Dynamics of long, flexible pipes during surface towing and drift loads on flexible floating structures

The dynamics of long, flexible pipelines during ocean surface towing and the drift loads on flexible structures are investigated. The towing problem is formulated in both vector and scalar forms. The structural behavior of a free-free beam under axial loads is first analyzed. A previously unreported natural frequency is found and confirmed numerically using both the finite-element method and a symbolic mathematics package. Several useful formulas are given to obtain this new natural frequency. Two hydroelasticity theories are developed for the analysis of a towed, long flexible pipeline. The hydrodynamic forces on a large Ocean Thermal Energy Conversion (OTEC) pipe are obtained by simplifying the three-dimensional problem to a two-dimensional problem. The simplification leads to using the strip theory. To consider the drag effects, a second hydroelasticity analysis is also performed based on Morison's equation. The hydroelastic problems are solved numerically in both regular and irregular waves. It is found that for large-diameter OTEC pipes, except in the head sea condition where strip theory fails to give good results because of its inherent limitations, the two methods give almost identical results for incoming waves of large periods. However, when the pipe diameter is small, such as marine risers, only Morison's approach is applicable.; To study nonlinear wave forces on flexible structures, the second-order mean wave loads on flexible structures are obtained. The derived mean drift load formulas are first applied to rigid bodies and the calculated results are compared with published work. New results are obtained for several large flexible floating structures. The mean wave drift forces on a pipeline of 2 in diameter are found to be very small. The newly derived formulas can deal with the symmetry properties of the structure and of the hydrodynamic problem.; In working with hydroelastic problems on large floating structures, several useful relations are derived for flexible structures in regular waves. These theoretical relations are believed to be useful in checking the numerical results and they have been verified numerically.