Design storm analysis simplification study for flexlay installation systems

Abstract

When doing installation analysis of flexible pipe systems one has to do dynamical analysis. The goal of said analysis is to find the limiting sea states that various operations or installations can take place in. To do this time domain analysis has to be utilized. The procedure is to make a design storm to check that the installed product can be installed safely. Due to the nature of flexible risers, the loads are often non-linear. Due to this non-linearity the time domain analysis is a heavy burden in terms of computer and engineering time. To get a good estimation of the design parameters it is normal to do 5 to 10 full 3 hour simulations for each wave parameter. When having to check a broad band of periods, wave heights and wave directions these calculations take too much time. The purpose of this thesis is to find faster methods of doing normal time domain analysis without reducing the accuracy of the results. Non-linear time domain analysis has been used to find benchmarking values for a given wave height. The numbers of realizations needed to accurately capture the statistical spread of the design parameters has been looked in to. A simplified method to shorten the time it takes to run a time domain analysis has also been looked into. The idea is to generate the full 3 hour wave trains, and finding the largest wave in this wave train. Then the analysis is completed for a small duration before and after the largest wave. The number of realizations needed for normal flexible lay operations is 5 to 6, when taking maximum axial tension and maximum curvature into consideration. If we go lower than 5 realizations the standard deviations are too large to have confidence in the results. The simplified method showed that 2.5 minutes of simulation before and after the largest wave gave results that mostly had less than 1% deviance from the full 3 hour simulation. For the curvature this was not the case for a few instances, but overall it seemed to give good results. This method should not be recommended if the generated design parameters are close to the critical values for the flexible pipe. For normal scenarios this method of doing the time domain analysis seems promising. The largest upside to using this method is that it can save around 90% of the computational time.