ABSTRACT

Several studies have incorporated the trench effect into the SCR's fatigue analysis based on the two main approaches: artificial insertion of a trench profile in the TDZ, and automated trench formation using non-linear hysteretic riser-seabed interaction models. There have been contradictory results with no coherent agreement on the beneficial or detrimental effect of the trench on fatigue life. The current study has been conducted to resolve existing challenges by proposing a reliable methodology by defining an equivalent stiffness to generate a consistent trench profile entirely compatible with the natural curvature of the SCR in the TDZ.

INTRODUCTION

Steel Catenary Risers (SCRs) are widely used in deep-water offshore facilities to convey hydrocarbon products from the seabed to the floating structures. Due to the cyclic motion of the floating vessel, the riser repeatedly makes contact with the seabed resulting in progressive soil degradation in touchdown zone (TDZ), leading to the gradual penetration of the riser into the seabed. Subsea surveys showed that this riser embedment further develops over the early years of riser operation (first 2-3 years of operation), reaching the ultimate profile with a maximum depth of around 2.5D to 5D, where D is the pipe diameter (Thethi and Moros, 2001; Bridge and Howells, 2007). Previous studies have widely investigated the influence of the trench effect on the fatigue analysis of catenary risers, particularly SCRs. Some of the studies have shown the fatigue life improvement in the TDZ due to trench formation (e.g., Langner, 2003; Nakhaei and Zhang, 2008; Elliot et al., 2013; Randolph et al., 2013; Sharma and Aubeny, 2011; Wang et al., 2016), while other studies have shown a reduced fatigue life (e.g., Leira, 2004; Giertsen, 2004; Shiri and Randolph 2010; Rezazadeh et al., 2012; Shiri, 2014a, b; Zargar 2017). Also, some studies have obtained scattered results showing improved or reduced fatigue life because of trench formation (Randolph et al., 2013; Dong and Shiri, 2019; Shoghi and Shiri, 2019; 2020). It can be seen that there is not a coherent answer among the researchers, and the beneficial or detrimental effect of the trench is still a point of the question. Shoghi and Shiri (2020) conducted a qualitative assessment of the trench effect based on the results reported in the literature and showed that some of these contradictory results are related to the methodology used to implement the trench profile underneath the riser.

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