Page | 104
20.5 Scalar (Expansion) Method
A third method is the scalar or expansion method. This method calculates the amount the uncertainty ellipses must be expanded or contracted (in cases where the ellipses overlap) in order to just meet. The separation factor is simply the factor by which the ellipses are expanded or contracted. The calculations required to generate the separation factor using the scalar method are best left to the computer. In the example the green ellipses are 40% bigger than the originals so the separation factor would be 1.4. When the geometries line up correctly this method produces the same number as the alternative methods. The same safety rules developed for use with the older methods can still be used. But the scalar method has the advantage of never calculating optimistic or pessimistic separation factors no matter what the 3D geometry.
Figure 90: Scalar (expansion) method.
This method produces numbers which are very similar to the other two methods and can be used with the same safety rules but it is never optimistic or pessimistic in any geometry.
20.6 Probability of Collision
There is a movement in the industry towards more probability based anti-collision reporting. As of April, 2012 there is no final recommendation from the SPE technical section (ISCWSA) on this. At present this section will present information on collision probabilities and their calculation.
20.6.1 Difference between Separation Factor and Probability Based Rules A separation factor based rule is purely geometric whereas a probability based rule relates to actual risk. For example, take two different uncertainty situations which both calculate a separation factor of 1.0. In the first case two 12-1/4” w llbo s h ve uncertainty envelopes which are hundreds of feet across. In the second case the uncertainty envelopes only have a radius of one foot from each well centre. Clearly the second case has a much higher probability of being an actual collision than the former. Rules merely using reported separation factor numbers will not distinguish between these two cases.
Figure 91: The uncertainty envelopes for two calculation methods.
Made with FlippingBook flipbook maker