Measurement of Earthquake Ground Acceleration and Structural Response of a Fixed Offshore Platform

Abstract

ABSTRACT The earthquake measurement system on Chevron U.S .A.'s Platform Grace, offshore California, successfully recorded the ground motion and structural response due to a significant earthquake. This paper describes the measurement system and presents some of the recorded data. The measurement system consists of 23 accelerometers located at three levels on the platform jacket, at the mudline, and two elevations below the mudline; the data acquisition and event recognition systems on the platform; and a remote data communication facility at an onshore laboratory. INTRODUCTION Measured structural response and ground motion are essential for verifying the seismic design and analysis procedures for offshore platforms. A measurement system consisting of seven stations was installed on Chevron's Platform Grace to obtain data for such verification. Platform Grace is located near the eastern end of the Santa Barbara Channel, offshore California (Fig. 1). The water depth at the site is approximately 318 ft. The soil condition at the platform site consists of 190 ft of sandy silt overlying clayey silt. Over consolidation ratios of the soils decrease from 2 to 3 near the mudline to 1 to 1.5 at about 250-ft penetration. The platform measurement system has successfully recorded five earthquakes since it was installed in 1981. All except one were of low intensity and short duration. The exception was the recording of 23 channels of platform and ground acceleration recorded data offered a unique opportunity to estimate the platform's dynamic parameters using system identification techniques. The purpose of this paper is to describe the measurement system, discuss some of its design features, and present samples of the data recorded during the Whittier Narrows earthquake. The structural analysis results are described in detail elsewhere (Mason et a1.,. 1989) and will not be repeated here. MEASUREMENT SYSTEM The measurement system consists of four components: the strong motion sensors, a data acquisition system, an event recognition and recording system, and a remote communication facility. Strong Motion Transducers The motion sensing transducers are 23 force-balance accelerometers with a natural frequency of 800 Hz. The sensing range is presently set at -0.45 g to 0.45 g, but the sensitivity can be reset externally if warranted. The accelerometers are located at three levels on the platform jacket, on the mudline, and at two elevations below the mudline. Their locations and orientations are shown in Figure 2. The accelerometers below the mudline are intended to sense the "free field" ground acceleration due to an earthquake. The two biaxial stations at elevation +44 ft were placed in watertight junction boxes rigidly attached to platform legs C1 and A4 at the northwest and southeast corners of the well deck. The two biaxial stations at elevations -55 ft and -155 ft were placed in 3-in I.D. steel tubes that extend from the top of the platform jacket to the mud1ine station is located at approximately 3 ft southeast of platform leg Cl. Coupling with the seabed is achieved by vertica1 spikes beneath the accelerometer housing.