THE EFFECTS OF CURRENT BLOCKAGE ON OFFSHORE STRUCTURES

Abstract

This thesis revisits the problem of hydrodynamic forces on fixed space-frame structures in combined waves and an in-line steady current. Because of current blockage, the actual drag force experienced by such structures is over-predicted by the standard Morison equation and the present industry standard practice. A set of analytical models of current blockage is formulated based on the actuator disc theory and proposed to represent the actual hydrodynamic drag force more accurately after taking into account the current blockage effects. This thesis tests and verifies the adequacy of the analytical model against series of experiments and full Computational Fluid Dynamics (CFD) numerical simulations, as well as demonstrating the novel use of a porous block as a simple representation for the complex geometry of real offshore structures in the numerical simulations.

Much of this thesis are devoted to validation of the proposed full current blockage model (FCB) for regular waves with an in-line current. For relatively small current speed compared with wave velocity amplitude, the drag force time history on obstacle arrays is well approximated by a summation of the wave drag and the current drag components independently, so there is no wave times current cross-term. The shape in time of the wave drag component is proportional to the standard Morison approximation, while that of the current drag component is found to be phase-locked to the oscillatory wave crests. The full model will be shown to fit the entire force time history as well as the peak force for a wide range of experiments and numerical simulations, requiring only calibration of the Morison type drag and inertia coefficients.

The FCB model and the use of a porous block model in numerical simulations in general work very well for statically-responding structures in regular waves. The drag force reduction is real and significant, and this has a direct implication and application to new-builds and reassessment of space-frame offshore structures, such as jackets and compliant towers.