The subject is concerned with the hydrodynamic forces due to flow separation over the cross section of the riser. Since the incident flow is non‑oscillatory, the position of the wake is not reversible to the upstream side of the riser. The characteristics of the flow may be classified by four different regimes corresponding to the subcritical, critical, supercritical and transcritical Re.15, 16 Boundary layer and flow separation theory provide the background regarding the relationship between the flow field and the hydrodynamic forces.17 Some factors which may affect the hydrodynamic loads are addressed in the following.