Fernando Jorge Mendes de Sousa

A Theoretical Approach to Predict the Fatigue Life of Flexible Pipes

This paper focuses on a theoretical approach to access the fatigue life of flexible pipes. This methodology employs functions that convert forces and moments obtained in time-domain global analyses into stresses in their tensile armors. The stresses are then processed by well-known cycle counting methods, and S-N curves are used to evaluate the fatigue damage at several points in the pipe’s cross-section. Finally, Palmgren-Miner linear damage hypothesis is assumed in order to calculate the accumulated fatigue damage. A study on the fatigue life of a flexible pipe employing this methodology is presented. The main points addressed in the study are the influence of friction between layers, the effect of the annulus conditions, the importance of evaluating the fatigue life in various points of the pipe’s cross-section, and the effect of mean stresses. The results obtained suggest that the friction between layers and the annulus conditions strongly influences the fatigue life of flexible pipes. Moreover, mean stress effects are also significant, and at least half of the wires in each analyzed section of the pipe must be considered in a typical fatigue analysis.

Flexible Riser Fatigue Re-Evaluation to Extend the Service Life

Some PETROBRAS fields are near mature now, around 15 years of production, and their production still important to the company portfolio, the possibility of extending the service life of these flexible risers becomes extremely attractive. This work addresses the re-evaluation of the fatigue life of old flexible risers aiming to extend their fatigue life at the same environment conditions or at new and less challenging ones. To fulfill this condition a special procedure is being applied to stretch the service life of the installed flexible risers, considering irregular wave analysis conditions, distributions of damage around the circumference and along the bend stiffener area and phase difference between tension and bending and in some cases a coupled analysis of the ship, mooring and risers systems. This kind of new fatigue procedure could also become of paramount importance to Petrobras to allow the design of conventional flexible risers for ultra deep waters.Copyright

Calibration of Design Conditions Based on Long Term Top Tension for Catenary Risers

Response based approaches are not common in riser design. Due to the high computational costs associated to these methodologies, it is usual to replace the calculation of extreme long term responses by the calculation of responses to a few number of artificial sea states, supposed extreme. However, this hypothesis may not always be applicable. The extreme response of a riser is influenced by several factors. For instance, vessel response motions resonance can occur for waves of periods lower than the ones associated to the desired long term period. In this way, this work has two main objectives. The first is to propose a computationally feasible methodology to calculate long term extreme responses; the second is to calibrate loading conditions, based on the long term responses, to be used when designing catenary risers. The parameter selected to represent the response is the centenary (100y) riser top tension. The utilization of the proposed methodology is illustrated by a case study where three possible positions for a turret in a FPSO hull were compared. The obtained results indicate that this methodology can contribute to substantial changes in the way risers are designed, focusing on the response instead of on the occurrence of extreme sea states.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

Neural Networks Applied to the Wave-Induced Fatigue Analysis of Steel Risers

Time domain stochastic wave dynamic analyses of offshore structures are computationally expensive. Considering the wave-induced fatigue assessment for such structures, the combination of many environmental loading cases and the need of long time-series responses make the computational cost even more critical. In order to reduce the computational burden related to the wave-induced fatigue analysis of Steel Catenary Risers (SCRs), this work presents the application of a recently developed hybrid methodology that combines dynamic Finite Element Analysis (FEA) and Artificial Neural Networks (ANN). The methodology is named hybrid once it requires short time series of structure responses (obtained by FEA) and imposed motions (evaluated analytically) to train an ANN. Subsequently, the ANN is employed to predict the remaining response time series using the prescribed motions imposed at the top of the structure by the floater unit. In this particular work, the methodology is applied aiming to predict the tension and bending moments’ time series at structural elements located at the top region and at the touchdown zone (TDZ) of a metallic riser. With the predicted responses (tensions and moments), the stress time series are determined for eight points along the pipe cross sections, and stress cycles are identified using a Rainflow algorithm. Fatigue damage is then evaluated using SN curves and the Miner-Palmgren damage accumulation rule. The methodology is applied to a SCR connected to a semisubmersible platform in a water depth of 910 m. The obtained results are compared to those from a full FEA in order to evaluate the accuracy and computer efficiency of the hybrid methodology.

A Methodology to Predict the Remaining Fatigue Life of a Flexible Pipe With Broken Tensile Armor Wires

In this work, an approach to predict the remaining fatigue life of flexible pipes with damages in their tensile armor wires is proposed. This approach relies on a previous proposed approach to calculate the fatigue life of intact flexible pipes. By relying on results from theoretical and experimental investigations, the previous proposed expressions were modified in order to account for damages in the tensile armor wires of these structures. Furthermore, the computation of the fatigue life was also modified in order to account for results from inspections in these pipes thus allowing the estimative of the remaining fatigue life of the pipe. The use of this methodology is illustrated in the analysis of a 9.13″ flexible pipe considering different conditions in its outer tensile armor wires: intact and with one up to ten wires broken along time. The results obtained indicate that the rupture of the tensile armor wires may significantly reduce the fatigue life of flexible pipes and, consequently, may lead to the premature failure of the pipe.Copyright

Long-Term Extreme Response of Marine Structures Considering the Combination of First and Second Order Wave Effects

In this paper, the long-term extreme response of a floating marine structure subjected to first and second order (slow-drift) wave effects is addressed. The proposed formulation is based on the long-term up-crossing rate, which is obtained by integrating the contribution of all short-term sea states. In order to speed up the evaluation of the long-term integral, an analytical expression for the short-term responses up-crossing rates was adopted. This expression is based on Hermite polynomials, and considers the response as a second order Volterra stochastic process. This formulation was applied to evaluate the 1-yr, 10-yr and 100-yr lateral motions of a circular-shaped monocolumn platform, and the non-Gaussian (nonlinear) results are compared with those obtained by assuming the response as a Gaussian process.Copyright

An Alternative Methodology to Evaluate the Fatigue Life of Risers Based on Regular Waves

In the last years, the increasing knowledge about environmental data has also increased the number of sea states that are used in the fatigue analysis of risers. These analyses demand high computational resources and, consequently, methodologies to reduce this cost become very attractive. One of the most studied alternatives in this area is the utilization of regular waves, represented by block diagrams. Based on distributions such as the one proposed by Longuet-Higgins, irregular waves are decomposed in regular waves, originating deterministic scatter diagrams. The Longuet-Higgins distribution, however, has a tendency to generate an unrealistic number of waves at high periods and, consequently, it is usual do set a maximum cutoff level for the periods. In this work, a different approach to generate the regular waves scatter diagrams is investigated. It is based directly on the generation of time series of sea surface elevations, allowing an easy identification of heights and periods of regular waves. The obtained results indicate the feasibility of the proposed method, based on comparisons with irregular waves results.Copyright

Fatigue Analysis of a 6” Flexible Pipe With Broken Tensile Armor Wires

This work focuses on a methodology to predict the fatigue life of flexible pipes with wires broken in their tensile armors. Initially, the mechanical behavior of these pipes is discussed. Relying on this discussion, a simple set of equations is proposed in order to calculate the stresses in the armors of these pipes. These equations employ pre-estimated linear coefficients to convert forces and moments that act on the pope into stresses. These stresses are then processed by well-known cycle counting methods and S-N curves are finally used to evaluate fatigue damage at several points in the pipe’s cross section. The use of this methodology is exemplified by the assessment of the fatigue life of a 6” flexible pipe in which 0 up to 5 wires of its outer tensile armor are broken. The results indicate a substantial reduction in the fatigue life of the pipe with the increasing number of wires broken.Copyright