Modeling of Seabed Interaction of Oceanographic Moorings in the Frequency Domain
Publication: Journal of Waterway, Port, Coastal, and Ocean Engineering
Volume 132, Issue 6
Abstract
A simple empirical model for the interaction of a catenary mooring with the seabed is investigated in the frequency domain. The moorings that are considered here are low-pretension moorings often used in shallow waters. In these moorings, the catenary stiffness is more important than the elastic stiffness. The lower portion of the mooring is replaced by a mass–spring–damper system to include the effects of inertia, damping, and stiffness whereas the inertia and damping terms often have been neglected in previous studies. In this paper, the definitions of low-pretension and high-pretension moorings are clarified, and the parameters that determine whether the catenary or elastic stiffness is dominant are identified. The paper discusses the determination of the equivalent mass, stiffness, and damping as functions of static variables in these moorings, and the contribution of these terms toward the dynamic tension. It is shown that without the inertia term, the quadratic trend of the dynamic tension amplitude with the forcing frequency cannot be reproduced. It is also found that the aspect ratio, the ratio of the horizontal to the vertical dimension of the suspended portion of the mooring, is the critical parameter that determines whether the mooring behaves as a low-pretension or high-pretension mooring. In addition, it is found that, in low-pretension shallow water moorings, the equivalent mass, damping, and stiffness are also functions of the aspect ratio only.
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Acknowledgments
The writers gratefully acknowledge the support of the Office of Naval Research (Ocean Engineering and Marine Systems) under Grant No. ONRN00014-92-J-1269 and the Deep Ocean Exploration Institute of Woods Hole Oceanographic Institution. Additional support for the first writer was provided by a Woods Hole Oceanographic Institution postdoctoral fellowship.
References
Brown, D. T., and Mavrakos, S. (1999). “Comparative study on mooring line dynamic loading.” Mar. Struct., 12(3), 131–151.
Chatjigeorgiou, I. K., and Mavrakos, S. A. (1998). “Assessment of bottom-cable interaction effects on mooring line dynamics.” Proc., 17th Int. Conf. on Offshore Mechanics and Arctic Engineering, ASME, Lisbon, Portugal, No. OMAE98-0355.
Gobat, J. I. (2000), “Dynamics of geometrically compliant mooring systems.” Ph.D. dissertation, WHOI/MIT Joint Program.
Gobat, J. I., and Grosenbaugh, M. A. (1998). “WHOI cable: Time domain numerical simulation of moored and towed oceanographic systems.” Proc., Oceans ’98, Vol. 3, IEEE, Nice, France, 1681–1685.
Gobat, J. I., and Grosenbaugh, M. A. (2000). “WHOI cable v2.0: Time domain numerical simulation of moored and towed oceanographic systems.” Technical Rep. No. WHOI-2000-08, Woods Hole Oceanographic Institution, Woods Hole, Mass.
Gobat, J. I., and Grosenbaugh, M. A. (2001a). “A simple model for heave-induced dynamic tension in catenary moorings.” Appl. Ocean. Res., 23(3), 159–174.
Gobat, J. I., and Grosenbaugh, M. A. (2001b). “Dynamics in the touchdown region of catenary moorings.” Int. J. Offshore Polar Eng., 11(4), 273–281.
Gobat, J. I., and Grosenbaugh, M. A. (2005). “Time-domain numerical simulation of ocean cable structures.” Ocean Eng., accepted.
Gobat, J. I., Grosenbaugh, M. A., and Triantafyllou, M. S. (2002). “Generalized- time integration solutions for hanging chain dynamics.” J. Eng. Mech., 128(6), 677–687.
Grosenbaugh, M. A., Anderson, S. P., Trask, R., Gobat, J. I., Paul, W., Butman, B., and Weller, R. A. (2002). “Design and performance analysis of a horizontal mooring for upper ocean research.” J. Atmos. Ocean. Technol., 19(9), 1376–1389.
Inoue, Y., and Surendran, S. (1994). “Dynamics of the interaction of mooring line with the sea bed.” Proc., 4th Int. Offshore and Polar Engineering Conf., ISOPE, Osaka, Japan, 317–323.
Kwan, C. T., and Bruen, F. J. (1991). “Mooring line dynamics- comparison of time domain, frequency domain, and quasi-static analyses.” Proc., Annual Offshore Technology Conf., OTC, Houston, 95–108.
Ong, P. A., and Pellegrino, S. (2003a). “Modeling of seabed interaction in frequency domain analysis of mooring cables.” Proc., 22nd Int. Conf. on Offshore Mechanics and Arctic Engineering, ASME, Cancun, Mexico, OMAE2003-37465.
Ong, P. A., and Pellegrino, S. (2003b). “Modeling of touchdown behavior of catenary moorings in frequency domain.” Proc., 5th Int. Symp. on Cable Dynamics, A.I.M., Santa Margherita, Italy.
Triantafyllou, M. S., Bliek, A., Burgess, J., and Shin, H. (1986). “Mooring dynamics for offshore applications: Part I, Theory.” TechnicalRep. No MITSG 86-1, MIT Sea Grant College Prog, Cambridge, Mass.
Triantafyllou, M. S., Bliek, A., and Shin, H. (1985). “Dynamic analysis as a tool for open-sea mooring system design.” Soc. Nav. Archit. Mar. Eng., Trans., 93(1), 303–324.
Webster, W. C. (1995). “Mooring-induced damping.” Ocean Eng., 22(6), 571–591.
Wu, S. (1999). “Investigation into three mooring line-seabed interaction models for frequency-domain mooring line dynamic analysis.” Proc. 9th Int. Offshore Polar Engineering Conf., Vol. II, ISOPE, Brest, France, 320–325.
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© 2006 ASCE.
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Received: Jan 31, 2005
Accepted: Nov 1, 2005
Published online: Nov 1, 2006
Published in print: Nov 2006
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