ライザー縦振動の周波数応答における不動点の存在と設計に及ぼす影響

概要

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The 4000 m class deepsea drilling vessel will be realized in Japan in near future. The drilling vessel is equipped with deepsea riser, and this paper discusses longitudinal dynamic response of the deepsea riser in hung-off condition. This paper shows that there exist fixed points in the frequency response of strain of hung-off deepsea risers. A fixed point is a pont in a frequency response which does not change irrespective of the change of some governing parameters of the dynamic system. The fixed point is originally found in the discreet mass-spring system, and this paper shows that the fixed point exist also for a continuum system, riser & BOP system. The fixed points are shown to exist in the frequency response of axial strain with respect to the damping of riser and also the damping of BOP. Considering the position of fixed points in the frequency response, it can be concluded that the increase of damping does not always improve the dynamic response of hung-off riser. When the external exciting frequency, namely heave motion of vessel, is lower than the fixed point, strain at riser top is increased with increase of damping. On the other hand, if the riser is excited at the frequency larger than the fixed point, response is significantly improved.