The influence of the soil at touch-down-point (TDP) for steel catenary riser (SCR) design is of paramount importance because it can control the feasibility of the SCR option as a possible deepwater riser solution. Previous analyses suggest that fatigue responses are significantly influenced by the riser-soil interaction at the TDP. Understanding this interaction is, therefore, critical for acceptance/qualification of a SCR. The feasibility of SCR often depends on how accurately the SCR response is predicted near the TDP. In this paper, the impact of soil stiffness modeling, trench depth and soil non-linearity on the overall SCR response is examined in detail. Another important aspect of the touch down point response involves the degradation of the soil stiffness due to cyclic loading of the soils. Although not explicitly addressed by the analyses performed for this paper, comments are made about the potential overall effect on the SCR response.
A SCR is considered as one of the most attractive riser solutions in deepwater. With advancement of exploration and production in deepwater around the world, the design pressure and temperature envelopes have increased from 34.5 MPa (5 ksi) to 103.5 MPa (15ksi) and 200oF (93oC) to 450oF (232oC), respectively. In addition, the presence of sour fluid content as well as reservoir souring during production poses potential problems for deepwater riser design. These challenges often result in a tedious design process to satisfy corrosion fatigue and strength requirements, specifically near the hang-off and touch down areas. Fatigue design near the TDP is often difficult to predict and can lead to a conservative design approach. While most of the fatigue damage is contributed by the vessel motions and vortex-induced- vibration (VIV), VIV due to steady current is very difficult to estimate due to its sensitivity to several input parameters. Responses due to the vessel motions in small or large sea-states are equally important since large sea-states control strength design whereas small to medium sea-states govern fatigue design.
