Rafael Loureiro Tanaka

Bending Stiffener Design Through Structural Optimization

Bending stiffeners are very important ancillary equipments of umbilicals or flexible risers, since they protect the lines from overbending. Their design however is a complex task, since many load cases must be taken into account; the structure itself has a section that is variable with curvilinear coordinate. To aid the designer in this task, optimization algorithms can be used to automate the search for the best design. In this work an optimization algorithm is applied to the design of the bending stiffener. First, a bending stiffener model is created, which is capable of simulating different load case conditions and provide, as output, results of interest such as maximum curvature, deformation along the stiffener, shear forces and so on. Then, a bending stiffener design procedure is written as an optimization problem and, for that, objective function, restrictions and design variables defined. Study cases were performed, comparing a regular design with its optimized counterpart, under varying conditions.Copyright

Predictions of Armour Wire Buckling for a Flexible Pipe Under Compression, Bending and External Pressure Loading

During installation and operation a flexible pipe may be subjected to high compressive forces, high cyclic curvatures and external pressures leading to high reverse end-cap loads. Under such loading conditions, which occur particularly in the touchdown region for deep water applications, the limiting condition for the flexible pipe can be the compressive stability of the tensile armour wires. Two potential instability modes are possible: radial mode (birdcaging) and lateral mode (lateral wire disorganization). Previous work on the subject has established the key factors which influence the onset of each buckling mode [1],[2],[3] and [4].In order to ensure the feasibility of flexible designs for applications with increasing water depth, it is important to improve the knowledge of the mechanisms which can lead to instability of armour wires and enhance the ability to predict with greater assurance, the particular conditions which increase the risk of wire instability.The focus of this work is the comparison of finite element prediction of radial buckling (birdcaging) with physical testing results under loading states that lead a pipe to birdcaging failure.The numerical model incorporates all tensile armor wires and their interactions with each other and adjacent layers. The outer sheath and reinforcing tape layers are explicitly represented, while the inner layers of the pipe (pressure armour and carcass sheath) are idealized using a homogeneous representation. The model also incorporates the effects of manufacturing pre-tension and hoop strength in the anti-birdcaging tape layers which are critical determinants for the onset of buckling.A key aspect of the method presented is the means by which the loading is applied. Specifically, the modeling handles the simultaneous and controlled application of end rotations, axial compression and radial resistance of the tapes through to the point of tape failure, pipe ovalisation and subsequent radial displacement and buckling of individual wires.In summary, in this paper a solid modeling approach is presented, which is compared with full a scale sample test data, that enables the simulation of a flexible pipe undergoing large combined compression, curvatures and pressure loading.© 2012 ASME

Simplified Finite Element Models to Study the Dry Collapse of Straight and Curved Flexible Pipes

Dry collapse is one of the possible failure modes of flexible pipes. It refers to the situation in which no damage occurs in the flexible pipe external sheath. In this scenario, all layers of the pipe withstand the external pressure loading in a deep-water application. Such a situation is addressed in this work, which proposes some simplified modeling techniques to represent straight and curved flexible pipes subjected to external pressure, undergoing dry collapse during simulation procedure. The results of the proposed models are compared to other reference results, from a fully three-dimensional (3D) finite element model. Good agreement has been got, even with the proposed simplifications with a large reduction in computational cost when compared to full 3D model.

A Nonlinear Analytical Model for Flexible Pipe Crushing Analysis

This paper discusses a nonlinear analytical model for flexible pipe crushing analysis, improving analytical models previously published in OMAE2003, 2010 and 2011. Following that path, and still based on the concept of equivalent pipes and rings, the present model discusses a strategy to cope with an elastic-to-plastic analysis. Such a model considers the combined action of squeezing and concentrated loads applied as representing those due to the caterpillar shoes. Experimental results and finite element analysis are used to assess the pertinence of analytical modeling hypotheses.Copyright

Crushing of flexible pipes under traction: a theoretical-experimental assessment

The paper presents a theoretical-experimental comparison concerning standard crushing-traction tests of flexible pipe prototypes. The theoretical model for crushing is analytical and based on classic assumptions of equivalent pipes, applying model previously published in OMAE2003, and OMAE2010. Such a model considers the combined action of squeezing, concentrated loads due to the caterpillar shoes as well as the effect of initial ovalization. The experimental measures include a detailed internal geometrical mapping of the deformed carcass, until plastic deformation becomes evident. Discussion is made on the pertinence of modeling hypotheses. Sensitivity analyses, regarding initial ovalization and helical pitch of the pressure armor are also addressed.Copyright

A Finite Element Model for Umbilical Cable Crushing Analysis

Umbilical cables are essential elements of offshore floating production systems. Due to their complexity, the offshore industry regularly counts on numerical tools to perform design assignments. One of these assignments is to evaluate strains and stresses states in all components due to distinct sets of external loads. The main purpose of this paper is to present a numerical model for prediction of the stress and strain fields in the umbilical cable components under crushing loads. Such loads, outcoming from the laying operation, comprise the caterpillar shoes load and the squeezing effects, associated not only to the tensile armours, but also to helical components under tension. The referred model comprises a joint analysis using a two-dimensional Finite Element Method (FEM) fed by an analytical model, which represent three-dimensional effects. A combined analytical-numerical approach is much easier to implement than a complete fully three-dimensional one and it is meant to obtain results efficiently, without the need of a large computational capacity. The paper presents and discuss modeling hypotheses and methodology, describing in which way three-dimensional effects and interactions among cable components were treated. Case studies with three umbilical cables are presented.Copyright

Validation of Solid Modeling and Analysis Techniques for Response Prediction of Deepwater Flexible Pipe

Accurate prediction of the load capacity of the constituent components of flexible risers and flowline structures is critical in demonstrating fitness for purpose, particularly as the envelope of application of flexible pipe is being extended to incorporate higher flow rates, greater water depths, more aggressive fluid contents and challenging installation conditions. Analytical techniques and numerical solid modelling methods have evolved significantly in recent years as a means of improving the ability to accurately predict flexible pipe load capacity.Due to the complexity of the flexible pipe structure and the load sources and load paths to which it can be subjected, different analysis methods are required, depending on the type of response being evaluated. Specific effects can be difficult to simulate using analytical approaches and require local stress analysis using numerical techniques. This paper presents numerical approaches to two such effects, the extreme loading of tensile armour wires within a flexible pipe end fitting and the stability of tensile armour wires in axial compression.Copyright

Structural Behavior of Umbilicals: Part I—Mathematical Modeling

Umbilicals for offshore application are very complex, since they combine elements of different mechanical behavior, such as steel tubes, thermoplastic hoses and power cores in a single structure, not to mention helically laid-up armouring layers and polymeric sheathes. This motivates continuous research on their mechanical modeling. This paper presents research undertaken in the structural behavior of umbilicals and focus on the mathematical modeling of the elements, which are gathered into concentric layers. Interaction between layers is included as well as helical lay-up of elements. The model here presented will be compared to a set of experimental results in a separate paper (Part II). An analytical model was developed combining equilibrium equations, geometric compatibility and constitutive relations to obtain a set of equations that describe the umbilical behavior under external loads. This set of equations is numerically solved to obtain contact pressures (or gaps) among layers, radial variations and strains in the elements (used to calculate the stresses). The model was built to be general in order to be able to cope with complex cross-sections designs often encountered. It was then implemented in a tailor made local analysis software, called UTILFLEX®. Both modeling and software resulted from a development program partnership between Prysmian Cables & Systems and University of Sao Paulo. The paper will present modeling hypotheses and structural models that were used for steel tubes, hoses (thermoplastic and reinforced) and power cables and how the interaction among them has been treated.Copyright

Thermal Effects on the Anchoring of Flexible Pipe Tensile Armors

In the design of flexible pipes, predict the anchoring behavior on end fittings is always challenging. In this sense, Prysmian Surflex has developed a finite element model, which should help the end fitting design as well the prediction of the structural behavior and the acceptable maximum loads. The current model considers that the contact between armor-resin is purely cohesive and has been suitable for the design of end fittings [1] and [2]. But tests and new studies [3] and [4] indicate that only cohesive assumption would not be the best approach. Experimental data from prototype tests also show that the current model would not predict acceptable results for loads higher than those used in previous projects.This document will describe a study developed considering the friction and thermal contraction, instead of the cohesive phenomenon in the anchoring behavior analysis. Small scale tests were conducted in order to understand the anchoring relation between the resin and the wire used in the tensile armor. For this purpose, a special test device was developed to simulate an enclosure system. A parametric study was also performed to identify the cooling temperatures, coefficients of friction and contact properties parameters taken from small scale tests. The finite element model considers the thermal effects during exothermic curing.Using the new parameters obtained, a second model was developed. This model consists of only one real shaped bended wire inside an end fitting cavity. To validate the model, samples were tested on laboratory according anchoring design. The results of this round of tests were studied and corroborate the argument that use friction and thermal effects is better than use only the cohesive condition.Copyright

Flexible Pipe Anchoring System: Resin Ratio Effects on Mechanical Properties

At the end of flexible pipe manufacturing, the assembly of end fittings ancillaries is necessary to provide the connection with the platform and the subsea systems. The assembly process uses epoxy resin to ensure the holding of the tensile armor layers inside the end fitting. The anchoring system has great importance in the integrity of the flexible pipes system, since their failure causes the separation between the flexible pipe and ancillaries, what can put on risk the production and the environment. During the qualification tests of new structures were observed that some variables of the process could be determinant on the mechanical strength resistance required for the product. One of these known variables is the ratio resin/hardener. Resin supplier specifies at the datasheet the proper ratio to obtain the full cure, but it was observed that small variations in the concentration alters the maximum tensile anchoring strength supported by the structure, as well the minimum curing time that should be obeyed, so it can be better used regarding their mechanical properties. In this way, laboratorial tests were conducted to enhance the performance of this resin in the tensile anchoring system, according to Prysmian Surflex products’ necessity. Compression tests varying the resin/hardener ratio were done to evaluate the resistance as a function of curing time. Besides this, tensile tests (tensile wire pulls over from resin) were conducted to establish a relation between maximum tensile load and the resin ratio, and so provide enough data to control the best proportion of resin during the end fitting assembly phase.Copyright