Frequency Domain Analysis of Truss Spar Platform

ABSTRACT
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.
Keywords: Kikeh truss spar, Dynamic response, Frequency domain, Wave spectrum.