Abstract

There are many unique challenging aspects in the structural analysis and design of a Truss Spar. This paper provides indepth discussions on inplace strength analyses of the truss structure based on the author's firsthand design experiences. Inplace structural strength design criteria are outlined, and an integrated procedure based on random wind and wave time domain analyses is presented. Technical issues related to structural and hydrodynamic modeling, computer software requirement, time domain environmental load simulation and extreme global load prediction for structural design, structural load mapping, and time domain design load case selection are discussed. Analysis results are presented for a generic Truss Spar design to illustrate the various loading conditions for the Gulf of Mexico environment and inplace structural behavior of the truss structure.

Introduction

The first two Truss Spars have been installed in 2001 in the Gulf of Mexico. More Truss Spars are currently in various design and fabrication stages, and will be installed in the future. Compared to the Classic Spar, the Truss Spar design replaces the cylindrical midsection of the hull with the truss section as shown in Figure 1, hence the name Truss Spar. The truss section is a space frame consisting of tubular members similar to a jacket type structure. The truss structure is inherently more efficient, with well over 50% less steel weight than the plated cylindrical midsection of the Classic Spar, as a structural link between the upper hard tank and the soft tank at the keel of the platform. The heave plates are integral parts of the truss design, normally consisting of a stiffened plate structure with girders supported by the truss horizontal braces, knee braces and vertical braces. The main function of the heave plate is to provide added mass and damping for heave motion of the Spar platform. The truss and heave plates also provide lateral support for the risers.

For inplace conditions, the truss structure is subjected to continuous dynamic loading due to wave excitations and Spar motions. Since the truss section has the least structural redundancy compared to the hard tank and the soft tank, considerable structural analysis and design efforts are required to ensure the adequacy of the truss and its connections for structural strength and fatigue. Unlike the tension leg platform and other floating platform concepts, the Spar platform is a truly compliant system, whose natural periods for all the six degrees of freedom of motions are significantly greater than the predominant wave period. Hence, the low frequency loads, such as slow varying wind load, wave drift force, and second order nonlinear wave excitations, will have significant impact on the dynamic response of the Spar. In particular, the time varying dynamic wind load contributes significantly to the pitch/roll motion, which in turn contributes the most to the global bending moment and shear force in the truss structure.

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