Steel catenary risers (SCR) have been an attractive choice for recent deep-water field developments. However, design of SCRs for harsh environment is a great challenge, especially from large motion host platforms such as semi-submersible platforms. The key driver for the design of SCR in harsh environment is the fatigue near hang-off and at touch down point. This paper describes the concept of weight optimized SCR design for deep water harsh environment fulfilling both strength and fatigue requirements. The concepts incorporate presently available materials and technology. For the fatigue critical cross-sections, some qualified cost-effective and easily installable solutions are proposed. These proposed concepts are demonstrated through case studies of a few common riser sizes for such deep water field development with large vessel motions.
Steel catenary risers (SCR) have been an attractive choice for recent deep-water field developments. However, design of SCRs for harsh environment has been a great challenge, especially from large motion host platforms such as semi-submersible platforms. The application of SCR from such semi submersible Floating Production Units (FPU) at harsh environment present design challenges due to the large motions of the vessel from waves and large vessel offsets from wind, current and slow-drift wave motions. There are buckling issues at touch down point (TDP) due to large heave and surge motions, and fatigue problems due to vessel motions and soil-riser interaction. These design challenges can be successfully addressed by introducing a SCR concept with varying weight along the riser and with lightest possible cross-sections in the Touch Down Zone (TDZ), Karunakaran et al. (2002). This concept is schematically shown in Figure 1. The variation in weight of the riser is achieved by applying different density coatings. In this paper, the availability of such coatings which are qualified and ready to use and their properties are discussed. Also, the choice of material for different applications and their influence on the fatigue performance is discussed. The effects of such weight optimized design of SCRs are demonstrated for North Sea condition for three different applications. These example studies are performed using nonlinear time domain analysis and the fatigue analyses are performed applying a comprehensive irregular wave time domain analysis procedure. The examples clearly indicate that there is a remarkable improvement in the dynamic behavior of SCRs with the application of such weight optimization along the riser length. The von Mises stress at TDP is reduced considerably. Furthermore, the effect of soil-riser interaction on the fatigue response of such lightweight coated SCRs is much reduced as compared to that of a bare SCR. Hence the weight optimized SCRs with lightweight coating at TDZ presents a very attractive solution for deepwater applications in harsh environments with large vessel motions.
Figure 1 SCR concept for harsh environment (Available in full paper)
A key issue in the present study is to restrict the weight optimization to use of qualified materials. When moving to deepwater applications, issues like long-term performance with respect to hydrostatic creep and water absorption become key factors.