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Spectral Fatigue Analysis for Topside Structure of Offshore Floating Vessel
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 Title & Authors
Spectral Fatigue Analysis for Topside Structure of Offshore Floating Vessel
Kim, Dae-Ho; Ahn, Jae-Woo; Park, Sung-Gun; Jun, Seock-Hee; Oh, Yeong-Tae;
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 Abstract
In this study, a spectral fatigue analysis was performed for the topside structure of an offshore floating vessel. The topside structure was idealized using beam elements in the SACS program. The fatigue analysis was carried out considering the wave and wind loads separately. For the wave-induced fatigue damage calculation, motion RAOs calculated from a direct wave load analysis and regular waves with different periods and unit wave heights were utilized. Then, the member end force transfer functions were generated covering all the loading conditions. Stress response transfer functions at each joint were produced using the specified SCFs and member end force transfer functions. fatigue damages were calculated using the obtained stress ranges, S-N curve, wave spectrum, heading probability of each loading condition, and their corresponding occurrences in the wave scatter diagrams. For the wind induced fatigue damage calculation, a dynamic wind spectral fatigue analysis was performed. First, a dynamic natural frequency analysis was performed to generate the structural dynamic characteristics, including the eigenvalues (natural frequencies), eigenvectors (mode shapes), and mass matrix. To adequately represent the dynamic characteristic of the structure, the number of modes was appropriately determined in the lateral direction. Second, a wind spectral fatigue analysis was performed using the mode shapes and mass data obtained from the previous results. In this analysis, the Weibull distribution of the wind speed occurrence, occurrence probability in each direction, damping coefficient, S-N curves, and SCF of each joint were defined and used. In particular, the wind fatigue damages were calculated under the assumption that the stress ranges followed a Rayleigh distribution. The total fatigue damages were calculated from the combination with wind and wave fatigue damages according to the DNV rule.
 Keywords
topside structure;spectral fatigue analysis;motion RAO;dynamic wind spectral fatigue analysis;
 Language
English
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 References
1.
American Petroleum Institute, RP 2A-WSD, "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms- Working Stress Design", Twenty-First Edition, December 2000.

2.
Bentley Systems Inc., SACS Manual (2012).

3.
Det Norske Veritas, DNV-RP-C203, "Fatigue Design of Offshore Steel Structures", October 2012.

4.
Det Norske Veritas, DNV-RP-C205, "Environmental Conditions and Environmental Loads", April 2007.

5.
Holmes, John D (2007). Wind Loading of Structures, Taylor & Francis, 340.