SOIL INTERACTION EFFECTS ON SIMPLE CATENARY RISER RESPONSE | 2H Offshore

Introduction

---

The use of steel simple catenary risers in deepwater developments is becoming more popular with a number of SCR’s already installed offshore Brazil and in the Gulf of Mexico. Upcoming deepwater field developments in the Gulf of Mexico, Brazil and West Africa have planned SCR configurations for large diameter export and subsea tie back production lines. A typical SCR configuration is shown in Figure 1.

The concept of the steel catenary is inherently simple and is often thought of as an extension of the flow line or pipeline. However, the dynamic movements experienced by the SCR from vessel motions and hydrodynamic loading result in a more complex behavior of the structure compared with flow lines and pipelines. This results in a requirement for sophisticated numerical tools to assist in the design process, in particular for the prediction of extreme storm stresses and long-term fatigue life due to vessel motions and vortex induced vibrations (VIV).

Current developments of steel catenary technology have focused on better understanding of these issues, one of which is the SCR touch down region and its interaction with the seabed. Studies carried out in the STRIDE JIP have shown that riser strength and fatigue response are influenced by the seabed soil and its local geometry in the touch down region. Their potential implications on SCR design have led the industry to investigate this area further. Recent initiatives include large and small scale testing programs carried out in the STRIDE JIP and CARISMA JIP [1] respectively.

This paper defines the mechanisms and uncertainties surrounding riser-seabed soil interaction, describes improved riser soil interaction models and their effects on SCR response, and outlines the STRIDE Phase III large scale testing initiative to better understand and model riser-soil interaction.

---

Conclusions

---

It is evident that current modeling of the seabed for SCR design using a flat rigid or elastic seabed is simplistic compared with the deep profiled trenches observed at the touch down zone of existing SCR’s in the Gulf of Mexico. Current modeling techniques do not account for the potential stress raising effects of soil suction and the passive soil resistance of the trench wall.

More rigorous modeling techniques using pipe-soil models to better account for seabed stiffness, soil suction and trench wall resistance can be used to improve prediction of riser fatigue life and strength, and also assess the significance of these mechanisms on overall SCR design. Critical load cases are found from parametric analyses, which show that seabed stiffness and soil suction can affect riser fatigue damage, and trench wall resistance and soil suction can cause a local increase in riser stress during a large vessel offset.

Uncertainties surrounding the mechanisms of riser-seabed interaction and validation of the  riser-soil models developed have been addressed in large scale testing within the STRIDE JIP Phase III program. Further investigation is required on the damping of vortex induced vibrations when the riser is trenched riser into the seabed.

Reference:

Thethi, Ricky. 2001. Soil Interaction Effects on Simple Catenary Riser Response. Texas: Deepwater Pipeline & Riser Technology Conference. www.2hoffshore.com