Ana Lúcia F. Lima Torres

A Frequency Domain Approach for Random Fatigue Analysis of Steel Catenary Risers at Brazil’s Deep Waters

The steel catenary riser was adopted by Petrobras as a cost-effective alternative for oil and gas export and for water injection lines on deepwater fields, where large diameter flexible risers present technical and economic limitations. The installation of the P-18 SCR was a pioneer project of a free-hanging steel catenary riser linked to a semi-submersible [1] and demonstrated the technical feasibility of the concept. Fatigue damage verification is an important issue in SCR design, demanding a high number of loading cases to be analyzed. The random time domain nonlinear analysis is considered an attractive and reliable tool for fatigue analysis as nonlinearities are properly modeled and the random behaviour of environmental loadings is considered. As time domain analysis is high computer time consuming, the frequency domain analysis has been considered as an alternative tool for the initial phases of riser design to be used mainly for fatigue damage verification. This paper presents a methodology developed to perform a linearized frequency domain analysis aiming at fatigue damage verification. Two drilling risers were analyzed with the frequency domain procedure developed. The model of a steel lazy-wave riser was analyzed both in frequency and time domain in order to compare fatigue damage results. The analyses were performed using the Petrobras’s in-house computer codes ANFLEX, ALFREQ and POSFAL developed and implemented as part of projects from CENPES/PETROBRAS with “COPPE/UFRJ -The Engineering Post-Graduating Coordination of the Federal University of Rio de Janeiro”.© 2004 ASME

Lazy-Wave Steel Rigid Risers for Turret-Moored FPSO

The free-hanging SCR (Steel Catenary Riser) was adopted by Petrobras as a cost-effective alternative for oil and gas export lines on deepwater fields, where large diameter flexible risers present technical and economic limitations. It is considered an available technology for semi-submersible application. There was interest in applying SCR’s attached to FPSO (Floating, Production, Storage and Offloading) units due to the trend of using these units for exploration and production in Brazilian deep waters. This alternative has to be carefully studied due to the high offsets and heave motions imposed by the vessel on the top of the riser. This work presents the approach and methodology adopted in Petrobras to study the structural integrity and feasibility of a lazy-wave SCR attached to a bow turret-moored FPSO at a water depth of 1290 m. The analysis was performed using the Petrobras’s in-house computer codes ANFLEX and POSFAL developed and implemented as part of projects from CENPES with “COPPE/UFRJ - The Engineering Post-Graduating Coordination of the Federal University of Rio de Janeiro”. For VIV (Vortex Induced Vibration) fatigue damage calculation SHEAR7 was used.Copyright

Steel Lazy Wave Riser Design: API-RP-2RD and DNV-OS-F201 Criteria

Several feasibility studies on the use of large diameters steel catenary’s risers for higher water depths have been made by PETROBRAS. It should be mentioned that the structural design criteria, traditionally employed for such risers under extreme and operational conditions, were based on the API-RP-2RD. This design criterion makes use of single safety factor on von Mises stress. Recently, a new design standard DnV-OS-F201 for metallic risers has been proposed. This standard, based on limit state design principles, are presented on the LRFD (Load and Resistance Factor Design) format, allowing different riser design alternatives to take into account the environmental conditions. This paper presents the main feasibility study outlines of an 18” Steel Lazy Wave Riser (SLWR), attached to the border of a spread-moored FPSO at 1800 water depth, for an extreme design condition, considering both the API and DnV criteria. All numerical analysis were accomplished by the PETROBRAS’s in-house computer code ANFLEX.Copyright

Influence of Bimodal/Bidirectional Characteristics of Campos Basin Sea-States on Riser Fatigue Damage Results

Petrobras R&D Center developed feasibility studies of the lazy-wave configuration of steel catenary risers (SLWR) connected to a turret-moored FPSO and to the border of a spread-moored FSO, in Campos Basin, at water depths of 1290m and 1800m, respectively. In both projects the fatigue analysis considered the bimodal/bidirectional characteristics of sea-states. Special methodologies for environmental data statistical treatment and riser fatigue analysis were developed. This paper presents the study performed to evaluate the importance of considering the bimodal/bidirectional characteristic of Campos Basin sea-states in the steel catenary risers (SCR) fatigue damage verification.Copyright

Lazy-Wave Steel Rigid Risers for FSO With Spread Mooring Anchoring System

Petrobras developed projects with European companies and Brazilian universities in order to study different configurations of steel risers using flexibilization elements. For the bow turret-moored FPSOs the lazy-wave configuration was considered the most adequate due to its structural behaviour and costs when compared to other configurations. A detailed study was performed by the Petrobras R&D Center to verify the structural integrity of a lazy-wave SCR (SLWR) attached to a turret-moored FPSO at a water depth of 1290 m. The results for the installed riser showed its feasibility. Petrobras continued the studies of the SLWR to verify its behaviour when connected to a FSO with a spread-mooring anchoring. This paper presents the approach and methodology adopted in Petrobras to verify the structural integrity of a SLWR attached to a FSO with spread-mooring anchoring at a water depth of 1800 m. The riser analysis was performed using the Petrobras’s in-house computer codes ANFLEX and POSFAL developed and implemented as part of projects from CENPES with “COPPE/UFRJ - The Engineering Post-Graduating Coordination of the Federal University of Rio de Janeiro”. For VIV (Vortex Induced Vibration) fatigue damage calculation SHEAR7 was used. Maximum stresses were verified through a deterministic non-linear time domain-analysis. The time-domain random nonlinear analysis was considered to be the most appropriate to be used for fatigue damage calculation due to the possibility of representing the existing non-linearities of the model and random characteristic of the environmental loading. For the fatigue damage analysis, a set of load cases that considers the bimodal / bi-directional characteristics of sea-states, probability of occurrence and energy content, was used.Copyright

A Frequency Domain Approach for the Random Fatigue Analysis of SCR Considering Bimodal/Bidirectional Characteristic of Campos Basin Sea States

Fatigue verification is an important issue in steel risers design, demanding a good representation of the loading conditions that will occur during the riser entire lifetime. PETROBRAS has carried out a series of measurement and acquisition programs over the past decade, including the Campos Basin simultaneous acquisition of waves, current and wind data. The campaigns are called the PROCAP1 in Marlim Field and PROCAP2 in Barracuda Field. Those programs provided simultaneous environmental data (wave, wind and current) containing multimodal / multidirectional sea-states that occur in Campos Basin, with two main peaks dominating the total energy content [20,21]. As fatigue damage calculation depends on the stresses variations during the lifetime of the structure, the set of loads used in the analysis should be complete enough to represent all possible situations. The high number of loading conditions used in riser fatigue verification associated with the random time-domain analysis that demands a high computer time for processing the analysis, impact the design schedule. The frequency domain approach, based on linearization techniques, is an alternative tool for riser analysis and has been studied mainly for structural fatigue verification applications. For this particular application, due to the low intensity of loadings, the geometric nonlinearity is considered by means of a previous nonlinear static analysis, followed by a dynamic frequency domain analysis on the deformed model. The nonlinearity of the drag part of Morison’s formula has to be conveniently treated by linearization techniques. This work presents a comparative study where the results using a frequency domain analysis are compared to the results of a time domain analysis. Both approaches were used in the analysis of a steel lazy-wave riser (SLWR) model connected to a spread-moored FPSO, submitted to fatigue environmental loadings considering the bimodal/bidirectional characteristic of Campos Basin sea-states. The analyses were performed using the PETROBRAS’s in-house computer codes ANFLEX, ALFREQ and POSFAL developed and implemented as part of projects from CENPES/PETROBRAS with “COPPE/UFRJ - The Engineering Post-Graduating Coordination of the Federal University of Rio de Janeiro”.Copyright

Methodology for Analysis of Installation of a Steel Lazy Wave Riser

The Petroleum Industry has been looking for a configuration for large diameter steel risers, which can be used either at FPSO’s or Semi Submersible Platforms. An interesting option with a low cost increase is the lazy-wave configuration, which is obtained by the addition of buoyancy modules along a limited length of the riser, close to its touch down area. The steel lazy-wave riser is usually checked for the standard failure modes in the operational phase. The present work shows that it is also necessary in the early stages of design, to define an installation procedure in order to assure configuration feasibility, as high stresses develop at riser hog and sag areas as a function of buoyancy modules installation. Riser configuration changes during the installation process demands managing top angle, launched length and distances between the platform and the installation vessel. The limitations of the installation’s equipment have to be taken into account as well. The methodology for the riser analysis in obtaining a feasible solution is outlined. The riser analysis is performed using the Petrobras in-house computer code, ANFLEX and considers a real application scenario for study.Copyright