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Procedures for establishing offshore position.
When spudding an offshore well or positioning a jackup or jacket, the final seabed coordinate will be the definitive position used for future drilling operations. This is unlikely to be exactly beneath the centre of the topside structure due to ocean currents, tilting, piling limitations or spud can penetration differences. In deep water, even a 0.5 degree tilt will create a shift of about 1% of water depth. Once the spud or the jacket upending is complete, the final coordinate is simply copied to surface and the wells are assumed vertical above the seabed. Not only do we inherit the coordinate from the subsea operation but we also
Figure 124: Wellhead and Platform coordinates.
inherit the uncertainty.
For Long Baseline acoustic positioning this is likely to be of the order of .5% of water depth at 1 SD and for USBL it is likely to be 1% of water depth at 1 SD. In the absence of a detailed marine survey report demonstrating otherwise, these figures should be used.
27.7 Points to Ponder
Subsea positioning accuracy is dependent on the equipment used, the water depth, and the absence of human error in the taping and recording of offsets. It is important that the SFP communicates well with the marine survey department to ensure that:
1. 2.
Both departments are using the same coordinate reference system and datum.
Vertical references are clearly understood and agreed.
3. The method of calibration of seabed arrays is understood and adequate for the job. 4. A clear report is produced of the final positions and their uncertainties. 5. Subsea well positions and not topside centres are used for well coordinates.
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