Structural modelling of offshore module for loadout, transportation and installation

Abstract

The demand for oil and gas, as well as the proceeding into new harsh areas in the north for exploring and producing oil and gas keep pushing the boundaries of offshore engineering into ever-deeper waters and harsher environments. The exploration and production activities require correct modelling of the offshore platforms to tolerate these challenges. This requires accurate modelling of offshore modules throughout all phases. This thesis will focus on modelling of an offshore module. A previous Master’s thesis in UiS, (Desta, 2012) has already considered loads for designing of an offshore module placed on a platform but neglected transportation, lifting and installation loads. The effects of these loads are significant on the Norwegian Shelf and they will be discussed in the present study. Available literature is reviewed and the modelling is performed by applying the software SAP2000. The modelling will be limited to modelling of a module on a fixed steel platform. The main focus of the thesis is to stress the many important considerations and conditions during the loadout, transportation, lifting and installation phases. During transportation, the motions of the transport barge is of key concern. During the lifting/ installation phase, the moments of the module caused by the lift arrangement, dynamics as well as possible impact forces must be considered. Moreover, the relative motion between the barge and the crane is of primary importance. These load conditions are evaluated and optimum dimensions are computed. These dimensions are compared to the results given in the Master thesis prepared by Desta (2012) to validate weather the dimensions evaluated by Desta (2012) are within acceptable limits for loadout, transportation, lifting and installation procedures. This thesis work is performed at the University of Stavanger. In the thesis work, we have in initially selected material properties and cross section geometries for the module from Desta, (2012). The report presents an introduction to the selection of design factors in the different operational phases and presents a comparison of these factors as given in NORSOK, ISO and DNV-GL standards. Structural analysis are carried out for the transportation, lifting and installation phases to check the load capacity of the module and to see if the structure can withstand the different effects during these phases. The design principles and methods will be discussed extensively. Subjective risk analysis of load-out, transportation and lifting/installation has also been performed and suitable risk reducing measures are proposed. At the end, the discussions and the main findings, challenges and recommendations based on the modelling results and experiences during the thesis work are highlighted. Recommendations on how to obtain an efficient design process backed by risk assessment are also presented. It should be noted that the words ‘action’ and ‘load’ are used interchangeably in this report.