This paper has investigated the effect of mooring failure on the global performance of a typical offshore structure under regular surface wave. A self-developed time domain program has been set up to capture the transient response of the system when one or several mooring lines are simultaneously disconnected from fairlead. The results indicate that the whole response could be divided into healthy stage, transient stage and steady stage accordingly. The low frequency of platform is almost decreased with increasing number of broken cables due to the reduced stiffness. The remaining mooring lines on the up-wave side and close to the broken position would exhibit profoundly increasing tension range. The riser tension is less affected by mooring failure due to its vertical configuration and low tension level compared to mooring tension.
A typical floating offshore system usually concerns three subsystems: the top platform, the mooring system and riser system. They influence each other and respond to environmental loading in a complex way. The positioning mooring lines are likely to be broken when subject to harsh conditions in deeper water, which could result in the instability of production system or even offshore accident. Therefore, an accurate description of the system dynamics under the failure of mooring lines could be a challenging but essential problem.
A majority of researchers have demonstrated the theory of the coupled floating system and conducted its performance under broken moorings / risers. Veritas (2010) introduced 2 methods for the dynamics of coupled system: fully coupled analysis, that is the mass/damping/stiffness matrixes of riser and platform are assembled together to realize an overall solution, and efficient analysis, which means that the forces and moments induced by the moorings or risers will act on the platform. Meanwhile, the motions of upper nodes from these slender structures will keep consistent with those of connection points on the platform hull. Liu, et al (2016) combined the ABAQUS modular for riser and self-developed FORTRAN modular for drilling platform and exchanged data with each other to consider the coupled effect. Caire (2012) employed the coupled analysis based on SESAM and compared the results between the coupled and de-coupled method. Hao, et al. (2020) mainly studied the hydraulic pneumatic tensioner in TLP-TTR system under its local failure based on AQWA. Cheng, et al. (2021) conducted the detailed response of TLP, i.e., time series, spectral analysis, phase diagrams and tendon stiffness, under the condition of tendon broken. Chuang, et al. (2020; 2021) investigated the mooring disconnection in the SEMI-mooring-riser system, aiming at the response of platform, tension of remaining lines and risk of riser clashing. More broadly, Bae, et al. (2017) carried out a complex simulation of floating wind turbine with broken mooring line considering the aerohydro-mooring coupled effect in the accident.
