Optimization analysis of the riser top tension force in deepwater drilling: Aiming at the minimum variance of lower flexible joint deflection angle

This paper presents an analysis approach optimizing the top tension force of deepwater drilling riser when the variance of the lower flexible joint deflection angle to the minimum value. The riser is modeled as an Euler-Bernoulli beam subjected to non-uniform axial force and transverse random force. In this paper, the riser transverse vibration behavior is described by transfer matrix method and spectrum analysis, and the distribution inertial force is calculated by the cubic Hermite interpolation polynomial. Finally, the variance of lower flexible joint deflection angle is given, and the minimum variance and the optimal top tension are figured out accordingly. Based on this, the influences of water depth, floating vessel top displacement, riser wall thickness and structure damping coefficient on the optimal top tension are discussed. Analysis results show that, for a given status, there is an optimal top tension making the deflection angle variance be minimum, and the optimal top tension increase with water depth, floating vessel top displacement and riser wall thickness. While the minimum value of the variance increase with water depth and the floating vessel top displacement, and decrease with riser wall thickness. However, the structure damping coefficient has almost no influence on the optimal top tension.