On the Modelling of Foundations for Jack-up Units on Sand
Authors: Cassidy M.J. , Houlsby G.T. 
Published: October 2008

There is a steadily increasing demand for the use of jack-up units in deeper water and harsher environments. To be confident of their use in these environments there is a need for jack-up analysis techniques to reflect accurately the physical processes occurring. The common modelling of spudcan footings as pinned or as linear springs is an over-simplification of foundation behaviour, and can therefore lead to overconservative design. This paper details the implementation of a work hardening plasticity model for spudcan footings on dense sand (named Model C) into a dynamic structural analysis program (named JAKUP). The motivation for this is to have a balanced approach to the analysis of jack-up units, with the non-linearities in the structural, foundation and wave loading models all taken into account. Wave loading is considered using NewWave theory, and the importance of random wave histories shown by constraining the deterministic NewWave into a completely random surface elevation. Examples are shown to emphasize the differences in predicted extreme response according to the various footing assumptions: pinned, linear springs and Model C. The role of plasticity theory for modelling foundations will be highlighted. Further uses of JAKUP in the understanding of the extreme response statistics and reliability of jack-up platforms are discussed.

This paper is directed to those interested in the long-term reliability analysis of jack-up units.

Introduction

Jack-ups were originally designed for use in the relatively shallow waters of parts of the Gulf of Mexico. Due to their economic importance within the offshore industry, there has been a steadily increasing demand for their use in deeper water and harsher environments1. There is also a desire for a longer lasting commitment of a jack-up at a single location, especially in the role of a production unit2. To be confident of their use in these environments, there has been a need for changes in analysis techniques to make them more accurate, avoiding unnecessary conservatisms which were once commonplace.

More realistic modelling of jack-ups, based upon the relevant physical processes has been developed in a number of areas, the most significant being;dynamic effects,geometric non-linearities in structural modelling,environmental wave loading,models for foundation response.

There has been considerable diversity in this development, with studies using varying levels of complexity for different aspects of the analysis. This is especially true for soil/structure interaction, with some studies using detailed structural models or advanced wave mechanics, whilst still using the simplest of foundation assumptions (i.e. pinned footings)3,4,5.

The purpose of this paper is to describe a more balanced approach to the analysis of jack-up units. The components of a dynamic structural analysis program, named JAKUP, which is capable of considering the major non-linearities in jack-up response, are described. The motivation for this development is to achieve confidence in all of the components affecting the response. The principal characteristics of the structural, foundation and wave loading models are detailed, and example calculations given. Lastly, a discussion of the applications of these models is presented.