Petrobras has been investigating the steel-catenary-riser (SCR) alternative since the beginning of the 1990s. Since then, fatigue verification has been an important issue, demanding good rep-resentation of the loading conditions that occur during the lifetime of the riser. The concern with fatigue has motivated Petrobras to research several areas, such as metocean data acquisi-tion, hull design for motion optimiza-tion, special touch-down-point (TDP) joints, accurate models for vortex-induced-vibration (VIV) analysis, and the corrosion-fatigue effect.

IntroductionInstallation of the P-18 SCR was a pioneer project of a free-hanging SCR connected to a semisubmersible, and it proved the technical feasibility of the concept. Although this riser was installed as a prototype, it is still working in the gas transfer from platform P-18 to platform P-26. It has been monitored since 1999, and the results are being compared with the design data and with simulations performed with in-house computer pro-grams and other commercial packages that include the complete design meth-odology. Other SCRs were studied, such as the 12-in. oil-export riser for the P-19

semisubmersible in 770 m of water and the 10-in. oil- and gas-export lines for the P-36 semisubmersible at a water depth of 1360 m.

The free-hanging SCR configuration is considered as an available technology for semisubmersible applications, and there is interest in the application of SCRs connected to floating production, storage, and offloading units (FPSOs) because of the trend to use these units for exploration and production in deep water. This has caused a need to study this concept carefully, given the high offsets and heave motions imposed by the vessel at the top of the riser.

Fatigue verification is an important issue that requires accurate evaluation of the loading conditions that occur during the riser lifetime, and it also requires a precise knowledge of con-struction aspects that could decrease or change riser-materials resistance.

Wave-Induced FatigueOver the past few decades, Petrobras has acquired Campos basin wave, cur-rent, and wind data, resulting in a metocean database containing more than 7,000 records. Within these data, the occurrence of multimodal/multidi-rectional sea states was identified. To use this database in riser design, the in-house software tools for structural fatigue analysis were upgraded to con-sider bimodal/bidirectional sea states.

Because fatigue verification is an important issue in steel-riser design, a good representation of loading condi-tions that occur during the riser lifetime is needed and use of the entire database is recommended. However, the riser design schedule can be affected if a random time-domain analysis is used. To minimize this, one solution adopted was to develop a statistical procedure to reduce the database to a reasonable number of representative loading cases

to be used in fatigue-damage verifi-cation. This method resulted in the adoption of approximately 150 fatigue-loading cases. The combined wave, current, and wind data are preserved in the loading conditions that were cho-sen to represent all the usual metocean situations in Campos basin.

Hull Design To Reduce Motion. In the Campos basin, the wave fatigue environment has been shown to be the limiting factor for SCR feasibility. The alternative of optimizing platform motions has been one of the ways to increase the possibility of SCR applica-tions. Among dynamic motions, heave has been identified as the most damag-ing. For the P-52 design, in 1800 m of water in the Roncador field, a limit for a maximum heave at an extreme point in the hull has been established, and these data determined the choice between existent hull models.

When starting a new hull design for the P-55 unit, more-complete criteria was used, with a set of operational waves chosen from the traditionally most damaging ones. The P-52 motions were taken as a reference. There was interest in knowing what level of minimized motions could be obtained considering a deep-draft-hull concept. A study was conducted with a large number of hull geometries, and the most adequate ones, in terms of con-structability and other naval aspects, were chosen. The amplitudes of heave motions for these hull models were ver-ified to be from 15 to 20% smaller than those obtained for the P-52 hull. On the basis of this, a set of maximum motions under medium- and high-fatigue wave conditions has been established.

The monohull concept also has been designed to achieve minimized heave motions. The same criterion has been applied as a guide to the models stud-

This article, written by Assistant Tech-nology Editor Karen Bybee, contains highlights of paper OTC 19249, In fluence of Fatigue Issues on the Design of SCRs for Deepwater Offshore Brazil, by A.L.F.L. Torres, M.M. Mourelle, S.F. Senra, E.C. Gonzalez, and J.M.T. da Gama Lima, Petrobras S.A., originally prepared for the 2008 Offshore Technology Conference, Houston, 58 May. The paper has not been peer reviewed.

Copyright 2008 Offshore Technology Conference. Reproduced by permission.

Design of Steel Catenary Risers for Deepwater Offshore Brazil

OFFSHORE FACILITIES

The full-length paper is available for purchase at OnePetro: www.onepetro.org.

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ied for the monobore design, with and without storage capacity.

Coupled Models. The use of coupled analysis tools in the design of SCRs becomes even more important because a large number of risers and mooring lines are connected to the platform and the system is in deep water like the P-18, P-52, and P-55 systems. Petrobras has developed in-house software programs to analyze the coupling between the nonlinear hydrodynamic behavior of the hull and the structural and hydrodynam-ic behavior of the lines. Hybrid methods that combine the use of programs based on coupled and uncoupled formulations have been considered the possible road-map toward a fully coupled analysis and design methodology. The coupled analy-sis carried out with the hybrid model is attractive in contrast to the excessive computer cost of the fully coupled meth-od because a large number of analyses for calculation of the fatigue behavior on risers are necessary with it.

In recent studies of SCR design for the P-55, the numerical model of the system was generated in three different coupled programs, and the results were compared with the empirical data from the model tests in terms of platform motions and line tensions. Calibrations

could be adjusted to obtain a more reliable numerical model of the entire system. Some investigations have been conducted related to riser fatigue-dam-age response, comparing the use of coupled and uncoupled methods.

Another issue identified in the model test of the P-55 was vortex-induced motions (VIMs) in this deep-draft semi-submersible platform. The VIMs can result in additional oscillations in riser and mooring-line tension, as well as additional fatigue loading of the riser TDP. These effects are under investiga-tion through towing tests and compu-tational-fluid-dynamics (CFD) calcula-tions to verify if it is necessary to use some mitigation device on the hull.

Frequency-Domain Approach. A non-linear random time-domain analysis has been adopted in fatigue-analysis verifica-tion because model nonlinearities are modeled properly and the environmental-loadings random behavior is considered. The disadvantage is the high computer time required. Because fatigue-damage calculation depends on stress variations during the lifetime of the structure, the set of loads used in the analysis should be sufficiently complete to represent all possible situations. Because Petrobras uses its own measured environmental

database that contains a large number of data points, the use of time-domain analysis may affect the design sched-ule. Another solution, besides the sta-tistical treatment procedure to reduce the database, was the development of a frequency-domain methodology, based on linearization techniques, that was implemented in in-house software to be used as an alternative tool for the initial phase of riser design.

In general, results indicated agreement between frequency- and time-domain approaches in identification of critical-joint and critical-loading cases. In terms of fatigue-damage calculation, frequen-cy-domain analysis when compared to time-domain analysis furnished better results for the lazy-wave SCR configu-ration. For the free-hanging configura-tion, larger differences were found and the frequency-domain approach tends to be more conservative. Research to evaluate the soil-structure interaction, aiming at representing the TDP varia-tion that is significant in the free-hang-ing configuration is ongoing.

VIV FatigueCampos Basin Currents. Campos basin current profiles for deep water are com-posed of two layers coming from differ-ent sources. These layers have different

Fig. 1Fatigue-damage contribution at the TDP.

Infield and Export RisersD-Class Weld at Outer Diameter

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Production A Production B Water Injection Gas Export Oil Export A Oil Export B

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age,

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VIV long term VIV short term Heave-induced VIV (1-year wave) *Heave-induced VIV

(100-year wave) *1st- and 2nd-order damage

* Relatively low damage values

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directions. The first layer covers the depths from sea level to approximately 300 to 400 m, going predominantly to the south and southwest directions. Below this level, another layer becomes dominant going to the north and north-east direction.

From the first VIV calculations, as a function of the 2D characteristic of the software, the current velocities along the depth were projected to the riser in-plane and out-of-plane directions. This was supposed to capture the charac-teristic of directionality of the Campos basin current profiles.

Short-Term Response. The idea of con-sidering a short-term response came from the necessity of predicting the riser response when facing a 100-year current event during its operational life. No extreme stresses were expected, but it was necessary to know the magnitude of the induced fatigue damage. The approach used assumes that the dam-age from the worst-possible extreme event will be resisted by the riser. The extreme events, however, are not very well characterized in terms of duration and the way the phenomenon evolves. In recent applications, the short-term damage represented a significant per-centage of the total damage, as the case for the P-55 design for an 1800-m water-depth application for the Roncador field, shown in Fig. 1.

Alternative CFD Model. As an alterna-tive to VIV traditional-model use, the initiative was to incorporate a CFD pro-cedure into the in-house riser-analysis package. The discrete-vortex method has been implemented and is being tested and compared to the traditional-model results for some real applications. The method brings the possibility of using the current profiles with their directional characteristics, and results obtained so far indicate some less conservative results.

MaterialsThe first SCR applications developed by Petrobras were related to import and export lines. When the use of SCRs started to be planned for production lines, as in the P-52 project, the problem of how to face the highly corrosive envi-ronment turned out to be a major one. The presence of carbon dioxide (CO2) and/or hydrogen sulfide (H2S) in the produced stream created doubt about the applicability of the S-N curves used.

P-18 SCR The P-18 SCR is the only operating SCR currently in the Campos basin. At the time it was installed in 1998, the riser was the first SCR to be installed on a semisubmersible unit. A complete monitoring system was installed, and the measurement campaign lasted for approximately 2.5 years.

Many issues were investigated, and today the generated database still is being used in studies. The confirmation of the expected riser behavior, char-acterization of platform motions, and identification of critical current profiles for VIV response have been some of the results obtained.

The measured data at strain gauges at the riser top, associated with the measured flex-joint-angle variations, are being used in the reassessment of flex-joint fatigue life. The data, besides including real values, include VIV-induced axial vibrations that were not included in the design phase of the flex joint, and today it is the main reason for revisional work regarding the top connection system.

The riser was installed without any suppressor device for VIV. The updated method regarding VIV and wave fatigue is being applied to define what will be considered as the riser updated-design fatigue life. The TDP is the main focus because it is the region that suffers damage from platform-induced wave motions and also from VIV.

The reassessment plan includes the generation of an updated engineering critical assessment and inspection of welds by two methods, one performed externally and another by umbilical pig. The accuracy of the field inspec-tion may not correspond to ideal val-ues; neither may have the same level of accuracy obtained during riser con-struction, but both will give important reference values that will support the riser-integrity evaluation. A permanent monitoring system has been designed for the top section of the P-18 SCR that, once field tested, will be considered as a model for other SCRs to be installed in the future.

General Comments When planning for a platform with a large number of SCRs, the interference between the adjacent risers can become an issue. Besides working on the dis-tance between supports and in the difference between the azimuths of the

risers, frequently it is necessary to con-sider a difference in top angles between neighboring risers. As a function of this, the necessity of using top-angle values of 15 to 17 for some risers, in the vicinity of other risers with 20 of top angle, became the usual practice dur-ing design. For reduced-heave-motion units, such as the deep-draft unit P-55, the adoption of 17 or even 15 did not cause problems.

Conclusion From the last results obtained with Petrobras design methodology and updated data, it is impossible not to consider the option of using VIV sup-pressors for an SCR. The question is the relative length to be used, but when analyzing total length variation of strakes, their VIV efficiency, and their location along the SCR, the results obtained through traditional modeling do not present monotonic results, thus making it difficult to make decisions about the length of the strakes.

The successive evolution in the set of design currents applied for long-term and short-term response calcula-tions has caused the design to be more robust and realistic, but up to now, always increasing the effect of VIV on the overall riser design.

The incorporation of CFD proce-dures, which can keep the Campos basin current profiles directionality characteristic, is a promising way to obtain less conservative results. Up to now, the design for production risers subjected to corrosive fluids has led to clad sections, corresponding to less than 20% of total riser length. This trend represents specific conditions of the Roncador field. More-severe situa-tions may occur that can force the use of longer clad sections. Application of clad pipes in the critical regions requires a better understanding of the weld behavior, geometrical imperfec-tions, and nondestructive-test results.

Monitoring new risers to be installed is a key point that can support the evolution of design methodology for VIV and also with respect to wave-induced platform motions. The moni-toring system can be planned to be composed of some equipment on the top section that will operate during the entire riser lifetime, and another set of equipment that will be used for a limited duration for evaluation of the design methodology. JPT