When installing subsea structures and pipelines, the ability to capture the worst case loading and limiting sea states during the installation analysis can improve safety offshore and lower the operational risks. Therefore, it is important to select the correct analysis method to obtain the most accurate results.
Time-domain irregular wave dynamic analysis is performed using OrcaFlex as this is representative of a real sea state as opposed to regular wave. For irregular wave analysis, DNVGL-RP-N103 recommends performing a 3-hour simulation to ensure adequate statistics. However, this is very computationally expensive to perform due to the large quantity of load cases required. Therefore, it is common practice to analyse a short simulation period (e.g. 100s) that incorporates the expected worst-case loading during a 3-hour simulation at the point of interest on the vessel, such as crane tip or tensioner exit.
A short simulation can be defined by the largest rise/fall of a wave or extreme vessel response (max/min heave, velocity or acceleration). Knowing which method will capture the worst-case loading and associated limited sea states is critical in the analysis. As such, a study will be performed using OrcaFlex to determine which method correlates most with the worst-case loading for subsea structure and pipeline installation.
The study will involve running a 3-hour simulation to determine the time at which each extreme amplitude occurs for a range of wave heading/period combinations, using a static model containing just the installation vessel. The analysis will then be re-run for a 100s duration, with the simulation starting at least 50s before the extreme amplitude time occurrence, using a static model of the complete subsea installation scenario.
The outcome of the study is to suggest the most suitable method for obtaining the worst case loading for installation of subsea structures, pipelines (rigid and flexible) and umbilicals.