Integrated Structural Analysis Methodology for Truss Spars

Eleven truss spars have been successfully installed in the deep water fields since late 2001. Compared with other floating systems, Truss spars offer significant advantages in motions, stability, and project schedule. One of the unique aspects of a truss spar is that it exhibits both high-frequency and low-frequency motion responses. The high-frequency motions, or wave frequency motions, are peaked around the wave spectral energy, while the low-frequency motions correspond to the natural periods of the spar’s rigid-body motions. Accurate structural design should include loads due to both wave and low frequency motions. The wave-frequency motions can be accurately estimated with potential/diffraction theory, but the low-frequency motions cannot be accounted for using the traditional spectral method. The traditional spectral method may be acceptable to other types of platforms such as Semi-submersibles and TLPs, but can become non-conservative for a spar structure. In the past, this challenge was overcome by performing time domain analysis to design the truss and a combined time and frequency domain analysis to design the other structural components. The procedure proved to be time consuming and inefficient, requiring extensive engineering hours. The hull design process was enhanced by developing an integrated structural analysis methodology. The methodology significantly reduces engineering hours and maintains accuracy in the estimation of loads by a combination of the wave frequency and low frequency motion responses. Efficient use of personnel for the labor-intensive structural modeling tasks was also achieved. Use of this methodology in two spar projects has proved to add significant value. The procedure is also applicable to a range of floating platforms such as Technip’s extended draft platform (EDP) and other deep draft floating platforms. Salient features of the integrated structural analysis methodology for both strength and fatigue analysis of the truss spar are discussed in the paper. Structural loads determined from the integrated methodology are compared with those from a complete time-domain analysis of the truss spar.