The marine drilling riser is a key outfit which links the wellhead above seabed with the drilling vessel together and it is also a fluid passageway during drilling. It can be regarded as a special beam-column for the reason that it is surrounded by seawater and filled by drilling fluid, while there is tension load which varies along its axial direction. Therefore, the hydrostatic pressure effect on riser must be considered by the LAME formula.

Thickness of the main tube can be reduced because the peripheral lines of hyperstatic integration (HSI) marine drilling riser may share axial loads. The structure change into HSI pattern will affect the bending rigidity of marine drilling riser which is a significant factor to calculate the natural frequencies. The factors affecting natural frequencies of the marine drilling riser include axial loads shared by peripheral lines and changing of the main tube thickness. This paper is focus on analyzing the influential factors by using finite element method, and solves the differential equation which includes influential factors by numerical method to achieve a simple formula to calculate natural frequencies of marine drilling riser. Because the natural frequencies of HSI marine drilling riser decrease, the marine drilling riser can avoid the frequency lock and resonance, and improve dynamic behavior so as to reduce operation cost and risk significantly.

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