Laboratory tests have been carried out in a pulsating water tunnel to determine the flow interference effects between cylinders in an array subjected to harmonic oscillating flow. The modification to the drag and transverse vortex shedding force on each cylinder due to the presence of neighbouring members has been measured in the form of interference coefficients. The results show reduction of drag on shielded cylinders but a significant increase (up to three times) of the maximum transverse force on a cylinder in the wake of another, the effects increasing with the flow orbit length. The primary area of application is intended to be to the design of conductor tube arrays for offshore oil production platforms, and the results are discussed in this context.
Offshore oil exploration and production in the North Sea in recent years has led to a need for further research into aspects of wave loading on structures. To meet this need a programme of research was recommended by the Offshore Structures Fluid Loading Advisory Group and this has now been carried through with funding from the Offshore Energy Technology Board of the Department of Energy. One of the projects was to determine through model tests to what extent wave loading on a fixed circular cylinder was modified by the presence of other nearby cylinders. The major application envisaged was to the vertical conductor tube array of a production platform, where a regular array of quite closely spaced circular cylinders is subjected to wave and current action. The purpose of this paper is to present the key findings of the project, the full report of which is given in Ref 1.
The experimental work was carried out in the pulsating water tunnel of the Hydraulics Research Station with horizontal 7.62 cm diameter cylinders subjected to a rectilinear oscillating flow with a range of periods and orbit lengths. The configurations studied were single cylinder, two cylinders and a 3 × 3 square array at varying angles to the flow, the cylinder spacing being three diameters between centres in all cases. One cylinder only was instrumented to measure forces, the other cylinders simply serving to produce the disturbed flow field. The intention was to compare the forces on a cylinder in such a configuration with those on an isolated cylinder under precisely the same external flow conditions and hence to determine some form of interference coefficient as a function of external flow parameters.
One problem with such small scale model tests is that the flow Reynolds number (VD/v) is smaller than for the real array. This is inevitable in any model and in fact using the pulsating water tunnel allows larger Reynolds numbers to be achieved than would be possible in any alternative apparatus. However, an attempt was also made to simulate high Reynolds number flow conditions in some tests by fitting 3.2 mm tripwires on both sides of each cylinder at ±45 degrees to the direction of flow.