To determine jackup specific touch-down operational limits, the spudcan-soil interaction needs to be considered and analyzed. Due to the complexity of this interaction during touch-down, a simplified spudcan-soil interaction model has been developed to simulate the interaction. The model uses a set of cylindrical pipes, whose properties are derived and validated by simulations that use actual spudcan geometry and accepted soil modeling techniques.
The objective of this paper is to introduce the procedure that has been developed to derive and validate the properties of the pipe set that is used in the simplified spudcan-soil interaction model. In the procedure, the derived properties of the pipe set are validated in both translational and rotational motions independently. In the translational motion validation, the properties of the pipe set are initially validated. Then in the rotational motion validation, the pipe set properties are further validated to assure the appropriateness of the simplified model.
In the translational motion validation, the actual spudcan-soil interaction is simulated using ABAQUS with the spudcan's geometry and 3D Mohr-Coulomb soil modeling for various spudcan penetration depths considering both vertical and lateral motions. The results of soil reaction from ABAQUS are then used to determine initial properties of the pipe set, which are then input to Orcaflex for the simplified simulation. By comparing the resulting vertical and lateral soil reactions between ABAQUS and Orcaflex, the initial properties of the pipe set are validated. The properties of the pipe set include: pipe arrangement, length, diameter, and hydrodynamic coefficients as well as the soil's normal stiffness and friction coefficients in the axial and lateral directions. The rotational motion validation uses the same procedure, but considers rotational motions.
This paper also presents an example analysis that illustrates the procedure that has been developed to perform the spudcan-soil-touch-down analysis using the simplified spudcan-soil interaction model. In the example, a fully coupled hull-leg-spudcan touch-down analysis is performed using Orcaflex. The analysis procedure can be used to develop jackup specific touch-down operational limits considering: different sea states, soil properties, water depth, and jack-up leg lowing speeds.