Multiphase forces on bend structures – critical gas fraction for transition single phase gas to multiphase flow behaviour

Piping structures are generally subjected to high dynamic loading due to multiphase 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. A 1.5D radius bend was fully equipped with force rings (2*8), piezo strain gauges, classic strain gauges, dynamic pressure sensors (11) and both upstream and downstream a resistance tomography and transparent section. Both single phase (gas and liquid) as well as multiphase conditions were measured with maximum gas velocities up to 40 m/s and liquid velocities up to 4 m/s. In this paper, results for the wet gas cases are presented in which the transition from single phase to multiphase characteristics is evaluated. This is done for two configuration: A single bend with a long horizontal inlet and for a configuration with a Ubend upstream of the measurement bend. In the forces, there does not seem to a strict break point. For the test bend there was little or no vortex formation at single phase conditions. Therefore also for single phase, the main excitation is turbulence. The transition in the forces, from single gas to multiphase, can be understood from the increased mass (momentum) flow. For the low pressure conditions, the transitions occurs typically at a liquid fraction of =1-10 %. The frequency spectrum does change from a continuous decay as function of frequency to a more ‘top-hat’ profile at higher liquid fractions.