Abstract

Alternatives to piled foundations, such as suction caissons, are becoming increasingly viable options for fixed platforms, particularly in the development of marginal fields. Whilst pile design procedures evolved smoothly from onshore experience and theory, design of shallow foundation systems has had to be re-examined in light of the intense offshore loading conditions. This is currently being undertaken through experimental investigations, which indicate the possibility of defining foundation response using plasticity theory.

This paper reports the response of circular foundations on very dense sand within the context of plasticity theory. The load and displacement paths are applied using a sophisticated three degree-of-freedom loading rig, designed specifically to explore plasticity related concepts. The tests are on dry sand to ensure drained behaviour. Eight tests are performed on each sand sample, thus minimising material variations between tests, which is particularly important at the high peak friction angles being explored.

The tests were at 1-g, and concentrate on four embedment ratios (0, 0.16, 0.33 and 0.66) on a sand of 95% relative density. The research indicates a change in shape of a yield surface with increase in embedment ratio. Whilst the tests concentrate primarily on the plastic deformations, the behaviour within the yield surface is also examined. A conceptual model, based on plasticity, is presented.

The findings of the research are particularly relevant to the offshore industry in the development of shallow and skirted foundations for dense sand deposits.

Introduction

Historically, offshore structures for oil and gas production have been anchored to the sea floor by large diameter piles. Recently, a novel foundation type has been used in place of these piles30, best described as upturned buckets, typically 15m diameter and 5m deep, they are usually called suction caissons. These foundations are installed by pumping out water to cause a slight suction to the inside of the caisson, once it has made a sufficient seal within the soil. In the North Sea, where a large part of the seabed is covered by a layer of very dense sand, the suction assists penetration by temporarily reducing the resistance of the sand as well as increasing the net downward force on the caisson.

Offshore structures are often exposed to severe loading due to wave, current and wind forces. Their foundations are exposed to higher horizontal and moment loading than are encountered onshore. Whilst pile design procedures have evolved from onshore experience and theory, design guidelines for shallow foundation systems, such as suction caissons, have yet to be devised for the intense loading conditions found offshore. In the particular case of suction caissons, there are no precedents in onshore experience, and at present there is little offshore experience, though this is rapidly changing8. This means that there are no accepted procedures, such as the API guidelines for piles, nor are there large amounts of published data available, as firms tend to keep any acquired knowledge in-house so as to gain competitive advantage.

One of the critical aspects of the design is the ability to resist tension, which may occur under extreme conditions. The important issue is the rate at which the caisson is loaded, compared to the rate of fluid flow within the soil, as the latter affects the soil response. The typical loading on the foundation will be random variations about some mean value, which results in a complex flow regime within the soil, which in turn dominates the response of the foundation. However, it is necessary to define the long-term drained loading limits of

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