Hydrodynamic loads and response of a Mid Water Arch structure

• A subsea Mid Water Arch (MWA) structure was studied in a range of steady flow conditions.
• Experimental techniques were developed to carry out model scale testing of the MWA.
• Numerical simulations were carried out and validated by model testing results.
• The experimental methods were found to provide an effective means of studying a MWA.
• The limitations of numerical analysis software emphasised the importance of experimental testing when investigating a complex MWA structure.

A Mid Water Arch (MWA) is a subsea structure used in flexible riser and umbilical systems. Understanding the hydrodynamic properties and response of the MWA to environmental conditions is important in the design of such systems. In this study, these areas have been investigated through both experimental model scale testing and numerical simulations. To carry out the model scale testing it was necessary to develop two experimental methods; captured testing to determine the drag forces on the structure, and tethered testing to enable the offsets, rotations and tether tension loads to be resolved. The numerical simulations are comprised of Computational Fluid Dynamics (CFD) with ANSYS CFX to explore the drag forces on the MWA, hydrodynamic diffraction analyses with ANSYS AQWA to find the added mass of the MWA, and dynamic analyses with OrcaFlex to study the offsets and rotations of the MWA in the tethered arrangement. The model testing results were used for comparison and validation of the numerical simulations; namely, the captured testing and CFD studies, and the tethered testing and OrcaFlex analyses. The findings from this study have shown the significance of experimental testing for the purpose of investigating the hydrodynamic loads on a MWA structure.