Measurement Of Ekofisk Subsidence

SUMMARY Subsidence of Ekofisk platforms was recognized late 1984. Several measurement programs were initiated to determine the subsidence bowl shape as well as the subsidence history at the seabed. The measurement programs are described, their apparent accuracies are shown and their contributions to specific knowledge of Ekofisk seabed subsidence is discussed. INTRODUCTION Phillips Petroleum Company Norway is the operator for the Ekofisk Field, located roughly in the center of the North Sea. Several platforms, located near the center of' this field, are connected by bridges. This group of platforms is referred to as the Complex. One of these platforms is a concrete tank structure, a perforated wall surrounds this structure. The field was discovered in 1969 and has produced oil and gas since 1971. The Subsidence of the platforms at the Ekofisk Complex was noticed late 1984. The most obvious indication of seabed Subsidence at Ekofisk was a change in the number of visible holes in the breakwater wall on the Tank. When the Tank was installed in the summer of 1973, five holes were visible (Figure 1). Today only 3 holes are visible. Another indication of subsidence was the obvious high water level with respect to the steel platform references. The boat landing of one of the older jackets (the 2/4-C jacket) was located at the water level originally. This boat landing was below water even at low tide in 1984. Certain inspection and maintenance work that could originally be done with scaffolding started to require divers. A map of the Greater Ekofisk Area is shown in Figure 2. These indications were stronger than could be expected from platform settlement. This led to the SUsplcl0n of subsidence. Several measurement programs were initiated to determine if a Subsidence bowl was present and what the rate of subsidence might be. The first measurements which clearly demonstrated the presence of a subsidence bowl were bathymetric measurements obtained along pipelines in the Ekofisk area during the summer of 1984. The pattern of the pipelines form a 10-armed starfish. It was possible to interpolate between pipelines to obtain a crude bathymetric map. A subsidence bowl was clearly evident from this mapping process indicating that the platforms were following the movement of the seabed and not just settling. The methods for precise subsidence monitoring started during the 1984/1985 winter. Searching of historical files and intensive subsidence monitoring, permitted Phillips Petroleum Company to piece together a picture of the process of subsidence to date. From a review of all available data, a picture has been created to show the historical subsidence trends and the accuracy associated with these estimates. DESCRIPTION OF MONITORING METHODS Several measurements systems were used in determining the total amount of subsidence. They are illustrated in Figure 3. The pre-1984-measurements were not intended for Subsidence monitoring, so their accuracy is not as good as would be desired. Other monitoring methods were initiated after the subsidence was discovered.