28.6 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.

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 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.