A large vertical cylinder with a deep draft which is moored red with linear springs in waves is analyzed both theoretically and experimentally. The theory is based on the second-order diffraction-radiation problem. A nonlinear damping term based on the viscous drag force is also included in the motion analysis. The theory is extended from regular waves to apply to irregular waves. The test consisted of measuring the six degrees of motion of the cylinder and the loads in the mooring lines in regular waves, wave groups and irregular waves. The theoretical results are correlated with the experimental data.


The sea keeping characteristics of a large floating moored vertical cylinder are investigated in this paper. The investigation includes a theoretical analysis and the corresponding experimental results. The results presented in this paper will have direct applications in the design and analysis of tension leg platforms, deep water spar-type loading and/or storage tanks and semi-submersibles. Moreover, the analysis and data presented here are general and basic in nature so that they should find wide application in the design of any large floating or fixed offshore structure.

The horizontal load and motion of the cylinder consist of three principal parts: a steady component, a high frequency oscillating component and a low frequency oscillating component near the system natural frequency the surge. The high frequency components are linear in nature and occur at the same frequency as the incident wave, whereas the steady and the low frequency components are a consequence of the second-order contributions. Since the low frequency drift force may excite the natural frequency of the system where linear damping is small, the dynamic amplification may be quite large resulting in responses often greater than the high frequency linear response. Investigation into nonlinear damping terms likewise is important. This study provides extensive test results and theoretical verifications in these areas which should aid in the design of such systems.

The calculation of wave forces on fixed vertical cylinders has been the subject of study for many years. Havelock (1) obtained a closed form expression for the first-order horizontal force and steady second-order force on a large vertical cylinder in deep water. Mac Camy and Fuchs (2) later extended the first order theory for finite water depths. They also obtained approximate expressions for the steady second-order force components on the cylinder. Flokstra (3) derived a shallow water approximation for the second-order steady force on a fixed cylinder using the expression of Havelock. Complete expressions for the second-order steady force on a fixed vertical cylinder have recently been derived by Chakrabarti (4).

The steady drift forces on a floating, moored vertical cylinder, on the other hand, must be obtained through a numerical technique by solving the first order diffraction-radiation problem. The method was developed by Pinkster (5) for a body of general shape.

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