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27.5 Boxing in This is simply a series of distance measurements taken from a surface vessel to a transponder on the seabed. The vessel will use DGPS at surface along with the Gyro and Pitch and Roll to determine a location for a hull mounted transponder when each measurement is taken. This provides a series of surface locations and 3D distances to the transponder. The boxing in process finds the best fit position for the seabed transponder to fit the multiple observations. To minimise systematic errors, the vessel usually sails in a circle around the transponder so that any scale factor error or depth error tends to cancel out as observations are taken from all directions. For critical operations like Relief well drilling it useful to ask the marine survey contractor for the standard deviation of the box in results. This will usually result in a circular uncertainty envelope that follows the same rules as the 1, 2 and 3 Standard Deviation (SD) probability distributions we are familiar with.
With Long Baseline arrays, a signal can also be sent to each transponder in turn to measure the distance to all the others. This is usually accurate to a few centimetres and is adjusted for a best fit network to match the box in and the internal baseline observations. However, it should be noted that the network adjustment only provides a very accurate relative position of the array.
Figure 122: Networked Acoustic Array.
The absolute position and the orientation (which can be crucial for relief well positioning) are still dependent on the quality of absolute positions of the individual transponders. The final absolute position may well be established by taking a weighted average of the transponder positions and fitting the final network to the same average. Similarly the orientations of each transponder may be calculated from the new centre and the network rotated to fit the average orientation. These processes will inevitably result in uncertainties for the array which will translate into surface position uncertainties for any ROV positions or spudding positions if a transponder is used on the drillstring.
27.6 USBL
Ultra Short Baseline systems measure the time taken for a signal to travel to the transponder and return but they also measure the phase difference of the incoming response between multiple receivers. This provides a 3D vector from the USBL position to the transponder on the seabed. If the transponder is a permanently positioned unit, multiple USBL positions can be taken and the vessel observing the seabed position will take readings with the vessel at two opposing headings. The effect of this is that any pitch or roll error will be the opposite if the heading changes by 180 degrees and the resulting seabed position is unaffected. The final scatter of results is averaged out and the first estimated position for the subsea transponder is established. This will be further improved by a subsequent network adjustment and so there is not much difference in the final accuracy of the seabed array.
Figure 123: Reading a permanent seabed AT by vessel.
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