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Some aspects of marine riser analysis Irani, Mehernosh Boman


The problem of the static and dynamic analysis of marine risers is considered in this thesis. The equations of motion for a pipe conveying fluid are derived using a variational approach. The nonlinear expressions for the bending and torsional curvatures are obtained for a beam undergoing finite rotations. The elastic strain energy is expressed in terms of these curvatures and Hamilton's principle used to derive the equations of motion valid for large deformations. The effect of internal flow is included in the variational formulation in a consistent manner. A finite element formulation is developed for the static analysis of compliant risers. Large deformations are handled by using a convected coordinate system fixed to the elements. The nonlinear problem is solved using a combined incremental-iterative approach based on an instantaneous linearization of the Taylor series expansion of the forces about the displaced configuration. The numerical results presented here show that the large displacement formulation can predict the static deflected shape of different types of compliant riser systems. The dynamic response of a typical marine riser system to excitations of the ocean waves, current and surface vessel motion is also studied. The hydrodynamic loading is represented by a general form of the Morison equation. The nonlinear drag force in the Morison equation is linearized using the method of equivalent linearization and a solution procedure in the frequency domain developed. Alternatively, the complete nonlinearity of the hydrodynamic loading is retained and the equations of motion are integrated in the time domain. The effect of this linearization on riser response predictions is evaluated. The method of equivalent linearization involves less computational effort and it approximates the response reasonably well. The effect of internal flow on the response of a typical production riser configuration is also studied. It is concluded that internal flow has a negligible effect on the dynamic response.

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