Analysis Methods for Mooring Systems with focus on Accidental Limit State

Abstract

During the last years, the oil and gas industry has moved into new frontiers, which require more complex mooring solutions. The failure rate is unacceptably high, with 43 incidents for the Norwegian Continental Shelf alone, between 2000 and 2013. It is therefore interesting to compare the mooring system design tools used by the industry, and to investigate the behaviour of a damaged mooring system. The focus has been split between a comparison of mooring analysis in time domain and frequency domain, and an investigation of an accidental limit state(ALS) mooring analysis. The goal of the comparison was to understand the theory behind both methods, to verify, adopt, and simplify numerical models, and to perform analyses with the numerical models and compare the results. The frequency domain software MIMOSA and the time domain software SIMO/RIFLEX coupled, with the SIMA graphical user interface, were used for the analyses. The numerical simulation was initially meant to be performed for two entirely dierent mooring systems, but due to problems, only a catenary mooring system has been analysed. The analyses show that the tension in the mooring lines and the oset of the moored vessel are comparable for both methods. The frequency domain yields the most conservative results, which is expected due to linearisations and simplications made. For the ALS analysis, the behaviour of a damaged mooring system was analysed. This was performed in time domain only, with a catenary mooring system, where one mooring line was broken. The analysis investigated the increased load on the remaining mooring lines, and also the eect of when the mooring line failed. The analysis show that for the remaining mooring lines in the cluster with a broken line, the mean tension increases by 21% and the max tension increases by 25%. Further analyses showed that the broken system was still robust against failure, with only a 0.03% chance of further failure, when entering a 100-year storm with one line broken and 100% minimum breaking strength (MBS). With a MBS degraded to 80%, the chance of failure was only 1.38%. The transient motion phase after a line failure was not necessarily the governing design criterion. For the specic condition analysed, the tension in the mooring lines was lower when a mooring line failed during the storm, than it was when the failure took place before the storm. This means that the ultimate limit state design criterion was governing for this condition.