.

The general relationships between the stator 40 and the rotor 41 in the modulator 32 are shown in fig. 3a. As indicated by the arrows, drilling mud flows through the housing 4 2 in a downward direction in the borehole, and rotation of the rotor 41 generates an acoustic signal which is sent upwards. In contrast to modulators according to prior art, which usually place the rotor against the current direction above the stator, the rotor in the modulator 32 is placed in the current direction below the stator.

As shown, both the stator 40 and the rotor 41 are provided with a plurality of radially extending tabs 71, 72 along the circumference, spaced symmetrically about coaxial hubs. The tabs form wedge-like protrusions, radially from the hubs. Each tab is defined by a top (the surface facing the flow), a base (the surface with the flow), opposite radially-facing sides (surfaces extending outward from the hub connecting the top and base),

and an end (surface farthest from and concentric with the hub, which abuts the inner wall of the housing). All the tabs 71 in the stator 40 are similarly constructed, and all the tabs 72 on the rotor 41 are similarly constructed. The same number of tabs is used for the stator and the rotor, and the number of six is found suitable. Choosing a different number is possible, but will change the characteristics of the generated signal.

For better rigidity, either the stator or the rotor 41 or both of these can be provided with an edge circumscribing the ends of their lobes. The stator 40 can also, alternatively, be integrated into the housing 42. This choice is based on convenient manufacturing.

The ports between adjacent lobes of the stator and rotor are defined by the periphery of the hub and the adjacent sides of adjacent lobes. It is considered advantageous, but not necessary, that the respective tabs and intermediate ports are dimensioned in such a way that they are of approximately the same size.

The 6 tabs 71 on the stator 40 (fig. 3a-3b, 4a and 4b) are evenly distributed around the stator hub. The tops and bases of the stator tabs 71 are parallel to each other and at right angles to the hub axis. The sides of the tabs 71 are generally radial to the nave axis, with opposite sides of each tab at an angle of 30°, and equal sides of adjacent tabs at an angle of 60° to the nave axis (Fig. 4a). The internal threads 50 on the inside of the stator hub (see Fig. 4b) provide, in addition to connecting the stator 40 to the support structure 51 as described above, a device for adjusting the amplitude of the generated acoustic signal by varying the opening between the base of the stator the tabs 71 and the tops of the rotor tabs 72. The stator tabs 71 are designed with an outer width Wl, and the surface of the tops of the tabs is equal to the outer width W2 and the surface of the base of the tab (Fig. 6). The stator ports are designed so that they have equal inlet and outlet openings, where the inner and outer widths Pl, P3 and the inlet openings are the same as the respective inner and outer widths P2, P4 of the outlet openings.

The R6.tor tabs 72 (Figs. 3a,3b and 5a-5c) are evenly distributed around the rotor hub so that the radial lines drawn from the hub axis and through the centers of the tabs 72 form angles of 60° with each other, and angles of 30° with lines drawn from the hub axis and through the centers of adjacent rotor ports (see Fig. 5a). Like the stator 40, the tabs 72 of the rotor 41 have parallel tops and bases which are at right angles to the hub axis. However, the sides of the tabs 72 slope outwards in the direction of the fluid flow (positive slope). The outer width W4 (see Fig. 5) and the base surface (back side) of each rotor blade 72 is greater than the corresponding outer width W3 and the surface of its top (front side). Fig. 5c illustrates a preferred positive uniform slope of 12° for the sides of the tabs 72. Other slopes of 8° to 30° are also applicable.

As shown in fig. 5a are the edges 74 and 75 of each rotor blade

72 (designed where the sides meet the top) at an angle of 2 7°, and so are the edges 76 and 77 (designed where the side meets the base). The tops of the rotor tabs 72 lie below the bases of the stator tabs 71, and the outer width W3 (Fig. 6) and surface of the tops of each rotor tab 72 is smaller than the corresponding outer width W2 and the surface of the base of each stator - tab 71. The rotor ports are designed in a complementary way, so that the internal width P5, the external width P7 and the surface of the inlet opening for each rotor port is greater than the corresponding internal width P2, external width P4 and surface of the outlet opening of each stator port (see fig. 6). As the rotor ports are formed by the space between the rotor tabs 72, the sides of the ports slope inwards in the direction of flow.

As shown in Figures 5a and 5b, each rotor tab 72 has a bore 80 to receive the machine screws 57 (Fig. 2) which serve to secure the tabs 72 to the rotor bushing 59.

The relative dimensioning of stator and rotor tabs 71, 72 as described causes the flowing drilling mud to exert dynamic forces on the rotor, so that the modulator 32 is biased in a stable open position. When the modulator 32 is in a non-equilibrium position as shown in fig. 3b, forces are generated that act on the geometry of the modulator to cause high pressure to be applied to one side of the rotor blades 72, and low pressure to be applied to the other side. These forces force the ro-tcr tabs 72 into positions directly below the stator tabs 71, thereby aligning the stator and rotor ports to provide the greatest passage of fluid through the modulator 32.

Examples of stator and rotor dimensions for a modulator, designed as shown in fig. 3a-5c, which exhibit stable open mode is shown below. These dimensions give an underlap between rotor and stator of 3.18 mm, and gave satisfactory efficiency with a rotor/stator separation of 1.59 mm. The dimensions are identified with reference to fig. 6.

Stator 40

Number of tabs = 6

Outer diameter = 114.3 mm

Depth = 15.9 mm

Width Wl = 23.9 mm

Width W2 = 23.9 mm

Thickness = 25.4 mm

Hub diameter = 5 7.2 mm

Port distance Pl = 15.9 mm

P2 = 15.9 mm

P3 = 23.9mm

P4 = 23.9mm

IN

Rotor 41

Number of tabs = 6

Outer diameter = 113.5 mm

Depth = 15.1 mm

Width W3 = 20.6 mm

Width W4 = 28.6 mm

Thickness - 15.9 mm

Hub diameter = 5 7.2 mm

Slope = 12°

Port distance P5 = 15.9 mm

P6 = 9.5 mm

P7 = 25.4 mm

P8 = 17.5 mm

It has been pointed out that stable open efficiency was achieved only for the liquid flow direction shown in fig. 3a.

For liquid flow in the opposite direction, the modulator 32 will exhibit the stable closed efficiency as devices according to prior art. Thus, for a freely rotating shaft 60 (not driven and not prevented from rotating), the modulator 32 will act as a check valve that opens as a result of fluid flow in one direction and closes as a result of fluid flow in the other direction.

Other designs of stable open type modulators 32 can be constructed following the same principles as used above. In general, the stator must be placed above the rotor against the direction of current. The stator tabs should preferably have straight (not sloping) radially extended sides, and be dimensioned so that the tabs and the intermediate ports have approximately the same size. The thickness of the rotor (fig. 6) should preferably be equal to or less than the thickness of the stator. The sides of the rotor blades should slope outwards in the direction of the flow, with a positive slope preferably of 8° to 30°. Underlap should be provided between the top of the rotor blades and the base of the stator blades (ie the surface of the top of the rotor blades should be less than the surface of the bases of the stator blades). The required amount of underlap will depend on the thickness and slope of the rotor. The thinner the rotor, the less underlap will be required. The rotor/stator distance should not be too small. Appropriate distance can be determined empirically. Smaller distances give stronger signals, while

larger distances provide better stable open efficiency.

Another embodiment of the modulator 32, constructed according to the preceding criteria, comprises a stator 85 and a rotor 86 as illustrated in figures 7a and 7b. The stator 85 has five tabs 87, evenly spaced around the periphery of a stator hub. Examples of stator and rotor dimensions for a stable open modulator, designed as shown in Figures 7a and 7b for operation with a rotor/stator spacing of 2.38 mm, are given below:

Stator 85

Number of tabs = 5

Outer diameter = 111.1 mm

Depth = 19.05 mm

Width Wl = 34.1 mm

WidthW2 = 34.1 mm

Thickness = 31.8 mm

Hub diameter = 71.4 mm

Port distance Pl = 20.6 mm

P2 = 20.6 mm

P3 = 32.5 mm

P4 = 32.5 mm

Rotor 86

Number of tabs = 5

Outer diameter = 110.3 mm

Depth = 19.05 mm

Width W3 = 37.8 mm

Width W4 = 4 7.6 mm

Thickness = 10.3 mm

Hub diameter = 71.4 mm

Slope = 30°

Port distance P5 = 23.8 mm

P6 = 13.5 mm

P7 = 46 mm

P8 = 19.05mm

Radial lines drawn through the centers of adjacent tabs 87 make angles of 72° with each other. The opposite sides of each tab 87 form an angle of 36°, and the opposite sides of adjacent tabs 87 also form an angle of 36°. The stator is thus symmetrical, and the size of its lobes

is the same as the size of its gates.

The rotor 86 is placed in the flow direction below the stator 85, and likewise has five lobes 88 evenly distributed around a central hub. The sides of the tabs 88 slope outwards in the direction of flow with a positive slope of 30°. The outer width W3 of each rotor top is somewhat larger than the outer width W2 of each stator tab base (see end section fig. 7b). Underlap is produced between the stator tabs 87 and the rotor tabs 88 by arranging a greater convergence angle for the top edges of the sides of the rotor tabs 88 than for the bottom edges of the sides of the stator tabs 87. As shown in fig. 7a, the lower edges 89 of the sides of each rotor tab 85 have an angle of 52°, and extend radially outward from a point on the center axis of the rotor hub. The upper edges 90 and 91 on the sides of each rotor blade 86, which also have an angle of 52°, extend radially from a point along the centerline of the blade, instead of from the hub axis. Consequently, the top of the rotor blade has a smaller surface than the base of the stator like 85:. Although the underlap at each adjacent edge of the ends of the rotor and stator lobes is somewhat negative (W2-W3 = -0.0625 mm in the above example), the underlap increases rapidly with lobe depth toward the hub.

Fig. 8 illustrates another embodiment of the present invention, and comprises a stator 100 placed upstream of a rotor 101. The stator 100 has six lobes, and is similar to the stator 40 as described above with reference to fig.

4a and 4b. The size of the stator tabs 102 and the intermediate stator ports are the same, and the widths W1, W2, P3 and P4 are all the same (see Fig. 6). The rotor 101 is so constructed that the outer width W4 of the base of each tab 103 is equal to the outer width P8 of the outlet for each port. The relationship between the dimensions of the stator 100 of the rotor 101 is such that Wl = W2 = W4

= P3 = P4 = P8. This design, where the inlet and outlet openings of the stator ports and the outlet openings of the rotor ports are of the same size, minimizes the disturbance of the rotor slope to the fluid flow when the modulator is in its open position. This brings with it the advantage of reduced wear and erosion of the rotor tabs 103.

To improve the acoustic signal, the thickness of the rotor can be reduced by milling the top of the rotor. However, this reduces the underlap between the tops of the rotor tabs 103 and the bases of the stator tabs 102. In order to ensure a stable open efficiency, an area 104 with an increasing slope is arranged by creating a cut in an internal part (near the rotor hub 105 )

in that edge of the sides of the tabs 103 that face the current. These notches 104 help the sloped sides establish dynamic forces in the fluid to produce a stable open characteristic for the modulator 32.

Fig. 9 shows a modification of a partial swath construction of the rotor loi in fig. 8. The rotor 106 in fig. 9 differs from the rotor 101 in fig. 8 in that the outer widths W2, W4 and P4, P8 are not equal. The sides of each rotor blade 107 each have a positive slope of approximately 12°, and each blade 107 is provided with partial slats 108 with increasing slope similar to the slats 104 on the rotor 101.

Further modifications to the foregoing embodiments can be made to produce a modulator which not only exhibits the desired stable open characteristic, but which will also produce a fluid flow induced agitation to remove rust which has become lodged between the rotor and the stator. Such a modification is illustrated in Figures 10a and 10b, where a modulator 34 comprises a stator 110 and a rotor 113, mounted in a housing 42. The stator 110 is similar to the 6-lobed stator 40 as previously described. The sides of its lobes 111 are non-sloping, and are generally radial to the stator hub axis. However, the sides of the tabs 111 on the rotor 113 comprise a central sloping area similar to the previously described designs, but also have non-sloping leading and trailing edge areas 115,116 which are parallel to the sides of the stator tabs 111 (see unfolded view in fig. 10b ). The outer width W3 (Fig. 6) of the top of the rotor tab 114 which abuts the non-sloping leading edge region 115 is smaller than the outer width W2 of the base of the stator tab 111, thus providing an underlap. The outer width W4 of the base of the rotor tab 114 abutting the non-sloping trailing edge region 116 is approximately the same as the outer width W3 of the top. The non-sloped trailing edge region 116 of each rotor flap side is formed by undercutting the sloped region across the full depth of the rotor flap 114. The edges between the rotor flap top and the leading edge region 115 on the rotor flap sides are preferably sharp to help in a cutting action on rust that has stuck

in the gap between the stator and the rotor.

The design of figures 10a and 10b generates dynamic fluid forces due to drilling mud through direct impact and vortex shedding acting on the rotor 113 to force the modulator into a stable open position. However, the reset forces in the <:>azimuth direction are proportional to the angular displacement, with the result that a periodic movement in the form of aerodynamic vibrations is set up when the rotor is not driven by the shaft. The amplitude and frequency of the vibrations depend on the fluid flow rate, the design of the modulator and the inertia of the shaft. This vibration causes the rotor flaps 114 to oscillate between partially and fully open positions, and also generates an acoustic signal whose frequency depends on the rate of vibration. As the vibration rate is a vibration of the current speed, the modulator construction in fig. 10a and 10b be used for monitoring the current rate, while the frequency of the generated signal is monitored in a known manner, e.g. by conventional frequency analysis circuits incorporated in the signal detector circuit 36 (fig. 1).

Although particular embodiments of the present invention have been shown and described in the preceding examples, it will be clear to those skilled in the field to which the invention relates that further changes and modifications can be made without deviating from the invention and its further aspects. The aim of the claims is therefore to cover all such changes and modifications that fall within the spirit and scope of the invention.

Claims (14)

1. Pressure pulse generator of the type used to communicate information between points in a wellbore via a fluid flowing in a tubing string, comprising: a housing arranged to be connected in the string so that a fluid flowing in the string at least partially will flow through the house; a stator mounted firmly inside the housing; a rotor rotatably mounted within the housing near the stator, the stator and the rotor each having a plurality of separate lobes with openings therebetween presenting a plurality of ports for the passage of fluid such that rotation of the rotor relative to the stator will shift the orientation of the respective stator and rotor ports from a position providing the greatest fluid flow to a position providing the least fluid flow, so that fluid flow through the housing will be interrupted to cause the generation and transmission against the flow of a pressure pulse signal ; characterized by : a device for establishing dynamic fluid forces as a result of the fluid flow through the housing which biases the rotor generally in such an orientation that the ports provide the greatest fluid flow. 2. Apparatus according to claim 1, characterized in that the force-establishing device comprises relative positioning of stator and rotor, and that the stator and rotor tabs are designed in such a way that the geometry of the generator directs the flow of fluid to produce the forces. 3. Apparatus according to claim 1 or 2, characterized in that the stator and rotor tabs have sides, where the rotor is placed in the flow direction below the stator, that the sides of the rotor tabs slope outwards in the direction of flow, and that the rotor tabs are underlapped in relation to the stator tabs. 4. Apparatus according to claim 3, characterized in that the sides of the rotor blades slope at an angle of 8° to 30°. 5. Apparatus according to claim 1, 2, 3 or 4, characterized in that the number of stator tabs is the same as the number of rotor tabs. 6. Apparatus according to one of claims 1 to 5, characterized in that the size of the stator and rotor tabs is generally the same as the size of the respective intermediate stator and rotor ports. 7. Apparatus according to one of claims 1 to 6, characterized in that both the stator and rotor blades have top surfaces facing the current and base surfaces facing the current, and where the bases of the tops of the rotor blades are smaller than the surfaces of the bases for the corresponding stator tabs. 8. Apparatus according to one of claims 1 to 7, characterized in that the outer widths of the tops of the rotor tabs are smaller than the outer widths of the bases of the corresponding stator tabs. 9. Apparatus according to one of claims 1 to 7, characterized in that the outer widths of the tops of the rotor tabs are the same as the outer widths of the bases of the corresponding stator tabs, and the edges of the rotor tabs formed where the sides meet the tops of the rotor tabs converge towards the hub with a convergence angle greater than that of the edges of the stator lobes formed where the sides meet the bases of the stator lobes. 10. Apparatus according to one of claims 1 to 9, characterized in that the rotor blades have peak surfaces facing the flow, and the rotor sides comprise regions of increased slope at the edges that are formed where the sides meet the peaks. 11. Apparatus according to one of claims 1 to 10, characterized in that the generator further comprises a device for establishing further dynamic fluid forces as a result of the fluid flow through the housing, which causes the rotor to oscillate with a movement in the form of aerodynamic vibrations when the rotor is placed in the orientation where the ports provide the greatest fluid flow. 12. Apparatus according to claim 11, characterized in that the device for establishing additional forces comprises designing the rotor blades in such a way that the rotor geometry directs the flow of fluid to produce the additional forces. 13. Apparatus according to claim 12, characterized in that the rotor blades have bases in the flow direction, and that the device for establishing additional forces comprises that the sides of the rotor blades have undercut, non-sloping trailing edge regions at the edges formed by the sides meeting the bases. 14. Apparatus according to claim 13, characterized in that the rotor blades have peaks against the flow, and that the sides of the rotor blades have non-sloping leading edge regions at the edges formed by the sides meeting the peaks.