Effects of Mooring Line Failure On the Wave Frequency Responses of Truss Spar Platforms

Mohamed, M.A.W. (Department of Civil Engineering, Universiti Teknologi PETRONAS) | Montasir, O.A. (Department of Civil Engineering, Universiti Teknologi PETRONAS) | Kurian, V.J. (Department of Civil Engineering, Universiti Teknologi PETRONAS) | Liew, M.S. (Department of Civil Engineering, Universiti Teknologi PETRONAS)



Experimental and numerical studies on a typical truss spar with intact and damaged mooring line conditions are presented. Physical model motions in surge, heave and pitch in addition to the mooring line tensions were measured. A MATLAB code named TRSPAR was developed for the dynamic analysis of truss spar. The numerical predictions agree very well with the measurements for the two structure conditions. Mooring line failure has shown insignificant effect on the wave frequency responses of the truss spar. However, for relatively low frequency waves, surge responses for mooring damage condition are lower than the corresponding intact mooring condition responses.


The challenging deepwater environment makes the traditional fixed offshore structures unsuitable. Therefore, alternative innovative platform concepts such as spar have been developed. In general, truss spar, which is the second generation, has low fabrication cost, less surge offset and less mooring line requirements compared to the first generation classic spar. In the late 1990s, development of truss spar concept advanced much with a large amount of research effort in model test (Prislin et al 1998, Troesch et al 2000), and theoretical study (Kim et al 1999, Luo et al 2001, Wang et al 2002). Spar platform has six degrees of freedom. However, the dominant motions are only three; i.e., surge, heave and pitch. It has natural frequencies of motions far below the dominant ocean exciting wave forces frequencies; this is due to its large mass and relatively small restoring stiffness. Mooring lines form an integral part of floating offshore structures. Many studies have been conducted on its effect on the structure motions. In general, methods of analysis of mooring line can be classified into two main categories known as quasi-static analysis and dynamic analysis. The main difference between these two methods is the effect of fluid mooring line interaction, which is considered only in the dynamic analysis.