DDTM_04

Directional Drilling Training ManualSection 4 - Surveying Document Type UOP Template (Word 6 PC) Software Source File Other Source File Microsoft Word 6.0 for Windows NT DDTM_04.DOC TM.DOT Author Author info Mike Smith Anadrill Technique 200 Gillingham Lane Sugar Land TX 77478-3136 Tel: + 1 281 285 8859 Fax: + 1 281 285 8290/4155 email: [email protected] Review & approval Revision History 04 Dec 96 06-Dec-96 2nd Revision Final review and approval MJS Provisory - 04 Dec 96 Confidential Directional Drilling Table of Contents 4 Surveying Page 4.1 MAGNETIC & NON-MAGNETIC REQUIREMENTS ................................................................4-1 4.1.1 Magnetic Fields ........................................................................................................4-1 4.1.1.1 Aspects of the transitory field .........................................................................4-2 4.1.2 Magnetic field strength .............................................................................................4-4 4.1.3 Magnetic Dip angle...................................................................................................4-5 4.1.4 Magnetic Declination Angle.....................................................................................4-6 4.1.5 Magnetic Interference ...............................................................................................4-7 4.1.5.1 Drill String Magnetic Interference ..................................................................4-7 4.1.5.2 Minimizing Errors ...........................................................................................4-10 4.1.5.3 External Magnetic Interference.......................................................................4-12 4.1.5.4 D&l Package Spacing......................................................................................4-13 4.1.6 Earth’s Gravitational Field........................................................................................4-13 4.2 MAGNETIC SINGLE SHOTS & MULTISHOTS........................................................................4-15 4.2.1 Survey Instruments ...................................................................................................4-15 4.2.1.1 Magnetic Surveys............................................................................................4-15 4.2.2 Magnetic Single Shot................................................................................................4-15 4.2.2.1 Power pack ......................................................................................................4-15 4.2.2.2 Timer or Sensor...............................................................................................4-15 4.2.2.3 Camera ............................................................................................................4-16 4.2.2.4 Angle unit, Compass .......................................................................................4-17 4.2.3 Magnetic Multi-shot Survey Instrument...................................................................4-18 4.2.3.1 The multi-shot timer........................................................................................4-18 4.2.3.2 The multi-shot camera.....................................................................................4-18 4.3 GYROSCOPES ......................................................................................................................4-19 4.3.1 Principles of Gyroscopic Surveying .........................................................................4-19 4.3.1.1 Historical Background ....................................................................................4-19 4.3.2 The Gyroscope..........................................................................................................4-19 4.3.2.1 Components.....................................................................................................4-22 4.3.3 Classification of Gyroscopes ....................................................................................4-23 4.3.3.1 Use...................................................................................................................4-23 4.3.3.2 Construction and Function. .............................................................................4-23 4.3.3.3 Restraints on the movement of the spin axis...................................................4-23 4.3.4 Evolution of Gyroscopes used in surveying oil-wells ..............................................4-25 4.3.4.1 The Surface Read out Gyro.............................................................................4-25 4.3.5 Forces acting upon Gyroscopes ................................................................................4-25 4.3.5.1 Precession........................................................................................................4-26 4.3.5.2 Nutation...........................................................................................................4-26 4.3.5.3 Fundamental Precession..................................................................................4-27 4.3.5.4 Origin of Precession ........................................................................................4-28 4.3.5.5 Gimbal Lock....................................................................................................4-28 4.3.5.6 Tumbling .........................................................................................................4-29 4.3.5.7 Caging .............................................................................................................4-29 Provisory - 04 Dec 96 Confidential Directional Drilling 4-i Table of Contents 4 Surveying Page 4.3.6 Directional Gyro .......................................................................................................4-29 4.3.7 Level Rotor Gyro......................................................................................................4-30 4.3.7.1 Apparent Drift .................................................................................................4-30 4.3.7.2 Temperature Effect..........................................................................................4-30 4.3.7.3 Intercardinal Tilt Error or Gimbal Error .........................................................4-30 4.3.8 Rate Gyroscopes .......................................................................................................4-31 4.3.8.1 Accelerometer Operation ................................................................................4-32 4.3.8.2 Rate-Gyro Operation .......................................................................................4-33 4.3.9 Other Rate Gyro Systems .........................................................................................4-35 4.3.9.1 Rate Gyro ........................................................................................................4-35 4.3.9.2 Torsion Bar Rate Gyro ....................................................................................4-35 4.3.9.3 Rate Integrating Gyro......................................................................................4-35 4.3.9.4 Rate Integrating Gyro Use...............................................................................4-36 4.3.9.5 Strap Down System.........................................................................................4-36 4.3.9.6 Accuracy, quality control and why Rate Gyros?.............................................4-36 4.3.9.7 Errors in Rate Gyros........................................................................................4-37 4.3.10 Gyroscope suspension ............................................................................................4-38 4.3.11 North Seeking Gyros ..............................................................................................4-38 4.3.12 Drift Values ............................................................................................................4-39 4.3.12.1 Nature and Source of Drift ............................................................................4-39 4.3.12.2 Acceleration sensitive drift ...........................................................................4-39 Provisory - 04 Dec 96 Confidential Directional Drilling 4-ii Table of Contents List of Figures Figure 4-1 Figure 4-2 Figure 4-3 Figure 4-4 Figure 4-5 Figure 4-6 Figure 4-7 Figure 4-8 Figure 4-9 Figure 4-10 Figure 4-11 Figure 4-12 Figure 4-13 Figure 4-14 Figure 4-15 Figure 4-16 Figure 4-17 Figure 4-18 Figure 4-19 Figure 4-20 Figure 4-21 Figure 4-22 Figure 4-23 Figure 4-24 Figure 4-25 Figure 4-26 Figure 4-27 Figure 4-28 Figure 4-29 Figure 4-30 Figure 4-31 Figure 4-32 Page Earth’s magnetic field - rotation of liquid core ...................................................... 4-2 Earth's magnetic field - dynamo theory.................................................................. 4-2 Earth’s magnetic field............................................................................................. 4-3 Fluctuation's in the earth's magnetic field .............................................................. 4-3 Magnetic field strength........................................................................................... 4-4 Magnetic dip angle ................................................................................................. 4-5 Magnetic dip angles at poles and equator .............................................................. 4-6 Magnetic declination angle..................................................................................... 4-7 Drill string magnetism............................................................................................ 4-8 Effect of hole angle on drillstring magnetic interference...................................... 4-8 Effect of azimuth on drillstring magnetic interference ........................................ 4-9 Drillstring magnetic interference at different latitudes ...................................... 4-10 Magnetic lines of force in the drillstring ............................................................ 4-11 Effect of magnetic hot spot in MWD collar ....................................................... 4-12 NMDC requirements. ......................................................................................... 4-13 Deviation of Universal Gravitation Constant ..................................................... 4-14 Simplified diagram of a typical gyroscope......................................................... 4-20 Realistic view of the configuration of a typical gyroscope ................................ 4-21 Gyro rotation around outer gimbal axis.............................................................. 4-22 Gyro rotation around inner gimbal axis.............................................................. 4-22 Single degree of freedom gyro ........................................................................... 4-24 Two degree of freedom gyro ............................................................................... 4-24 Representation of nutation.................................................................................. 4-26 Relationship of celestial and ecliptic poles ........................................................ 4-27 Origin of precession ........................................................................................... 4-27 Free gyro............................................................................................................. 4-28 Two degree gyro................................................................................................. 4-29 Rate gyro............................................................................................................. 4-31 Rate gyro accelerometer operation..................................................................... 4-32 Rate gyro accelerometer principle of operation ................................................. 4-32 Three step process to calculate survey from rate gyro ....................................... 4-33 Rate gyro survey axes......................................................................................... 4-34 List of Tables Table 4-1 Table 4-2 Page Common relative values of total magnetic field strength ........................................ 4-5 Common relative values for dip angle..................................................................... 4-5 Provisory - 04 Dec 96 Confidential Directional Drilling 4-iii Explain what influences the amount of non-magnetic material needed in a directional BHA. These fluids are subjected to internal circulation currents similar to phenomena observed at the periphery of the sun. It is designed to give the DD an appreciation of the various telemetry systems used in different MWD tools. Both single shot and multishot instruments are described. 4.Surveying 4 Surveying About this chapter This chapter describes the various survey methods used in the oilfield. A magnetic field results from the electrical currents generated by the relative motion between the liquid core and the mantle. Africa) where the Anadrill DD is trained to run both single shot and multishot gyro surveys. An explanation is given of how the signal is transmitted to surface in each case. An introduction to MWD tools is included in this chapter. The conclusion that there is a liquid portion of the core is compatible with available data (Figure 4-1). There are various types of survey instruments available. 2. The internal circulation of these fluids acts as the source of the Earth’s magnetic field according to the principle of a self excited dynamo (Figure 4-2).1 Magnetic & Non-Magnetic Requirements There are several theories to explain the Earth’s magnetic field: Theory #1: Rotation of the Earth’s solid exterior relative to its liquid iron core is believed to induce a slow rotation of the core. Magnetism and non-magnetic requirements are discussed. There are parts of the world (e.1 Magnetic Fields Provisory . The DD must familiarize himself with each type. a good introduction to gyros is given in this chapter. magnetic survey instruments are covered. Maintenance of the survey instrument is a necessary task for the DD. As a logical progression from this. While MWD tools are in wide use today. Explain the principle behind gyro surveys. Objectives of this Chapter On completing this chapter the directional driller should be able to do the following exercises: 1. Theory #2: Similar to theory #1. However. 4.1. The center portion of the Earth is largely composed of iron and has the mechanical properties of a fluid.04 Dec 96 Confidential Directional Drilling 4-1 . Gyroscopic surveys are necessary in certain situations.g. It is not possible to cover all the gyro procedures in this manual. W. every DD must know how to take magnetic single shot surveys. The transitory field generated outside the Earth.1 Aspects of the transitory field The transitory field is responsible for the following variations of the magnetic field.dynamo theory The total magnetic field is the sum of two fields of different origins: • • The principal field which originates within the fluid nucleus of the Earth.04 Dec 96 Confidential Directional Drilling 4-2 . Magnetic storms which may reach several hundreds of gammas .rotation of liquid core Figure 4-2 Earth's magnetic field . • • • • Secular variations of approximately 15 gammas per year .1. The cyclical "Eleven Years" variation .a major effect.a minor effect.a minor effect. This field is caused by the rotation of the Earth relative to the Sun and by the cycles of the Sun’s activity.Surveying Core Figure 4-1 Earth’s magnetic field . Provisory .1. 4.a minor effect. Diurnal solar variation on the order of 30 to 40 gammas per day . In Alaska and some parts of the North Sea. Solar flare particles reach the Earth in approximately two days.Surveying The Earth’s own magnetic field extends out to approximately 8 times the radius of the planet. 10 5 5 10 Figure 4-3 Earth’s magnetic field These particles collide with the Earth’s magnetic field at a speed of 640 miles per second and are slowed down at the shock front to 400 miles per second. The Main Phase (MP) produces a drop in the magnetic field strength due to an opposing field generated by the energized particles in the magnetosphere. Beyond this prevails the Magneto Pause. The shock wave preceding the cloud of plasma from the solar flare compresses the magnetosphere and rapidly intensifies the geomagnetic field at ground level (Figure 4-4). the Earth’s magnetosphere is compressed by high energy particles from the solar wind (Figure 4-3). a region in space where the Earth’s magnetic field contacts the Solar Wind. Solar Wind 15 10 5 5 10 15 20 25 il Magneto Ta Solar Wind Figure 4-4 Fluctuation's in the earth's magnetic field Provisory . It is followed by the Initial Phase (IP) which lasts from 30 minutes to a few hours. This compression takes place over a few minutes and is called the Sudden Storm Commencement (SSC). this has serious effects. On its sunward side. however. This is normally not a problem for locations in the Gulf of Mexico and at lower latitudes.04 Dec 96 Confidential Directional Drilling 4-3 . Variations in the solar wind produce changes in the Earth’s magnetic field. Some useful conversions: • • • • • • 1 gamma 1 micro tesla 1 tesla 1 gauss 1 gauss 1 gauss = = = = = = 1 nano tesla 1000 gammas 109 gammas 105 gammas 10-4 tesla 1 oersted The magnetic field intensity recorded at ground level is of a much smaller magnitude than that prevailing around the Earth’s core.G.Surveying 4. the field strength reaches 800. At the periphery of the core (approximately 3500 kilometers outward from the center of the Earth). The total magnetic field intensity is the vector sum of its horizontal component and its vertical component (Figure 4-5). The C. 57. Extreme total field values at the surface which you are unlikely to see range from 63. Electromagnetic Units are used for measuring the strength of the Earth’s magnetic field and are called Gammas.6) = 9392 gammas Gulf of Mexico: 50.000 gammas.000 gammas close to the North Pole to 27.04 Dec 96 Confidential Directional Drilling 4-4 . The vertical component of the magnetic field points toward the ground and therefore contributes nothing to the determination of the direction of magnetic north. magnetic field strength or total field.1.000 gammas near the equator (on the east coast of Brazil).510 gammas x cos (80. HFH. X Z Y Horizontal Component of Magnetic Field Strength Figure 4-5 In Alaska: Magnetic field strength. The horizontal component can be computed from the following equation: Magnetic Field Strength (HFH) x cos (Magnetic Dip Angle) = Horizontal Component Definition of Dip Angle can be found in Figure 4-6.2 Magnetic field strength The total magnetic field strength may be referred to as the H value.S.7) = 25.250 gammas Provisory .450 gammas x cos (59. Uncertainties induced by temporal variations in the magnetic field. Errors from the tool electronics.Surveying MWD instruments measure the three components of the magnetic field vector H. The expected value can be obtained from a previous acceptable survey. Magnetic North Magnetic Dip Angle Figure 4-6 Table 4-2 Gulf of Mexico 59 degrees Magnetic dip angle Common relative values for dip angle. East Canada 70 degrees Beaufort Sea 84 degrees North Sea 70 degrees Provisory . These are due to local anomalies and are called "dip holes". (On the latest M1 specifications.04 Dec 96 Confidential Directional Drilling 4-5 .1. the value has been reduced to 500 gammas).500 gammas North Sea 50. Uncertainty in the measured value of the magnetic field. 4. Temperature sensitivity of the magnetometers. Differences observed between the measured HFH value and the value derived from Geomag may be due to the following factors: • • • • • Uncertainties in drill string magnetism.000 gammas Acceptance limit is ± 660 gammas between the expected value and the measured value.000 gammas Gulf of Mexico 50. This is not to be confused with repeatability or change from one survey to the next. Extreme values which you are unlikely to see for dip angle range from 90 degrees close to the North Pole to almost zero degrees at the equator (see Figure 4-7).000 gammas Beaufort Sea 58. This is also the angle formed between the total magnetic field vector (HFH) and the horizontal vector. There are also several other points on the Earth's surface where the dip is equal to 90 degrees.3 Magnetic Dip angle The magnetic dip angle is equal to the angle between the tangent to the Earth's surface and the magnetic field vector (Figure 4-6). from the "Geomag" program or from the Anadrill district office. Table 4-1 Common relative values of total magnetic field strength East Canada 54. 0. and the less the dip angle. then the direction of the Earth’s magnetic field relative to true north can be calculated.1. Angles of declination to the west of geographic north are negative and magnetic declinations to the east of geographic north are positive. This is dependent upon the location (both in latitude and longitude) and can vary in areas of high magnetic activity (such as Alaska).75 for horizontal holes and +/.) 4.Surveying NORTH POLE or TOTAL magnetic field vector DIP = 0° Equator Tangent at the Equator Angle formed with magnetic vector is equal to 0° NORTH POLE or TOTAL magnetic field vector Tangent at the North Pole DIP = 90° Equator Figure 4-7 Magnetic dip angles at poles and equator The acceptance limit is +/. For example.0. 5° west can be written as -5° and 5° east can be written as +5°. the higher the horizontal component. All magnetic surveys require a conversion to geographic direction by adding or subtracting this angle (Figure 4-8). This is not to be confused with repeatability or change from one survey to the next. The angle between magnetic north and geographic north (true north) is defined as the magnetic declination or the angle of declination (Figure 4-8). Also remember. the value is +/.04 Dec 96 Confidential Directional Drilling 4-6 .0.50 in other cases. If magnetic declination is known. Magnetic declination can vary and the total magnetic field strength may vary greatly during extreme sun spot activity. This dipole does not correspond with the Earth’s rotational axis.75 degrees between the normal expected value and the measured value. Provisory . (On the latest M1 specifications. the closer to the equator: • • • the lower the total field strength.4 Magnetic Declination Angle The Earth can be thought of as having a magnetic dipole running through its center with north and south poles at either end. A magnetic "hot spot" in the drill collar Fluctuation in the Earth’s magnetic field. Even if the components of a drilling assembly have been demagnetized after inspection. There are certain instances where a gyro survey may need to be used if the well requires steering out of casing or if a possible collision exists with another well. the steel section of the drill string will become magnetized by the presence of the Earth’s field (Figure 4-9). All surveying instruments using magnetometers will be affected in accuracy by any magnetic interference.1. There are also cases where magnetic interference may be corrected or at least taken into account until a different BHA is used. Certain formations (iron pyrite. which can include interference from: • • • • • A fish left in the hole. External magnetic interference can occur as the drill string moves away from the casing shoe or from the casing window.5 Magnetic Interference There are two types of magnetic interference: • • Drill string magnetic interference. 4. Nearby casing. Any deviation from the expected magnetic field value can indicate magnetic interference. External magnetic interference.04 Dec 96 Confidential Directional Drilling 4-7 . hematite and possibly hematite mud).1.Surveying TRUE NORTH MAGNETIC NORTH ANGLE OF DECLINATION Figure 4-8 Magnetic declination angle 4. gyroscopic (gyro) measurements will have to be used.1 Drill String Magnetic Interference The drill string can be compared to a long slender magnet with its lower end comprising one of the magnetic poles.5. Provisory . It can also occur as another cased hole is approached. In such a case. Correction programs for magnetism of the drill string exist. This may happen as the angle builds from vertical (Figure 4-10) or as the azimuth moves away from a north/south axis (Figure 4-11). The discussion for Drill String Magnetic Interference uses the Slim 1 example throughout.04 Dec 96 Confidential Directional Drilling 4-8 . This would be the X axis for all other Anadrill tools. DRILL STRING MAGNETISM Magnetic Flux Lines Magnetic Flux Lines Z Y X Earth’s Magnetic Flux MAG Z Y X INC Figure 4-9 Drill string magnetism Drill string magnetism can be a source of error in calculations made from the supplied magnetometer data. Anadrill uses the well known Shell correction technique. Horizontal component of Z axis error smaller with no inclination Z Z Y X Y M Z Y IN X C AG X Horizontal component of Z axis error larger with increased angle.Surveying Note in Figures 4-9 through 4-12 that the Z axis reference is for the Slim 1 tool. MAG Z Y X INC Figure 4-10 Effect of hole angle on drillstring magnetic interference Provisory . Also. changing the composition of the BHA between runs may change the effects of the drill string. the error from a magnetized drill string is relatively greater than that experienced in lower latitudes (Figure 4-12). Z Y X MAG Z Y X INC Horizontal component of Z axis error largest in the East-West direction.) Provisory . This is why experience has shown that the magnetic survey accuracy worsens as the hole angle increases (especially with drill string magnetic interference). Y X Figure 4-11 Effect of azimuth on drillstring magnetic interference It is because of drill string magnetism that non-magnetic drill collars are needed. When drill string magnetism is causing an error on the Z-axis magnetometer. only the horizontal component of that error can interfere with the measurement of the Earth’s magnetic field (see Magnetic Field Strength section). The total H value should remain constant regardless of the tool face orientation or depth as long as the hole inclination. is affected. normally creating a greater magnetic field effect along this axis. The horizontal component of the Zaxis error is equal to the Z-axis error multiplied by the sine of the hole deviation.20% in the Gulf of Mexico. (Some gyros derive true north from the Earth’s rotation. Thus.53% error in Alaska compared to only 0. Since this is in effect one long dipole magnet with its flux lines parallel to the drill string. The increased value of the Z-axis due to drill string magnetism will normally cause all calculated azimuths to lie closer to north. The magnitude of this error is dependent on the pole strength of the magnetized drill string components and their distance from the MWD tool. Non-magnetic drill collars are used to position the compass or D&I package out of the magnetic influence of the drill string. 0. This error will show up when a gyro is run in the well. Since the horizontal component of the Earth’s magnetic field is smaller on the Alaskan Slope. The error will normally appear in the calculated survey as an increased total HFH value (higher total field strength than the Earth alone). The magnetometers are measuring the resultant vector of the Earth’s magnetic field and the drill string. azimuth and BHA remain relatively constant.04 Dec 96 Confidential Directional Drilling 4-9 . This increase is due to the larger value of the Z-axis magnetometer. only the Z-axis of the magnetometer package (Z-axis is usually the axis of the surveying tool). a 50 gammas error has a larger effect on a smaller horizontal component. All MWD surveys will be positioned (magnetically) north of the gyro survey stations.Surveying MAG Z Y X INC Z Horizontal component of Z axis error smallest in the North-South direction. A steel stabilizer may be satisfactory on the Equator. there is a very slight ridge at the point where the two bores come together. but not as far north as Alaska.400 GAMMAS 25. When a collar has been bored from both ends. This is detrimental to the accuracy of the survey.2 Minimizing Errors Drillstring magnetic interference at different latitudes One way to minimize the error caused by the drill string is to eliminate as much of the magnetism as possible. whether magnetic or not.04 Dec 96 Confidential Directional Drilling 4-10 . Provisory .5. This is done by isolating the magnetometer package with as many non-magnetic drill collars as possible. do not space exactly in the center of a nonmagnetic collar. never space within 2 feet of a connection. this effect can be removed by trepanning the collar bore. however. Usually.Surveying DRILL STRING MAGNETISM Horizontal Component of Drill String Magnetic Field M Z Y IN X C AG Z Y X Horizontal Component of Earth's Magnetic Field in Alaska Horizontal Component of Earth's Magnetic Field in Gulf of Mexico 50 GAMMAS 9. In Alaska all stabilizers used in the BHA are non-magnetic. Obviously the presence of a steel stabilizer or steel component between two non-magnetic collars results on a pinching of the lines of force (Figure 4-13). The length of the non-magnetic collars implies a uniform and non-interrupted non-magnetic environment.1.200 GAMMAS Figure 4-12 4. This becomes magnetically hot due to the cyclic rotation stresses to which the collar is subjected during rotary drilling. since a conventional steel stabilizer located between two non-magnetic collars results in an interfering field which may reach 250 gammas. Therefore. This. Additionally. As much as 40% of azimuth error has been seen due to this effect. Each connection in a drill string. is magnetic due to the effects of the mechanical torque of the pin in the box. This mechanical stress causes the local metal around the connection change its magnetic properties and can actually cause a survey azimuth reading error in the tens of degrees in some cases. is not true in practice. Note that with 120 feet of non-magnetic material above the magnetometer package the effects of drill string magnetism in places like ALASKA may still be seen. • • • The further away from the Equator (in latitude). The larger the hole inclination. Anadrill has performed jobs in Alaska with as much as much as 165 feet of non-magnetic material (Motor .45 feet M1 collar . azimuth and BHA remain fairly constant.Surveying Non Magnetic C ollar Steel Stabilizer Non Magnetic Collar Non Magnetic C ollar Length of Non Magnetic Collars implies a uniform. These are also examples in which the azimuth accuracy will likely decrease. hard metal facing and matrix used on stabilizers can be very magnetic. Remember: • If magnetic interference is encountered from the drill string. the total H value should remain constant regardless of tool face orientation or depth as long as the hole inclination.04 Dec 96 Confidential Non Magnetic C ollar Directional Drilling 4-11 . non-interrupted non-magnetic environment.90 feet Monel).30 feet Monel . The following are circumstances where more non-magnetic drill collars are necessary to counter drill string magnetism effects. The further away from a north/south hole azimuth. Figure 4-13 Magnetic lines of force in the drillstring Even non-magnetic stabilizers are actually magnetic near the blades. In fact. Never space inside a non-magnetic stabilizer. At a minimum. Provisory . (The total H value will also vary when the D&I package is close to casing joints. • • Do not mistakenly interpret change in total H value as a failed magnetometer sensor. Best results can be achieved by using a combination of Monel collar and one of the magnetic connection algorithms such as developed by Shell. H Earth H measured Figure 4-14 Effect of magnetic hot spot in MWD collar Provisory . Therefore.Surveying • The horizontal component of the Z-axis error is equal to: – • • [(Z-axis error) x sin(drift)]. Hot spot All 3 axis measurements are affected. Normal Earth Magnetic Field Hy Y Hx Hz X Z Hot spot H total Magnetic Hot Spot Rotating With MWD Collar On thisdrawing hot spot is perfectly aligned with X axis.04 Dec 96 Confidential Directional Drilling 4-12 . This is why magnetic survey accuracy declines as hole angle increases (especially with drill string magnetic interference). the total magnetic field will vary. Calculated azimuth will be wrong but will be repeatable with the same tool face. in places such as Alaska. it may be due to a tool face dependency. Remember that drill string interference is more pronounced in areas of high dip angle.). but will be repeatable when the BHA is placed in the same orientation (Figure 4-14). Fluctuation in total field is observed when MWD tool is rotated. Remember. our total H value will change with varying tool face settings.1. all three axis of the D&I package will be affected.5. It may be caused by magnetic interference. If a hot spot occurs on a non-magnetic collar. 4. Do not mistakenly interpret a change in a survey with a failed magnetometer or inclinometer. total field strength can routinely vary by 100 gammas.3 External Magnetic Interference When magnetic interference from external sources is encountered (such as from a fish in the hole or from nearby casing). the change may not be problem as long as the operator and directional driller are aware of the change and take it into account. due to magnetic interference from the motor. continue adding lengths of non-magnetic drill collars both above and below the MWD collar until the AE value is below 0.1.4 D&l Package Spacing In order to avoid magnetic interference.5 degrees. but these charts should not be used for reliable answers. It may even be necessary to use a non-magnetic orienting sub in some areas of the world.5 degrees. Other formulas exist for D&I spacing but this is probably the most accurate. a non-magnetic short drill collar (of 10 to 15 feet) should be placed between the motor and D&I package. These charts were valid at the time because most wells were kicked off to less than 10 degrees of inclination and often without a mud motor (whipstock.1.Surveying 4. For horizontal drilling. This formula is relatively easy to use and interpret.6 Earth’s Gravitational Field Newton’s Law of Gravitation: Every particle of matter in the universe attracts every other particle with a force which is directly proportional to the product of the masses and inversely proportional to the square of the distance between them.5. If it is not. Some operators may prefer to drill with a predicted error of one degree during the build up phase of the well and then correct for it later. it may be impractical to achieve a predicted azimuth error of less than 0. A simple way would be to resurvey the corrected path with a different spacing or a different BHA. anytime a mud motor is run. As a rule of thumb.04 Dec 96 Confidential Directional Drilling 4-13 . and especially for well paths with a medium radius of curvature. Provisory . 4. empirical charts were used to estimate the length of non-magnetic material needed. non-magnetic drill collars must be used. Figure 4-15 NMDC requirements. In the past. jetting).5 degree. If a mud motor is used to correct the well azimuth (on a slant hole) and a change in the magnetic field is observed. Gravitational force is a function of distance from the center of the bodies in question (Figure 4-16). The formula in Figure 4-15 can be used to accurately predict errors in azimuth due to magnetic interference from the drilling assembly. The absolute value of the predicted azimuth error (AE) should be less than 0. Experiments have shown that mud motors produce a magnetic field from 3 to 10 times greater than components such as steel stabilizers and short drill collars. The empirical charts are still useful to obtain a rough estimate of the non-magnetic material needed in a particular area. Surveying The gravitational field (G) is primarily a function of: • • • Latitude (main factor). The G value changes from 0. The rate of change is approximately 0.001.000 on the surface. Some of the changes in the measured value of G over the Earth are attributed to the Earth’s rotation. if the G value was exactly 1.04 Dec 96 Confidential Directional Drilling 4-14 .997 at 0 degree latitude (Equator) to approximately 1.999 at 20000 feet. Provisory .003 at 90 degree latitude (a 0. it would be 0.0005 per 10000 feet.000 feet to see 0. Regional fluctuations in the density of the Earth’s crust are practically negligible. (This can be observed when surveying with “time option"). the equatorial radius is larger than the polar radius.006 change ). These can be attributed to: • • • • Temperature sensitivity. Errors due to electronic circuitry. In other words. Therefore. Shifts in the sensor operating parameters which occur when the inclinometer is exposed to the shocks and vibrations of the drilling environment. A decrease in G can also be seen with increasing hole depth. You would have to be at 20. The rotation has given the Earth a slightly flattened shape. Errors due to bad axis alignment. Earth's Gravitational Field Mass = m GmM e g= r2 g G = Universal Gravitational Constant r = radius between centers Mass of Earth = Me Figure 4-16 Deviation of Universal Gravitation Constant Other reasons for discrepancies in the measured G value are due to instrumentation errors in the inclinometer. Depth/Altitude: referenced to mean sea level (MSL) Regional fluctuations in the density of the Earth’s crust. 2.2. The battery tube may have a snubber for use with top landing running gear. These four elements are assembled together and usually inserted into a carefully spaced protective barrel (running gear) before being lowered or dropped.1 Survey Instruments The inclination and azimuth of the well bore at specific depths can be determined by one type of survey called the "single shot survey". The protective casing can be thermally insulated for wells where the downhole temperature exceeds the tolerance of the photographic film used. Provisory . The orientation of deflection tools. 4. 4. while “multiple shot" surveys are used to record several individual readings at required depth intervals. 4.2 Magnetic Single Shot The magnetic single shot instrument is used to simultaneously record the magnetic direction of the course of an uncased well bore and its inclination from vertical.2 Timer or Sensor The timing device is used to operate the camera at a predetermined time. to bottom.Surveying 4. 4. The location of possible dog legs or excessive hole curvatures.1.2. causing lost time while the survey is re-run.2. Care should be taken to identify the correct polarity prior to loading batteries into the battery tube.04 Dec 96 Confidential Directional Drilling 4-15 . The surveyor must estimate the time it will take for the instrument to fall to bottom whether lowered on wire line or dropped (go deviled). Wireline steering tools give continuous survey readings while drilling. The instruments consist of four basic units: • a power pack or battery tube • a timing device or sensor • a camera unit • a compass .2. 4.1 Magnetic Surveys Magnetic survey instruments must be run inside non magnetic drill collars or open hole.2 Magnetic Single Shots & Multishots Directional surveying permits • • • • The determination of bottom hole location relative to the surface location or another reference system. It is also used to determine the toolface of a deflection device when deviating the well. Failure to do so can lead to a "mis-run" survey.2.2.inclinometer unit. inside the drill-pipe.1 Power pack The size and number of batteries required varies with the instrument as does their polarity. The monitoring of the azimuth and inclination during the drilling process. the motion sensor will detect the loss of movement and fire the camera resulting in a mis-run. In the past. mechanical timers have been considered more robust. Provisory . or non-magnetic collar sensor. 4. The benefit of the timer is that it can be used when dropping or "go deviling" the survey. A "Monel". From a floating rig. particularly at shallow depths. more commonly. timing devices are being replaced with electronic sensors which detect either the lack of movement as with a motion sensor. Problems arise when using either type of timer which are not necessarily due to instrument malfunction. If the time delay expires before the instrument has seated inside the non-magnetic drill collar. as well as unnecessary risk of stuck pipe resulting from not moving the drill string. Most Monel sensors must be in a non-magnetic environment for a set time. the usual solution to this problem is for the operator to overestimate the time required. and anticipate problems with wire-line units or other surface equipment. the presence of non magnetic materials. before activating the camera unit. may affect a motion sensor. usually from thirty seconds to one minute before firing the camera unit. is not subject to these limitations. This serves to ensure that the instrument is actually seated in the non-magnetic collar and allows the compass card and inclinometer in the angle unit to settle before the picture is taken. For Magnetic single shot surveys taken on wireline.2.3 Camera The magnetic single shot camera has three main components: • the film disk seat • the lens assembly • the lamp assembly. the resulting survey will be invalid.Surveying The timers available today are either mechanical. if the descent of the survey instrument is interrupted for any reason below surface. the operator knows exactly when the lights will come on and can minimize the length of time that the pipe is still.04 Dec 96 Confidential Directional Drilling 4-16 . or electronic. Electronic timers allow the operator to preset the time delay on the instrument. or. The motion sensor detects when all motion has stopped for a given time (usually about thirty seconds). It senses the change in the surrounding magnetic field as it enters the non magnetic drill collar. as a safety factor. The motion sensor is to some extent mechanical: it employs a movable element to detect motion and this may stick or lose sensitivity again resulting in a mis-run. This system has several drawbacks. as with a "Monel" sensor. The most common problem results from timer miscalculation. Timers and sensors should always be surface tested before use. With modem solid state electronics this is no longer true and mechanical timers are now rarely used. This then results in time lost waiting for the timer to expire with the instrument in place.2. often to the nearest second before loading it into the running gear. a wireline problem for example. although less accurate than the electronic timers. the downhole movement of the drill pipe imparted by the heave of the ocean. Since it is quite difficult to accurately predict the time involved in lowering the instrument to bottom. affected by motion and magnetic interference from the drill string. "just to be safe". 04 Dec 96 Confidential Directional Drilling 4-17 . This is normally identified by a stamp on the angle unit itself. the exposure of the film is controlled instead by the timing of the light illumination. The survey disk is read as correct. In most instruments.compass This type of angle unit utilizes a compass ball floating in fluid. used not to measure inclination. to depict the direction as it should be on the high side. and the compass measures the direction or azimuth of the well. A detailed description of the survey output and operating procedure will be discussed in chapter five. and using the same principle. Scale inclinometer . Care should be taken to establish the correct method of determining gravity toolface. such as a mud motor. rather the compass ball tilts and rotates beneath it.4 Angle unit. it is very sensitive to low inclinations and is often used to survey vertical holes such as those drilled for conductor pipe where absolute verticality can be critical. but to identify the “low side" of the hole with a small metal ball enabling the gravity tool-face of a deflection tool. This type of angle unit is normally used for higher inclinations ( above twenty degrees).compass Similar in principle to the pendulum cross-hair. weighted floats or air bubbles. They may measure inclination only. Depending on the manufacturer. Provisory . Just as simple. before using the single shot for downhole orientation. The more commonly used angle units fall into three basic categories: Cross-hair pendulum . These are the simplest but least used inclinometers as they apply only to special cases. Floating ball inclinometer. they may use pendulums. These devices are nominally designed for a specific application and vary in design and principle. the bubble inclinometer. this angle unit has an independent weighted inclinometer which appears as a scale superimposed onto the compass card on the survey photo disc.compass One of the most common types of angle unit for inclination and direction up to twenty degrees. the single shot camera unit has no shutter mechanism. Perhaps the simplest inclinometer is one which is used for measuring very low inclinations. The inclinometer measures the inclination of the well bore. The ball is inscribed with both azimuth and inclination.Surveying Unlike normal cameras. to be measured in an environment where magnetic interference precludes the use of conventional angle units. We will try to discuss most of the commonly used angle units in this section. Because the inclination and azimuth are not read independently. Care should be taken when interpreting gravity tool face using this type of angle unit. 4. a combination of inclination and direction. The cross hair sight is centered in the instrument and does not move. The compass card is free to rotate inside the housing and maintain a reference to magnetic north. The inclinometer is an independent and free swinging pendulum cross-hair.2.2. Somewhat like a round carpenter’s level. which naturally falls to the low side. The compass card is printed in reverse in order for the pendulum. high side (for use with mud motors). is the "low ball" type inclinometer. gravity toolface is interpreted either “as read" or is reversed. the lens assembly is prefocussed and no field adjustments are necessary. the angle units must be manufactured Geographically specific for the area or zone in which they will be used. Compass This is the measurement device. some are unreadable due to pipe movement. In some types of tools.3. the common interval between surveys is equivalent to the length of a stand of drill pipe (90 ft). one survey per minute would be acceptable. of course. can match individual shots with given depths.3. longer periods between shots can extend the running time of the instrument and allow a full survey in one run. Provisory .Surveying 4.04 Dec 96 Confidential Directional Drilling 4-18 . the film spools fit into separate cartridge-type magazines which can be preloaded and interchanged outside the darkroom without fear of exposure. 4. and in normal applications. Because the multi-shot takes continual surveys. or a portable developer bag (often supplied with the tool) prior to development.2.ratchet type. a guide spool which passes the film across the focus of the camera lens. This interval is commonly in the one to three shots per minute range.1 The multi-shot timer Depending on the manufacturer.2. in most cases. and the compass units are usually interchangeable. The drive mechanisms are usually simple worm-drive devices or solenoid plunger . As the instrument is dropped or "go deviled" inside the drill pipe. is adequate. or reciprocated for long periods. some tools allow the operator to specify the interval between shots. 4. The operator.2. when developed shows as a series of shots spaced along it. Because of this. The photographic film is. but do not differ much in principal. and the surveys taken when the pipe is placed in the slips on tripping put of the hole.2 The multi-shot camera These also vary with manufacturer. The capacity of the Multi-shot to store data depends upon the amount of photographic film that can be stored in the camera unit. and calculate the survey using this data. The battery tube is often lengthened in order to accommodate a greater number of batteries. by carefully recording bit-depth against time. which is loaded by the operator and installed in the tool. The running gear used is normally the same for both types of survey. Basically the camera consists of a film magazine spool.3 Magnetic Multi-shot Survey Instrument The Magnetic Multishot survey tool differs from the single shot tool in that the timer is programmed to take a series of readings separated by a preset time interval. light sensitive and must be handled either in a darkroom. The other feature of the multi-shot camera is the drive mechanism which turns the film spools in synchronization with the exposure-timer. In the case where the pipe is pulled extremely slowly. and where the hole depth dictates a lengthy trip out of the hole. and the take-up spool which stores the exposed film. The valid surveys are found at the points where the pipe was set in the slips for a connection and the compass was still. and the camera unit is designed to take a series of recordings instead of just one as in the single shot. The film. while others are fixed. 2 The Gyroscope A Gyroscope is basically a balanced. Provide a significant enhancement in survey accuracy. Rate-Gyro Systems Other terms used in the industry to describe rate-gyro systems include: inertial navigation. 2. single / multi-shot. its background. it would stay vertical for as long as the motor ran. which is free to rotate on one or more axes.1 Historical Background The industry began developing what is now most commonly referred to as “rate-gyro surveying systems" in the late 1970's. MWD. As long as the top spins fast enough.1 Principles of Gyroscopic Surveying 4. If the top were propelled by a spin motor at a particular speed designated by its mass. There is no attempt to compare systems or provide expert technical description of any company’s technology.Surveying 4. The intent is rather to provide a basic understanding of gyro technology. in a few instances with funding from the major oil companies. which was adapted from the navigation system in the Harrier Jump Jet. gyrocompassing. it attempts to hold its vertical orientation. The first system developed applying modem aerospace techniques was the Ferranti FINDS tool. magnetic-based and free-gyro systems. Provisory . Wellbore survey technology can be classified into four groups. 4. north-referencing and continuous guidance.04 Dec 96 Confidential Directional Drilling 4-19 . It had been found that the existing surveying methods. as follows: 1.3. dip-meter) 3. Free-Gyro Systems (film-based/electronic) 4. Now five companies offer rate-gyro service in various areas of the world. spinning mass. could not provide a reliable means of quality assurance for the level of accuracy wanted by the industry .1. north-seeking.3. steering tools. Inclination Only Device (Totco) 2.approximately 1% of hole depth. These are: • • • • • Gyrodata Schlumberger Baker Hughes Inteq Sperry Sun Scientific Drilling 4. The basic operation of a gyroscope can be compared to a spinning top. Magnetic-Based (film-based / electronic. The goal of the overall development was to adapt modern aerospace guidance techniques for oil industry applications with the following objectives: 1. Provide a means of quality assurance. and place among other surveying methods. that is.3.3 Gyroscopes This section discusses gyroscopic wellbore surveying services available today in the oil industry beginning with basic gyroscopic theory and leading up to Rate Gyro technology. if no external forces acted on it. goals. like all gyros. How a gyro reacts to external force is a major topic in this discussion. or drift. in order to understand the forces working upon them.04 Dec 96 Confidential Directional Drilling 4-20 . for a perfect gyro cannot be built. off its orientation. The classic example of a natural occurring gyroscope is the planet Earth-a spinning mass attempting to hold a particular orientation in space established long ago. Fortunately. we will look at simplified gyroscopes initially. It reacts to external forces with some movement. the drift is very small. the same kind used in the oil industry listed in category 3 above. The term "resistant to external forces" is important. Figure 4-17 shows a simplified gyroscope within its housing in a typical well surveying configuration. The forces of the spinning Earth-Gyro will also become important to this discussion. And. The next step in basic gyro understanding is the two-degree-of-freedom gyroscope. that will not be acted upon by external force and react by movement.Surveying This is the simple basis of all gyroscopes used in navigation. nor is the Earth a perfect one. Spin Rotor Gimballing System Gimbal Angular Pick-off Torquer Gyro Case Resolver Figure 4-17 Simplified diagram of a typical gyroscope Provisory . a spinning mass which through its momentum becomes resistant to external forces and attempts to maintain an orientation like the top in space. The frames supporting the gyroscope. and allowing this freedom of rotation are referred to as Gimbals. Free-gyros have been used in wellbore surveying since the 1930’s. Because gyroscopes can be extremely complicated. as shown in Figure 4-19.04 Dec 96 Confidential Directional Drilling 4-21 . the spinning mass can attempt to maintain its original orientation no matter how the base moves. As the probe moves downhole through different directions and inclinations. the gimballing allows the gyro to attempt to maintain a horizontal orientation in space. Note that a compass card is aligned with the horizontal spin axis of the gyro. so throughout the survey the spin axis attempts to hold its surface orientation.Surveying Figure 4-18 shows a more realistic view of the configuration of an actual gyroscope. Survey data is collected downhole by affixing a plumb-bob assembly over the compass . the gyro is pointed in a known direction prior to running in the well. In performing a wellbore survey. The gyroscopes shown in Figure 4-19 and Figure 4-21 are two-degree-of-freedom gyros. INNER GIMBAL MERCURY SWITCH INNER GIMBAL ASSEMBLY OUTER GIMBAL CAM CAGING ROD BEARING INNER GIMBAL ASSEMBLY OUTER GIMBAL TORQUER JOURNAL OUTER GIMBAL ASSEMBLY Figure 4-18 Realistic view of the configuration of a typical gyroscope The gimbals isolate the gyro from the base so. Provisory . resulting in readings of wellbore azimuth and inclination.04 Dec 96 Confidential Directional Drilling 4-22 . The plumb-bob always.) Figure 4-19 Gyro rotation around outer gimbal axis OUTER GIMBAL AXIS INNER GIMBAL AXIS SPIN AXIS Figure 4-20 Gyro rotation around inner gimbal axis Provisory . surface read-out free-gyro systems which eliminate the plumb-bob. it points out the inclination of the well on the concentric rings and the azimuth by correlation with the known direction of the gyro spin axis established at surface. When the tool is inclined off vertical. points down toward the Earth’s center. (Note: There are also electronic. as a pendulum.Surveying At each survey station a picture is taken of the plumb-bob direction with respect to the compass card. 3. and between the outer gimbal and the inner gimbal. the major components of the gyro are comprised of: • • • The Spin Motor. namely: 4. a Torquer which enables compensation for certain types of errors and processing the gyro at desired rates. the spin axis may have: • An elastic restraint (rate gyro or gyrometer which measures the input angular velocity). The Gyro Case which is the outer enclosure. For the sake of simplicity.1 Use • • • Instrument gyros such as artificial horizons and gyro compass are used for measuring and indicating purposes.3. The Gimbal suspension.3. which includes: the ball bearings (or gimbal bearings) between the gyro-case and the outer gimbal. 4.3. Control gyros are used to generate signals. the rotor bearings holding the spinning rotor in the inner gimbal. In a single degree of freedom gyro.1 Components A gyroscope is a spinning wheel whose spin axis can move relative to some reference mount. the spin axis is stabilized against rotation around the gimbal axis but is disturbed by rotations about the quadrature axis. Provisory .3 Classification of Gyroscopes Gyroscopes are usually classified according to various characteristics. Two degree of freedom gyro in which the rotor spin axis can move with respect to the case around two axes in an uncontrolled manner (Figure 4-22).3.3.2. The gimballing system isolates the spinning rotor from the gyro-case: – – – • • • • If the gyro-case turns around the outer gimbal axis (Figure 4-19). If the gyro-case turns around the inner gimbal axis (Figure 4-20).2 Construction and Function. The Gimballing System which is the structure carrying the spin motor. the main characteristic of which is "angular momentum". Two major types: Single degree of freedom gyro which requires only one coordinate axis to locate the SPIN axis with reference to the instrument mount (Figure 4-21) .Surveying 4.3.3. an Angular Pick-off which senses relative angular displacements between the gyro gimbal and the case. 4. Stabilizing gyros are used to generate torques for stabilizing purposes.3 Restraints on the movement of the spin axis In a Single degree of freedom gyro.04 Dec 96 Confidential Directional Drilling 4-23 . 4. INNER GIMBAL AXIS In a Two degree of freedom gyro.04 Dec 96 Confidential Directional Drilling 4-24 . No restraint (integrating gyro). except for unavoidable frictional restraints. the spin axis may be: SPIN AXIS Base Plate Figure 4-21 Single degree of freedom gyro OUTER GIMBAL AXIS INNER GIMBAL AXIS SPIN AXIS Figure 4-22 Two degree of freedom gyro Provisory . Completely free. Supplied with torquers for correction or measurement purposes.Surveying • • • • A viscous restraint (rate integrating gyro which measures the input angular displacement). These surveys supply accurate readings when carefully operated by an experienced surveyor.3.3. Sensitive axes of the rate integrating gyro and the accelerometer scan components of the earth’s rotation and earth’s gravity. Accelerometers instead of Angle-Units are used to measure hole inclination. Four gimbal gyroscopes comprised of a small conventional directional gyro mounted on a pair of gimbals in such a way that the outer case can be moved to any position without disturbing the position of the directional gyro. Problems with battery powered mechanical cameras are eliminated and survey data is supplied in real time. tool face and monitors probe temperature. A wire line supplies power and connects the probe with a surface computer that monitors probe performance and prints survey data as it is gathered. 4. gyroscopes react to external force by movement or drift off their orientation. Survey data is read by a downhole electronics package and transmitted to the surface computer via a single conductor wireline. The surface computer can monitor probe performance. The computer calculates azimuth. A particular aspect of all gyros is that they react 90 degrees with respect to the applied torque. North Seeking Gyroscopes. comprised of a two degree of freedom gyro and an accelerometer. The gyro must maintain this heading throughout the survey. However the system still relies on conventional two degrees of freedom gyros for directional data. Downhole a small camera regulated by a timer and powered by a battery pack takes pictures of the plumb bob superimposed on the gyro compass card. Continuous guidance tools. With this type of gyro the inclination is given by a plumb bob located inside an Angle-Unit and a camera records the survey data. The second generation provides progress in the recording of survey data. Reliable directional date depends on two things: • • The gyro must be accurately aligned to some known direction before being run down hole. Provisory . therefore time wasted by mis-runs is reduced. 4.Surveying 4.4.5 Forces acting upon Gyroscopes As mentioned earlier. inclination.04 Dec 96 Confidential Directional Drilling 4-25 .1 The Surface Read out Gyro. comprised of a rate integrating gyroscope and an accelerometer. The system requires no surface orientation and is not subject to such problems as gimbal lock and gyro tumbling sometimes encountered with conventional gyros. • • • A down-hole electronics package replaces camera angle-unit and timer.4 Evolution of Gyroscopes used in surveying oil-wells The First Generation of gyro survey instruments used a conventional two degree of freedom gyro to set a directional reference point.3. It is designed to eliminate gimbal error corrections and to survey the true hole direction at any slant angle from one degree to horizontal. Spin axis of gyro is secured in the horizontal position after being aligned to NORTH. Conversely. the gyro will rotates (precess) about an axis in quadrature to both the spin vector and the torque vector. in a two degree of freedom gyro. Provisory . the gyro will rotate about the inner gimbal axis.Surveying In the case of a free-gyro survey system. Therefore.5. Once gyro reaches maximum speed. During a free-gyro survey. Caging is achieved by an electromagnet which turns the outer and inner gimbal ring through special guides always to a certain position in relation to the outer case axis. Figure 4-23 Representation of nutation In contrast to precessional motion. attempts are made to monitor drift and correct for it.3.04 Dec 96 Confidential Directional Drilling 4-26 . If the spin vector tries to move into the torque vector.5. bearing wear and the one inescapable force Earth rotation. nutation would persist indefinitely.1 Precession If we exert a torque on the inner gimbal. It is a self sustaining oscillation which physically represents a transfer of energy from one degree of freedom to another and back again. The rotation of a gyro spin axis. In reality. 4. the gyro will begin to rotate around the outer gimbal axis. In a frictionless system. nutation needs no external torques to sustain it. inner gimbal axis and outer gimbal axis are mutually orthogonal. Caging is used to maintain spin rotor axis stationary while speed is increasing from 0 to maximum. in response to an applied torque is called the Precession. gimbal bearing friction serves to damp out nutation. forces causing the gyro to drift off its surface orientation lead to azimuth error.2 Nutation Nutation (Figure 4-23) is a wobbling of the rotor spin axis. Caging locks the rotor to the case so that the spin axis.3. 4. Nutation is more important at low spin rotor speeds. if torque is applied about the outer gimbal axis. nutation is nil. Typical causes for drift include system shocks. to the influence of the moon. to the influence of the sun on to the swollen periphery of the earth at the equator. With respect to the Ecliptic Pole. and. the plane of the earth's equator is inclined 23º27' to the plane of earth's orbit around the sun. the celestial pole of the Earth (Figure 4-24) travels a circle whose radius makes an angle of 23º27' with the ecliptic pole. to a lesser degree. As a result. primarily.3.Surveying 4. (Figure 4-25) Ecliptic Pole Celestial Pole 23° 27' Ecliptic Equator Celestial Equator 23° 27' Figure 4-24 Relationship of celestial and ecliptic poles Moon Ecliptic Pole North Pole 23° 27' B Ecliptic Plane A Equ ator South Pole Figure 4-25 Origin of precession Provisory .5.3 Fundamental Precession The precession of the earth in a slow and circular movement of the earth’s axis of rotation around the poles of the ecliptic due.04 Dec 96 Confidential Directional Drilling 4-27 . OUTER GIMBAL AXIS INNER GIMBAL AXIS SPIN AXIS Figure 4-26 4. mechanical stops are utilized to restrict angular motion about the inner gimbal axis. This precession is slow requiring a period of 25. rather than referenced only to earth’s gravity or North. However. a free gyro indicates the Amount of Input motion rather than a Rate of motion. Hence. the gyro would lose one degree of freedom. By application of the principle of gyroscopic inertia.04 Dec 96 Confidential Directional Drilling 4-28 . A two degree of freedom gyro or Free Gyro (Figure 4-26) is so named because the Spin axis may be set to any desired direction.5. Should outer gimbal axis and spin rotor axis become parallel.3.3. To prevent gimbal lock. thereby acting to erect the earth’s axis.800 years to complete a single cycle. As opposed to a single degree of freedom gyro. a torque is generated which attempts to pull the earth’s equator into the plane of the ecliptic.Surveying 4. Because the earth spins.5° per minute. a free gyro tends to maintain the orientation of the spin axis fixed in space.5 Gimbal Lock Free gyro In a two degree of freedom gyro in which the spin rotor axis is supported by gimbals. because the distance between A and the moon is a little shorter than between point B and the moon. A typical free gyro drift rate is . a free gyro is a short term device which functions accurately for less than five minutes because of high drift rates.4 Origin of Precession The moon exerts a slightly greater gravitational attraction on the earth’s point A than on point B. the spin rotor cannot arrive at a position parallel to the outer gimbal axis.5. Provisory . it reacts to the torque by precessing. the outer gimbal turns through 180°. is called tumbling and results in the loss of the orientation reference. axis. Directional gyros are used to establish an arbitrary reference in a horizontal plane. Provisory . Therefore.04 Dec 96 Confidential Directional Drilling 4-29 . the gyro is then pointed to some reference direction and will be uncaged only after the desired initial orientation is secured. In this caged position.6 Tumbling The use of "stops" to prevent gimbal lock may cause a problem. inner gimbal axis and outer gimbal axis are mutually orthogonal.7 Caging The Caging mechanism locks the rotor assembly to the case so that spin. So.6 Directional Gyro A directional gyro (Figure 4-27) is a two degree of freedom gyro which has its spin axis set in a horizontal plane and which is used to measure angular motions around the Vertical. 4.3.5. In a free gyro . about its gimbal axis. the ‘O' of the compass card points to the reference direction. Gyro Card Outer Gimbal Inner Gimbal Torque Direction of Torque Precession Figure 4-27 Two degree gyro The three most common operating modes of a directional gyro are: Free Directional Gyro Mode A free directional gyro does not have torquers and is not slaved to any sensing device. This outer gimbal axis movement. careful attention must be exercised for selection of the spin axis reference. Its spin axis. This mode is used for navigation in polar regions where magnetic headings are likely to be erroneous.5. the Reference is not reset once the gyro is uncaged.Surveying 4. Thereafter. any deviation of the compass card from the initial setting is referred to as drift. resulting in a gyro whose spin axis is stationary with respect to the earth .3. acts as an inertial reference rather than an earth reference. 4. Latitude Corrected Mode permits the gyro to be precessed by an amount sufficient to cancel out the effect of earth's rotation. accordingly. when the gyro is uncaged. When the inner gimbal strikes one of the inner gimbal axis stops.3. the rotation of-the earth would indicate an apparent 360 turn of the axis in 24 hours.2 Temperature Effect Warming of the gyro can cause slight dislocations of the center of gravity due to the varying expansion coefficients of the different materials. such as copper and steel. 4. If. An adjustable weight in the form of a screw is attached to the inner gimbal ring and has the effect of a vertical power on the gyro axis. 4. The gyro axis is thereby returned to a horizontal position.3. 2. This arrangement keeps the gyro spin axis aligned with the magnetic meridian. this force turns the outer gimbal ring.3. or an apparent drift of 15º per hour. Due to the phenomenon of precession.1 Apparent Drift The apparent drift of a gyro is caused by the influence of the earth rotation. the inner gimbal suspension is equipped with a Mercury Switch (or an Electrolytic level) which is operated at even the slightest deviation of the gyro axis from the horizontal and gives a corresponding impulse to a small motor (torque motor ) mounted on the vertical axis.3. 4. In this gyro mode. By adjustment of the screw. either to the right or to the left. to the effect that the gyro turns simultaneously with the rotation of the earth. Gimbal errors occur when the angular motions of gimbals do not correspond to the actual motion occurring about their reference axes. it can be set to offset the apparent drift at any geographic latitude by an identical counter acting force. in order to avoid a tilting of the gyro due to unbalance and or the effect of friction.7.04 Dec 96 Confidential Directional Drilling 4-30 . The screw is set for the particular latitude where the gyro is used. A gravity sensing device (Erector) to maintain a spin axis horizontal to the earth. 4. so that its axis of rotation would be at right angles to the earth axis.Surveying Slaved Mode uses an external reference such as: 1. the same would be observed but in reversed direction. the gyro axis would be parallel to the earth axis and the gyro would not show any apparent drift.7 Level Rotor Gyro The level rotor gyro is a gyro in which the spin axis is maintained level in a plane parallel to a tangent to the earth’s surface.3 Intercardinal Tilt Error or Gimbal Error The gimballing error encountered in a directional gyro is also known as intercardinal tilt error. A flux gate transmitter to provide a continuous azimuth torquing signal.7. for instance. At the Equator. At the South Pole. The apparent drift caused by the rotation of the earth is corrected by applying a special force to the inner gimbal ring. This motor then turns the outer frame by a very minute amount.3. Provisory . a perfectly balanced gyro were located at the North Pole in a horizontal position. Possible errors caused by rising temperature are compensated by a piece of bimetal which is mounted on the inner gimbal frame and offsets sufficiently the unbalance caused by temperature through a bending effect. depending on which side of the switch is operated.7. such as a Rate Gyro. the whole angular deflection is transmitted to the spin rotor axis.8 Rate Gyroscopes When the gyro case moves around the gyro spin axis. when the gyro case moves around the input axis. its output measures relative motion between gimbals. Potentiometer Take-off Output Axis Torsion Spring Gyro Rotor Gimbal Can Insrument Case Spin Axis Figure 4-28 Rate gyro Provisory . the accelerometer assembly measures the force of gravity making it point toward the Earth’s center. again. Unlike the free-gyro.Surveying When a gimbal axis transducer is used. However. Figure 4-28 provides a simplistic illustration of one rate-gyro and accelerometer configuration in a survey tool. In order to minimize such errors.04 Dec 96 Confidential Directional Drilling 4-31 . When the gyro case moves around the output axis. their operation can be most easily described through comparison. and. The combined readings of the accelerometer and rate-gyro at a survey station allow calculation of wellbore azimuth and inclination. be positioned in a plane parallel to the overall well direction anticipated. As long as the gyro case is caused to rotate. and the pivot-point for the accelerometer pendulum. Although rate-gyros and accelerometers collect survey data in a completely different manner than the free gyro. the universal joints act as the gimballing mechanism for the gyro. the rate-gyro system is one that measures the forces acting on the gyro. eventually. Conversely. unlike the plumb-bob. there is no direct transmission of angular deflection to the gimbal assembly. the spin rotor axis will precess around the output axis until the torque vector direction and the spin rotor axis are co-incident. The gimbal error depends upon borehole inclination and the hole direction related to the reference direction. In this case. the gyroscopic torque is proportional to the angular rate of gyro rotation. the spin rotor axis should. so as to result in a difference as little as possible between: 4. It is this property which is used to construct a one degree of freedom gyro.3. The rate of precession is: • • directly proportional to the applied torque: inversely proportional to the angular momentum. there is no angular deflection transmitted to the spin axis. The rotation of the case around the input axis will then result in a very strong torque applied to the spin axis. when the surface orientation is carried out. which is not necessarily the actual angular motion of the base. Although the assembly can be said to have a pendulum like the plumb-bob. Pick-off coils measure movement of the magnetized mass (M) and send a signal to the torque coils. Torquer Gyro Rotor Figure 4-29 Rate gyro accelerometer operation The amperage signal.3. As shown.1 Accelerometer Operation Figure 4-29 provides a more detailed illustration of accelerometer operation. Torquer Accelerometer Outer Case Spin Motor Spin Axis Bearings Universal Joint Pick-off. Accelerometer case Pivot Pendulum Permanent magnet Torque coils Pick . in this instance the pendulum is forced to maintain its case orientation.04 Dec 96 Confidential Directional Drilling 4-32 . the accelerometer calculates wellbore inclination at a survey station. This is illustrated in Figure 4-30. the force then varies with inclination until reading zero when the pendulum is vertical. Since the tool-accelerometer axis is aligned with the wellbore axis. When the pendulum (M) is horizontal. measured by the accelerometer when the probe is in a well allows calculation of wellbore inclination. Universal Joint Pick-off. or force.8. the force component of gravity is 1 g.offs M Sensitive axis Figure 4-30 Rate gyro accelerometer principle of operation Provisory .Surveying 4. which apply an equal and opposing force to keep the pendulum aligned. When the tool is placed in the test stand. The sensitive axes planes in Figure 4-31 relate to the same ones shown in Figure 4-32. Calculating a survey point can be seen as a three step process utilizing the combined readings of the rate-gyro and accelerometer. then.Surveying 4.04 Dec 96 Confidential Directional Drilling 4-33 .2 Rate-Gyro Operation The rate-gyro. Combining the true north reading from the gyro. then. once the accelerometer measures gravity to calculate wellbore inclination. as shown in Figure 4-31. the rate-gyro reading of the component of earth spin-rate will correspond to a particular true north reference as modeled in the test stand. so that the wellbore/tool axis is coming out of the page. the values for the Earth rate and true north vectors vary with latitude. meanwhile. Just as the force components of gravity vary to resolve wellbore inclination. for a given latitude the system can also calculate the true-north force component (TN) due the relationship of the vectors. inclination and the direction of the tool. Since the rate-gyro instead measures the Earth-rate force.3. which is aligned with the tool axis. with latitude and inclination known. Similar to the accelerometer. one of the forces acting on it is the spinning Earth force. pick-off and torque coils measure the forces acting on the gyro and keep it aligned with the case. When the tool is stopped at a survey station. as with the accelerometer. measures the Earth spin-rate vector. the rate-gyro tool is calibrated in a highly precise test stand at the service company’s facility. Accelerometer Gyro Y Combined TN X Y HS TN HS Y AZ X G X Inclination HS Toolface True North Hole Azimuth Figure 4-31 Three step process to calculate survey from rate gyro Provisory .8. the way the rate-gyro accomplishes this can be compared to the accelerometer’s operation. When the tool is at a survey station in a wellbore. is at each survey station to calculate the true north direction with respect to the wellbore azimuth. Although more complex. Prior to a survey. Once the gyro is set spinning and becomes free in space. the spin force causes the gyro to move or drift (gyro precession) off the surface orientation as mentioned earlier. In the case of the free-gyro system. As illustrated. provides wellbore azimuth as the angle between true north and high-side. tool high-side is also known. when the rate-gyro is pointed in different directions in the stand it measures varying component values for earth spin-rate on its sensitive axes. it is turned in a range of directions while its measurements of Earth forces are modeled with respect to a known reference. The purpose of the rate-gyro. discussed below. Since the gyroscopic torque is proportional to the rate at which the gyro case is rotated. Depth is derived from wireline measurement and the system can perform singleor multi-shot surveys. the spin rotor axis will precess until the restraining spring torque is equal to the gyroscopic torque developed as a reaction to the rotation of the gyro case. Single degree of freedom gyros can be classified depending upon the type of restraint (spring) between gyro case and gimbals. In a one degree of freedom gyro. During a multi-shot the tool is stopped at periodic stations and a mathematical formula is applied for the overall survey calculation. Under these conditions. A Rate Gyro is a single degree of freedom Gyro in which the precession is limited and controlled by a restraining spring attached between the gyro case and the gimbal suspension (Figure 4-28). the gimbal assembly is afforded to move only a few degrees (2 or 3 degrees) . Provisory .on either side of a ’0’ reference point.Surveying High Side Hole AZ X1 Horizontal Plane TN = True North Y2 AZ High side Toolface Horizontal Plane Y1 Y2 I X1 I I Gyro accelerometer Sensitive axis plane Wellbore Ho Sp le a in/ xis too la x is ER TN Figure 4-32 Systems of the type described require an electric wireline and provide real-time data at surface.04 Dec 96 Confidential Gravity Rate gyro survey axes Directional Drilling 4-34 . There are three basic types. the angle through which the gyro axis precesses is directly a measure of the angular rate of gyro case rotation. As in aerospace. In this case. a torsion bar.9.). 4. They vary in size. almost always.9. more force measuring as explained above. this system measures its three-dimensional movement through space (or down the well) to calculate a survey. A set of Balancing Nuts is often included in the shaft supporting the rotor can assembly so as to balance the can and rotor after sealing. The Ferranti system is the only full navigation system as it is adapted from aerospace technology. these systems also calculate the changes in wellbore azimuth and inclination by measuring the forces acting on rate-gyros and accelerometers to determine changes in tool direction. adequate sensibility.9. via the angular displacement. there are several different types of rate-gyro surveying systems available.3. Provisory .9 Other Rate Gyro Systems As mentioned previously. In a Rate gyro.3. 4. locking means. configuration.2 Torsion Bar Rate Gyro In such a gyro. The rate integrating gyro. One end of the main shaft is anchored to the outer instrument and the other end is supported in a bearing mounted in the outer casing.3. the system measures the change in direction of the platform and the distance it moves. provides counter torque to stop rotation. The Ferranti tool is the only tool which does not use wireline measurement for depth. There are other rate-gyro tools available which perform a survey while moving but use the wireline for depth measurement. uses the floated rotor construction. These balancing nuts must have adequate capacity. (Essentially.1 Rate Gyro A Rate Gyro measures the rate of precession (precession velocity). The Rate Gyro uses a spring restraint on the output axis.3 Rate Integrating Gyro In most particulars. Stops located about 3 degrees from the zero position minimize cross coupling errors caused by abnormal input rates about the spin axis when excessive precession occurs. The rate integrating gyro is customarily built so that very little angular motion takes place about the precession axis.04 Dec 96 Confidential Directional Drilling 4-35 .3. the rate integrating gyro is constructed in the same manner as the rate gyro. In the conventional use as a sensor. The angle of twist which measures the input velocity is picked up by a pick-off device. 4. nor more than ± 2° in floated rate integrating gyroscopes. The torsion bar twists and by so doing. The only opposition to motion of the gimbal can is that of the fluid viscosity. diameter and utilizes three gyros and accelerometers mounted on a stabilized platform. A precessional angular velocity input around the input axis causes a rotation of the shaft around the output axis.Surveying 4. namely a necked down section on the main shaft is used to provide elastic (or spring) restraint. It has a 10 5/8 in. the precession angle motion rarely exceeds 3° for non-floated rate integrating gyro. and mode of operation. both the inertial torque and the frictional torque are negligible compared with the string torque. Modern aerospace guidance techniques employing rate-gyros and accelerometers provide the only current means of both providing this range of survey accuracy and qualifying the information.05 degrees. northern latitudes.4 Rate Integrating Gyro Use In a rate integrating gyro. the gyro. the input and spin axis can easily become misaligned. For this reason the rate integrating gyro is mostly used as error sensor and nulling device. The strap down system implies the use of wide angle gyros with an angular freedom of ± 10 degrees. 4. by simple drifting.000 feet of hole. gyro drift and tool misalignment. quality control and why Rate Gyros? To achieve a high range of accuracy as stated earlier and.1 degree that’s resolution . let's say the accuracy goal is one foot per 1. These systems can accomplish this through extensive quality control procedures because rate-gyros and accelerometers can be calibrated for a level of performance and monitored and checked for data quality. no film-based survey device has an opportunity to achieve this level of accuracy with assurance because the film cannot be read to the accuracy required. This means that in a 10. 4.04 Dec 96 Confidential Directional Drilling 4-36 . the operator is to be assured of bottom-hole location by plus or minus 10 feet.6 Accuracy. although the technology has seen much improvement. devise a means of assuring it-is a significant. the terms accuracy and resolution of readings are confused. is fixed to the instrument case. the rate integrating gyro responds to an angular input displacement with an angular output displacement. there is also no automatic centering device. since there is no spring. be used for angular measurement. For simplicity's sake. for instance. In the case of magnetics.but providing that level of precision is a completely different matter. Very often. the Rate Integrating Gyro measures the integral of angular velocity (angular displacement) via an output angular displacement. In fact.3. A survey system may be able to read survey data to 0.3.Surveying The viscous restraint can be provided by the mechanical action of the flotation liquid.1 and 0.3.000 feet requires azimuth and inclination accuracies in the range of 0. Basically. Therefore. The free-gyro's major error sources are surface orientation.9. In the strap down system the coordinate system is not stabilized with respect to inertial space but. declination corrections. Although other survey technologies (magnetic and free-gyro) may achieve this range of accuracy some percentage of the time. and expensive task.9.000 foot wellbore survey. error variables such as magnetic interference. instead of being stabilized in space by gimbals is mounted directly to the instrument case. To get in the range of one foot/1.9. The cross coupling error becomes prohibitive and the instrument can. 4. instead. no longer. furthermore. difficult. in reducing the damping gap to a few tenths of millimeters. Provisory . respectively. even Sun spot activity pose difficult quality control problems. The output displacement is then a measure of the input displacement. they have no available means of quality control to assure it. The reason for the name Integrating Gyro becomes apparent when one considers that: • • the Rate Gyro measures Angular velocity via an angular displacement.5 Strap Down System With the Strap Down System. Coupling Errors Derivation of the equation for Rate Gyros depends upon mutual perpendicularity between the three significant axes.7 Errors in Rate Gyros Gyros may be susceptible to various errors. Horizontal acceleration applied .Surveying However. the accuracy of available systems varies. Anisoelastic Errors Iso-elasticity implies the equal elastic suspension of the rotor in all directions. for example. the displacement of the center of gravity of the rotor will be the same amount and in line with the applied force regardless of the direction of the force. Alignment and hysteresis errors Initial alignment can be mechanically adjusted so that the error in reading is no more than one per cent This residual error can be nulled out by the use of compensating voltages. and running procedures can also degrade survey quality. for instance. Some systems degrade. For example. Cylindrical Errors Are due to the rotational motion of the input shaft resulting in a deflection of the gimbal structure. To be acceptable. If a force is applied to a rotor which is isoelastically supported. above 75 degrees of latitude and inclination because the Earth and gravity vectors become smaller and more difficult to resolve.9.04 Dec 96 Confidential Directional Drilling 4-37 . Conical errors are due to the geometry of the instrument and are inherent in the instrument. accurate readings degrade in the overall survey calculation. However. the mass center no longer lies along the bearing center line. the spin axis and input axis must become misaligned and some of the spin velocity becomes coupled with the input velocity to affect the output reading. When the instrument operates.the mass center moves horizontally Mass center and output axis no longer coincide. if a survey probe is misaligned in the well. a gyro should have an anisoelastic coefficient in the order of: 0. Reviewing a service company’s procedures for quality control and data verification is important to assigning a specification to a particular system.mass center and output axis are coincident. Because of this deflection. Rate-gyro and accelerometer quality also varies in its ability to achieve accuracy. Vertical acceleration applied . The error can be minimized by making the torsional spring constant large. Conical Errors Anisoelastic and cylindrical errors are due to frictional effects and imperfect mechanical properties. 4.3.05/hour/g2. Three possibilities: • • • No acceleration . Rate-gyro system accuracies can also vary according to inclination and latitude. because of mechanical hysteresis. Provisory .Anisoelastic coefficients are given as degrees/hour/g2. when a torsion bar does not return to its ’O’ original Transducer and torsion bars are misaligned and an alignment error results. some alignment error results during operation. The damping fluid is thus necessary to preclude the possibility of sustained oscillations. From the previous examples we know that torques can be generated around the output axis because of vibratory effects for instance.3.Surveying Drift Errors or drift rates represent the output reading which may be obtained when no input signal is applied. although designed mostly for accelerometers is also used in gyro instruments. So. The high speed of the rotor causes a severe wear problem so that after a relatively short period of operation. As noted previously. in turn. Here. If no damping were present. In spite of elaborate precautions. This unbalance. the output axis indicates the North direction. The function of the bearing is thus reduced to alignment and centering. The instrument must operate with a damping factor of 0. a single degree of freedom gyro requires a restraint of some sort. Ideally. the friction does not cause an operating problem but rather one of longevity.10 Gyroscope suspension Gyros currently use two types of suspension: • • Ball Bearings Flotation Ball Bearings used in gyroscopes are about the finest that can be made. The North Seeking instrument has an input axis which is constrained to sweep out the horizontal plane in order to determine the East/West direction. Aside from providing buoyancy.7. the flotation fluid also serves a cushion for the gimbal and provides a viscous damping medium.11 North Seeking Gyros The North Seeking Gyroscopes are involved with the rotation of the earth. Bearings are assembled in an air conditioned room in which the air is lint-free. the instrument would have an extended oscillatory response.3. when the input axis is pointing to East or West (reading equal to zero). Flotation The flotation principle. 4. the buoyancy effect is adjusted so that the main shaft bearings are not required to support any axial load. The most critical bearing appears to be that supporting the gyro. As stated previously.5 to 0.04 Dec 96 Confidential Directional Drilling 4-38 . These torques will cause an output reading and the equivalent input turning rate is the Drift Error. Provisory . Some torques may be present even when the instrument is standing still. wear in the bearings causes dynamic unbalance. causes vibrations. the level of Coulombs friction and stiction (vibration generated by scraping effects) is high enough so that additional schemes are necessary to reduce the frictional effects further. tendency to drift and further wear. Flotation is realized by simply filling the instrument casing with a fluid so that the Gimbal Can is supported by fluid buoyancy. These are: • • • torques due to thermal convection currents electromagnetic torques stemming from the signal generators torques due to the flexible leads feeding the gyro motor 4. the earth rotation component is nil in the East/West direction. all of which are time variable.12 Drift Values Drift values may range as follows: • • • • 0. Error Torque such as gimbal error. friction. With the spin rotor axis slaved in vertical position. supported by electrical fields 4. for which adjustment or compensation may be applied. Drift and Drift Correction. Acceleration sensitive drift 4. temperature gradients. The manner of measuring and compensating for both drift and gimbal error are discussed elsewhere. The random drift is due to small uncertainty torques such as caused by bearing noise. For instance a gyroscope. Uncertainty Torque the random components of drift which bear no correlation with any inputs.04 Dec 96 Confidential Directional Drilling 4-39 . However most common gyroscopes do not maintain absolute immobility but drift from their initial fixed position.Surveying Prior to starting any measurement. it is prerequisite that the spin rotor axis be secured in Vertical Position.5° to 1° per minute for cheap gyros a few degrees per hour for directional gyros 1/100th degree per hour for inertial gyros using gimbal flotation 1/1000th degree per hour for some inertial gyros with spherical spinning rotors. a sweeping motor constrains the input axis to sweep out the horizontal plane. Random drift is generally determined by statistical analysis of a large number of drift tests. uncorrected for the rotation of the earth and viewed by an observer on earth.12. The torque which may cause drift may be separated into two main categories.torques due to the non orthogonality of the principal axes and wheel speed change.3.12. Provisory . 4. The drift due to error torques is of three types: • • Non acceleration sensitive drift – – Generally caused by elastic or magnetic torques.3.2Acceleration sensitive drift Other sources of systematic drift rate errors may be temperature sensitive torques due to differential expansion . The Drift Rate is the best and most important single figure of merit used to describe the performance of a gyroscope.1Nature and Source of Drift Apparent drift caused by the rotation of the earth. at a latitude of around 45° North appears to be drifting at the rate of about ten degrees per hour. Generally caused by mass unbalance.3. Gyroscopes are used because of their property to remain immobile in inertial space.