Experimental investigation on vortex-induced vibrations of a hang-off evacuated drilling riser
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ORIGINAL PAPER
Experimental investigation on vortex-induced vibrations of a hang-off evacuated drilling riser Liangjie Mao . Song Zeng . Qingyou Liu
Received: 10 July 2020 / Accepted: 19 October 2020 Ó Springer Nature B.V. 2020
Abstract A model experiment has been conducted for a hang-off evacuation riser considering the lower marine riser package (LMRP) based on the strain gauge test technology. The finite element eigenvalue method is used to analyze the natural frequency and mode shapes of the hang-off riser considering the axial tension force and the weight of LMRP. The modal analysis method is used to restructure the motion and analyze the vortex-induced vibration characteristics of the hang-off riser. Then, the influence of evacuation velocity is discussed. Results indicate that the mode shapes amplitude of the experimental riser with LMRP is significantly larger than that of the ordinary cantilever. The dominant frequency of each measuring point is the same, and the end-cell-induced vibration small frequency is observed near the bottom end. The vortex shedding in the cross-flow (CF) direction produced a large lift force, which resulted in the vibration frequency of in-line direction dominated by the CF direction. When the reduced velocity (Vr) is L. Mao (&) S. Zeng Q. Liu (&) State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Sichuan, Chengdu 610500, China e-mail: [email protected] Q. Liu e-mail: [email protected] L. Mao S. Zeng Petroleum Engineering School, Southwest Petroleum University, Sichuan, Chengdu 610500, China
less than 40, the frequency response of the hang-off riser model increased with the evacuation velocity, which was subjected to the Karman vortex shedding frequency. When the frequency response of the riser model reached, the second-order natural frequency with Vr exceeds 40. Consequently, the ‘‘lock-in’’ phenomenon was observed and the riser exhibited vibrations with the second-order model. Keywords Hang-off riser Evacuation Lower marine riser package Vortex-induced vibration
1 Introduction The riser is the pipeline connecting the drilling platform to the subsea wellhead. This equipment is indispensable but an extremely fragile part of the deepwater drilling system. The riser must be disconnected from the lower marine riser package (LMRP) and evacuated with the platform during its recovery process or when facing extreme weather conditions, such as typhoons. The riser is subjected to excitation by platform motion, wave, and current load in this case. On the one hand, the platform motion or water flow load may cause large deformation of the riser. On the other hand, the fluid flowing through the sides of the riser periodically generates vortices, thereby causing the riser to swing pendulum motions. This phenomenon is called vortex-induced vibration (VIV).
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This vibration may cause rapid deterioration in fatigue damage and increase the resistance of the
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