Abstract

Accurate estimate of the steel catenary riser's (SCR) fatigue damage in the touchdown zone (TDZ) is heavily dependent on the floater's motion and the interaction between SCR and seabed. This paper aims to incorporate the floater's motion and the soil's resistance into the integrated dynamic analysis model, and investigates the influence of riser's conveying medium and soil's shear strength on SCR's fatigue damage in time domain. The case study indicates that the SCR's fatigue damage is severely affected by the soil's shear strength in TDZ and it is significantly underestimated if the riser's conveying medium is not taken into consideration.

Introduction

Steel catenary risers (SCRs), which are steel pipes slung from floater to seabed in a catenary, provide a technically feasible and commercially efficient solution for deep-water hydrocarbon exploitation (Quintin et al., 2007; Shiri, 2010). The fatigue performance of SCR in touchdown zone (TDZ) is still a challenge for SCR's design, where the most uncertainly comes from the complex nature of riser-seabed interaction in TDZ. It is therefore important to develop a riser-seabed interaction mechanism to provide a realistic technique for determining dynamic response and fatigue performance in TDZ (Elosta et al., 2013; Shiri, 2014).

The interaction mechanism of riser-seabed has been investigated experimentally in field and in laboratory by many scholars (Willis and West, 2001; Bridge and Willis, 2002; Bridge et al., 2003; Clukey et al., 2005; Hodder and Byrne, 2010). Experiments show that the riser-seabed interaction involves complex nonlinear process including trench profile, nonlinear soil stiffness, finite soil suction and breakaway of the riser from the seabed. Therefore, using a rigid surface to simulate the seabed and ignoring the nature of trenching development process and the effect of soil's suction are not suitable.

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