Yield interaction model of jack-up considering nonlinear load-displacement relationship before yield

Abstract

Abstract Offshore structures with jack-up systems can be operated at depths up to 150\xa0m and are used not only as drilling rigs and production rigs but also as support and accommodation units. Jack-up operation is carried out under environmental loads such as wind and wave, for which it is essential to understand jack-up behavior and structural response. As the boundary condition, the foundation model of offshore structures affects the vibration mode of a structure and, consequently, the behavioral and structural analysis results as well. Typical simple foundation models such as pinned and linear spring do not reflect soil-structure interaction (SSI) in jack-up analysis. As an alternative, the International Organization for Standardization (ISO) guidelines have suggested this SSI model as a simple secant model, a yield interaction model, and a time-consuming but accurate soil continuum model. In the present study, a structural analysis of jack-up is performed using the yield interaction model and the soil continuum model. The yield interaction model, Model B for clay, derived for consideration of the nonlinear behavior of soil, has been studied and continuously improved until recently. The existing model has generally assumed a linear load-displacement relationship in the elastic region; but this relationship may overestimate the load, as soil plasticity occurs gradually in practice. In this paper, the Hyperbolic Model B is proposed, based on which the horizontal and rotational load-displacement material curves in the region before yield are assumed to have a hyperbolic relationship. The regression equation for the initial stiffness accompanying the model is also suggested for single clay and soft-over-stiff clay. A fully coupled SSI analysis with a soil continuum model is performed to validate the proposed model. Large soil deformation accompanied by deep penetration is considered simultaneously in the structural analysis of jack-up. As a result, inside the yield envelope, the existing model may overestimate the moment acting on the soil, thereby underestimating the bending moment at the hull-leg joint. The Model B developed and proposed in this paper well predicts the soil response and bending moment distribution of the leg, which results are validated in comparison with those of the soil continuum model. The proposed model can also simulate the hysteresis soil response for a sinusoidal load like in the soil continuum model. These effects make that behavior of soil and structure before yield is well simulated, therefore, efficient design is available in terms of the fatigue strength assessment.