Frequency Domain Analysis of Truss Spar Platform

Spar is a type of deepwater floating type of platform used in ultra deep water. Malaysia has recently installed its first spar at Kikeh field near Sabah and is the first one ever installed outside the Gulf of Mexico. In this study, dynamic analysis of a typical truss spar in frequency domain has been conducted and the motion responses in surge, heave and pitch have been evaluated. The truss spar has been modeled as a rigid body with three degrees of freedom (i.e. surge, heave, pitch) at its centre of gravity, connected to the sea floor by ten multi-component catenary mooring lines attached to the truss spar at the fairleads. Frequency domain analysis has been performed by choosing a suitable wave spectrum model to represent an appropriate density distribution of sea water at the site under consideration. The motion response spectra have been determined based on the wave spectrum for each of the motion and the motion response profiles are evaluated from the spectra. The maximum amplitudes obtained were 0.25 m for surge, 0.02 m for heave and 0,013 radians for pitch. These results have the same trend but lower amplitude compared to the responses obtained by time domain dynamic analysis.