José Alfredo Ferrari

Dynamic Behavior of a Vertical Riser and Service Life Reduction

In the last years, the most of offshore oil and gas reserves discoveries in Brazil are placed in ultra-deep water depths. Petroleum production from these offshore fields needs developments with novel solutions in terms of necessary technologies and economical viability. The use of vertical rigid risers such as top tensioned risers (TTR) and others like combined systems as self standing hybrid risers and steel catenary risers for ultra-deep waters have shown viable from both, technical and economical aspects. However, there are needs for detailed studies on their dynamic behavior in order to improve, particularly, the understanding of influence of the environment as wave and current, and floating platform oscillations at the riser top. The present work presents studies on vertical top tensioned riser dynamic behavior through time domain simulations of its displacements and respective, bending moments and stresses. Influences of the vortex induced vibrations (VIV) and waves on the riser service life reduction are analyzed. Maximum and minimum envelops for displacements and stresses along riser length are shown.Copyright

Experimental and Numerical Study of Damaged Tubular Members Under Lateral Loads

Structural behavior of dent damaged tubular members by local indentation remains a key issue for the safety and failure assessment of critical structures, including marine and nuclear facilities, oil and gas pipelines. This failure mode most often arises from very large localized plastic deformations caused mainly by excessive or accidental loads such as, for example, during the collision of adjacent risers in deepwater floating production systems (FPS). The complex interaction between the local deformation in the dented region and global bending of the tubular member may severely reduce the plastic collapse load while, at the same time, strongly affecting its load-deflection behavior. This study presents an experimental and numerical investigation of the structural behavior of a dented tubular member under lateral load which is applicable to marine risers. Experimental load-deflection curves measured using a 4 1/2″ O.D. (114 mm) API N80 pipe (580 MPa yield stress) with varying length characterize the plastic response during local indentation and global bending. 3D finite element models are employed to generate numerical solutions describing the large deformation, non-linear behavior for the tested pipes. The experimental results agree well with the numerical results. The analyses provide further insight into the structural response of tubular members and risers which dent damage effects.Copyright

Investigations on the Behavior of Vertical Production Risers

The present paper describes the behavior of a vertical riser in waves, currents and which is excited by displacements at the riser top. Fundamental equations for riser behavior are described and the Ferrari&Bearman model [4] is applied to estimate hydrodynamic load for “in-line” and transverse directions of the riser. Solution for riser behavior is obtained in time domain, calculations are performed for deepwater field production scenario and discussions are addressed. The influence of variation of riser diameter and drag coefficient along its length, the presence of floaters and effect of petroleum fluid flowing in the riser are investigated. Moreover, effects of the upper and bottom end conditions of the riser on its behavior are also taken into account in the analysis.Copyright

Dynamic Behavior of a Vertical Production Riser by Quasi 3D Calculations

A study on dynamics of a vertical production riser under environmental loads is carried out. Current and wave loads along the riser, and surface vessel motions due to the environment at the riser top are taken into account. Based on descriptions of fundamentals of the Quasi 3D model previously proposed by Ferrari&Bearman (1999), numerical simulations for riser displacements have been performed. In line and transverse hydrodynamic current and wave loads on the riser are estimated and riser displacements thought Quasi 3D fashion are estimated in the time domain. Comparisons with literature results and experimental ones are carried out.Copyright

Numerical Simulations of Vortex-Induced Vibration of Flexible Cylinders

The main purpose of this paper is to acquire a better understanding of the hydroelastic interactions, which take place between oscillating flexible cylinders and fluid forces. The cylinders are subjected to currents and shear flow, and the hydrodynamic forces are estimated by CFD tools. This article presents the results of an investigation being carried out at the University of Sao Paulo, in which a discrete vortex method is used to simulate the flow around a flexible cylinder. The calculations are compared with results obtained employing the quasi-steady theory, as proposed by Ferrari [2]. Also, the calculations are compared with experiments of a cantilever flexible cylinder immersed in a current, see Fujarra [6]. The reduced velocity vs. non-dimensional amplitude curve obtained in our calculations is compared with the experimental results. Visualizations of the wake indicate a hybrid mode of vortex shedding along the span. A 2S mode is found in regions of low amplitudes, changing to a 2P mode in the regions of larger amplitudes. The position of the transition of the modes varies with the reduced velocity. Our intention is to apply this model to problems occurring in the offshore industry. In this industry fluids are conveyed from the seabed to the platform through slender structures called risers. These risers are subject to shear and oscillatory flows due to currents and waves, respectively, flows with a very high degree of complexity, with changes of intensity and direction the deeper the water depth. A finite element structural model based on the Euler-Bernoulli beam theory was developed. In order to evaluate the dynamic response, a general equation of motion is solved through a numerical integration scheme in the time domain. The hydrodynamic forces are evaluated in two-dimensional strips. The technique used is the Discrete Vortex Method, which is a Lagrangian numerical scheme to simulate an incompressible and viscous fluid flow. A practical case of marine risers is also presented. In this case the results for various uniform currents acting on a single, flexible cylinder, representing a riser of 120m with 100m under water, are shown. Envelopes of maximum and minimum in-line and transverse displacements are presented. There is also a comparison of a shear flow case between the CFD numerical code with the quasi-steady theory code developed by Ferrari [2].Copyright

Dimensional Study for Brazilian FPSO

After the boom of converted floating, production, storage and offloading systems, based on the old VLCC design, many engineering institutes started thinking about optimum dimension for new units. However, these new FPSOs designs carried out worldwide concerns about good seakeeping behavior when considering general weather conditions, in order to apply their project to different locations around the globe. Analyzing the Brazilian specific conditions, it was verified that, considering waves, current and wind characteristics, the dimension proportions found in the projected units were not the best options, mostly because of the swell waves influence. Thus, in a cooperative project between University of Sao Paulo and PETROBRAS, the best dimensions for a specific case were studied, based on real premises from Campos Basin. During the study, the roll motion, which usually creates operational limits during hard environmental conditions, was focused. It was possible not only to evaluate the best breath and draught relations, but also the inclusion of a structured skirt in the ship bilge. The influence of different shapes in the ship’s bow and stern were also evaluated, showing interesting results regarding the forces applied on the vessel. All the analyses were conducted considering numerical analysis, and the final dimensions were applied to a scaled model, which allowed to verify the real behavior of the projected unit in a test basin. As a conclusion, it was possible to define an optimized hull for the PETROBRAS premises, giving them a real design to be used in future explorations.Copyright

Parallel Simulation of a Marine Riser Using MPI

In this paper the dynamic response of a marine riser due to vortex shedding is numerically investigated. The riser is divided in two-dimensional sections along the riser length. The Discrete Vortex Method is employed for the assessment of the hydrodynamic forces acting on these two-dimensional sections. The hydrodynamic sections are solved independently, and the coupling among the sections is taken into account by the solution of the structure in the time domain by the Finite Element Method. Parallel processing is employed to improve the performance of the method. The simulations are carried out in a cluster of Pentium IV computers running the Linux operating system. A master-slave approach via MPI — Message Passing Interface — is used to exploit the parallelism of the present code. The riser sections are equally divided among the nodes of the cluster. Each node solves the hydrodynamic sections assigned to it. The forces acting on the sections are then passed to the master processor, which is responsible for the calculation of the displacement of the whole structure. Scalability of the algorithm is shown and discussed.© 2004 ASME

Riser Technology and Industry-University Cooperation: Part II — Risers Loading — VIV, Heading and Dynamic Positioning

This is the second of two companion papers intending to summarize and pointing out some of the main engineering achievements obtained through a continuous cooperative research effort, jointly conducted by Industry and University. Part I covered Riser Mechanics and design issues. Part II is dedicated to Riser Loading and Dynamic Positioning. In Part I (Pesce et al., 2003 [1]), SCR dynamics, touch down point dynamic interaction loading, catenary riser dynamic compression under twist, risers and umbilical cables structural mechanics, were treated and modeled through analytical and numerical methods. SCR design issues and fatigue assessment of flexible pipes were also addressed. In this second paper, Part II, VIV, extreme dynamic loading, nonlinear dynamics of FPSOs and turret positioning are presented. Nonlinear control techniques are addressed aiming to optimize FPSO heading, constrained to a cost function that takes into account risers loading, mooring tension, roll motion and fuel consumption.Copyright

Numerical Representation of the Collision Interaction Mass for Dual Riser Systems

This work presents a numerical procedure to represent interaction masses for the collision of adjacent risers. The approach is based on the concept that cyclic interaction between two mechanical systems can be described by representative masses which are associated with the energy dissipated during collision. Development of the numerical formulation employs key concepts connected to group theory for mechanical systems to define an expression for the collision interaction mass incorporating the dissipative behavior of each riser system. An application then follows to predict the contact (collision) regions and collision participating masses for a pair of adjacent risers subjected to different load and environment conditions.© 2002 ASME