Three-dimensional finite element analysis of offshore jack-up structures accounting for non-linear soil-structure interaction

Abstract

Jack-up structures are widely used in the offshore industry. Not only in the field of oil and gas, but also in the installation for offshore wind turbines. These jack-up structures typically consist of three trusswork legs, where the foundation is formed by inverted conical cones that are penetrated into the soil, called spudcans. The spudcans transfer the combination of weight and environmental loading to the underlying soil. Jack-up structures are assessed for each particular offshore site, where their stability is checked for a 50 year return period storm as prescribed by the current guidelines. The behaviour of the jack-up structures is strongly influenced by the restraint given by the spudcan footings. This phenomenon is generally described as fixity and is a complicated effect, governed by the interaction between soil and structure. To properly take into account this effect, computational models are needed that incorporate the complicated non-linear behaviour of the jack-up foundation in a structural framework. This paper describes the three-dimensional finite element modelling of a generic jack-up structure, that takes into account the interaction of soil and structure. Through calibration, the complicated non-linear behaviour of the soil is captured in the model. A benchmark study is performed, where the prediction of the finite element model is compared to experimental tests, as well as a macroelement model. This comparison shows that the finite element model is capable of describing the global behaviour of the jack-up unit under loading, as well as the behaviour of the individual spudcan footings. Furthermore, it has been shown through parametric analyses that emphasis should be placed on the calibration of soil parameters under compression, especially at high stresses. The dilative behaviour of the soil has a positive influence on both the global capacity and stiffness, as does an increase of preloading of the structure. The loading direction along the axis of symmetry provides the highest global capacity and stiffness of the rig during loading. Torsional loading of the structure predicts a significant torsional moment at the spudcan footings and predicts a rotation of these footings in the soil. The incorporation of a large-deformation finite element framework has shown to be crucial in the proper prediction of the behaviour of the jack-up structure during push-over loading.