Method and apparatus for providing a magnetic direction reference

A magnetic observatory device is deployed at a fixed location such as the sea bed to provide data measuring variations in the local earth magnetic field. This data is used to correct information from magnetic direction sensors, for example in a measurement-while-drilling system of a drill string. Also described is a reference apparatus for initial calibration of the device or for marine magnetic mapping purposes.

This invention relates to methods and apparatus for use in establishing
 directional information by reference to the earth's magnetic field.
 The earth's magnetic field allows relatively simple and inexpensive methods
 to be used to establish direction with reference to north. As is well
 known, however, at any given point on the earth's surface there is a
 discrepancy ("variation") between magnetic north and true north, and the
 variation changes slowly over time. In addition, the variation at any
 given point is subject to considerable short term alterations including a
 noticeable diurnal pattern, and can be significantly affected by proximity
 to masses of magnetic material, whether natural or manmade. These factors
 limit the usefulness of magnetic techniques in providing accurate
 positional or directional information.
 The present invention is particularly, but not exclusively, concerned with
 the control of directional drilling of oil wells and the like. In the
 offshore oil industry, requirements for enhanced recovery and recovery
 from deep fields have led to an increasing requirement for long range
 directional drilling of high accuracy, but these techniques are limited by
 a lack of information as to the position and direction of the working end
 of the drill string.
 It is increasingly common to make use of measurement while drilling ("MWD")
 techniques in which instruments located immediately adjacent the drill bit
 transmit measured information to the surface by means such as mud pulse
 telemetry. It is possible to make use of magnetic sensors in MWD in an
 attempt to monitor the orientation of the drill bit with respect to true
 north, but the accuracy obtainable by magnetic measurement is not
 sufficient to provide the required degree of positional and directional
 accuracy.
 It is also known that a bore hole can be surveyed to a high degree of
 accuracy by means of mechanical or solid state gyros. These however cannot
 be used while drilling, and therefore a survey of this nature requires the
 drilling operation to be stopped while the bore hole is surveyed with a
 gyro instrument package inserted on a wireline through the drill string.
 One object of the present invention is to provide a method and apparatus
 which enables the accuracy of magnetic directional sensors to be
 substantially increased.
 The present invention, from one aspect, provides a method of providing a
 magnetic reference for use in determining direction, the method
 comprising:
 providing a magnetic observatory device which includes magnetometer means
 for detecting the direction of the earth's magnetic field and telemetry
 means for transmitting information derived from the magnetometer means to
 a remote location;
 positioning said magnetic observatory device at a fixed location relative
 to the surface of the earth;
 establishing the orientation of the magnetic observatory device with
 respect to true North;
 deriving from said information the instantaneous variation between true
 North and magnetic North in the vicinity of said location.
 From another aspect, the present invention provides a magnetic reference
 apparatus comprising magnetometer means responsive to the earth's magnetic
 field to provide output signals representative of the orientation of the
 apparatus with respect to the earth's magnetic field, and heading
 reference means providing a directional reference related to true North.
 A further aspect of the invention resides in a magnetic reference apparatus
 comprising magnetometer means responsive to the earth's magnetic field to
 provide output signals representative of the orientation of the apparatus
 with respect to the earth's magnetic field, and heading reference means
 providing a directional reference related to true North.
 In a particularly preferred form of the invention, the magnetic observatory
 device is deployed at a desired location, typically on the sea bed, and
 provides data for correcting the output of a magnetic MWD system, and the
 orientation of the magnetic observatory device with respect to true north
 is determined at the time of deployment by use of the magnetic reference
 apparatus.

Referring to FIG. 1, a magnetic observatory device has a housing 12 which
 may for example be formed by a length of stainless steel drill collar with
 suitable end caps. The device is powered by battery packs 14 and contains
 3-axis magnetometers 16 arranged to measure local magnetism on x, y and z
 axes. Suitable forms of magnetometer are well known in the art. The output
 signals from the magnetometers 16 are transmitted via an electronics
 module 18 and acoustic transducers 20 to a remote location.
 Referring to FIG. 2, in one application of the invention the device 10 is
 placed on the seabed 22 in the vicinity of a drilling rig 24. The device
 10 therefore supplies to the drilling rig 24 data defining the
 instantaneous characteristics of the earth's magnetic field in the
 locality. The data is suitably in the form of the declination and dip of
 the local magnetic field, which can readily be derived from the triaxial
 magnetometer signals by processing either in the device 10 or on the rig
 24. It may also be useful to derive the magnetic field strength.
 A drill string 26 extends from the rig 24 for directional drilling and is
 terminated in a bottom hole assembly 28 which includes a drill bit and an
 MWD apparatus. The MWD apparatus includes a magnetic sensor of known type
 which makes use of the earth's magnetic field to derive a directional
 signal, the signal being transmitted to the surface by mud pulse
 telemetry.
 It is therefore possible to use the data from the observatory device 10 to
 correct the directional signals from the bottom hole assembly 28 to
 provide accurate, substantially real-time, directional information and
 thus to define accurately the position of the bottom hole assembly 28 in
 three dimensions as drilling progresses.
 It will be appreciated that the observatory device 10 should be deployed on
 the seabed 22 in a position remote from localised magnetic anomalies, and
 in this connection it has been found that the large quantity of steelwork
 in a typical drilling platform or rig produces a surprisingly large local
 anomaly.
 The observatory device 10 will normally be deployed on the seabed by
 lowering it on a cable, or simply by dropping it, with the result that the
 orientation of the device 10 will be unknown. The apparatus used in FIG. 3
 may be used in an initial calibration step to establish accurately the
 orientation of the device 10 with regard to true north.
 FIG. 3 shows an apparatus in the form of a tower body 30 comprising a
 latticework tower 32 with a flotation spar 34 and a ballast keel 36. The
 top of the tower 32 mounts an accurate, stiff aluminium bar 38 at either
 end of which is mounted a GPS receiver 40. The GPS signals received by the
 receivers 40 are compared and the phase difference between the signals is
 measured to derive measurements of change of heading, pitch and roll, in a
 signal processing module 35. The module 35 may also receive heading
 information from an accurate gyro source 37. These parameters can
 typically be measured to accuracies of about 0.025 degrees and are not
 subject to drift.
 A 3-axis magnetometer arrangement indicated generally at 42 is positioned
 at the keel 36 of the apparatus for measuring the x, y and z components of
 the earth's magnetic field.
 The apparatus 30 is used as follows.
 The apparatus 30 is first calibrated on-shore at a magnetically clean site
 where multiple readings are taken of heading, pitch, roll and magnetic
 readings on each axis. The data collected for each axis will consist of
 the correct dot product of the earth's magnetic field vector with the
 attitude vector of the magnetic axis. If the earth's magnetic field is
 accurately known at that site, the misalignment of the magnetometer axis
 can be established with respect to the pitch, roll and heading axes.
 The apparatus 30 is then checked for magnetic interference by rotating the
 apparatus to see if any magnetic component is constant on any axis as the
 apparatus is turned.
 Any other earth magnetic vector can now be monitored with confidence as
 long as the apparatus 30 is not damaged and is held stationary. At sea
 this is not possible so a software program is used whereby the attitude
 sensors and the magnetic sensors are read by connecting them to a
 computer. The data all arrives at different times but an accurate
 satellite clock is used to time stamp all the data which is fed into a
 Kalman Filter to produce running curves to model the motion of the buoy
 with time. When a magnetic reading arrives, its exact age is known in
 milliseconds and can be entered into the Kalman Filter to "deskew" the
 times of the data, by interpolating the motion curves and bringing it to a
 single point in time.
 In a prototype, an accurate GPS Gyro was used along with an inertial motion
 reference unit which provided data up to 50 times per second, allowing
 accurate curves around the time of generation of the magnetic data. Having
 established the earth's magnetic filed accurately at the surface, this
 allows us to use the seabed system 10 which can subsequently monitor for
 changes to that field with time at that location. In practice, since the
 earth's magnetic field varies during the course of the day (the diurnal
 variations), it is useful to obtain data from the seabed system while
 using the surface system so that these diurnal variations from the day's
 average can be corrected for in the surface system data.
 It would also be possible in principle to use a 3-axis set of magnetometers
 with only a gyro reference, but the accuracy of such an arrangement
 suffers from time drift of the gyro and possible magnetic interference
 between the gyro equipment and the magnetometers.
 In the situation in FIG. 2, the reference apparatus of FIG. 3 can be
 deployed temporarily in the vicinity of the observatory device 10. The
 reference apparatus is used to derive instantaneous magnetic data and
 relate this to the direction of true north, and thus by matching the
 magnetic data from both devices the orientation of the observatory device
 10 with respect to true north can be determined. The relatively complex
 reference apparatus can then be removed for other uses.
 The reference apparatus of FIG. 3 may be used in other ways. In particular,
 it may be towed behind a suitable vessel for the purpose of conducting
 magnetic surveys in general, and in one particular use of this nature it
 may be used to establish very accurately the location of anomalies in the
 magnetic field caused, for example, by shipwrecks. The vector analysis
 determines the vector of addition to the earth's magnetic field, and by
 sailing over a wide area and monitoring the additional vector the centre
 of the magnetic influence can be pinpointed very quickly.
 For a general survey, the towing vessel may be equipped with a reference
 gyro and with a fan beam laser which cooperates with reflector prisms at a
 fixed location, such as an oil rig. A base line is established from the
 rig by theodolite survey, and the fan beam and gyro are used to steer the
 towing vessel on a predetermined route, suitably at about 5 degrees to the
 baseline.
 By providing the apparatus in the form of a buoy which may be towed behind
 a vessel, the effect of the magnetic structure of the towing vessel may be
 minimized.
 Where magnetic MWD is being used in directional drilling in a deep water
 field, it may not be convenient to deploy an observatory apparatus on the
 seabed. In this event, the apparatus of FIG. 3 may be moored in the
 vicinity to provide the reference information.
 Although described with particular reference to marine use, the present
 invention may also be used in terrestrial applications.
 Modifications may be made to the foregoing embodiments within the scope of
 the invention, as defined in the claims. For example, in the case of a
 seabed observatory device the acoustic telemetry system could be replaced
 by wireline transmission to a surface buoy and thence by radio.