Ney Roitman

Application of some damage identification methods in offshore platforms

Abstract The present work reports on the evaluation of the performance of some damage identification methods applied on two different small scale hydroelastic models of fixed offshore platforms designed and constructed according to the Similitude Theory, where Model 1 shows greater redundancy than Model 2. Experimental tests were carried on Model 1 in order to verify the behavior of the structure due to damage and to deck mass changes and in order to evaluate the feasibility of the application of the used methods.

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.

Identification of offshore platform structural damage using modal analysis techniques

Abstract This paper reports on some important preliminary developments using an experimental modal analysis technique applied to small scale models to identify tubular joint damage which very often occurs in offshore framed structures.

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

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

DYNAMIC RESPONSE ANALYSIS OF A SMALL-SCALE MODEL TENSION LEG PLATFORM

Experimental tests performed on a hydroelastic small-scale model of a tension leg platform for deep water are reported. Comparison between the processed experimental dynamic signals and theoretical responses obtained with a simplified six degrees of freedom model are made. Moreover, some effects of fluid-structure interaction are briefly discussed in the light of results from both impact and wave loading tests.

Vibration Reduction in Steel Catenary Risers by the Use of Viscoelastic Materials

The increasing need for petroleum is leading to an exploration in waters each time deeper, leading the structures to have a complex behavior. One of the main components in this area are the risers. These structures are submitted to dynamic loads, among them, it can stand out the one that it is induced by the detachment of vortexes when the structure is submitted to sea currents. These loads may reduce their lifetime due to fatigue. Therefore, many methodologies are being developed to increase the damping of the risers and reduce the vibration effects. One of the ways to reduce the vibrations is the use of viscoelastic materials associated with constraining layers, known in literature as “sandwich structures”. One possible application for this concept is related to the control and the reduction of vibrations in steel catenary risers. The main purpose of this work is present a methodology in order to increase the structural damping factors using the concept of sandwich structures with viscoelastic materials. This application differs from the usual because offshore structures work in lower frequencies than the civil ones, leading the need of an own development for its application. It was developed a numerical model of the sandwich tubes, and its results are evaluated through experimental tests in simple structures accomplished at the Structures Laboratory at COPPE/UFRJ. Through this analysis it is observed a great addition of damping that would allow a reduction of the vibration levels and an increase of the structures lifetime, for example, in steel catenary risers.Copyright

Damage Identification in Offshore Platforms Using Frequency Response Functions

The damage identification problem in offshore platforms, specifically in jacket platforms, has been studied since the seventies. In spite of the effort dispended, there is still no spread out methodology that can be applied in this kind of structure, due its specific characteristics (structural complexity, load and operational conditions, etc.). The development of such methods would be extremely useful to preview structural failures, once this kind of structures is almost completely under water. Most of the developed methods for damage identification use modal parameters, by comparisons of different structural time conditions, and the damage may be expressed through the observed discrepancy. This paper presents a methodology to locate damage based on an iterative method. It was developed an optimization software which uses the Goal Programming Technique to choose the best set of variables that minimizes the difference between experimental and numerical Frequency Response Functions (FRFs), which can better represent the structure situation. Some numerical simulations were performed on a scaled fixed oil platform model to verify the efficiency of the method. The results show that such method is efficient when damages are restricted to a set of optimization variables.Copyright

An Experimental Investigation on the Bending Behaviour of Flexible Pipes

A flexible pipe is a composite structure, built up of several steel and plastic layers, which has been increasingly used in floating offshore petroleum production systems. It is characterized by presenting low bending stiffness and high capacity with regard to internal and external pressure and tension. In order to determine some physical properties of a flexible pipe specimen, which is useful for a global analysis, and to better understand the behaviour of its tensile armour layers, when the pipe is submitted to low tension loads, a series of experimental tests were performed by COPPE/UFRJ and CENPES/PETROBRAS. In this context, bending tests, with internal pressure variation, were carried out on a 4” internal diameter flexible pipe. In some specific cross sections, the outer plastic sheath of the specimen was removed to enable the installation of electrical extensometers in the tensile armour layer. Some experimental results were compared to those obtained through analytical models, and the discrepancies are discussed.Copyright