Findings From a Joint Industry Study on Foundation Fixity of Jackup Units
Authors: Osborne J.J. , Trickey J.C. , Houlsby G.T. , James R.G. 
Published: October 2008

During the design storm conditions the soil supporting jack-up unit footings may be close to, or at plastic failure. Accordingly, the use of elastic solutions to the problems of foundation load response during load conditions which approach the ultimate design conditions are inappropriate.

The rotational restraint, or moment fixity, offered by the footings under these conditions will reduce the moment load seen in the lower leg guides. Because of the lack of knowledge relating to this subject an industry sponsored research project was conducted. This paper briefly presents the study conclusions.

INTRODUCTION

Before operating a jack-up platform at a specific location the ability of the unit to withstand the envisaged 50 year storm conditions is usually assessed to ensure that structural failure does not occur. The structural strength of the unit is checked and the performance of the foundations evaluated.

In certain cases excessive bending moments can occur in the legs at the position of the lower guide. In the past it has been widely accepted that, for these analytical purposes, the foundation behave as a pin joints and cannot, therefore, sustain any moment. Intuition and practical experience suggest that this is a conservative approach. Inclusion of foundation moment restraint, or fixity, into the structural model results in a redistribution of a proportion of the leg bending moments at the lower guide to the bottom of the leg. Thus for a given unit, under specific conditions, the unit may be suitable for operations in harsher environments. Prior to this study engineers have generally considered the spudcan elastic rotational displacement as a rigid circular footing on an isotopic homogenous elastic soil. (API RP2A (1986), Borowicka (1936) Gerrard (1974)).

However, as the foundation loads approach failure under storm conditions elastic theories clearly become inappropriate. To investigate these nonlinearities jack-up unit foundation model tests and numerical analyses were performed on sand and clay soils. Calculation procedures were developed for the evaluation of the moment restraint, or fixity, offered by the foundation. Where possible these methods were validated against finite element analyses. Four jack-up unit structural F.E. models were subjected to storm loading conditions to evaluate the study findings.

Standard nomenclature are presented at the end of the paper and relevant dimensions are illustrated in Figure 1.

The bulk of the work was performed during March 1985 to March 1987 at Oxford and Cambridge Universities and Fugro Ltd. Due to the brevity of this paper it is not possible to report on all aspects covered during this wide ranging study. Attention is therefore focused on the research conclusions. Copies of the full three volumed report, which include analyses, worked examples and discussion, are available from the principal author.

MODEL TESTS - CLAY

The model tests used two sets of footings, of 50 mm and 100 mm diameter, in a 450 mm diameter tank, on consolidated kaolin clay.