Performance of Artificial Offshore Islands Under Wave and Earthquake Loading: Field Data Analyses
Authors: Finn W.D. Liam , Lai Susumu , Ishihara Kenji 
Published: October 2008

Two methods of analysis are described for evaluating the effects of waves on the slopes of an artificial offshore island constructed of cohesion less material. One method considers the effects of the instantaneous stress and porewater pressure fields on stability; the other evaluates the cumulative effects of the many waves in a storm. A method is also presented for determining the acceleration and porewater pressure responses of an island to strong earthquake shaking. Field verification is presented for the analysis for predicting instantaneous pore water pressures due to waves. No field data is available to check the predictions of the analysis of cumulative effects of waves. Analyses of typical designs by this method indicates the importance of maintaining permeability?s higher than 10−4 em/sec in island fill to avoid stability and liquefaction problems under wave loading.

A full scale verification of the earthquake response analysis is provided by the recorded acceleration and porewater pressure responses of a man-made island in Tokyo Bay during a recent large earthquake.

Introduction

Over the past ten years, man-made islands of sands and gravels have been used extensively as drilling platforms for oil and gas exploration in the Beaufort Sea. Most have been constructed in less than 40 ft of water with design lives of a few years at most. Only the simplest geotechnical design assessments were made on most islands. As exploration moves to deeper waters and the need for secure designs for production platforms develops, more complex forms of island construction have been evolving. The caisson-retained island shown in Fig. 1 is a recent development to minimize the amount of fill required and to eliminate some of the hazards due to island beaches being subject to direct attack by waves and ice1. The longer life times needed for production platforms and the-newer complex forms combining sand beams with water piercing structures require sophisticated geotechnical engineering and an ability to quantify the probable response of the islands to environmental loads. The chief environmental loads are due to ice, waves and earthquakes. The relative importance of the risk from each of these sources depends on the location of the island.

The purpose of this paper is to describe some recent technological developments that permit analysis of the effects of waves and earthquakes on artificial islands. Two methods for analyzing wave effects and one for determining earthquake response are described briefly. The analyses are incorporated in readily available computer programs. Results from typical analyses are discussed. One case history involving the response of a man-made island in Tokyo Bay to a recent large earthquake is discussed in detail.

There is very little quantitative data available on the response of artificial islands to environmental forces. To remedy this lack of information, two major long-term studies have recently been initiated by the Alaska Oil and Gas Association and Esso Resources Canada Ltd. to provide the kind of test data required to check analytical procedures and provide a framework for design concepts. The methods of analysis and associated computer programs discussed in this paper will be tested further in these studies.