Page | 176 12. Correcting for Sag...................................................................................................................................................... 68 13. Correcting for Magnetic Interference ....................................................................................................................... 70 13.1 Drilling magnetisation ......................................................................................................................................... 70 14. Multi Station Analysis ............................................................................................................................................... 74 14.1 Calculation and background ............................................................................................................................... 74 15. Correcting for Pipe and Wireline Stretch ............................................................................................................ 79 15.1 Forces on the drillpipe ........................................................................................................................................ 79 15.2 Thermal Effects ................................................................................................................................................... 81 16. Human Error v Measurement Uncertainty ......................................................................................................... 83 16.1 Common Human errors ...................................................................................................................................... 83 16.2 Misapplication of Uncertainty in Top Hole ......................................................................................................... 83 17. Understanding Error Models .............................................................................................................................. 86 17.1 Error Models and Instrument Performance Models .......................................................................................... 86 17.2 Modelling Uncertainty ........................................................................................................................................ 87 17.3 Probability in two dimensions ............................................................................................................................ 88 17.4 How can we determine the size and shape? ...................................................................................................... 89 18. The ISCWSA Error Model: Introduction .............................................................................................................. 93 18.1 Some background to ISCWSA ............................................................................................................................. 93 18.2 Covariance........................................................................................................................................................... 94 18.1.1 How the errors affect the observations....................................................................................................... 94 18.3 The Variance Covariance Matrix ......................................................................................................................... 96 18.4 Eigen Values and Eigen Vectors .......................................................................................................................... 96 18.5 Collision Risk........................................................................................................................................................ 96 18.6 Definition of Ellipse Axes..................................................................................................................................... 97 18.7 How errors propagate......................................................................................................................................... 98 Ap1. The ISCWSA Error Models: Explanation and Synthesis ........................................................................................ 100 Ap1.1 Introduction .................................................................................................................................................. 100 Ap1.1.1 Assumptions and Limitations of the ISCWSA Model ............................................................................ 137 Ap1.1.2 References in this chapter .................................................................................................................... 137 Ap1.1.3 Abbreviations ........................................................................................................................................ 138 Ap1.1.4 Nomenclature ....................................................................................................................................... 138 Ap1.1.5 Definition of Axes .................................................................................................................................. 138 Ap1.2 Framework of the Error Model ..................................................................................................................... 140 Ap1.2.1 Overview of the Error Model ................................................................................................................ 140 Ap1.2.2 Error Sources......................................................................................................................................... 141 Ap1.2.3 Error Magnitudes .................................................................................................................................. 141 Ap1.2.4 Weighting Functions ............................................................................................................................. 141 Ap1.2.5 Error Propagation .................................................................................................................................... 142 Ap1.2.6 Summing Error Terms and Propagation Modes ...................................................................................... 143 Ap1.2.7 Transformation to Borehole Axes............................................................................................................ 144 Ap1.2.8 Bringing It All Together ............................................................................................................................ 145 Ap1.2.9 Bias Terms................................................................................................................................................ 145 Ap1.3 MWD Error Model......................................................................................................................................... 146
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