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).

The interest in the application of a SCR connected to a FPSO for exploration and production in deep water, has motivated the need to carefully study this concept due to the high offsets and vertical motions imposed by the vessel at the top of the riser. Petrobras has developed through its Research Center the study of different steel riser configurations. For bow turret-moored and spread-moored FPSOs based on VLCC converted hulls, the steel lazy-wave riser (SLWR) has been considered as an adequate solution due to its structural dynamic behavior and costs when compared to other configurations. Although the SLWR furnishes acceptable results for fatigue and extreme environmental conditions, the search for the best configuration is very demanding as any changes to a geometric parameter affect its whole structural dynamic behaviour. The search for configurations that meet all the code criteria for the riser project required meticulous detail that has not always lead to the best results because the number of variables involved is quite significant. Another important aspect is the installation procedure that can also influence the final configuration. In order to reduce the engineering time in generating and analyzing several configurations, optimization tools were studied and used in association with Petrobras in-house software to help define a model that could achieve all design verification phases more easily. This paper presents the experience with the use of an optimization procedure applied to facilitate the design of a SLWR connected to a FPSO unit offshore Brazil. The process of optimization begins with a set of preliminary geometric variables and constraints that are associated with multiple objectives related to economic, construction and safety factors. The result of the optimization process is a set of feasible configurations from which, through careful selection, the "one of the best" configuration is chosen.

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The work was focused in the chase for alternative configurations that could resist to the high FPSO motions in the Brasil’s Pre-Salt harsh wave environment, and that could also be less compliant laterally when compared to the SLWR solution. A case study was taken where an infield 8 inch SLWR configuration has been taken for comparisons. After adjusting the SSWR (Steel Steep Wave Riser) main characteristics such as top angle, buoyant section length, buoyancy modules geometry and spacing, feasible configurations have been obtained. For a configuration to be considered as feasible, a set of verifications have been carried out including extreme events, wave fatigue, vortex induced vibration and installation. The verification was performed considering several riser top connection positions and azimuths along the FPSO riser support balcony. The interference with neighboring risers has been also taken as an important issue, but was taken solely for comparison with the SLWR configurations. The installation phase has been focused including the stages of bottom connection, normal pipe lay and the connection at the FPSO. The main problems associated to the installation phase of the steep wave configuration were identified and addressed in the discussion presented. As the SSWR configuration has a fixed point at the sea bottom, two different solutions for this connection have been studied, and the final choice is described. The main differences between SSWRs and SLWRs, and the possible advantages of the SSWR configuration are discussed and a direct comparison is presented.

One of the main challenges in rigid riser design for Brazilian Pre-salt is the fatigue limit state. At this new production frontier, some key points are imposed as a challenge for riser designers, mainly due to the high level of motions imposed by the FPSO at the riser top in a coupled system with water depth around 2200 meters, and thicker riser’s thermal insulation demanded for flow assurance (which worsens the dynamic response of production risers). Additionally, high contaminant levels in the fluid (CO2 & H2S) demands CRA materials. Within this context, Petrobras has been considering Steel Lazy Wave Riser (SLWR) configuration as a base case scenario for rigid riser projects, since this configuration is able to absorb part of the FPSO motions that would reach the touch down zone (TDZ) and, consequently, making this region much less demanded when compared against Steel Catenary Risers (SCR). In its pioneer deepwater SLWR [1], Petrobras adopted a conservative approach for fatigue assessment that involved degenerated SN curves from DNV-RP-C203, i.e. D curve in cathodic protection with the slope changing point (SCP) shifted to 5 × 106 for external wall and F1 curve in air with SCP at 5 × 107 for internal wall. More recently, both DNVGL and BSI have reviewed their fatigue assessment codes and no longer holds parity between SN curves. BS-7608 Ed. 2014 introduced different SCPs in order to account for a possible non-conservativeness in the assessment of low stresses under variable amplitude in the loading spectra. DNVGL-RP-C203 Ed. 2016 now presents three different bilinear SN curves for the internal wall of pipelines and risers that depends on weld misalignment, while it keeps SCP unchanged. This paper presents a recent case study for a typical SLWR configuration in pre-salt, in order to evaluate the impact of the changes proposed by the new versions of these design codes in the fatigue life of riser girth welds. Results of this work showed that the impact of different positioning of slope changing points in SN curves can have a great importance for riser design, since typical load spectrum lies around this region. Fatigue life could be increased up to twice or three times if one of these codes are adopted instead of the Shifted SN curves. However, the effect of low stresses under variable amplitude loading spectra is still a concern and it should be further investigated.