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Summary. Phillips Petroleum Co. Norway pioneered the application of pressure gauges to measure offshore platform subsidence histories. Four pressure gauges to measure offshore platform subsidence histories. Four pressure-gauge systems were installed in the Greater Ekofisk area. pressure-gauge systems were installed in the Greater Ekofisk area. Prototype data prove that the progression of subsidence can be measured Prototype data prove that the progression of subsidence can be measured with pressure gauges with a relative uncertainty of 9 mm.

Introduction

Phillips Petroleum Co. Norway is the operator for the Ekofisk field, Phillips Petroleum Co. Norway is the operator for the Ekofisk field, roughly located in the center of the North Sea (Fig. 1). Several platforms near the center of this field are connected by bridges. platforms near the center of this field are connected by bridges. This group of platforms is known as the Ekofisk Complex. The field was discovered in 1969 and has produced oil and gas since 1971. In Nov. 1984, subsidence was discovered to take place in the central Ekofisk field. Several measurement systems were used to obtain information on the total amount of subsidence and the subsidence rate. An experimental measuring system based on pressure gauges was installed. The system consisted of pressure gauges suspended from the Hotel and Tor platforms. Fig. 2 is a sketch of this experimental system. More than 10 weeks of uninterrupted, usable data were obtained and interpreted. After the measurement system was determined to be attractive for further development, permanent pressuregauge subsidence-monitoring systems were installed on pressuregauge subsidence-monitoring systems were installed on Platforms Hotel 2/4-H, Tor 2/4-E, Albuskjell 2/4-F, and Edda Platforms Hotel 2/4-H, Tor 2/4-E, Albuskjell 2/4-F, and Edda 2/7-C during the summer of 1987. Seabed pressure gauges serve the following purposes. 1. They track the progression of subsidence accurately so that a reliable subsidence rate is computed. 2. Data collection and interpretation are economical and not labor intensive. 3. The seabed-pressure-gauge system provides continuous measurements so that discontinuous subsidence can be seen.

Measurement Principle

Fig. 3 illustrates the principle of how to measure relative subsidence between two platforms with pressure gauges. Pressure gauges are installed on selected platforms near the mudline, resting on a firm base on a lower platform brace. The pressure gauges are built to measure absolute pressure (i.e., hydrostatic plus hydrodynamic plus barometric pressures, including any disturbing influences). In plus barometric pressures, including any disturbing influences). In the ideal case, an average pressure is measured for every 0.67-second period and polled every 3 seconds by the platform computer. If gauges are broken, the remaining gauges on a platform are polled every 10 or 20 seconds, depending on whether one or two sensors are broken, because even broken gauges are polled for up to 10 seconds in each polling cycle. The computer averages the available measurements from each 30-minute period on the hour and the half hour, and subsequently time tags and transmits that average to a central computer data-gathering system (Fig. 4). The barometric pressure, measured between cellar deck beams, is polled, averaged, transmitted, and stored in the same way. The data are retrieved for checking and interpretation at our Tananger base. An IBM PS/2 Model 70-386(TM) with Lotus 1-2-3(TM) software is used to check and interpret the data. The relative subsidence between two platforms can be computed by comparing the 6- or 10-day average value of the observed pressures on one platform with a similar value for another platform. pressures on one platform with a similar value for another platform. A change in this pressure difference indicates a change in subsidence. The gauges on the Albuskjell, Edda, and Tor platforms are used as bench marks. Geological and geophysical reservoir properties from the area indicate that these platforms are suitable to serve as bench marks for these types of measurements.

Other Methods Used

Other methods were previously used to derive subsidence properties. For instance, satellite measurements were used properties. For instance, satellite measurements were used until Feb. 1990, after which time we discontinued their use because of cost, logistic complexities, and disappointing accuracy for offshore subsidence-type measurements. Because the distance from subsiding Ekofisk platforms to reference platforms is on the order of 10 to 12 km, optical methods platforms is on the order of 10 to 12 km, optical methods are not accurate enough to measure platform elevations.

Hardware Installation and Maintenance Requirements

Fig. 5 shows the major components of the hardware installed on Platforms Hotel 2/4-H, Tor 2/4-E, Albuskjell 2/4-F, and Edda Platforms Hotel 2/4-H, Tor 2/4-E, Albuskjell 2/4-F, and Edda 2/7-C. The hardware installed to support the pressure-gauge subsidence-monitoring system consists of a deck frame, from which a sensor cage is lowered along guide wires into the stub of a lower brace clamp with a pneumatic winch. The top of the clamp stub is the reference plane of measurement. The sensor cage is weighted with lead to ensure continuous contact of the sensor cage and the stub. The winch cable is also the signal cable. Cable spacers separate the guide wires from the signal wire. The pressure sensor is a Paro-scientific Digiquartz Depth Sensor Model 8130(TM), intended to measure pressures up to a hydrostatic pressure of 130 m water. The barometric pressure is measured by a Paroscientific Digiquartz Model 215AT(TM) barometer. The air inlet for the barometer is located between the cellar deck beams of the platform. A sensor power-supply unit and a data-gathering and -interpretation system is installed on each platform. Two pressure sensors were installed next to one another on the Hotel platform so that a backup gauge would be available. In addition, a comparison of the observed pressure will give information on any differential sensor drift. Twin gauge systems were installed on all subsidence-measurement stations as well as the reference platforms during 1989. This was done primarily to increase the reliability of all systems by having backup systems.

Background Theory

Basic Theory. Wiegel rewrote the Bernoulli equation and appended it to include the influence of tides, barometric pressure, and other disturbing influences to give the pressure of one location, Location A:

Pa = pgy+p t+pgT-[0.5p(vx+vy)]-[pu(vx+vy)] -pu+pv+p+pgS,.............................(1)

where y = undisturbed hydrostatic head above the pressure sensor to the main sea level.

SPEFE

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