Abstract

The spectral analysis approach is a very elegant and computationally efficient method of analyzing the fatigue life of offshore jacket platforms. The primary limitation of the approach is that it assumes linearity of both the structural system and the wave-loading mechanism. The approach is now widely used for the analysis of deepwater, dynamically responsive platforms where nonlinearities are usually not serious. There are also advantages associated with using the approach for shallow water platforms although nonlinearities then become significant, particularly the wave-loading mechanism. Various methods have been proposed to enable the spectral method to be used for some nonlinear situations, including a new approach which uses the Longuet-Higgins wave height-period joint probability density function in order to obtain a better linearization technique. This linearization process is associated with the particular wave heights chosen for producing the transfer functions. The new approach provides a better method for choosing the appropriate height of each so-called base wave case. In order to verify the new approach, a time series analysis, including wave-loading nonlinearities, has been adopted to obtain a reference fatigue life. The sea surface elevation spectrum has been decomposed into a set of equivalent harmonic components. The water particle velocities and accelerations were then individually evaluated and the appropriate (Morison’s) wave loading was computed for each time step in the sea surface time history. The structural stress response time history was then calculated, from which a fatigue life estimate was obtained. This paper presents the results obtained using this new approach, as well as comparative results obtained using the deterministic, spectral, and time domain approaches applied with a representative sea state. The results show that the deterministic-spectral method has a considerable amount of potential, especially for new design work where weight savings and/or increased confidence levels may be achieved.

Issue Section:
Research Papers
Topics:
Fatigue analysis, Water, Waves, Fatigue life, Seas, Density, Design, Emission spectroscopy, Linearization techniques, Offshore platform jackets, Particulate matter, Probability, Spectroscopy, Stress, Time series, Transfer functions, Weight (Mass)
1.
Battjees, J. S., 1970, “Long-Term Wave Height Distribution at Seven Stations Around the British Isles,” Deutsche Hydrographische Zeitschrift Jahrgang 25, pp. 179–189.
2.
Bishop, N. W. M., 1991, “Dynamic Fatigue Response of Deepwater Offshore Structures Subjected to Random Loading,” Structural Engineering Review, SER 76/11, Aug.
3.
Bishop, N. W. M., and Sherratt, Frank, 1989, “Fatigue Life Prediction From Power Spectral Density Data. Part 1, Traditional Approaches,” and “Part 2, Recent Developments,” Environmental Engineering, Vol. 2, Nos. 1 and 2.
4.
Bishop, N. W. M., Witz, J. A., Feng Q., and Kirkwood, M., 1994, “An Appraisal of the Deterministic-Spectral Method for Estimating Fatigue Damage of Offshore Structures,” 7th International Conference on the Behaviour of Offshore Structures, Vol. 3, July.
5.
Dirlik, T., 1985, “Application of Computers in Fatigue Analysis,” University of Warwick, Ph.D. thesis.
6.
Hasselman, K. et al., 1973, “Measurements of Wind Wave Growth and Swell Decay During the Joint North Sea Wave Project (JONSWAP),” Deutsche Hydro. Zeitschr, Reihe, A8.
7.
Longuet-Higgins
 
M. S.
,
1983
, “
On the Joint Distribution of Wave Periods and Amplitudes in a Random Wave Field
,”
Proceedings Royal Society
, Vol.
A389
, London, U.K., pp.
241
–
258
.
8.
Marex Technology Limited, 1991, “A Reassessment of Environmental Conditions in the Rough Field,” Report No. 1091, Oct.
9.
Martin, C. J., Grant, C. K., and Sproson, R. A., 1988, “Wave Height Distribution in Shallow Water,” Advances in Underwater Technology, Ocean Science and Offshore Engineering, Vol. 16, Oceanology 88, pp 77–88.
10.
Pierson, W. J., and Moskowitz, L., 1964, “A Proposed Spectral Form for Fully Developed Wind Seas Based on Similarity Theory of S. A. Kitaigorodskii,” Journal of Geophysical Research, Vol. 69, No. 24.
11.
Williams
 
A. K.
, and
Rinne
 
J. E.
,
1976
, “
Fatigue Analysis of Steel Offshore Structures
,”
Proceedings, Institute of Civil Engineers
, Part 1, Vol.
60
. pp.
635
–
654
.
This content is only available via PDF.
Copyright © 1996
 by The American Society of Mechanical Engineers
You do not currently have access to this content.