ABSTRACT

Wake shielding has a siginicant impact on deepwater marine riser design, particularly in the context of wake induced oscillation and ensuing riser clashing. Its effects are not well understood, nor have these effects been investigated thoroughly. In the present study, a computational investigation into the wake shielding effects is carried out. The application of a finite volume method is adopted, in conjunction with a hybrid unstructured mesh generation technique, for simulating two-dimensional cross flows around single and twin cylinders. The numerical results of the mean wake flow behind an isolated cylinder as well as the mean drag force on the downstream cylinder situated in the wake of an upstream cylinder are presented in comparison with some test data. The paper also examines the validity of the set of wake formulae based upon the Schlichting's work which is currently used in the offshore riser cluster design.

INTRODUCTION

For deepwater top tensioned vertical riser clusters, as used in TLPs and SPARs, one of the key design concerns is to determine/optimise the minimal spacing between individual risers such that the riser motions induced by horizontal ocean currents do not result in detrimental riser clashing with each other. The clearance problem is more pronounced in deepwater as the riser lateral deflection under a current increases in proportion to the square of water depth. The presence of strong ocean currents at some deepwater development locations exacerbates the problem even further. A cylinder placed in the wake of an upstream cylinder is subjected to different types of forces, which are generally divided into the following three groups:

  • A broadband buffeting force generated by the turbulent flow shed from the upstream cylinder inducing broad band buffeting vibrations.

  • A periodic vortex shedding induced force causing high frequency

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