Piping structures (topside and subsea) are generally subjected to high dynamic loading due to internal flow. Especially for multiphase systems, the induced forces can lead to high vibration amplitudes. 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 amplitude. These forces differ with flow regime and are still difficult to predict. For single phase flow, enlarging the bend radius significantly reduces the induced forces.

In this paper, experimental results are discussed in which the dynamic forces due to multiphase flow are compared between two bends. One with a bend radius of 1.5D and one with a radius of 3.3D. For both bends, experiments with single phase (gas and liquid), wet gas (up to a liquid fraction of 1%) and more general multiphase conditions have been done. At single phase flow conditions, the fluid forces are a factor 5-10 (gas only) or a factor 3 (liquid only) lower in the 3.3D bend compared to the 1.5D bend. However, in the case of the multiphase conditions, the induced forces are on average 25% higher at comparable conditions.