The blue arrows represent the water currents that actuate over the riser, while the red arrows represent the direction of the movement of the riser (i.e. instantaneous velocity). Observe that the greater the alignment of those two groups of arrows the greater the influence of the water currents in the final movement of the riser. For that situation we can see that there is no such alignment, which means that other environmental forces (winds and waves) have a greater influence in the final movement of the riser.