Fluid-structure interaction in non-rigid pipeline systems

Abstract

Fluid-structure interaction in non-rigid pipeline systems is modelled by water hammer theory for the fluid coupled with beam theory for the pipe. Two different beam theories and two different solution methods in the time domain are studied and compared. In the first method, the fluid equations are solved by the method of characteristics and the pipe equations are solved by the finite element method in combination with a direct time integration scheme. In the second method, all basic equations (fluid and pipe) are solved by the method of characteristics. The solution methods are applied to a straight pipeline system subjected to axial and lateral impact loads and to a one-elbow pipeline system subjected to a rapid valve closure. In comparing the beam theories, the effects of rotatory inertia and shear deformation for practical pipe geometries and loading conditions are investigated. The significance of fluid-structure interaction is demonstrated. The fluid-structure interaction computer code flustrin, developed by delft hydraulics and which solves the acoustic equations using the method of characteristics (fluid) and the finite element method (structure), enables the user to determine dynamic fluid pressures, structural stresses and displacements in a liquid filled pipeline system under transient conditions. To validate flustrin, experiments are performed in a large scale 3D test facility consisting of a steel pipeline system suspended by wires. Pressure surges are generated by a fast acting shut-off valve. Dynamic pressures, structural displacements and strains (in total 70 signals) are measured under well determined initial and boundary conditions. The experiments are simulated with flustrin. The agreement between experiments and simulations is shown to be good: frequencies, amplitudes and wave phenomena are well predicted by the numerical simulations. It is demonstrated that the results obtained from a classical uncoupled water hammer computation, would render completely unreliable and useless results.