Current Blockage: Reduced Forces on Offshore Space-Frame Structures

Abstract

ABSTRACT The standard method of estimating the fluid loading on an offshore space-frame structure makes use of the Morison equation to calculate the force exerted on each structural element and ignores the influence of the rest of the structure. However, the use in bad calculations of the steady current measured or estimated for the open sea far away from the structure (platform) leads to a significantoverestimate of the current-induced components of the bading because the average current within the structure is lower than that far upstream. This study describes a method for calculating this blockage of the free-stream current at the structure and the resulting global loading on the structure. This simple method is validated against measurements from a wide range of experiments. INTRODUCTION The extreme environmental bading on a typical steel offshore platform is dominated by the fluiddra g due to waves and currents' For a given sea state the bad is maximum when the waves and the current are propagating in the same direction. The standard method of estimating the fluid loading on a fixed offshore space-frame structure makes use of the Morison equation to calculate the force exerted on each element. The total bad is just the sum of the forces on all of these elements, each calculated as if the rest of the structure were removed. Thus it is assumed that the presence of the structure within the flow field does not modify this flow. This is not unreasonable for the wave-induced water motion. However, the use in bad calculations of the steady current measured or estimated for the open sea far away from the platform leads to a significant overestimate of the currentinduced components of the loading. The presence of obstacles in a steady current alters the structure of the flow field (Fig. 1). Upstream of the obstacles the flow velocity is reduced by the divergence of the flow. This is a direct consequence of the net force exerted by the obstacles on the fluid equal and opposite to the drag. Wihin the obstacle array, each obstacle generates a wake. Downstream, there is a global wake associated with the whole array followed by wake spreading produced by mixing between the wake and the external flow. The aim of this work is to estimate the degree of conservatism inherent in standard design practice for offshore structures resulting from the omission of current blockage and to suggest an improved calculation procedure. A simple method of allowing for blockage of the current is validated against both field and laboratory-scale experimental data in steady flows. Further analysis has shown that there is additional shielding associated with the interactive effect of waves and current. The magnitude of this additional force reduction depends upon the type of wave, e.g., a single large wave or regular waves. A comparison of the new regular wave model and published peak force measurements on a scale-model structure shows excellent agreement for a wide range of wave heights and current speeds. Large ocean waves do not occur in isolation. Thus the peak force exerted by the extreme wave in a storm is likely to be bounded by the models outlined above.