Steel Catenary Riser (SCR) is the most cost effective option for numerous deep water field developments. Despite its structural simplicity, the design and analysis of SCR’s can be complex especially due to the hydrodynamic forces. This paper selects three of the unexplained observations in SCR analyses, solves the mysteries analytically, and provides recommendations accordingly for improving riser design.

The three selected observations that are not well understood occur at three distinct locations along the SCR: the hang-off, the middle section, and the Touch Down Zone (TDZ). The riser maximum stress at the hang-off can be lower when the wave particle motion is considered in the dynamic simulation compared with using vessel motion time trace only. The TDZ fatigue damage is observed to be lower when background current is applied along riser in the motion fatigue analysis, even if the current direction is perpendicular to the riser motion direction. Finally, the motion fatigue response is observed to be better with a shorter strake length compared with that of the configuration wherein the strake coverage is extended to the seabed. 

The first two phenomena are generally explained by the “damping” effect, however, how the damping works was never clearly demonstrated. The third observation is actually contradictive to the damping effect as less strake length results in smaller drag forces and thus less damping. All of the above phenomena are related to the hydrodynamic forces that are exerted onto the riser by the surrounding sea water. Those forces are calculated by Morison’s equation which includes drag force and inertia forces. In this paper, a detailed analytical study using Morison’s equation illustrates how the drag force or inertia forces will affect the riser motion, and which force is the dominating one, for each of the three observations aforementioned. 

The understanding of these riser hydrodynamic behaviors leads to improved riser design. The applications of wave hydrodynamics and background currents present more realistic riser responses and reduce conservatism and thus cost. A riser with shorter strake length but more fatigue life will result in reduced cost too provided that adequate VIV fatigue life is preserved.