Modeling the Touchdown Zone Trench and Its Impact on SCR Fatigue Life
Authors: Mekha Basim , Randolph Mark , Bhat Shankar , Jain Sunny 
Published: May 2013

Abstract
Field observations show that a steel catenary riser (SCR) connected to a floating production platform will dig itself a trench of a few pipe diameters in depth through the touchdown zone (TDZ). This appears to happen during the early stages of the SCR life. The trench has a curved vertical profile that is shaped around the SCR in the touchdown zone (TDZ). The trench changes the geometry of SCR contact with the sea floor and potentially exposes the riser to stiffer soil at the bottom of the trench. The seabed trench may therefore be expected to have a significant effect on the SCR touchdown zone fatigue life. However, studies published to date have given contradictory indications as to whether the net effect is to increase or decrease the SCR fatigue damage in the TDZ. This paper presents a study of the main aspects that may impact fatigue damage for a given set of sea states: the trench vertical profile, seabed soil stiffness model (linear, non-linear) and the directionality of the predominant fatigue sea states relative to the orientation of the SCR. Three different approaches for analytical modeling of trench vertical profiles are considered, two based on previous literature and one new. For validation, the modeled trench profiles are compared with a measured trench profile for a 12-inch gas export SCR connected to a TLP in about 1000 m water depth. Fatigue damage is evaluated for realistic fatigue sea state bins for Gulf of Mexico and Western Australia, using either a conventional linear seabed model, or a hysteretic non-linear model, with soil parameters that are appropriate for the two different regional soil types. The overall conclusion, for both soil types, is that trenches have a beneficial effect, extending the fatigue life of the SCR in the touchdown zone.

Introduction
Steel catenary risers (SCRs) are often the most attractive riser solution for a deepwater field development, with fatigue life in the touchdown zone (TDZ) representing one of the main feasibility issues. As a result, the effect of soil stiffness variation and trenching on the long term fatigue life of SCRs has generated increasing interest, and many publications, in recent years. Field observations of SCR touch-down zones (Bridge and Howells, 2007) showed that a trench will almost always form in the seabed soil, since the top surface of the seabed soil is usually soft enough for self-trenching to occur. However, studies published to date have given contradictory indications as to whether the net effect of the touchdown zone trench is to increase or decrease the SCR fatigue life.
Touchdown fatigue studies are routinely performed using a linear model for the seabed response, even though it is accepted that the actual seabed response is non-linear, with the operative stiffness varying with the displacement amplitude. Advancement in understanding of the soil-structure interaction has been achieved through a combination of analysis, centrifuge model testing, field experiments and trench observations for existing SCRs. Studies have included two joint industry projects, STRIDE and CARISIMA (Bridge et al., 2004). This body of work has led to development and implementation of numerical soil structure interaction models in software packages available for SCR analysis. A summary of some of the work relevant to this paper is presented here as background literature in respect of (a) seabed response models, and (b) analytical modeling of SCR trenches. Before that, key papers that address SCR trenches and their potential effect on touchdown fatigue are summarized.