Global Response Analysis for Semisubmersible Offshore Platform

Abstract

As we are moving to the deeper and harsher environments in search of oil and gas resources, it has been more challenging to ensure the structural integrity of the offshore platform being capable of withstanding the extreme environmental loading. For such deep and harsh environmental conditions the wave induced loads constitute the major part of overall loading on the offshore structure. Therefore, it is important to quantify wave-induced load effects to ensure a reasonable, safe and robust design of the offshore systems. One of the approaches to accomplish this task is by performing a global response analysis with the extreme hydrodynamic loading on the offshore floating platform.

The main objective of the thesis has been to present a case study of performing a global response analysis for a semisubmersible platform which is one of the common offshore floating structures in the industry. The approach of the case study has been to model a global structural model of the platform and perform the global response analysis with the applied hydrodynamic loading of the waves recommended by the offshore design codes, checking the impact of the wave induced loading on the structural integrity in extreme environmental loading conditions in the critical structural members of pontoon and column sub assembly exposed to the wave loading. The structural response is evaluated in terms of motions response of the platform and stress distribution on terms of element average Von mises stress on to the structural components due to the load effects induced by these environmental loads. A global structural model of semisubmersible platform consisting of beam and shell finite elements has been used to perform the required global response analysis of the structure. For the structural modeling and analysis of the semisubmersible platform under consideration a standard finite element modeling software named SESAM GeniE has been used. Moreover to perform the hydrodynamic response analysis another software tool, HydroD Wadam has been used which is a general purpose hydrodynamic analysis program based on 3D potential theory for calculation of wave loading and wave induced responses.

To evaluate the wave induced loads for the semisubmersible platform a comparative study regarding the strength analysis concepts following two different design philosophies namely LRFD and WSD methods has also been discussed in this master thesis. This study has been performed by reviewing the different offshore codes & standards requirement for extreme environmental loading by the classification societies like DNV, ISO & API for the offshore floating column stabilized structure.

The results of the analysis have been presented as the motion response of the global structural model and checking for the structural integrity in terms yield utilization or stress distribution of the structural components exposed to wave loading. This allows us to identify the critical highly stressed region for the platform and to suggest about these critical zones requiring further detailed local analysis for the assurance of robust design. Moreover it has been concluded with the analysis performed that among WSD and LRFD methods, which is more conservative design approach for the Ultimate Limit State with application of extreme environmental loading for the case study of semisubmersible platform.