INFLUENCE OF SOIL'S VERTICAL STIFFNESS ON ESTIMATION OF FATIGUE LIFE FOR STEEL CATENARY RISERS (SCRS)

Fatigue damage estimation is a key part in a Steel Catenary Riser (SCR) system design. The riser-soil interaction has a significant influence on the fatigue assessment for a SCR system at the Touch-Down Zone (TDZ) because the soil's vertical stiffness is a critical parameter. The conventional methods developed to calculate the soil's vertical stiffness, for example, DNV method [10], Bridge method [4] and Audibert method [3], were based on the assumption that at the TDZ area the soil mass being interacted by the pipe is always under intact condition. Recently, Chen et al. published a newly developed method [5, 6], which was based on the laboratory test results carried out by Texas A&M University in the late 1980s and was further verified by recent laboratory displacement-controlled tests by Texas A&M University [2], resulting in softer soil's vertical stiffness. In order to understand the difference between Chen method and the conventional methods as well as the influence of Chen method on the SCR design, a study was carried out for a given soil condition and three typical sizes of riser pipes and is documented in this paper. In this paper, firstly, various methods used to calculate soil's vertical stiffness, i.e., DNV method, Bridge method, Audibert method, and Chen method, were briefly introduced. Secondly, a discussion was performed on the model of the conventional methods and the model of Chen method. Thirdly, as an example for a given soil condition, the soil's vertical stiffness was estimated using these methods respectively for 6"-dia., 12"-dia., and 18"-dia. riser pipes. Finally, the fatigue damages for various riser pipes were computed using the estimated soil's vertical stiffness and a comparison on the estimated fatigue damages was performed. The study results showed that the soil's vertical stiffness estimated by Chen method was about one order magnitude weaker than these estimated by the conventional methods. In other words, the soil's vertical stiffness estimated by the conventional methods was about 5 to 11 times that estimated by Chen method. The pipe's fatigue life estimated using the soil's vertical stiffness of Chen method was about 1.5 times (i.e., 50% longer than) that estimated using the soil's vertical stiffness of the conventional methods. These study results could improve a SCRs designer's understanding on: 1) the difference between the model of the conventional methods and the model of Chen method, 2) the impact of Chen method on the estimated fatigue life, and 3) that as the model of the conventional methods is a closer representative of the prototype and their results are more conservative, it is recommended to use the conventional methods for SCRs design. Eventually, these will help the designers to select an appropriate method for calculating the soil's vertical stiffness in the SCRs design.