Earthquake Criteria for Platforms, in the Gulf of Alaska
Authors: Bea Robert G. 
Published: October 2008

INTRODUCTION

The offshore industry stands on the threshold of a significant challenge the timely, safe, and economic development of the Gulf of Alaska oil and gas reserves. This area is frequently swept by severe storms, generating waves and currents which rival those of the North Sea and North Atlantic. In addition, the area is bordered by the earth's most active seismic zone, the Circum-Pacific Belt. Some of the largest recorded earthquakes have occurred on the perimeter of the Gulf of Alaska.

This paper will focus primarily on the second major environmental loading threat earthquakes Utilizing concepts, background, and data presented at the 1975 Offshore Technology Conference (1,2,3,4,5,6,7), an attempt is made to synthesize this technology in the framework of a reliability analysis (8,9,10,11) to arrive at a definition of earthquake ground motion criteria for pile-supported, drilling and production platforms.

CONCLUSIONS

Depending primarily on location and type of soil-foundation condition, elastic design level earthquake motions producing maximum effective ground velocities in the range of 50 to 70 cm/sec (20 to 28 in/sec) are indicated. To produce the desired level of loading and superstructure-foundation characteristics, a design wave height of approximately 120 feet is suggested in conjunction with application of API RP 2A member sizing guidelines. These results are very contingent on the type of platform, design procedure, and analytical model and assumptions used in this study. Design criteria are intended to be strength determining parameters. In an environment such as the Gulf of Alaska where there may be multiple environmental threats which may produce loadings of comparable magnitudes design criteria for one threat should not be chosen without explicit consideration of the other threats. The potential loading effects of severe ground motions developed by earthquakes on pile supported platforms are very different than those due to intense wave and current action. While loading patterns may be similar, one loading system is fundamentally load limited (earthquakes), and the other load-unlimited (waves and currents). As earthquake ground motion intensity increases, the amount of load which is transmitted "to the platform is limited by the ability of the foundation elements and soils to transmit the energy to the platform. In contrast, as wave-current action increases, the amount of imposed load is essentially unlimited. Two fundamental efforts are identified in this paper which should be a focus of engineering work in this general area of technology. The first is the recording of strong ground motions at offshore sites in the Gulf of Alaska. Such records would be used to determine the applicability of attenuation and local soil-geology modulation relationships which are used to describe the effects of distant earthquakes on a given location. No measurements have been made of strong ground motions offshore. The second is the development and calibration of a realistic soil-pile-superstructure interaction model applicable to moderate and intense levels of earthquake ground motions.