ABSTRACT
The effect of trenches due to riser-soil interaction on the fatigue life of SCRs has attracted attentions of scholars. In this work, the formation of trenches under SCRs was simulated rapidly by applying extreme sea states. By considering variations of water depth, floater heave and riser parameters in a certain range, a database of numerically simulated trench profiles was established. The parameters of polynomial trench profiles were fit and more general function coefficients for trench profiles were put forward. The results show that trench limits the fatigue damage of SCRs in the touch down zone (TDZ).
Steel catenary risers (SCRs) have become the first choice for deep sea oil and gas development due to their high temperature and pressure resistance, relatively low cost and ease of production and installation. SCRs have a unique structure style that integrates subsea pipeline with riser, and it is applicable for various types of floating structures, such as tension leg platforms, Spars, semi-submersible platforms and Floating Production Storage and Offloading systems (FPSO), etc. In practice, SCRs are connected to the floating structure at one end and to the subsea device at the other end. The hang-off point at the top and the touch down point (TDP) at the seabed are the key points of its structural design and fatigue design. Studies have shown that, in general, the TDP is the crucial point of fatigue damage of SCRs (Bhat et al., 2004). The possibility of fatigue failure is relatively large at this location, which may endanger the entire riser system. Therefore, researchers have carried out extensive researches on the fatigue of SCRs at the TDP (Wang et al., 2014, Wang et al., 2013, Fu and Yang, 2010, Li, 2012, Li and Low, 2014).
Some projects in practice have shown that due to the softness of the deep-sea soil, trenches may be generated under the interaction between the riser and the seabed. Remote operating vehicles (ROV) were used to survey the contact area under SCR and trenches with a maximum depth of 4-5 times the outer diameter of SCR (Willis, West, 2001) were found under SCR. In addition, the seabed deformation in the contact area can affect the stress distribution and then the fatigue life of the SCR. Researchers also conducted a series of studies on the shape of trenches and their effect on the fatigue life of the riser, but there are currently two opposite conclusions. Leira et al. (2004) conducted a study on the fatigue of the riser by establishing a trench model and found that the formation of trench increases the fatigue damage of the riser; however, researches from Langner (2003), Nakhaee et al. (2008), and Li and Low (2012) showed that trench can reduce fatigue damage in the touch down zone (TDZ). The reason for the existence of the above two opposite conclusions is that mathematical expressions were used to calculate the trench profiles and perform the fatigue analysis of SCRs. However, because the accuracy of the mathematical expressions for the trench profiles cannot be guaranteed at present, there must be a deviation from the actual trench profiles. Even a minor change in the trench geometry will result in the change of SCR configuration and further affect the SCR fatigue.
