Hydrodynamic Forces on Flexible Offshore Structures
Publication: Journal of Engineering Mechanics
Volume 110, Issue 3
Abstract
The extension of Morison's equation to allow for structural motion in predicting the hydrodynamic force on offshore steel jacket platforms may be based on two different hypotheses: (1) The relative velocity model which replaces the fluid velocity by the relative velocity between the fluid and the structure; and (2) the independent flow fields model which considers the flow to be a superposition of two unrelated flows, one due to the wave‐current action on a rigid cylinder, and the other due to the structural motion in still water. An iterative computational procedure that combines time domain and frequency domain analysis techniques is developed to solve the nonlinear governing equations for both models. Comparison studies are carried out for seastates ranging from the drag dominant through the inertia dominant regimes. Results indicate that the independent flow fields model always predicts a higher structural velocity response, the difference increasing with wave height. There is negligible difference for the inertia dominant range. At intermediate seastates, which are of primary concern for fatigue analysis, the relative velocity model appears to overestimate the damping. Consequently, its use in fatigue life prediction may be questioned. For typical offshore platforms, the applicability of the independent flow fields model diminishes as the seastate approaches extreme values.
Get full access to this article
View all available purchase options and get full access to this article.
References
1.
Angelides, D. C., and Connor, J. J., “Response of Fixed Offshore Structures in Random Sea,” Journal of Earthquake Engineering and Structural Dynamics, Vol. 8, Dec., 1980, pp. 503–526.
2.
Berge, B., and Penzien, J., “Three‐Dimensional Stochastic Response of Offshore Towers to Wave Forces,” Proceedings of the Offshore Technology Conference, Paper No. 2050, 1974.
3.
Borgman, L. E., “Ocean Wave Simulation for Engineering Design,” Journal of Waterways and Harbors Division, ASCE, Vol. 95, No. WW4, Nov., 1969, pp. 557–583.
4.
Burg, J. P., “Maximum Entropy Spectral Analysis,” thesis presented to Stanford University, at Palo Alto, Calif., in 1975, in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
5.
Dunwoody, A. B., “The Role of Separated Flow in the Prediction of the Dynamic Response of Offshore Structures to Random Waves,” thesis presented to the Massachusetts Institute of Technology, at Cambridge, Mass., in 1980, in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
6.
Fish, P. R., and Rainey, R. C. T., “The Importance of Structural Motion in the Calculation of Wave Loads on an Offshore Structure,” Proceedings of the Second International Conference on the Behavior of Offshore Structures, Vol. 2, Paper No. 50, Aug., 1979, pp. 43–60.
7.
Gudmestad, O. T., and Connor, J. J., “Linearization Methods and the Influence of Current on the Nonlinear Drag Hydrodynamic Force,” Journal of Applied Ocean Research, Vol. 5, No. 4, 1983, pp. 184–194.
8.
Hogben, N., Miller, B. L., Searle, J. W., and Ward, G., “Estimation of Fluid Loading on Offshore Structures,” Proceedings of the Institution of Civil Engineers, Part 2, 63, Sept., 1977, pp. 515–562.
9.
Kawamoto, J., Shyam Sunder, S., and Connor, J. J., “An Assessment of Uncertainties in Fatigue Analysis of Steel Jacket Offshore Platforms,” Journal of Applied Ocean Research, Vol. 4, No. 1, 1982, pp. 9–16.
10.
Keulegan, G. H., and Carpenter, L. H., “Forces on Cylinders and Plates in an Oscillating Fluid,” Journal of Research of the National Bureau of Standards (United States), Vol. 60, No. 5, May, 1958, pp. 423–440.
11.
Laya, E. J., “Effect of Structural Motion on the Hydrodynamic Forcing of Offshore Steel Structures,” thesis presented to the Massachusetts Institute of Technology, at Cambridge, Mass., in 1980, in partial fulfillment of the requirements for the degree of Master of Science.
12.
Malhotra, A. K., and Penzien, J., “Nondeterministic Analysis of Offshore Structures,” Journal of the Engineering Mechanics Division, ASCE, Vol. 96, No. EM6, Dec., 1970, pp. 985–1003.
13.
Mes, M. J., “Why Some Structures Jerk in Stormy Seas,” Journal of Ocean Resources Engineering, Sept., 1977, pp. 24–33.
14.
Moan, T., Haver, S., and Vinje, T., “Stochastic Dynamic Response Analysis of Offshore Platforms,” Proceedings of the Offshore Technology Conference, Paper No. 2407, 1975.
15.
Moe, G., and Verley, R. L. P., “An Investigation into the Hydrodynamic Damping of Cylinders Oscillated in Steady Currents of Various Velocities,” Report of the River and Harbor Laboratory, Norwegian Institute of Technology, June, 1978.
16.
Moe, G., and Verley, R. L. P., “Hydrodynamic Damping of Offshore Structures in Waves and Currents,” Proceedings of the Offshore Technology Conference, Paper No. 3798, 1980.
17.
Morison, J. R., O'Brien, M. P., Johnson, J. W., and Schaff, S. A., “The Force Exerted by Surface Waves on Piles,” Petroleum Transactions, American Institute of Mining Engineers, Vol. 189, 1950, pp. 149–154.
18.
Oppenheim, A. V., and Schafer, R. W., Digital Signal Processing, Prentice‐Hall, Inc., Englewood Cliffs, N.J., 1975.
19.
Proceedings of the Third International Ship Structures Congress, Oslo, Norway, Sept., 1967.
20.
Sarpkaya, T., “Vortex Shedding and Resistance in Harmonic Flow About Smooth and Rough Circular Cylinders at High Reynolds Numbers,” Technical Report No. NPS‐59SL76021, Naval Postgraduate School, Monterey, Calif., Feb., 1976.
21.
Sarpkaya, T., “Vortex Shedding and Resistance in Harmonic Flow about Smooth and Rough Circular Cylinders,” Proceedings of the First International Conference on the Behavior of Offshore Structures, Vol. 1, 1976, pp. 220–235.
22.
Sarpkaya, T., and Isaacson, M., Mechanics of Wave Forces on Offshore Structures, Von Nostrand Reinhold, New York, N.Y., 1981.
23.
Sarpkaya, T., Coliins, N. J., and Evans, S. R., “Wave Forces on Rough‐Walled Cylinders at High Reynolds Numbers,” Proceedings of the Offshore Technology Conference, Paper No. 2901, 1977.
24.
Shyam Sunder, S., and Connor, J. J., “Sensitivity Analyses for Steel Jacket Offshore Platforms,” Journal of Applied Ocean Research, Vol. 3, No. 1, 1981, pp. 13–26.
25.
Wu, S. C., and Tung, C. C., “Random Response of Offshore Structures to Wave and Current Forces,” Sea Grant Publication No. UNC‐SG‐75‐22, Department of Civil Engineering, North Carolina State University, Sept., 1975.
Information & Authors
Information
Published In
Copyright
Copyright © 1984 ASCE.
History
Published online: Mar 1, 1984
Published in print: Mar 1984
Authors
Metrics & Citations
Metrics
Citations
Download citation
If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download.