Abstract
So far, off the coast of Brazil, steel catenary risers (SCRs) are only used in the subsea gathering system when connected to a semi-submersible FPU (Floating Production Unit) or a Buoy Supported Risers system (BSR). This paper presents a technical feasibility study for the use of SCRs in the ultra-deepwaters of Pre-salt field (Santos Basin) connected to a spread-moored FPSO (Floating Production Storage and Offloading).
Environmental loads acting on FPSOs cause higher movements than on other types of FPUs (such as semi-submersible, SPAR and TLP). The wave-induced floater motions under the harsh conditions of Santos Basin may generate high fatigue damage at the riser TDZ (touchdown zone). In addition, extreme events may cause a significant compression load on the SCR, inducing global buckling in the same region and, thus, making the TDZ a region of special interest for the design.
To address the issues mentioned above, advanced engineering techniques and effective technologies available in the offshore industry were employed. The study considered different safety classes along the riser, allowing greater flexibility for the definition of safety requirements, and calibrated the fatigue safety factors according to a risk-based methodology. The proposed configuration made use of hydrodynamic dampers and fatigue performance improvement techniques such as upset end pipes and stress relief on critical welds.
The study results indicate the feasibility of SCRs for the operational conditions considered. The use of SCRs potentialy enhances the competitiveness of rigid riser systems for the Pre-Salt fields of Santos Basin when compared with Steel Lazy Wave Risers (SLWRs).
