Multiphase forces on bend structures – overview of large scale 6” experiments

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. For the low to mid gas fractions, the amplitude of the forces agree with literature, indicating no large diameter scaling effects. At very low liquid fractions, the comparison with literature starts to deviate. The amplitude of the forces scale well with the pressure at low liquid fraction whereas the mixture momentum is dominant at higher liquid fractions. The frequency spectrum is characterized by a top-hat shape of the PSD. The center frequency corresponds to the wave and slug frequencies from literature, although potentially a void wave interaction might be present at very low liquid fractions. The main slope of the PSD (m2) is in line with the larger diameter measurements from literature , although a clear relation at very low gas fraction is measured which is not covered in literature. This indicates also, that no large diameter scale effects play a role, although system setups might still play a role.