Theoretical modeling and analysis for mechanical properties of deepwater workover riser system

Low-cost midsize workover operation based on deepwater workover riser system is attractive. However, a comprehensive theoretical mechanical model for the complex workover riser system has not been found in present studies. In this paper, a coupled mechanical model of the workover riser system is established based on Euler-Bernoulli beam theory, hydraulic-pneumatic principles and the coupling relationship among workover risers, split tensioner system and workover platform. An analysis method for the coupled mechanical model is proposed based on the finite element method, Newmark-β method and Newton-Raphson method. A mechanical analysis program is finally developed in MATLAB and applied in mechanical property analysis of a deepwater workover riser system. It turns out that the bending moment of the workover riser system near the top part, tension ring and bottom part reaches extremes. Dynamic response envelopes of the workover riser system are obviously larger than static responses due to the dynamic excitation of wave loads and platform motions. It is also found that tension distribution ratio has obvious influence on mechanical properties of workover riser system. Mechanical response of the workover riser system decreases with TDR, whether in static or dynamic analyses.