Current blockage in a numerical wave tank: 3D simulations of regular waves and current through a porous tower

Abstract

This paper introduces a new numerical approach for the estimation of the global hydrodynamic loads on space-frame offshore structures exposed to combined waves and current. We provide numerical evidence for reduced fluid loading on offshore structures – current blockage, which serves as an extension to the analytical, computational and experimental work of Taylor et al. (2013) and Santo et al. (2014a, 2014b). A full 3D free-surface turbulent flow is simulated for a porous tower in a numerical wave tank. This is intended to model waves and current through a jacket or compliant tower, both space-frame structures. Comparisons are made between the numerical simulations and experiments conducted by Allender and Petrauskas (1987) on a scale-model jacket structure from the Gulf of Mexico, and the current blockage model presented previously in Taylor et al. (2013). Three different flows are simulated: steady current, regular waves with no current and regular waves with an in-line current. Overall, good agreement in terms of peak total forces is achieved, showing that the force reduction on such structures due to current blockage effects is real and significant. Additional information on force time history and flow visualisation are presented from the numerical simulations. Flow visualisation for waves and current reveals that the form of the global mean wake is simple at the structure but becomes complex well downstream. The simple form of the flow at the tower is responsible for the global force reduction being predictable using a modified version of the Morison equation (Morison et al., 1950). This paper also demonstrates the novel use of a porous tower as a simple representation for the complex geometry of real space-frame structures when exposed to combined large waves and significant in-line current, an approach which could be considered for possible incorporation into offshore design practice.