This paper presents the results of an extensive experimental investigation of hydrodynamic forces acting on production risers consisting either of a single pipe or of a larger pipe surrounded by a ring of smaller pipes. The tests were made with models to scale 1:2 exposed to steady currents, flow of constant acceleration, oscillating flow and oscillating flow in combination with a steady current, either parallel or at right angles. Reynolds numbers ranged from 10 4 to 1.5.106 and Keulegan-Carpehter, numbers from 20 - 70. Drag arid inertia coefficients were determined for the individual pipes in the riser by means of least square analysis. Further transverse forces and Strouhal numbers were analyzed.
The results show that even with a distance greater than 3.5 diameters between the pipes in the outer ring and a relatively small central cylinder, a composite riser behaves largely as a closed body when exposed to currents or oscillating flow. This does not apply to the transverse forces, which are completely dominated by strong eddy shedding from the outer pipes. Both for the single pipe and for the composite riser a reduction in drag and inertia coefficients was observed when a current was superimposed on oscillating flow, parallel as well as perpendicular.
In recent years rather large efforts have been made in design and development, of flexible risers for use in deep-sea production systems for gas and oil. An example of such a system is schematically shown in Fig. 1. A bundle-type riser is commonly designed with a central pipe, which carries the oil, surrounded by a circular arrangement of smaller pipes which may be used for various purposes such as carrying gas or slurry for prevention of blow-ups (Fig. 2).
A riser is exposed to a variety of hydrodynamic loads. In the upper stratum of the sea the loads are caused mainly by waves, surface currents and movements of the production platform, whereas further down the forces are due mainly to currents caused either by large scale oceanographic circulations or by heavy density currents. This means that the upper part of the riser is exposed to a combination of acceleration and drag forces, whereas the lower part is exposed mainly to current forces.
In order to resist the forces over large depths (100-800 m), the riser has to be flexible. This requires a very small moment of inertia so that the loads are taken by tension only. Therefore, under the influence of severe environmental loads, the movements of the riser will be rather large and, due to its low natural frequency, the riser may be exposed to resonance with the loads unless, by a suitable pretensioning it is tuned to be out of the critical frequency range.
