Comparison of Åsgard B platform's field measurement and time domain numerical simulations

Abstract

Moored floating platforms are used for drilling and exploration activates and they require high degree of precise positioning to perform optimally with associated facilities. They are subjected to combined environmental loads of waves, wind and current while in service, which affects their stability in addition to positioning. Dynamic analysis of a floating moored platform is therefore carried out to determine its response to environmental loads. This analysis could be done in the time domain or the frequency domain. A floating platform is associated with lots of non-linearities, which are linearised in the frequency domain simulations. In the time domain analysis however, the non-linearities are modelled with the intention of making more accurate analysis but this makes the computation complex and require a great deal of computer time. GVA Consultants have developed software, CASH (Coupled Analysis Software for Hydrodynamics) for time domain analysis of a moored floating platform. The program performs dynamic analysis in a combined state (low-frequency and wave –frequency) in the six degrees of freedom. The program had earlier been verified against other software and a good agreement has also been obtained between CASH and model test. The objective of the thesis work therefore is to compare CASH simulation with measured field data STATOIL has measured the environmental loads, motion responses and line tensions of Åsgard B semi-submersible platform in the North Sea and data are to be used for CASH simulation and results compared afterwards . The model test performed on Åsgard B was used to calibrate wind, current and wave drift force coefficients which form part of the input to CASH for the field environmental load analysis. Three different sea states (in increasing order) were selected from the measured field data to perform CASH analysis. CASH numerical simulations agreed with the model test results and measured field data in terms of mean offsets and mean line tensions. The disparity in standard deviation is suggested for further investigation.