Earthquake Criteria for Platforms in the Gulf of Alaska
Authors: Bea R.G. 
Published: December 2007

This paper describes earthquake criteria for platforms in the Gulf of Alaska. The effects of strong earthquake ground motions are determined by the dynamic response characteristics of the soils and attached structures. The design of the structures to resist extreme wave loadings has an important influence on these characteristics.

Introduction

The offshore industry is 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 swept frequently by severe storms that generate waves and currents rivaling 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 world's largest recorded earthquakes have occurred on the perimeter of the Gulf of Alaska. perimeter of the Gulf of Alaska. This paper focuses primarily on the second major environmental loading threat - earthquakes. Using concepts, background, and data presented at the Seventh Annual Offshore Technology Conference, this technology is synthesized in the framework of a reliability analysis to help define earthquake ground-motion criteria for pile-supported drilling and production platforms.

1. Depending primarily on location and type of soilfoundation condition, elastic design level earthquake motions producing peak 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 ft is suggested along with the application of API RP 2A member-sizing guidelines. These results are applicable to the type of platform, design procedure, and analytical models used in this study.

Design criteria are intended to be platform-system, strength-determining parameters. In an environment like the Gulf of Alaska, where there may be multiple environmental threats that produce loadings of comparable magnitudes, design criteria for one threat should not be selected without explicit consideration of other threats.

2. The potential force effects developed by severe ground motions on pile-supported platforms are very different from those caused by intense wave and current action. While loading patterns may be similar, one loading system fundamentally is force-limited (earthquakes) and the other is load-unlimited (wave and currents).

As earthquake ground-motion intensity increases, the amount of transmitted load is limited by the ability of the foundation elements and soils to transmit that energy to the platform. In contrast, as wave-current action increases, the platform. In contrast, as wave-current action increases, the amount of imposed load essentially is unlimited.

3. Two fundamental efforts are identified that should be a focus of engineering research in this general area of technology. First, strong ground motions should be recorded at offshore sites in the Gulf of Alaska. Such records could be used to determine the applicability of attenuation and local soilgeology modulation relationships that describe the effects of distant earthquakes on a given location. No measurements have been made of strong ground motions offshore.

JPT

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