EP0793000A2 - Méthode pour corriger les mesures de la direction - Google Patents

Méthode pour corriger les mesures de la direction Download PDF

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Publication number
EP0793000A2
EP0793000A2 EP96303391A EP96303391A EP0793000A2 EP 0793000 A2 EP0793000 A2 EP 0793000A2 EP 96303391 A EP96303391 A EP 96303391A EP 96303391 A EP96303391 A EP 96303391A EP 0793000 A2 EP0793000 A2 EP 0793000A2
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EP
European Patent Office
Prior art keywords
magnetic field
measurements
magnetic
components
parameters
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Granted
Application number
EP96303391A
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German (de)
English (en)
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EP0793000B1 (fr
EP0793000A3 (fr
Inventor
Michael Yuratich
Graham Mcelhinney
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Halliburton Energy Services Inc
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Halliburton Co
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Publication of EP0793000A3 publication Critical patent/EP0793000A3/fr
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    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
    • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B47/00—Survey of boreholes or wells
    • E21B47/02—Determining slope or direction
    • E21B47/022—Determining slope or direction of the borehole, e.g. using geomagnetism

Definitions

  • the present invention relates to the determination of the orientation of a borehole, using measurements of gravity and magnetic field.
  • the measurements used for determining the orientation of the borehole come from survey packages taken at different levels in the borehole.
  • the survey packages referred to in this invention refer to a combination of magnetometers and accelerometers which measure the earth's magnetic field vector and gravity vector respectively. These measurements can be used to determine the orientation of a survey package and hence of the equipment in which it is installed, the orientation being the azimuth (“Azi”) with respect to North, the inclination (“Inc”) with respect to the horizontal and the bodily rotation (“Rot”) about a specified axis of the package (see Figure 1).
  • Figure 3 depicts a typical arrangement of components in a drilling assembly, where the survey package is installed in a supposedly non-magnetic environment which distances it from the magnetic components above and below. While there are many other applications and circumstances of use, this application to drilling is the one with respect to which the present invention will be described in most detail.
  • the survey package When drilling, the survey package is installed in a special section of drilling pipe made from 'non-magnetic' material and known as a non-magnetic drill collar (NMDC).
  • NMDC non-magnetic drill collar
  • This environment near the drilling bit, is subject to much shock and vibration, and to winding and unwinding of the pipe arising from drilling torque.
  • the measured azimuth is adversely affected by the influence of magnetic material in proximity to the measurement instrument. Examples of this are the steel of the drill string, drill collars, stabilisers, motors, turbines and other magnetic materials (permanent and magnetically permeable).
  • Magnetically permeable material includes distortions in the earth's field.
  • the earth's field is itself subject to daily variations and magnetic storms and is often inaccurately known at the drilling location.
  • NMDC non-magnetic materials throughout the drilling assembly, so the major sources of magnetic interference will always be present.
  • the NMDC remains expensive and is a mechanically weak material compared to normally used steels, and so it is desirable to reduce its length, which in turn makes it necessary to correct the survey for increased amounts of magnetic disturbance.
  • NMDC non-magnetic drill collar
  • the major interference comes from magnetic materials essentially distant and on-axis (ie along the direction of the drill pipe).
  • US 4819336 and US 4999920 teach correction based on the assumption that interference is purely axial, ie they consider only the axial magnetometer reading to be affected. They ignore this reading and use the unmodified cross-axial (transverse) readings in conjunction with a priori knowledge of the earth's magnetic field's strength and/or dip.
  • EP 0,384,537 teaches how to obtain corrected basic accelerometer and magnetometer measurements from one survey level using constraints to adjust these values to meet a priori earth's magnetic field and dip data and the a priori magnitude of the earth's gravity vector. These corrected measurements are then processed in a separate step using standard formulae to obtain azimuth etc.
  • W0 94/16196 teaches how to compute a permanent magnetic interference correction for all three components of the measured magnetic field using surveys from two or three depth levels and a priori data on the earth's field.
  • the formulation balances an equal number of measurements with unknowns.
  • An ad hoc method is used to check the consistency of the results, the sine and cosine of the azimuth having first been estimated as if independent variables rather than different deterministic functions of one variable.
  • US 92/01804 teaches how to combine the outputs of several methods, some patented in their own right, in such a way as to give an azimuth which is claimed to be a good estimate.
  • the individual methods eliminate the axial magnetometer reading and differ principally in being considered better at various orientations relative to Magnetic North and inclination from the vertical.
  • Yet another object of the present invention is to reduce the number of required a priori parameters, total magnetic field and magnetic dip, when sufficient survey data is available.
  • a method for calculating the direction of a borehole comprising the steps of taking measurements of gravity and magnetic field, whereby to provide an estimated vector for the earth's gravitational field and magnetic field at the point of measurement, and
  • the technique has its principal application to survey packages run within a drilling pipe, but can be used in many other situations, such as survey packages used during wireline logging.
  • Magnetic surveying packages generally comprise a tri-axial accelerometer and a tri-axial magnetometer.
  • the accelerometers measure three independent components of the earth's gravity vector G plus disturbances
  • the magnetometers measure three independent components of the earth's magnetic field H plus disturbances. There are thus six independent readings at any time, and each such set of readings is generally termed 'a survey'.
  • Figure 1 shows the arrangement and notations for a survey package.
  • the local axes x, y, z form a right handed coordinate system with the z-axis pointing in the direction drilled and the x axis aligned within the drilling pipe to a position on the pipe known as the tool face.
  • the accelerometer and magnetometer axes are aligned along x, y and z.
  • the sensors are calibrated to produced a positive reading when the component of gravity or magnetic field measures points along the corresponding axis.
  • the sensor axes may not be precisely orthogonal or calibrated to read positively as stated above. By calibration it is possible to derive a matrix of coefficients which when applied to the readings converts them to equivalent orthogonal readings.
  • the drill pipe may have a slight sag or deliberately introduced bend, so that the z-axis of the package is not strictly aligned with the hole axis.
  • the method of the present invention is to compare the survey package measurements with those predicted by a model which takes into account known types of disturbance, and to adjust the parameters of the model in such a way as to reduce the residual differences to a minimum.
  • small residuals will show that the model with inferred parameters has indeed fitted the measurements well.
  • the sensitivity of the residuals to small changes in the inferred parameters indicates whether the data and model are well conditioned, ie that the results are trustworthy.
  • Figure 2 provides an example of azimuth corrected using the method disclosed herein, compared with the uncorrected azimuth and two different types of gyro surveys over a long section of hole (1,400 metres).
  • the hole inclination was 40-50 degrees.
  • Gyros have in the past been considered to be of high accuracy but it is now understood in the art that they are very susceptible to horizontal reference and misalignment errors for subterranean use.
  • the separation seen between the gyro curves is typical, as is their correct tracking of changes in hole azimuth.
  • the survey package correction method disclosed herein has both corrected the clearly wrong uncorrected azimuth, but equally importantly follows the details on the changes in hole direction. (The gyros were run in the hole after the magnetometers and after hole cleaning and reaming which smoothed the hole changes in direction.) This tracking of detail is not possible when only axial or simply cross-axial permanent magnetic interference is taken into account, as in much prior art.
  • the method disclosed herein is seen to be capable of making a valuable improvement in accuracy to the level where gyro and corrected surveys have to be scrutinised equally closely to determine the reasons for their differences.
  • the types of magnetometer disturbance to be modelled are those due to permanent magnetism, induced magnetism and electrical currents.
  • the total earth fields and the magnetic dip are subject to local differences from published tables, and to solar storms and regular variations, as is well known. It is therefore desirable to be able to treat these fields themselves as parameters to be determined, eg not to use suspect a priori values of magnetic dip and total magnetic field, although no particular model of their origin or variation is required.
  • the first term on the right hand side is the earth's magnetic field, which depends on the five parameters B, MDip, Azi, Inc, Rot, B and MDip.
  • the second term represents the net vector due to permanent magnetism and is independent of hole directions and the earth's field.
  • the third term is an approximation to the effects of induced magnetism. As this is due to the earth's magnetic field, it varies as the hole direction and tool rotation changes between surveys.
  • the final term is the effect of electrical currents.
  • the predicted measurement therefore depends on many parameters, collected as a list of arguments on the left hand side of the expression.
  • the permanent magnetic interference bias or perturbing vector
  • the other terms are important it can be seen that ignoring them will give rise to erroneous estimates of permanent magnetism.
  • NMDC non-magnetic drill-collar
  • Drilling mud may also contain magnetised particles, which constantly circulate through and around the drill pipe. These particles may show an affinity for the drill pipe, building up a coating. The combined effect over consecutive surveys is one of an essentially constant magnetisation.
  • Magnetically permeable materials also distort the earth's field directly by induced magnetism.
  • the amount of distortion seen at the magnetometer depends on the size, shape, orientation, relative position and permeability of these materials.
  • the magnetic field induced in a permeable ellipsoid can be expressed in terms of Legendre functions.
  • Prolate ellipsoids approximate the cylindrical shapes of the highly permeable ('magnetic') drill collars above and below the magnetometer.
  • the so-called non-magnetic drill collar in which the survey package is placed may in fact be weakly permeable, so may the package container itself, and the circulating mud may contain permeable materials as illustrated in Figure 5. The effect of these is to partially shield the survey package from the earth's field, as may be noticed in a workshop by the difference in readings seen between the survey package housed and unhoused in so-called non-magnetic collar.
  • the relative influence of the induction around the survey package and that from the distant but highly permeable materials depends on the particular construction of a bottom hole assembly. For example it is desirable to reduce the length of non-magnetic collar, as it is expensive, relatively weak in mechanical strength, and increases the distance between survey package and the drilling bit. By reducing this length the 'end' effects of permeable materials is increased, whereas with long non-magnetic collars the end effects are weak.
  • the preferred formulae for the induction effect depends on several parameters, an effective scale factor K p for the near-package induction, and an effective scale factor K s and effective direction vector s v for the end-effect induction:
  • a general induction correction will involve a 3 x 3 matrix of coefficients multiplying the earth's field. However this is unnecessary in practice and introduces many parameters to be found. The above preferred expression fits the circumstances well with fewer parameters.
  • the second part of this expression represents the distortion due to a distant permeable object lying in a direction v s , with a scale factor K p .
  • it results in correction along each axis which each depend on all the field components in be. If, as is often the case v s is found to be axial, then there is no such mixing of field components. In this case the total correction may be best understood by writing it in the form which shows clearly an induced differential correction of the axial and cross-axial field.
  • Survey packages are associated with electronic systems that require electrical power, and hence wires, generators and other components which carry electrical current.
  • the net constant or slowly varying current in any part of the system will generate a magnetic field which may be detectable by the magnetometer.
  • the modelled disturbance from any one such part of the system is the product of a direction vector v c scaled by K c and the current I, and for the entire system is the sum of such parts:
  • v c and K c may be determined by experiment by applying the currents in turn; it is important to note that v c is not the direction to the part of the system carrying the current, but the vector which describes the resultant magnetic field at the magnetometer. It can be seen that in the model either the currents are to be treated as parameters or to be measured and treated as a priori values.
  • the stored torque in the drill pipe may be released at the survey depth during the survey, in addition to changes in torque caused by reaction from the drilling assembly when mud is circulating.
  • the present invention allows for a model of accelerometer disturbances.
  • One such model is for a small but constant rate of turn and of longitudinal displacement. Then the x- and y- accelerometers will each see a centripetal component of acceleration and the z- axis accelerometer will see an offset.
  • the individual time sampled measurements can be treated as a separate survey with parameters for the rate of turn and longitudinal movement.
  • the main benefits of such a model may be obtained independently of the magnetic field model, ie improvements to the accelerometer readings can be obtained by means independent of the present invention or by the means disclosed in the present invention with terms relating to magnetometers struck out.
  • a minimisation method is preferred, in which parameters are chosen so as to minimise the difference between the actual measurements and the predicted measurements.
  • the means of measuring this difference is preferably that of the sum of the squares of the differences between each measurement and the predicted value.
  • the differences are individually scaled to make them comparable with each other.
  • the index j runs over the N different surveys used in the measurements and the index runs over the three measurement axes.
  • the noise standard deviations for the magnetometer and acceleromater scale factors ⁇ b and ⁇ g respectively are known levels of instrument noise.
  • Figure 6 illustrates the relationship of the parts of this expression, the parameters and the survey data. It shows how the survey measurements are compared to the predicted fields, the differences squared and summed to compute Fit, which is then passed to the minimiser. The latter iteratively makes a new estimate of the parameters, computes the expected fields in the absence of disturbances and uses these with the remaining parameters to update the predicted fields. The iterations are stopped when the Fit is judged to be good. While it is preferred to work in local coordinates, nothing in the method requires this choice, which could be a mix of coordinate systems consistently applied. Furthermore, the illustration of sources of interference can be refined in many ways as described previously without affecting the minimisation process.
  • the constraints include requiring
  • the present invention calculates the change in root mean square Fit that results from small changes in each parameter.
  • a large change in Fit indicates that the parameter is well resolved whereas a small change in Fit indicates that the choice of parameter is for example one of the induced magnetism coefficients, then this merely indicates that induced magnetism is unimportant for the given surveys.
  • the parameter concerned is one of the quantities Azi, Inc, Rot, B, MDip or G then the data is insufficient to provide reliable survey results.
  • the corrected hole direction at the last survey depth k say, necessarily can only be combined with shallower surveys k- 1, k- 2.... However, when the next survey k + 1 is taken, the hole direction at k can be reassessed using the data k + 1, k, k - 1...
  • This preferred method has the merit that it takes into account the later state of the survey system, and helps to confirm that nothing untoward has occurred. In general it is desirable to compute a corrected hole direction using surrounding surveys. In the expression for Fit given above, preferably only the parameter corresponding to the central survey should be used.
  • surveys are made at similar depths but with different bottom hole assemblies and hence different model parameters.
  • surveys from each run have been corrected satisfactorily by means of the invention as so far disclosed, or by other means if any, it is possible to effect a further but smoothed improvement in the hole direction estimate.
  • surveys from separate runs can be combined into one minimisation, where only those parameter than can sensibly be considered constant between trips are the same in the predicted fields for two runs. For example, Azi and Inc will be made the same and often, G, B and MDip.
  • Figures 7 - 11 illustrate various aspects of the invention obtained using actual survey data over a continuously drilled section of 1,400 metres.
  • the estimated values of B and MDip are compared to a priori reference data obtained from the well-known IGRF [expand name] model. The differences are less than the expected local variation for the geographical area concerned, and are essentially constant over the full drilled section.
  • Figure 9 shows the three components of permanent magnetic flux, showing firstly the larger axial component and secondly that the cross-axial components are not negligible.
  • Figure 10a shows the off-axis angle of the vector v s and K s in the end-effect induced magnetism correction, which as expected is small. All the magnetic parameters of the drilling assembly are essentially constant as anticipated in the formulation of the invention.
  • Figure 10b shows how the azimuth of the induced magnetism varies with depth, which as expected is reasonably constant as the magnetic parameters of the drilling assembly are essentially constant as anticipated.
  • Figure 11 shows the results of two runs with different drilling assemblies, corrected separately over an overlapping section of hole . While the corrected azimuths are very similar, the permanent magnetism and the uncorrected azimuths are markedly different.
  • Figure 12a shows how the induced magnetism inclination varies with the rotation of the drilling assembly
  • Figure 12b shows how the induced magnetism direction varies with the rotation of the of the drilling assembly.

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  • Physics & Mathematics (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
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EP19960303391 1995-05-15 1996-05-14 Méthode pour corriger les mesures de la direction Expired - Lifetime EP0793000B1 (fr)

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GB9509819 1995-05-15
GB9509819A GB2301438B (en) 1995-05-15 1995-05-15 Method for correcting directional surveys

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EP0793000A2 true EP0793000A2 (fr) 1997-09-03
EP0793000A3 EP0793000A3 (fr) 1998-02-04
EP0793000B1 EP0793000B1 (fr) 2001-10-04

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Cited By (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1999064720A1 (fr) * 1998-06-12 1999-12-16 Baker Hughes Incorporated Procede d'etalonnage de mesures magnetiques et d'estimation d'incertitude
US6508316B2 (en) 1998-05-14 2003-01-21 Baker Hughes Incorporated Apparatus to measure the earth's local gravity and magnetic field in conjunction with global positioning attitude determination
WO2005124102A1 (fr) * 2004-06-21 2005-12-29 Halliburton Energy Services, Inc. Levee de puits de forage
US7027926B2 (en) 2004-04-19 2006-04-11 Pathfinder Energy Services, Inc. Enhanced measurement of azimuthal dependence of subterranean parameters
US7103982B2 (en) 2004-11-09 2006-09-12 Pathfinder Energy Services, Inc. Determination of borehole azimuth and the azimuthal dependence of borehole parameters
US8195400B2 (en) 2009-05-08 2012-06-05 Smith International, Inc. Directional resistivity imaging using harmonic representations
US8271199B2 (en) 2009-12-31 2012-09-18 Smith International, Inc. Binning method for borehole imaging
WO2014092938A1 (fr) * 2012-12-10 2014-06-19 Schlumberger Canada Limited Fonction de pondération pour calcul d'inclinaison et d'azimut
EP2818632A2 (fr) 2013-06-25 2014-12-31 Gyrodata, Incorporated Techniques de positionnement dans des environnements à puits multiples
US8947094B2 (en) 2011-07-18 2015-02-03 Schlumber Technology Corporation At-bit magnetic ranging and surveying
US9297249B2 (en) 2011-06-29 2016-03-29 Graham A. McElhinney Method for improving wellbore survey accuracy and placement
EP2895888A4 (fr) * 2012-09-14 2016-06-15 Scient Drilling Int Procédé de détermination de variations locales du champ magnétique terrestre
CN106522924A (zh) * 2016-11-15 2017-03-22 北京恒泰万博石油技术股份有限公司 一种随钻测量中方位角的获取方法
US9658360B2 (en) 2010-12-03 2017-05-23 Schlumberger Technology Corporation High resolution LWD imaging
US11175431B2 (en) 2017-06-14 2021-11-16 Gyrodata, Incorporated Gyro-magnetic wellbore surveying
US11193363B2 (en) 2017-12-04 2021-12-07 Gyrodata, Incorporated Steering control of a drilling tool

Families Citing this family (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2317454B (en) * 1996-08-14 2001-03-07 Scient Drilling Int Method to determine local variations of the earth's magnetic field and location of the source thereof
US5960370A (en) * 1996-08-14 1999-09-28 Scientific Drilling International Method to determine local variations of the earth's magnetic field and location of the source thereof
GB9717975D0 (en) 1997-08-22 1997-10-29 Halliburton Energy Serv Inc A method of surveying a bore hole
US7681663B2 (en) * 2005-04-29 2010-03-23 Aps Technology, Inc. Methods and systems for determining angular orientation of a drill string
US8280638B2 (en) * 2009-02-19 2012-10-02 Baker Hughes Incorporated Multi-station analysis of magnetic surveys
US8600115B2 (en) 2010-06-10 2013-12-03 Schlumberger Technology Corporation Borehole image reconstruction using inversion and tool spatial sensitivity functions
US9982525B2 (en) 2011-12-12 2018-05-29 Schlumberger Technology Corporation Utilization of dynamic downhole surveying measurements
US9273547B2 (en) 2011-12-12 2016-03-01 Schlumberger Technology Corporation Dynamic borehole azimuth measurements
US11852007B2 (en) 2021-09-30 2023-12-26 Halliburton Energy Services, Inc. Drilling system with directional survey transmission system and methods of transmission

Citations (3)

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Publication number Priority date Publication date Assignee Title
EP0384537A1 (fr) * 1989-02-21 1990-08-29 Anadrill International SA Procédé pour améliorer la précision lors de la mesure de la position
US5321893A (en) * 1993-02-26 1994-06-21 Scientific Drilling International Calibration correction method for magnetic survey tools
US5398421A (en) * 1990-12-12 1995-03-21 Institut Francais Du Petrole Et Societe Method for connecting magnetic measurements performed in a well through a measuring device in order to determine the azimuth thereof

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0384537A1 (fr) * 1989-02-21 1990-08-29 Anadrill International SA Procédé pour améliorer la précision lors de la mesure de la position
US5398421A (en) * 1990-12-12 1995-03-21 Institut Francais Du Petrole Et Societe Method for connecting magnetic measurements performed in a well through a measuring device in order to determine the azimuth thereof
US5321893A (en) * 1993-02-26 1994-06-21 Scientific Drilling International Calibration correction method for magnetic survey tools

Cited By (24)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6508316B2 (en) 1998-05-14 2003-01-21 Baker Hughes Incorporated Apparatus to measure the earth's local gravity and magnetic field in conjunction with global positioning attitude determination
US6179067B1 (en) 1998-06-12 2001-01-30 Baker Hughes Incorporated Method for magnetic survey calibration and estimation of uncertainty
GB2358251A (en) * 1998-06-12 2001-07-18 Baker Hughes Inc Method for magnetic survey calibration and estimation of uncertainty
GB2358251B (en) * 1998-06-12 2002-09-04 Baker Hughes Inc Method for magnetic survey calibration and estimation of uncertainty
WO1999064720A1 (fr) * 1998-06-12 1999-12-16 Baker Hughes Incorporated Procede d'etalonnage de mesures magnetiques et d'estimation d'incertitude
US7403857B2 (en) 2004-04-19 2008-07-22 Pathfinder Energy Services, Inc. Enhanced measurement of azimuthal dependence of subterranean parameters with filters and/or discretely sampled data
US7027926B2 (en) 2004-04-19 2006-04-11 Pathfinder Energy Services, Inc. Enhanced measurement of azimuthal dependence of subterranean parameters
NO338056B1 (no) * 2004-06-21 2016-07-25 Halliburton Energy Services Inc Fremgang for å bestemme nærvær av magnetiske skjermingseffekter ved overvåking av en brønn
WO2005124102A1 (fr) * 2004-06-21 2005-12-29 Halliburton Energy Services, Inc. Levee de puits de forage
US7143521B2 (en) 2004-11-09 2006-12-05 Pathfinder Energy Services, Inc. Determination of borehole azimuth and the azimuthal dependence of borehole parameters
US7103982B2 (en) 2004-11-09 2006-09-12 Pathfinder Energy Services, Inc. Determination of borehole azimuth and the azimuthal dependence of borehole parameters
US8195400B2 (en) 2009-05-08 2012-06-05 Smith International, Inc. Directional resistivity imaging using harmonic representations
US8271199B2 (en) 2009-12-31 2012-09-18 Smith International, Inc. Binning method for borehole imaging
US9658360B2 (en) 2010-12-03 2017-05-23 Schlumberger Technology Corporation High resolution LWD imaging
US9297249B2 (en) 2011-06-29 2016-03-29 Graham A. McElhinney Method for improving wellbore survey accuracy and placement
US8947094B2 (en) 2011-07-18 2015-02-03 Schlumber Technology Corporation At-bit magnetic ranging and surveying
EP2895888A4 (fr) * 2012-09-14 2016-06-15 Scient Drilling Int Procédé de détermination de variations locales du champ magnétique terrestre
US9134452B2 (en) 2012-12-10 2015-09-15 Schlumberger Technology Corporation Weighting function for inclination and azimuth computation
WO2014092938A1 (fr) * 2012-12-10 2014-06-19 Schlumberger Canada Limited Fonction de pondération pour calcul d'inclinaison et d'azimut
EP2818632A2 (fr) 2013-06-25 2014-12-31 Gyrodata, Incorporated Techniques de positionnement dans des environnements à puits multiples
CN106522924A (zh) * 2016-11-15 2017-03-22 北京恒泰万博石油技术股份有限公司 一种随钻测量中方位角的获取方法
CN106522924B (zh) * 2016-11-15 2020-01-07 北京恒泰万博石油技术股份有限公司 一种随钻测量中方位角的获取方法
US11175431B2 (en) 2017-06-14 2021-11-16 Gyrodata, Incorporated Gyro-magnetic wellbore surveying
US11193363B2 (en) 2017-12-04 2021-12-07 Gyrodata, Incorporated Steering control of a drilling tool

Also Published As

Publication number Publication date
GB2301438B (en) 1999-04-21
NO961966L (no) 1996-11-18
EP0793000B1 (fr) 2001-10-04
NO316336B1 (no) 2004-01-12
EP0793000A3 (fr) 1998-02-04
GB9509819D0 (en) 1995-07-05
NO961966D0 (no) 1996-05-14
GB2301438A (en) 1996-12-04

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