A Mid-Water Arch (MWA) is used to support and restrain flexible risers in mid-water column. This paper investigates the hydrodynamic properties of a complex MWA shape. A simplistic method for determining added mass on a complex multi-component body is presented and evaluated using numerical analysis. The results of a second numerical investigation into the drag coefficients of the MWA are also presented. These coefficients are used in simulating the response of a flexible riser system.
With the increase in developments of floating oil and gas production systems, the use of flexible riser systems is increasing. A flexible riser system is used to connect subsea equipment and structures to the surface floating structures in dynamic environments. A MWA is a buoyant structure often installed in mid-water column on the flexible risers. The MWA structure is tethered to the sea floor whilst flotation tanks provide enough net buoyancy to support the risers and associated marine growth. Understanding the hydrodynamic loads and subsequent motions of the MWA structure is important when designing flexible riser systems. This is critical in the design of both the installation and operational scenarios. Design tools such as OrcaFlex are capable of analysing the loads and motions of complex flexible riser systems when given the hydrodynamic parameters of the MWA structure. Two of the most important and influential parameters on the loads and motions of the MWA are drag coefficient and added mass. This paper focuses on quantifying these two parameters by first proposing a simplistic method of approximating the added mass, then verifying this method through numerical analysis of the structure.
A recent paper by Russell & Vignaud (2011) analysed a common semicylindrical shaped MWA to determine the hydrodynamic coefficients and their effects on the motion of the MWA.