Piping structures are generally subjected to high dynamic loading due to multiphase fluid forces. In particular subsea structures are very vulnerable as large flexibility is required to cope for instance with thermal stresses. The forces due to multiphase flow are characterized by a broadband spectrum with high amplitudes. These forces differ with flow regimes and are still difficult to predict. Therefore a joint industry project was setup to perform both large scale (6") flow tests and to qualify CFD procedures and settings to calculate these forces with sufficient accuracy.
In various publications [1-12] forces on bends due to multiphase flow has been published. The measurement set-ups in these publications range from small size piping of 6mm up to large size piping of 100mm diameter. Most publications report the measured force for different liquid void fractions, as different results are obtained for low or high liquid void fractions. Different scaling rules are proposed with respect to hold-up, mixture momentum and Weber-number. The amplitude (root-mean-square of transient part of the force), combined with predictions of the power spectral density (PSD) function and centre frequency give a basis for a good force prediction. In general for multiphase conditions, the PSD shows a top-hat configuration.
With the PSD, the force on a bend can be estimated. However, bends are never present isolated from the rest of the piping. For a complete structure, the forces on all bends (and valves, reducers, Tjoints, …) need to be defined. At each bend the force spectrum must be known. That is, the total dynamic force amplitude (root means square (RMS)), the frequency spectrum, the phases between the frequencies and finally, the phase relation between the forces at the difference bends. The question is therefore how representative the PSD measured at a single bend is for a bend in larger structures. Focus in this paper is on showing and discussing the differences in behaviour between a single bend compared to the behaviour of a bend with an upstream disturbance. This is done by installing a Ubend configuration upstream of the measurement bend. Measurements have been performed with the Ubend configuration in both the horizontal and vertical position.