José Renato M. de Sousa

Undrained Load Capacity of Torpedo Anchors Embedded in Cohesive Soils

This paper presents a numerical based study on the undrained load capacity of a typical torpedo anchor embedded in a purely cohesive isotropic soil using a three-dimensional nonlinear finite element model. In this model, the soil is simulated with solid elements capable of representing its nonlinear physical behavior and the large deformations involved. The torpedo anchor is also modeled with solid elements, and its geometry is represented in detail. Moreover, the anchor-soil interaction is addressed with contact finite elements that allow relative sliding with friction between the surfaces in contact. A number of analyses are conducted in order to understand the response of this type of anchor when different soil undrained shear strengths, load directions, and number and width of flukes are considered. The results obtained indicate two different failure mechanisms: The first one involves significant plastic deformation before collapse and, consequently, mobilizes a great amount of soil; the second is associated with the development of a limited shear zone near the edge of the anchor and mobilizes a small amount of soil. The total contact area of the anchor seems to be an important parameter in the determination of its load capacity, and, consequently, the increase in the undrained shear strength and the number of flukes and/or their width significantly increases the load capacity of the anchor.

An Experimental and Numerical Study on the Axial Compression Response of Flexible Pipes

This paper deals with a nonlinear three-dimensional finite element (FE) model capable of predicting the mechanical response of flexible pipes subjected to axisymmetric loads focusing on their axial compression response. Moreover, in order to validate this model, experimental tests are also described. In these tests, a typical 4 in. flexible pipe was subjected to axial compression until its failure is reached. Radial and axial displacements were measured and compared to the model predictions. The good agreement between all results points out that the proposed FE model is effective to estimate the response of flexible pipes to axial compression and; furthermore, has potential to be employed in the identification of the failure modes related to excessive axial compression as well as in the mechanical analysis of flexible pipes under other types of loads.

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.

Undrained Load Capacity of Torpedo Anchors in Cohesive Soils

This paper presents a numerical based study on the undrained load capacity of a typical torpedo anchor embedded in a purely cohesive isotropic soil using a three-dimensional nonlinear finite element (FE) model. In this model, the soil is simulated with solid elements capable of representing its nonlinear physical behavior as well as the large deformations involved. The torpedo anchor is also modeled with solid elements and its complex geometry is represented. Moreover, the anchor-soil interaction is addressed with contact finite elements that allow relative sliding with friction between the surfaces in contact. Various analyses are conducted in order to understand the response of this type of anchor when different soil undrained shear strengths, load directions as well as number and width of flukes are considered. The obtained results point to two different failure mechanisms: one that mobilizes a great amount of soil and is directly related to its lateral resistance; and a second one that mobilizes a small amount of soil and is related to the vertical resistance of the soil. Besides, the total contact area of the anchor seems to be an important parameter in the determination of its load capacity and, consequently, the increase of the undrained shear strength and the number of flukes and/or their width significantly increases the load capacity of the anchor.Copyright

Flexible Riser Resistance Against Combined Axial Compression, Bending, and Torsion in Ultra-Deep Water Depths

The experience in the Brazilian offshore production systems is to adopt the traditional riser solution composed of unbonded flexible pipes at a free-hanging catenary configuration. In deep waters, the tendency has been to use different pipe length sections (normally two), each of them designed to resist typical loadings. At the bottom, pipe structure is dimensioned against external pressure, axial compression, bending and torsion, for example. The theoretical prediction of riser responses under the crescent combined loading conditions is a key issue at the TDP region. The potential failure modes are buckling of the armour tendons and also rupture of the high resistance tapes. Much effort has been done in order to have available, from the market, larger envelopes of certified methodologies and qualified products, applicable to the Brazilian ultra-deep scenarios. Since 2002, an extensive R&D Program has been conducted (i) to improve current design evaluation tools & criteria and (ii) to establish representative test procedures and scope, for prototype qualification against the potential failure modes associated with combined axial compression, bending and torsion, at the TDP regions of bottom riser sections in ultra-deep water depths. This paper describes the main steps of the R&D Program, as below: I. Improvement of computational tools to better represent the behavior of the tendons, II. Consolidation of a new strategy for structural analysis, under more realistic conditions, III. Issue of a more adequate set of pipe technical specifications, and IV. Review of both theoretical and experimental results obtained from Feasibility Technical Studies and offshore field tests, respectively. Some examples and results are showed to illustrate, step by step, the whole process covered by the cited Program. Finally, the authors document their main conclusions for further discussion.Copyright

A Study on the Response of a Flexible Pipe to Combined Axisymmetric Loads

In this work, the response of a 2.5″ flexible pipe to combined and pure axisymmetric loads is studied. A set of experimental tests was carried out and the results obtained are compared to those provided by a previously presented finite element model. The pipe was firstly subjected to pure tension. After that, the response to torsion superimposed with tension combined or not with internal pressure and the response to internal pressure combined with tension were investigated. In all these cases, the induced strains in the tensile armors were measured. Moreover, the axial elongation of the pipe was monitored in the pure tension test, whilst the twist of the pipe was measured when torsion was imposed and the axial reaction force was monitored when internal pressure was applied. The experimental results obtained agreed very well with the theoretical estimations indicating that the response of the pipe to tension and internal pressure is linear, whilst its response to torsion is nonlinear due to friction between layers.© 2013 ASME

A FE Model to Predict the Stress Concentration Factors in the Tensile Armor Wires of Flexible Pipes Inside End Fittings

In this work, a bidimensional finite element (FE) approach is proposed to estimate the stresses induced in the tensile armor wires inside end fittings (EF) of flexible pipes. This approach accounts for the residual stresses caused by the mounting procedure and the deformed configuration of the wire. The resin and its interaction with the wires are also addressed. A parametric study was performed aiming at investigating the influence of three parameters on the stress state along the wire, i. e., the contact conditions between the resin and the wire inside the EF, the stress levels induced during the factory acceptance test (FAT) or the offshore leak test (OLT) and the resin elastic properties. The study pointed that high stress concentration is induced in the transition between the flexible pipe’s body and the EF and the stress distribution along the wire may be significantly affected by these parameters. Moreover, the apparent axial stiffness of the wire is also modified by its anchoring conditions, which may lead to non-uniform stress distributions among the wires of the tensile armor layers.© 2013 ASME

Numerical and Experimental Study of a Flexible Pipe Under Torsion

The torsional behavior of a 4″ flexible pipe is here studied. The pipe was subjected to clockwise and anticlockwise torsion and also to torsion combined with tension. For pure torsion, two different boundary conditions were considered: ends free to elongate and prevented from elongating. When tensional and torsional loads are imposed to the pipe, only analyses with ends prevented from elongating are carried out. In all cases, the response of the pipe is predicted with a three-dimensional nonlinear finite element (FE) model and with a classical analytical model. Experimental tests performed at COPPE/UFRJ are also employed to validate the theoretical estimations. The obtained results point out that the pipe is torque balanced for clockwise torsion, but it is not balanced for anti-clockwise torsion. Moreover, analytical models for axissymetric analyses assume that the layers of a flexible pipe are subjected to the same twist and elongation, but the FE results state that this hypothesis holds only for anti-clockwise torsion. Therefore, some differences were found between the FE and analytical models mainly when clockwise torsion is considered. Finally, due to its ability to deal with friction and adhesion between layers, the FE estimations agreed quite well with the experimental measures.Copyright

On the Coupled Extensional–Torsional Response of Flexible Pipes

In this paper, the coupled extensional-torsional behavior of a 4″ flexible pipe is studied. The pipe was subjected to pure tension and two different boundary conditions were considered: ends free and prevented from axially rotating. The response of the pipe is predicted with a three-dimensional nonlinear finite element (FE) model. Some aspects of the obtained results are discussed, such as: the effect of restraining the axial rotation at the extreme sections of the model; the effect of friction or adhesion between the layers of the pipe on the induced axial rotation (or torque) and elongation; and the reduction to simple plane behavior usually assumed by analytical models. The numerical results are compared to the ones measured in experimental tests performed at COPPE/UFRJ. Reasonable agreement is observed between all results pointing out that the analyzed pipe is torque balanced and that friction mainly affects the axial twist or torque led by the applied tension. Moreover, the cross-sections of the pipe remain straight with the imposed load, but different axial rotations are found in each layer.Copyright

A Study on the Holding Capacity Safety Factors for Torpedo Anchors

The use of powerful numerical tools based on the finite-element method has been improving the prediction of the holding capacity of fixed anchors employed by the offshore oil industry. One of the main achievements of these tools is the reduction of the uncertainty related to the holding capacity calculation of these anchors. Therefore, it is also possible to reduce the values of the associated design safety factors, which have been calibrated relying on models with higher uncertainty, without impairing the original level of structural safety. This paper presents a study on the calibration of reliability-based safety factors for the design of torpedo anchors considering the statistical model uncertainty evaluated using results from experimental tests and their correspondent finite-element-based numerical predictions. Both working stress design (WSD) and load and resistance factors design (LRFD) design methodologies are investigated. Considering the WSD design methodology, the single safety is considerably lower than the value typically employed in the design of torpedo anchors. Moreover, a LRFD design code format for torpedo anchors is more appropriate since it leads to designs having less-scattered safety levels around the target value.