Structural analysis of a jack-up platform in extreme and operational conditions

Abstract

The site-specific in-place analysis of jack-up vessels must be carried out before each of its offshore wind turbine installation operations. This analysis is necessary to ensure the safe operational limits of the jack-up vessel for a site of operation. In this thesis, the site-specific analysis of a jack-up vessel in the operational and survival condition has been carried out by complying with the established offshore industry standards for site-specific analysis such as ISO 19905-1\cite{iso201619905} and SNAME 5-5A\cite{SNAME5-5}. The goal of this thesis is to develop an in-house Python toolbox for conducting a quick preliminary site-specific analysis of the jack-up vessel.

The first step of site-specific assessment is modelling the structure. Among the different modelling methods proposed in ISO 19905-1, the equivalent stiffness modelling technique is selected for developing the model using the 3D beam finite element method. The modal analysis of the simplified platform is conducted by solving the eigenvalue problem using the inverse iteration power method and the natural periods were obtained. The obtained natural periods were checked with the values from the in-house reference report.

In the next part of the thesis, the environmental load acting on the jack-up vessel are considered to be from wave/current and wind actions. These loads are determined following the industrial standards such as ISO 19905-1 and SNAME 5-5A. Morison equation is used for calculating the hydrodynamic forces due to wave and current acting on the jack-up vessel as recommended by ISO 19905-1. The current profile for different service conditions of the vessel is adopted from the in-house reference report. The wind actions on the jack-up vessel are determined from the windage area, wind profile and the wind pressure acting in the projected area. The wind profile for the load calculation is derived from the wind reference velocity, adhering to the ISO 19905-1 guidelines.

Finally, the responses of the jack-up vessel due to the environmental actions such as the base shear and overturning moments are determined using the empirical single degree of freedom (SDOF) method. The SDOF enables rapid estimation of structural responses in a conservative manner. The still water reactions on the jack-up legs are also an output of the program developed, which can be used for preload checks of the vessel. In addition to the current SDOF model, the complex random wave analysis method(Drag Inertia parameter method) is implemented in the python program, which is presently in the analysis and troubleshooting phase.