Bed Shear Stress Distribution Around Offshore Gravity Foundations

Abstract

Offshore gravity foundations are often designed with complex geometries. Such structures interact with the local hydrodynamics and generate enhanced bed shear stresses and flow turbulence capable of scouring the seabed or destabilizing bed armour where deployed. In the present study a novel bed shear stress measurement method has been developed from the camera and laser components of a Particle Image Velocimetry (PIV) system. The bed shear stress amplification was mapped out around six models of gravity foundations with different geometries. Tests were repeated for two bed roughness conditions. The structures tested included uniform cylinders, cylindrical base structures and conical base structures. The flow field around the models was also measured using PIV. The results of this study reveal that the conical base structures generate a different hydrodynamic response compared to the other structures. For uniform cylinders the maximum bed shear stress amplification occurs upstream, at an angle of 45° relative to the flow direction, and measurements were found to agree well with numerical results obtained by Roulund et al. (2005). In the case of the cylindrical base structure the maximum amplification occurs upstream at a similar location to the uniform cylinder case. For the conical base structures the maximum amplification of the bed shear stress occurs on the lee side of the structure, with the magnitude dependent on the side slope of the cone. The bed shear stress results were validated against stresses derived from analysis of the flow fields obtained by the PIV measurements performed under the same test conditions. Conclusions from the study are that the structure with the cylindrical base foundation produces the lowest bed shear stress amplification and that an increase in the bed roughness results in an increase in the amplification of the bed shear stress. These findings have direct implications for design of scour protection. In addition the flow reattachment point behind the foundation is dependent on pile Reynolds number (ReD). This suggests that the results of this study may be extrapolated for higher pile Reynolds using the method described in Roulund et al. (2006).

Scour development around structures with non-uniform cylindrical geometries

“…The deepest scour then gradually propagates towards the upstream of the structure and reaches the leading face of the structure and then the deepest scour point stabilizes at a location adjacent to the structure at an angle approximately 45° relative to the flow direction. This point of initiation of scour agrees with the findings of Tavouktsoglou et al (2015) who found that the maximum amplification of the bed shear stress occurs at the same location for all conical based structures which also agrees with the observations of Khalfin et al (1983) who observed that the maximum scour for some conical structures occurred at the lee of the structure. This behaviour is also reported in Petersen (2014) who examined the behavior of edge scour around protected piles with the difference that the maximum scour depth did not move towards the front of the scour protection scheme.…”

“…This translates into an increase in the local pressure field induced by the structure which is responsible for the amplification of the bed shear stress which is the driver of the scour phenomenon. This is clearly indicated in Figure 8 which shows that there is clear correlation between the values of the equilibrium scour depth and the amplification of the bed shear stress as shown in Tavouktsoglou et al, 2015. • For the uniform cylinder, the effect of a decreasing flow depth is not as apparent as the total flow interacting with the cylinder remains very similar thus not effecting the scour process significantly as long as the water depth is not in the "shallow water scour" regime Guo et al (2012).…”

“…Comparison between results of equilibrium scour from present tests and amplification of the bed shear stress (Source:Tavouktsoglou et al, 2015).…”

Equilibrium Scour-Depth Prediction around Cylindrical Structures

“…This in turn would result in a smaller downflow on the face of the structure, a reduced amplification of 167 the bed shear stress and thus, smaller scour depths. This statement is verified by Tavouktsoglou et al (2015) who 168 measured the amplification of the bed shear stress for the same flow conditions and structures for which the 169 pressure gradient distribution is calculated in Figure 2. The pressure gradients were calculated for two of the small 170 scale structures listed in Figure 3 and for a mean flow velocity of 0.39 m/s and a water depth of 0.165 m. They 171 found that there is a significant increase in the amplification of the bed shear stress between a conical base structure and a monopile, which agrees qualitatively with the pressure gradient profiles depicted in Figure 2.…”