Claudio Marcio Silva Dantas

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

Efficient Probabilistic Fatigue Analysis of a Riser Suspended and Moored by Chains

Fatigue analysis of an offshore structure usually requires the numerical simulation of a huge number of loading cases to compute the long-term integral associated to the accumulated fatigue damage. Papadimitriou et al. [1] and Low and Cheung [2] proposed two distinct approaches to solve the long-term fatigue damage more efficiently. These methods are known as Asymptotic Approximation Method and Perturbation Method, respectively. This paper investigates the efficiency of these two methods in the fatigue analysis of the steel pipe of a riser suspended and moored by chains (RSAA). Since there is an analytical solution for the stress spectrum of this riser, both time and frequency domain fatigue assessments approaches are considered. The accuracy and computational efficiency of the aforementioned methods are compared with the “brute force” direct integration methodology.Copyright

Parametric Study on the Axial Vibrations of Riser Suspended and Moored by Chains (RSAA) Configurations

Recently, in order to minimize the influence of the vertical motions in the risers and consequently allow the utilization of FPSOs in deep waters, a new riser configuration called RSAA (riser suspended and moored by chains - in Portuguese), composed of a rigid vertical riser, flexible structures and mooring line segments (top and bottom) was proposed. This configuration presents solutions to the most critical points in a riser design: the top tensions are dissociated from the bending moments at the top region, and the curvatures at the TDP are reduced by utilization of floaters. Feasibility analyses have shown that the vertical riser is the most critical part of the proposed system due to the FPSO high level of vertical motions. These motions are transmitted by the top chains, leading to high levels of axial stress variation due to dynamic tension. Faced with this, a parametric study is vital in order to understand the system’s behavior as well as to establish the main parameters which influence its structural behavior. Analytical methods may require some slight simplifications of the problem to be applicable, but they generally lead to compact formulas that do explain which parameters influence the results and why and how it does so. This work proposes an analytical model to determine axial stress and tension variations at the vertical riser, considering some simplifying hypotheses, like the flexible structures and mooring line segments at the bottom will be replaced by a mass and a spring. Neglecting some nonlinearities but considering the coupling between axial and transversal vibrations, a random dynamic analysis in the frequency domain can be performed to evaluate the maximum stresses and tensions levels with considerably lower computational costs.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

Analytical Methodology for Wave Fatigue Analysis on Riser Suspended and Moored by Chains (RSAA) Configurations

As oil exploration moves into fields located in deep water, the need for developing new concepts of riser systems aiming at reducing the influence of environmental loads consequently increases. Therefore, a new riser configuration called RSAA (riser suspended and moored by chains - in Portuguese) is proposed. This structure consists on a rigid vertical riser connected on its top to a floating unit and on its bottom to flexible and mooring lines segments. This new configuration improves the structural response for some of the most critical points in riser design: top tensions and bending moments at the touch down zone.Feasibility analyses have shown that the vertical riser is the most critical part of the proposed system due to the high level of vertical motions and dynamic tension. Therefore, a parametric study is vital in order to understand the system’s behavior as well as to establish the main parameters which influence its structural behavior. Analytical methods may require some slight simplifications of the problem to be applicable, but they generally lead to compact formulas that do explain which parameters influence the results and why and how it does so.This work proposes an analytical model that evaluates the vertical motions and axial tension along the vertical riser subjected to wave loads applied to the floating unit. This procedure could enable parametric studies and the evaluation of the stress amplitudes and, consequently, the fatigue damage along the vertical riser of the RSAA configurations. Furthermore, this work aims to show that this methodology is a fast and effective pre-design tool by comparing it with a nonlinear time domain finite element method analysis.Copyright

Riser Analysis of a New Dry Completion Concept for a Monocolumn Platform

In recent years, PETROBRAS has developed the concept of a monocolumn platform to be used as a floating production unit in deep and ultra-deep water oil fields. This platform, referred as MonoBR, is characterized by low heave and pitch induced motions, even in hurricane conditions, allowing its use as an alternative for locations such as Campos Basin and the Pre-Salt Area in Brazil, and also for the Gulf of Mexico. Despite their low vertical motions, this platform is not generally regarded as a dry completion unit. However, PETROBRAS and two public institutions in Brazil, UFRJ/COPPE and USP/POLI, have studied and designed an innovative mechanism to be installed in MonoBR, called Inverted Cup, which is a sliding floating body installed inside the MonoBR moonpool. In such way, the platform transmits only horizontal motions and rotations to the Inverted Cup; the heave responses of both bodies are decoupled. The Inverted Cup is then designed and dimensioned to receive dry completion X-mas trees and risers. The aim of this paper is to present the results of the riser analysis performed under this new concept — Inverted Cup system — to allow dry completion in the MonoBR. The environmental conditions refer to fields in the Gulf of Mexico and in the Tupi Field (Pre-Salt). The results point to a technically feasible mechanism to allow dry completion, according to adopted criteria.Copyright

A New Top Connection System for Steel Catenary Risers

PETROBRAS has been developing several studies in order to verify the structural feasibility of SCRs connected to production units such as FPSOs, semi-submersible and monocolumn platforms. Due to the great stresses developed at the top region as a consequence of the platforms rotation movements, robust stress joints or flex joints are always required to connect the SCRs to the platforms. However, those components may add high costs to any project, mainly where titanium is required. In attempt to avoid this fact, the objective of this work is to present a new top connection system called “Suspender”. This system is composed by the SCR, a mooring line segment, a flexible jumper and a “Y” shaped steel connector, and has the advantage of reducing the transmission of bending moments from the platform to the SCR. The performance of this alternative was compared to a more conventional one, composed by a SCR and a flex joint, and the obtained results indicate that this new system reduces the maximum stresses at the top region without changing the SCR behavior at the TDZ. It is also possible to extend the fatigue life by changing the SCR configuration.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