Engineers Australia

Predicting the load-displacement response of a mobile jack-up drilling rig on sand

Britta Bienen

Abstract

Before a mobile jack-up drilling rig is approved to be deployed at an offshore location, a site specific assessment is required to be carried out to show the rig’s capacity to withstand the design storm conditions. The wind, waves and current acting on the structure in addition to its self-weight result in complex loading applied to the foundations, which must be properly understood if the behaviour of the structure is to be correctly predicted. This paper discusses the foundation loads and benchmarks predictive models currently available. Jack-up load paths (up to ultimate failure) were obtained from experiments on a scaled model rig carried out in the beam centrifuge facility at the University of Western Australia. Compared to the experimental measurements, it is shown that the current guidelines SNAME (2002) are excessively conservative in predicting failure of the jack-up. Numerical modelling with the currently available models, however, achieved a good prediction of the experimental response, both in terms of ultimate failure and stiffness under working loads. Instead of running computationally expensive finite element analyses with the soil being modelled using continuum elements, the footing-soil interaction in the numerical analyses presented here is encapsulated into a point element, which is attached to the bottom nodes of the structural model. Numerical predictions of the experimentally measured response is shown for different loading directions of the jack-up, illustrating the implications of the footing load paths for the overall response of a jack-up to failure. The results show the ability of existing models to predict the load-displacement behaviour of jack-up foundations and their importance in predicting the overall response of the system.