Time Domain and Frequency Domain Analysis for Truss Spar Platform

ABSTRACT
The spar platforms for offshore oil exploration and production in deep and ultra deep waters are
increasingly becoming popular. A number of concepts have evolved, among them the ‘classic’
spar and “truss” spar being the most prevalent. Time domain and frequency domain analysis
were used to determine the dynamic responses for a typical truss spar platform. The hyperbolic
extrapolation wave theory and modified Morison Equation were used for simulating the sea state
and for determining the dynamic force vector. The platform was modeled as a rigid body with
three degrees of freedom, surge, heave, and pitch restrained by mooring lines. In time domain,
Newmark Beta method was used to solve the equation of motion. Static offset tests were
numerically conducted to obtain the mooring line stiffness curves. In frequency domain analysis, the steady state responses were obtained. Time domain results were compared to frequency domain results in terms of response amplitude operator. These results were discussed and compared with numerical and experimental results in the available literature.
Keywords: spar; dynamic analysis; time domain; frequency domain; hydrodynamic responses;
rigid body; mooring line