Investigations of Suction Caissons in Dense Sand

Prof. Guy Houlsby and Dr Byron Byrne

This work was sponsored by EPSRC (£35k) and the Rhodes Trust (Byron Byrne)

Suction caisson foundations are a novel alternative to piled foundations for fixed offshore structures. They take the form of large upturned "buckets", typically 10m-15m in diameter and 3m-5m deep. Their name arises from the fact that, after initial contact with the seabed is established, they are installed by applying a suction to the water trapped within the caisson. They offer cost savings over more conventional foundations both in terms of material costs (steel) and in terms of the time and equipment required for installation. They have already been used for two jacket structures on dense sand in the North Sea.

This project was concerned with the performance of the foundations once they are in place. Under typical storm loading the foundations are subjected to a combination of vertical, horizontal and moment loading. It is therefore necessary to understand their behaviour under cyclic combined loads. It is worth noting that design practice for offshore piled foundations has evolved gradually from onshore practice, and has for many years been codified in the API guidelines. Equivalent onshore experience does not exist for suction caissons, nor have design codes yet been established, so design must be based on fundamental research. The purpose of this research was therefore to carry out generic research on the behaviour of suction caissons on dense sand, with the ultimate purpose of establishing more general design procedures. The method used for the research was small-scale laboratory model testing.

The first phase of the work was to modify existing equipment to allow a wider range of tests to be carried out on model foundations. Following this a series of monotonic loading tests were carried out. The tests were compared with existing approaches to combined loading on foundations, which use plasticity theory. Broadly the existing framework was found to be appropriate, with some minor modifications being necessary.

Since the loading on offshore foundations is cyclic and transient in nature, cyclic testing was then carried out at a number of different loading rates. These tests provide an important database for design. An important feature of these tests was that pseudo-random cycling using the "constrained NewWave" approach was used to simulate extreme events within a random background. The cyclic loading tests show a pattern in which a stiff response to small amplitude cycling progresses through a gradual transition to a response with a large amount of hysteresis at large cycles. This type of transition cannot be modelled with conventional plasticity theory, but a new approach called "continuous hyperplasticity" has been developed which can model the main features of the cyclic response.

A number of publications are available about this work, including a paper presented at OTC 1999 (756k), a paper presented at OTC 2000, the Final Report to EPSRC (162k), and Byron Byrne's DPhil Thesis

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