Hydrodynamic Loading on Mid Water Arch Structures

In the design and analysis of subsea structures, the hydrodynamic coefficients play a significant role in determining the behaviour of the system, particularly when they are under the combined action of steady and oscillatory flow. For small regularly shaped structures and slender members there is a significant amount of literature that provides assistance in the selection of suitable values, however this is not the case for larger and irregular shaped structures where there is a scarcity of information. This paper details the results of a study to develop techniques to derive hydrodynamic coefficients for representative mid water arch configurations commonly used in shallow water flexible riser system applications. The results of a literature review are presented outlining where these structures fit into the different theoretical models. The use of the potential flow approach is verified by means of computational fluid dynamic simulations for a representative structure and both methods are utilised to assist in the generation of appropriate coefficients. A case study on a typical shallow water North Sea project is presented that demonstrates the importance of selecting appropriate hydrodynamic values to represent the structure. The influence of each of the coefficients on the arch motion, which has an impact on the riser response, is studied. It is shown that it is not always possible to be conservative and instead it is important to select appropriate coefficients for the structure in question, which may differ from those typically used. The method developed has wide ranging applicability and can in principle be applied to the determination of hydrodynamic coefficients for other large structures subject to hydrodynamic loading, including disconnectable turret buoys and hybrid riser tower systems.