Automatic Rejection And Substitution Of Suspect Oceanographic Subsidence Data Obtained With Pressure Gauges

Abstract

SUMMARY : Phillips Petroleum Company Norway, the operator for the North Sea Ekofisk Field, relies on seabed pressure gauges to measure platform subsidence. These gauges record primarily hydrostatic plus air pressures. Noise in the data is caused by high power electrical equipment and power breaks. To calculate platform subsidence, continuous pressure records are needed as well as an automatic data rejection capability to remove suspect data. The development of these procedures to remove and substitute suspect data and fill in missing data are presented. Oceanographic data relating to subsidence are automatically screened for suspect values. Missing and suspect data are substituted and tagged for later scrutiny. This development has resulted in significant labor savings in subsidence and environmental data processing. An automatic data rejection criterion was formulated based on the numerical properties of tides and tidal residuals. A mathematical procedure was defined to substitute suspect and missing data. Guidelines for general application of these procedures were formulated. The proposed data rejection and substitution and filling in procedures can be applied generally to oceanographic data histories. The procedures are suitable as a general oceanographic data screening, preprossesing and editing tool. INTRODUCTION Ekofisk is a hydrocarbon producing field which is located in the center of the North Sea (see Figure 1). In 1984 it was observed that seabed subsidence had taken place. It was concluded that this was a direct consequence of hydrocarbon production took place. After the discovery of Ekofisk subsidence, many questions had to be answered, not the least of which was at what rate subsidence was progressing. Initially we used the so-called Navistar satellite based on Global Positioning System to measure the progression and rate of subsidence, and found the system expensive and insufficiently accurate at the time. We also experimented with pressure gauges to measure subsidence, and found them to be more accurate and more operator friendly (References [1] and [2]). An advantage of the pressure gauge system is that the system is totally controlled by the operator and independent of outside consultants. The system is not available "off the shelf". This implies one disadvantage of the system: the operator had to undertake the development of the measurement system and data interpretation procedures. As time passed, our demands for continuous and accurate measurements of ever lower tolerance limits increased. For instance it is necessary to have continuous and reliable surface subsidence data in order to relate net reservoir voidage to seabed subsidence. This paper describes our effort to control the quality of the raw pressure gauge measurements on which we base our subsidence measurements. As is the case in any measurement system, bench marks must be chosen. To measure subsidence at the Ekofisk Complex, which is a group of platforms in the central Ekofisk field area connected by bridges, we use the platform elevations of the Albuskjell (2/4-F), Edda (2/7-c) and Tor (2/4-E) platforms as bench marks and reference elevations, these platforms are located at a distance of less than 10 miles from the Ekofisk Complex (Figure 2).