Fatigue Analysis of Semi-submersible

Abstract

Semi-submersible rig is widely used in offshore oil and gas industry, mostly for drilling, exploration and accommodation purpose. It can be operated in deep waters and harsh environments. In comparison to Jack-Up rigs using in shallower waters, semi-submersible rigs are with higher payload capacity and deck area. The design of semi-submersible has evolved much since the very first generation and for this report, a braceless ring pontoon semisubmersible with four columns are studied. Some modules from Sesam, a software suite developed by DNV-GL are used to perform the analyses. Finite element models are prepared based on technical drawings with some reasonable simplification and hydrodynamic analyses are performed to estimate the global responses of the structure. The global responses of the unit in terms of motion and loads are studied.

By using charts and graphs available in the documents of classification societies as a reference, a quick screen is performed to find out the allowable stress range and then together with the results of hydrodynamic analyses, the critical locations with stresses higher than the allowable stress range that need further analyses such as the connection points between pontoons and columns are selected. Stochastic fatigue analyses are then performed to estimate the fatigue life and usage factor of the location concerned.

In addition, the existing semi-submersible is compared with the original design without sponsons added on columns to check the possible impacts of sponsons in motion, hydrodynamic load distribution and fatigue life