This paper discusses a time-domain method for fatigue analysis of truss Spars' critical connections. The method is part of an integrated procedure used in the design of truss Spars. Critical connections include truss-to-hard tank, truss-to-soft tank and hard tank-to-topsides connections. Truss-to-hard tank connections require particular attention since these connections constantly sustain a significant tensile load. The procedure requires wave analysis of a global structural model by time-domain simulations and local finite element analysis of connection details. Force time histories of members framing into a connection are combined with stress influence factors (SIFs) calculated with local FE models to produce time traces at selected hot spots. Fatigue damages at these hot spots are then calculated by a rainflow counting algorithm. Example results are presented based on a generic truss Spar design.
The truss Spar structural concept is now entering the market for offshore oil and gas exploitation, classified as large floating vessels. The Kerr McGee's Nansen and Boomvang Spars in the final stages of construction are the first two truss Spars in the world, to be followed by BP's Horn Mountain Spar. Similar to classic Spars, tress Spars are designed for deep water, the three first ones covering water depths from around 1045m to 1650m. A truss Spar consists of four parts: soft tank, tress, hard tank and topsides. The upper section is a cylindrical hard tank providing buoyancy for the Spar. The soft tank at keel is designed to accommodate fixed ballast required for stability and keeping pitch motion within reasonable limits. The truss section links the hard tank and soft tank, also gives support to heave plates that are designed to increase heave damping. Topsides area and weight are mainly predetermined by drilling/production capacity requirement.
