Wave run-up on columns of deepwater platforms

Structural design of novel deepwater platforms is an outcome of innovation and optimization. Structures such as the extended Tension Leg Platform have columns of rectangular cross section, while platforms like spars are circular. An important operational requirement of these platforms is to have restricted heave motion to support vertical rigid risers. An unfortunate consequence of this requirement is the so-called breakwater effect, which results in significant wave run-up on the structure. In high sea states, considerable water could run up along the columns hitting the underside of horizontal deck. Estimating wave run-up on offshore structures is an essential part of deciding the minimum air-gap requirements of such structures. This article examines the run-up on circular and square cylinders using experimental and numerical methods. Experiments were performed in a wave tank on scaled models of prototype circular and square columns. Numerical simulations were conducted using the commercial software FLUENT. It was found that that the results for wave run-up from simulations and experiments were in good agreement and were consistently greater than linear diffraction theory. Numerical comparison between circular and square cylinders showed that the run-up was higher for square geometry for all conditions simulated. This increase was higher in steeper waves due to various nonlinearities in the flow around the structure. An attempt has been made to understand the various nonlinearities in the run-up profile using numerical flow visualization.