The influence of nonlinear hysteretic seabed interaction on slug-induced stress oscillations in steel catenary risers

Steel catenary risers (SCRs) are usually cost-effective solutions in the development of offshore fields and the transferring of the hydrocarbons from the seabed to the floating facilities. These elements are subjected to the fatigue loads particularly in the touchdown zone (TDZ), where the oscillating SCR is exposed to cyclic contact with the seabed. The slug-induced oscillation is a significant contributor to the fatigue loads in the TDZ. The cyclic seabed soil softening under the wave-induced riser oscillations and the gradual penetration of the SCR into the seabed are widely accepted to have a significant influence on SCR fatigue performance. However, this has never been investigated for slug-induced oscillations due to the lack of integrated access to comprehensive numerical models enabling the simulation of the riser slugging and nonlinear hysteretic riser-seabed interaction at the same time. In this paper, an advanced interface was developed and verified using the multi-point moving tie constraint in order to examine the influence of cyclic seabed soil softening on slug-induced oscillations of SCR. The interface was integrated with a pre-developed user subroutine for modeling of the nonlinear hysteretic riser-seabed interaction and incorporated into a global SCR model in ABAQUS. A comprehensive parametric study was conducted to investigate the influence of slug characteristics and nonlinear seabed soil model on slug-induced, wave-induced, and combined wave/slug induced oscillations of SCR in the TDZ. It was observed that the nonlinear seabed model could significantly affect the embedment of the SCR into the seabed under the slug-induced oscillations and consequently improve the fatigue life. The developed user interface was found to be a strong framework for modeling riser slugging.     

钢悬链式立管与非线性海床土相互作用分析方法研究

对基于大挠度柔性梁理论的立管动力分析程序CABLE 3D改编,将原程序中立管受到线性海床的弹性支撑力扩充为立管受到的海床垂向力充分考虑管土非线性相互作用,使新程序中立管与海床土的相互作用遵循p-y曲线。采用伽辽金方法在空间内离散立管的动态方程,最终采用Newmark-β法进行时域内迭代求解。利用改编后的新程序分别研究了立管与线性海床土和非线性海床土相互作用的对比以及不同垂荡幅值情况下立管的动态响应。研究表明,非线性海床土能够更加准确地模拟真实的管土相互作用,触地点区域的节点会经历不同的管土相互作用过程。     

Modeling touchdown point oscillation and its relationship with fatigue response of steel catenary risers

The riser-seabed interaction resulting in a trench formed in the touchdown zone (TDZ) of steel catenary risers (SCR) has a significant influence on accumulated fatigue damage. Several studies have used different trench modeling approaches to investigate the trench effect on fatigue. However, contradictory observations have been reported with no coherent agreement on the beneficial or detrimental effect of the trench on fatigue. In this study, the significance of trench geometry in fatigue damage evaluation was investigated. Using analytical and numerical approaches, a meaningful relationship was observed between the trench slope in different zones and the peak fatigue damage. A new set of rules was proposed for the qualitative assessment of the overall trend of trench effect on the variation of fatigue damage. The proposed assessment rules were validated by performing comprehensive numerical fatigue analysis. A comparison with samples of published experimental and numerical studies was also completed. It was observed that depending on the direction of the low-frequency vessel excursions, the peak fatigue damage may increase towards the near offsets and decrease towards the far vessel offset. This implied that the case dependency of the trench effect on fatigue response in different geographical locations with various environmental loads was a potential source for the contradictory results reported in previously published studies.     

Response-based environmental conditions for the design of steel catenary risers

This work presents a semi-analytical methodology to select design environmental conditions based on long-term cross-section utilization ratios at the TDZ (Touchdown Zone) for steel catenary risers. This approach uses simplified analytical models to calculate time series of short-term utilization ratios, defined according to the DnV-OS-F201 (2010) standard. After processing these time series, long-term utilization ratios can be determined with relatively low computational cost. By evaluating long-term utilization ratios, it is possible to define short-term design environmental conditions, defined as short-term conditions for which the extreme riser responses are equal to the long-term ones. This kind of methodology may represent a substantial change to the traditional focus given to riser design, which is based on responses obtained from extreme environmental conditions, instead of on the extreme responses.