Ekofisk automatic GPS subsidence measurements

A fully automatic procedure for the measurement of subsidence of many platforms in almost real time was developed. The procedure gives reliable subsidence and rate measurements. These are essential for confirming platform safety, planning of remedial work and verification of subsidence models. Automatic GPS satellite based subsidence measurements are made continuously on platforms in the North Sea Ekofisk Field area. A description of the system is given. The derivation of those parameters which give optimal measurement accuracy is discussed. GPS satellite based measurements are more attractive than seabed pressure gauge based platform subsidence measurements, but they are much cheaper to install and maintain. In addition, GPS based measurements are not subject to drift of any gauges. GPS measurements were coupled to oceanographic quantities such as the platform deck clearance. This leads to less complex offshore survey procedures.