Theodoro A. Netto

Strength Analyses of Sandwich Pipes for Ultra Deepwaters

Design requirements for pipelines regarding both ultimate strength and flow assurance in ultra deepwater scenarios motivated the development of a new sandwich pipe which is able to combine high structural and thermal insulation properties. In this concept, the annulus is filled with low cost materials with adequate thermal insulation properties and good mechanical resistance. The aim of this research work is to perform small-scale laboratorial tests and to develop a finite element model to evaluate the structural performance of such sandwich pipes with two different options of core material. After calibrated in view of the experimental results, a three-dimensional finite element model incorporating nonlinear geometric and material behavior is employed to perform strength analyses of sandwich pipes under combined external pressure and longitudinal bending. Ultimate strength envelopes for sandwich pipes are compared with those generated for single-wall steel pipes with equivalent collapse pressures. The study shows that sandwich pipe systems with either cement or polypropylene cores are feasible options for ultra deepwater applications.

A Phenomenological Description of the Thermomechanical Coupling and the Rate-dependent Behavior of Shape Memory Alloys

Shape memory alloys (SMAs) present a rate-dependent behavior, which means that the thermomechanical response depends on the loading rate. Therefore, although martensitic transformation can be considered as a non-diffusive process, the phase transformation critical stresses are temperature dependent and, since heat transfer process is time dependent, it affects the thermomechanical behavior of SMAs. This article deals with the rate dependence of SMAs, proposing a 1D constitutive model to describe this effect. The proposed model is formulated within the framework of continuum mechanics and thermomechanical coupling terms of the energy equation are incorporated in the formulation in order to describe the rate-dependent behavior. Numerical simulations are carried out comparing results with experimental data available in literature for different loading rates and environmental media, presenting a close agreement. Afterwards, numerical tests are performed in order to evaluate the model capabilities showing that it is capable to capture the general thermomechanical behavior of SMAs.

Buckle arrestors for deepwater pipelines

Two different buckle arrestor configurations were tested experimentally in order to evaluate the feasibility of using them for deepwater pipelines. Long steel alloy pipes with diameter-to-thickness ratios of 16 and 23 were provided with buckle arrestors and tested under quasi-static conditions in a hyperbaric chamber. Empirical formulae were obtained from the experimental results taking into account both pipe and buckle arrestor geometries as well as material properties.

Fatigue analysis of aluminum drill pipes

An experimental program was performed to investigate the fundamental fatigue mechanisms of aluminum drill pipes. Initially, the fatigue properties were determined through small-scale tests performed in an optic-mechanical fatigue apparatus. Additionally, full-scale fatigue tests were carried out with three aluminum drill pipe specimens under combined loading of cyclic bending and constant axial tension. Finally, a finite element model was developed to simulate the stress field along the aluminum drill pipe during the fatigue tests and to estimate the stress concentration factors inside the tool joints. By this way, it was possible to estimate the stress values in regions not monitored during the fatigue tests.

Effect of the piezoelectric hysteretic behavior on the vibration-based energy harvesting

Vibration-based energy harvesting has received great attention over the last years. The evaluation of the power output of the energy harvesters for different excitation frequencies and amplitudes of vibration has an important role in the design of the devices. In this regard, a wide range of nonlinear effects is observed having considerable influence on the generated power. The main goal of this contribution is to investigate the effect of the piezoelectric hysteretic behavior on the vibration-based energy harvesters. An archetypal model is employed to this aim by considering a one-degree-of-freedom mechanical system coupled to an electrical circuit by a piezoelectric element. Different hysteretic behaviors are investigated by considering the Bouc–Wen model. Numerical simulations are carried out establishing a comparison among hysteretic, nonlinear, and linear piezoelectric behaviors showing their influence on system dynamics.

A Simple Alternative Method to Estimate the Collapse Pressure of Flexible Pipes

Offshore oil and gas production worldwide constantly moves to deeper water with increasing flexible pipe operational severity. Failure mechanisms, i.e., sequences of events which may lead to failure, are nowadays more likely to happen. Therefore, it is important to develop reliable numerical tools that can be used in the design stages or during service-life to assess the structural integrity of pipes under specific operational conditions. This work presents a methodology to develop simple finite element models capable to reproduce the behavior of structural layers of flexible pipes under hydrostatic pressure up to the onset of collapse. The models use beam elements and include contact between layers, nonlinear kinematics and material behavior. Different configurations were analyzed: carcass-only, and carcass plus pressure armor with dry and wet annular. The dependability of the numerical models is assessed in light of experimental tests on flexible pipes with 4 and 8 inch nominal internal diameters. Relevant geometric parameters and material properties of each specimen were measured and subsequently used in the models to reproduce the physical experiments. The metallic inner carcass and pressure armor layer manufacturing processes cause a high degree of stress-induced material anisotropy. Due to the inherent difficulty to determine the non-homogeneous elastic-plastic material behavior of the wires’ cross-sections, a novel alternative method was used to estimate their average stress-strain curves up to moderate strains (2%). Good correlation was obtained between experimental and numerical results. The applied methodology proved to be simple and yet efficient and reliable for the estimation of the collapse pressure of flexible pipes.Copyright

Dynamic Propagation and Arrest of Buckles in Pipe-in-Pipe Systems

This paper deals with the problem of buckle propagation and arrest in pipe-in-pipe systems. A recent experimental study illustrated that if the geometric integrity of the carrier pipe is compromised, local collapse can result. In a pressure controlled environment such as that encountered in the sea, collapse will propagate dynamically usually collapsing both pipes. A new buckle arrestor consisting of a ring placed inside the annulus between the two pipes has been proposed. A methodology for designing such rings has been developed based on quasi-static buckle propagation and arrest experiments. Here we evaluate experimentally the adequacy of this design methodology under the more realistic case of dynamic buckle propagation. This is achieved by initiating collapse in a constant pressure environment similar to that encountered on the sea floor. The study involves first establishing the effect of the presence of the inner pipe on the velocity of buckles initiated at various external pressures ranging between the propagation pressure of the two-pipe system and the collapse pressure of the carrier pipe. Subsequently, the arresting efficiency of several internal ring buckle arrestors designed by the quasi-static criteria developed is established for buckles travelling at high velocities. Details of the experimental procedures used are presented along with the experimental results and discussion of their implications in practice.Copyright

On the Nonlinear Behavior of the Piezoelectric Coupling on Vibration-Based Energy Harvesters

Vibration-based energy harvesting with piezoelectric elements has an increasing importance nowadays being related to numerous potential applications. A wide range of nonlinear effects is observed in energy harvesting devices and the analysis of the power generated suggests that they have considerable influence on the results. Linear constitutive models for piezoelectric materials can provide inconsistencies on the prediction of the power output of the energy harvester, mainly close to resonant conditions. This paper investigates the effect of the nonlinear behavior of the piezoelectric coupling. A one-degree of freedom mechanical system is coupled to an electrical circuit by a piezoelectric element and different coupling models are investigated. Experimental tests available in the literature are employed as a reference establishing the best matches of the models. Subsequently, numerical simulations are carried out showing different responses of the system indicating that nonlinear piezoelectric couplings can strongly modify the system dynamics.

On the structural integrity of damaged columns of semi-submersible platforms

Offshore platforms are usually designed to sustain moderate or severe damages caused, in most cases, by the impact of a supply vessel due to an accidental drift or manoeuvre. As the available design codes do not specifically establish repair procedures for damaged columns, operators have been forced to perform major and costly restoration of the affected areas. In this work, the structural behaviour under compression of intact and damaged cylindrical shells with reinforcements is studied through a series of small-scale experiments and through a non-linear numerical model based on the finite element method. In light of the obtained experimental and numerical results, a very simple and effective repair technique is evaluated.

Residual Strength of Corroded Pipelines Under External Pressure: A Simple Assessment

The loss of metal in a pipeline due to corrosion usually results in localized pits with various depths and irregular shapes on its external and internal surfaces. The effect of corrosion defects on the collapse pressure of offshore pipelines was studied through combined small-scale experiments and nonlinear numerical analyses based on the finite element method. An extensive parametric study using 2-D and 3-D numerical models was carried out encompassing different defect geometries and their interaction with pipe ovalization. This paper briefly summarizes these results, which are subsequently used to develop a simple procedure for estimating the collapse pressure of pipes with narrow defects.Copyright