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27. ubs os on ng
For Offshore wells, the accuracy of the well coordinate is based on the accuracy of the surface and underwater positioning systems. These days, the surface positioning is invariably done using DGPS and this contributes very little (<1m) to the overall accuracy. However, this accuracy only describes the accuracy of the DGPS antenna position. This will not therefore be the accuracy of the final wellhead position.
This section describes the process and the likely accuracies of the process depending on water depth.
27.1 GPS positioning GPS positioning uses orbiting satellites as a reference. On board each satellite is a very accurate clock and a transmitting radio. These are powered by batteries with solar panels which charge up the batteries while the orbit is in sunlight. Once a second, each satellite emits a signal with its ID and time. (The clocks are deliberately set fast to compensate for the curvature of space time to keep up with time on the earth. If they were not, GPS would drift by about 4km per day).
In each receiver, software knows the equations of the orbits of each satellite and calculates exactly where the satellite was when the signal was transmitted. It is also time synchronised with the satellites and therefore can measure the time difference from transmission to reception which is converted to a distance in 3D space. This distance from the satellite location defines the radius of a sphere with its centre at a known latitude, longitude and height above the centre of the earth. Two spheres intersect with a circle and three or more define unique points on the surface of the earth. This raw position is good to about 5m. For oilfield positioning this is not sufficiently and so usually, the survey company will employ differential GPS.
Figure 119: Satellite positioning.
27.2 DGPS Differential GPS uses a local (within 500km) fixed GPS receiver as a reference station. The fixed station observes its position over a much longer period and knows its own coordinates very well. When a new signal is received, it observes the position and calculates a local correction which is then transmitted to all other DGPS receivers in the vicinity. In this way, our dynamic unit can observe its position as above but then listen for a correction signal from the fixed station. Once the latitude and longitude are corrected, the position is usually good to about 0.1m. This is usually adequate accuracy for oilfield positioning applications. If greater accuracy is required, a more expensive system known as Real Time Kinematic (RTK) can be used which also makes use of the observed frequencies of the incoming signals whish are affected by Doppler shift based on the relative velocity of the satellite orbit to the receiver. This is used in the mix to improve the observation accuracy and often results in position uncertainties of the order of 0.01m.
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