ABSTRACT:

This paper deals with the dynamic responses to top end excitation of marine risers/pipes conveying internal fluid. The marine riser is often used as a flexible link between undersea bore head and subsurface offshore platform. The tidal waves and the changes of sea level consistently excite its top end connected to a floating vessel. In order to carry out the performance-based design of the marine risers, the evaluation of their dynamic responses to top end excitations is imperative. In this study, the marine riser is simulated using twodimensional beam elements. Energy functional of the marine risers conveying fluids is derived from variational principle. Nonlinear equations of motion influenced by the nonlinear Morison waveform are obtained through Hamilton's principle. Investigation of the dynamic responses of marine risers to top end excitation is achieved using the finite element method and Newmark Average Acceleration Method. Interestingly, either beating or resonant phenomenon can be observed from the responses. It is also found that the top tension plays a major role in the increment of undamped frequencies of marine risers, while either the internal flow rate or the external hydrodynamic drag force remarkably affects the displacement amplitudes of the marine risers' dynamic responses.

INTRODUCTION

There are a number of different types of offshore platforms, which are depending on the functional uses. All of the offshore structures are generally designed against the dynamic load actions due to the severe wind and wave interactions. Marine risers/pipes are part of the offshore platform, generally attached to the platform structures or floating vessels as a transportation means for the hydrocarbon resources underneath the sea bed e.g. crude oil, natural gas, and so on. Generally, the dynamic excitations acting on the marine risers/pipes are due to the ocean waves resulting in the complicated movements of the vessels.

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