Morison Equation in Practice and Hydrodynamic Validity
- Jin S. Chung (ISOPE)
- Document ID
- International Society of Offshore and Polar Engineers
- International Journal of Offshore and Polar Engineering
- Publication Date
- March 2018
- Document Type
- Journal Paper
- 11 - 18
- 2018. The International Society of Offshore and Polar Engineers
- offshore structures, wave force, engineering practice, hydrodynamic force, risers, pipelines, velocity potential, Morison equation, hydrodynamic validity
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- 39 since 2007
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This paper presents a practical interpretation of the hydrodynamic force equation developed based on the velocity potential. It expresses the equation in an original version of the Morison equation as well as in its subsequent numerous extended or modified versions in the industry design practice. It is demonstrated that the hydrodynamic force equation derived from a velocity potential for slender structures can be expressed term-by-term in a form of the original as well as modified versions of the Morison equation as used for floating and flexible structures in motion in waves. From this expression, the validity range of Morison equation is presented and discussed from the point of view of the hydrodynamic force equation. Also further discussed are: (a) the drag term empirically added to the hydrodynamic force equation, similar to the term in the Morison equation; (b) the use of the frequency-dependent, added mass and radiation (or wave) damping accounting for free-surface effect; (c) periodic time-dependency in practice; and (d) member diameter relative to incoming wave length.
Since the late 1940s, oil and gas drilling and production activities have been moving from coastal to offshore areas. The industry faced a new challenge in the design of offshore bottom-fixed pile structures in accounting for current and water waves in shallow depth: wave and current-induced forces on the submerged part of the pile structures.
Since the 1950s, the Morison equation has been applied to the design of numerous jacket platforms, risers, and pipelines with only few failures reported. This indicates that the Morison equation has worked as a safe design tool for the offshore structures and equipment for more than half a century. Even these days, however, there have been occasional debates over the validity of the equation, from some theoretical hydrodynamics points of view, and on the accuracy or applicability of various extended or modified versions of the equation. Expressions of periodic forces in the Morison equation used in the industry have been disputed by some. This prompted the author to make the connection between the Morison equation (Morison et al., 1950) and theoretical hydrodynamic equations (Chung, 1975, 1976).
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