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Dynamic Analysis of a Drillstring-Riser System Drilling in Deep Water
Last modified: 2017-11-26
Abstract
A drillstring-riser model is developed to analyze the dynamic response of a drilling riser in the deep water. The main motivation is to precisely predict the distributions of deflection, bending moment, and stress state along a drilling riser in ocean environment; so that a proper operation strategy can be determined once ocean environment loads change drastically.
In the developed model, Ekman’s drift theory, Airy wave theory, and dynamic Morison’s formula are applied to describe ocean environment loads. Top tension, shear force, gravity, and inertia force are considered for both the drilling riser and the drillstring. The hydrodynamic forces induced by drilling fluids flowing inside and outside of the drillstring are included. In particular, the interaction between the drilling riser and the drillstring is simulated by introducing a series of spring-damper-friction units. Numerical simulation of the developed mathematical model is carried out by Abaqus in which the finite element method is adopted. According to the result of the numerical simulation, the dynamic response of a drilling riser submerged in the deep water is quite different from that submerged in the shallow water. In the deep water, the maximal riser deflection emerges below a third long of the riser but above its middle position. Large bending moment and large stress along the riser are observed within 20% of the water depth below sea level. In order to control the riser deflection, the operations, such as the increase of the top tension, the thickening of the riser wall, and the drift limitation of the riser top, can be applied. In addition, compared with the calculated deflection of the previous models which exclude the influences of the drill string, the maximal riser deflection obtained by using the developed drillstring-riser model is relatively small; namely, the movement of the drillstring restricts the deflection of the riser via their interaction. This finding indicates that, in a given safety margin of the riser deflection, the string- riser system has a stronger capacity of anti-deflection.
In conclusion, the presented model takes the drillstring-riser interaction into consideration; hence, it can provide more accurate dynamic response of a drilling riser in deep water. This improvement can be benefit for drilling engineers to choose proper operations to avoid a variety of risks when drilling in the deep sea.
In the developed model, Ekman’s drift theory, Airy wave theory, and dynamic Morison’s formula are applied to describe ocean environment loads. Top tension, shear force, gravity, and inertia force are considered for both the drilling riser and the drillstring. The hydrodynamic forces induced by drilling fluids flowing inside and outside of the drillstring are included. In particular, the interaction between the drilling riser and the drillstring is simulated by introducing a series of spring-damper-friction units. Numerical simulation of the developed mathematical model is carried out by Abaqus in which the finite element method is adopted. According to the result of the numerical simulation, the dynamic response of a drilling riser submerged in the deep water is quite different from that submerged in the shallow water. In the deep water, the maximal riser deflection emerges below a third long of the riser but above its middle position. Large bending moment and large stress along the riser are observed within 20% of the water depth below sea level. In order to control the riser deflection, the operations, such as the increase of the top tension, the thickening of the riser wall, and the drift limitation of the riser top, can be applied. In addition, compared with the calculated deflection of the previous models which exclude the influences of the drill string, the maximal riser deflection obtained by using the developed drillstring-riser model is relatively small; namely, the movement of the drillstring restricts the deflection of the riser via their interaction. This finding indicates that, in a given safety margin of the riser deflection, the string- riser system has a stronger capacity of anti-deflection.
In conclusion, the presented model takes the drillstring-riser interaction into consideration; hence, it can provide more accurate dynamic response of a drilling riser in deep water. This improvement can be benefit for drilling engineers to choose proper operations to avoid a variety of risks when drilling in the deep sea.