ABSTRACT

An experiment is described which was designed to investigate the forces normal to an isolated cylindrical member of an offshore structure. The presence of random directionally-spread waves, current, marine growth and structural motions were all modeled. This complex environment was simulated in the Maritime Dynamics Laboratory of SSPA Maritime Consulting AB (Swedish Maritime Research Centre at the time) by moving a 1 meter diameter instrumented cylinder along prescribed paths. Reynolds numbers over 2 × 106 and amplitudes of oscillation up to 15 meters were studied. It is observed that interaction of the cylinder with its own wake is extremely important. Strong vortex shedding effects throughout all of the experiments were evident. Large transverse forces were observed at Reynolds numbers over 106 " The Morison equation proves to be a good model of the forces. Comparisons are made with data from the Exxon Ocean Test Structure.

INTRODUCTION

Prediction of hydrodynamic forces on welded tubular structures in the ocean continues to be an area of active research. The force coefficients must be determined empirically due to the intricacy of the flow around a bluff body. Analytical models (l) of such flows are intractable and as yet unproven. The force coefficients for a fixed structure in storm-driven seas are not fully understood; this can be attributed to the difference between the regular harmonic fluid motions used in the laboratory and the random multidirectional motions found in the offshore environment. Reproducing the complex environment of oscillating flows in three dimensions, with mean flows due to currents and simultaneous structural motions at scales appropriate for design is extremely expensive. Developing theoretical tools for such problems has also proven elusive. Researchers have therefore concentrated on understanding simpler flows. It has been necessary to make in situ measurements and extrapolate these and laboratory results to the design environment. Hogben et al. (2) cite some 17 experiments relevant to the study of forces on cylinders in time varying flows. A recent review of the field is provided by Sarpkaya (3). Most laboratory experiments have been restricted to oscillations in one direction, and most of these experiments do not achieve scaled design conditions. Older field experiments, such as Wave Force Project II (4), did not have the benefit of measured fluid motions so that wave theories were necessary to deduce fluid motions and then force coefficients. Analysis of Ocean Test Structure (5, 6) data has determined force coefficients which are significantly larger than those previously observed in the field, especially for barnacle roughened cylinders. The Reynolds numbers and Keulegan-Carpenter numbers observed during OTS were somewhat less than those used in extreme wave design. And some differences in the force coefficients may exist at prototype conditions. As with all field experiments, the environment could not be specified for the OTS so that some conditions of interest were not observed. For instance, Reynolds numbers over 106 were not reached. And no measurements in simultaneous large current and waves were made.

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