Automatic Gps Satellite Based Subsidence Measurements For Ekofisk

Abstract

SUMMARY: A fully automatic procedure for The measurement of subsidence of many Platforms in almost real-time are presented. Such A method is important for developments which May be subject to subsidence and where Reliable subsidence and rate measurements are Needed for 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 Echoism Field area. A description of the system is given. The Derivation of those parameters which give Optimal measurement accuracy is described, The results of these derivations are provided. GPS satellite based measurements are Equivalent to 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 wereMulled to oceanographic quantities such as The platform deck clearance. These quantities Now follow from GPS based measurements. INTRODUCTION Ekofisk is an oil and gas field in the southern Portion of the Norwegian North Sea, It is one of Several fields operated by Phillips Petroleum Company Norway in that area, such as the Nearby Tor, Deal Elfish and Albuskjell fields. Late in 1984, it was noticed that the Echoism Platform decks were closer to the sea surface Than when the platforms were installed. Subsidence was the only logical explanation for These observations. After the subsidence Phenomenon was recognized, an accurate Measurement method was needed to measure Progression of subsidence and the associated Subsidence rate. One available system for which. No further development was needed, was the NAVSTAR GPS (Navigation System with Time And Ranging Global positioning System). The Measurements started in March 1985. For a Period of almost three years this system was the Back bone of the methods used to derive Subsidence rates and the progression of Subsidence. At approximately monthly intervals The elevation difference between three benches Mark platforms which do not subside (Albuskjell, Eddo and Tor) and the subsiding Hotel (2/4H) Platform was measured. Figure 1 shows their Relative locations. In 1988 we stopped taking GPS surveys on a Monthly bases and continued with subsidence Measurements based on pressure gauges Because this method gave more accurate results. Since then we have used GPS surveys only to Confirm measurements based on pressure Gauges, in 1991 and 1993. The need developed to have independent Confirmatory subsidence measurements. We decided to develop an automatic and continuous subsidence monitoring system Based on GPS. This system is the only available, technically acceptable system available for such measurements with expected accuracies similar to that of the pressure gauge Monitoring system (see References [1],[2],[3]. The system we chose to develop for Echoism is similar in principle to the Development of what Elf Aquitaine uses to Measure subsidence of their Zooidal platform In the Netherlands (Reference [3]). The advantage of a GPS system is primarily. That maintenance of the system is much Cheaper than of a pressure gauge system, as cNo vessels or divers are needed for repairs.