Ricardo Franciss

An Experimental Investigation of the Flow Around Straked Cylinders

This paper presents experimental results concerning the response of circular cylinders with and without strakes. The longitudinal and transverse fluid forces (drag and lift), amplitude response and wake structures of plain and helically straked cylinders are compared. Six different configurations of straked cylinders with pitches (p) equal to 5D, 10D and 15D and heights (h) equal to 0.1D and 0.2D are investigated. Measurements on the dynamic response oscillations of an isolated plain and straked cylinders and flow visualization employing a PIV system are shown. Fixed cylinder drag measurements are also shown. The models are mounted on an elastic base fitted with flexor blades and instrumented with strain gauges or in an air bearing base. The base is fixed on the test-section of a water channel facility. The flexor blades possess a low-damping and the flexor blades base an the air bearing base are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments ranges from 2000 to 10000, and reduced velocities, based on natural frequency in still water, vary up to 13. The drag coefficient is increased by 20% for the h = 0.1 D cylinder, and 60% for the h = 0.2 D cylinder, comparing both with the plain cylinder. The smaller height strokes (h = 0.1 D) do not prevent vortex formation in the region very close to the body, resulting in a decrease of about 50% of the amplitude response compared with the plain cylinder. Lowest amplitude response was found to the p = 10 D and h = 0.2 D case. The analysis of the vorticity contours shows that the shear layer does not roll close to the body (same result for the other cases with h = 0.2 D).Copyright

VSIV (Vortex Self-Induced Vibration) Kinematics

The Vortex Induced Vibration (VIV) of cylindrical lines that may occur when the lines are submitted to currents has been extensively discussed in the past few years and its behavior has become well known. However, it is not so well known that the vibrations may occur in a current-less situation, induced by the lateral motion of the structure itself. The present work refers to the last as the Vortex Self-Induced Vibration, the VSIV. This occurrence has been made clear in the LOC/COPPE/UFRJ (Laboratory of Waves and Currents of COPPE, the Graduate School of Federal University of Rio de Janeiro) by specifically designed tests. In these tests, a totally submerged horizontal cylinder was submitted to harmonic forced oscillations, being free to move in the transverse direction of the forced excitation. The VSIV then showed up, with the cylinder segment, describing vertical trajectories in two (vertical 8-shape), three, four, etc., almost circular trajectories (called the rings in the work). Subsequently, the work shows that the measurements in full scale with the VIV bottle on a Steel Catenary Riser in the PETROBRAS 18 platform also indicate the existence of the VSIV. The tests were carried out with Keulegan-Carpenter equal to 10, 20 and 30 and for several amplitudes. The response of the cylinder was represented in non-dimensional parameters corresponding to the amplitude, the excitation and the response frequencies.© 2008 ASME

Whipping Phenomenon Caused by the Internal Flow Momentum on the Catenary Risers of Offshore Petroleum Fields

A laboratory-scale model was designed to investigate the influence of the internal flow of two-phase oil and gas mixtures on the motion of slender risers hanging in catenary configuration used for offshore petroleum production in deep waters. The behavior of the riser arises from the interplay of various dynamic phenomena: the long length and relatively small diameter of the pipeline confers a cable-like elasticity to the system, which, under static loading, assumes a catenary shape; dynamic excitation caused by environmental conditions generates oscillations. The internal flow momentum may impose a natural whipping displacement — compounding swinging and bending — adding to the concerns of stress and fatigue. The internal flow may display different two-phase patterns (bubbles, slugs, intermittent, annular or stratified mixtures) possessing completely different characteristics; also, the flow-induced dynamic loading depends on the flow rates of both oil and gas phases. Although computer codes have been developed to simulate the motion of risers, there is much need for experimental validation. This research attempts to discern the effects of the internal flow, discriminating it from the other dynamic phenomena. Accelerometers and video acquisition were employed to verify the phenomenon and to determine the frequency spectrum of the oscillations.Copyright

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

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

Model Test of a Steel Catenary Riser in a Towing Tank

The objective of the present work is the study of the dynamic behavior of steel catenary risers (SCRs), focusing on the contribution of vortex-induced vibration (VIV), through model test in a towing tank. Nowadays, a great deal of effort is being spent in order to better understand VIV’s contribution in the dynamics of riser structures through experiments, analytical analysis and numerical predictions. In the present work, the design of a SCR model test, along with its setup in a towing tank, will be described in detail and discussions of main results from the experiments will be presented. The experiment has been conducted under several simulated environmental condition combinations, varying the towing speed, riser top forced oscillation amplitudes, waves amplitudes and periods. Very promising results have been observed from the experiment. Riser oscillations due to high harmonics of vortex shedding were observed. Analysis of the experimental results, coupled with the support of numerical tools, showed the influence of the phenomena of traveling waves in the cross-flow response as is reported from the literature.Copyright

Studies on VIV Fatigue Behavior in SCRS of Hybrid Riser Systems

The main objective of this work is to demonstrate the application of an analysis methodology for hybrid riser configurations. Such configurations are based on submerged buoys that are connected by jumpers to a floating production unit (a FPSO vessel for instance), and by risers to the seabottom. The focus is on VIV-induced fatigue behavior of the steel catenary risers that connect the buoy to the seabottom. The methodology employs numerical tools for the nonlinear static analysis, modal analysis, VIV and fatigue behaviour. The application of the methodology is illustrated with the analysis of a practical case study located in Campos Basin.Copyright

A Comparative Study of a Free Span Pipeline Through Numerical Simulations

A subsea pipeline has an important role to produce oil and gas from an offshore petroleum field, connecting a petroleum facility at the open sea and a near shore terminal at the coast. Very often, the pipeline passes over areas with uneven seafloor, and it may present free span portions. The main aim of the present work is improvements on the understanding of undesirable effects of vibrations in a subsea pipeline which presents free span portions along its length. This understanding is fundamental for the safe design and operation of the pipeline with possible reduction of its fatigue life.Dynamic loads can occur as a consequence of the presence of sea currents acting on portions of the pipeline with free spans. Due to this hydrodynamic current loads, the pipeline structure may oscillate in the same direction of the current (In-line) and, in its transverse direction (Cross-Line). This dynamic response at the free span is mainly caused by the Vortex Induced Vibration (VIV). It is very important for the pipeline design because it can result extreme unacceptable stresses as well as in exceeding limits for the fatigue damage of the pipeline. And, this problem of VIV is still not been completely understood.In the present paper, different models to estimate VIV forces due to sea current are discussed. For this purpose, different computer programs were used to predict vibrations in the transverse direction of the current incidence direction, caused by the vortex shedding in a free span of the pipeline. Simulations of the dynamic behavior of a free span portion of the pipeline were carried out by two approaches, respectively: an empirical hydrodynamic VIV force model, in frequency domain and, a semi-empirical VIV force model based on the lift coefficient and Strouhal number, in time domain. Simulations results are analyzed through comparisons with experimental data and also limitations of the each model are discussed.Copyright

COBRA Riser Concept for Ultra Deepwater Condition

Offshore ultra deepwater field is being promising as the future of oil and gas reserves. However, the development of ultra deepwater field posed many challenges, in particular, on the selection of the riser concept. Long suspended length of riser will significantly increase the vessel payload. High external hydrostatic pressure on the riser will increase the probability of collapse failure. Large dynamic motions of the vessel and large vessel offset yields potential buckling issues at the touch-down-point (TDP). In addition, potential fatigue problems due to vessel motions and soil-riser interactions also present at TDP area. Large current speed in deepwater field might also lead to vortex induced vibration (VIV) which eventually will contribute to significant fatigue damage for particular riser sections. By looking into these challenges, it is very important to select the most appropriate riser concept for the ultra deepwater field.Catenary Offset Buoyant Riser Assembly (COBRA) as newly developed hybrid riser concept offers a solution to overcome the challenges in ultra deepwater field. In general, COBRA consists of a catenary riser section with a long-slender sub-surface buoyancy module on top which is tethered down to sea bed via two mooring lines. The catenary section from top of the sub-surface buoy is connected to the floater by a flexible jumper. This flexible jumper can effectively absorb the floater motions, which give significant improvements for both strength and fatigue performance on the overall system. As a hybrid riser concept, this concept offers cost effective solution by avoiding all the expensive bottom assemblies that normally needed for a hybrid riser concept.This paper focuses on COBRA riser concept specifically for Santos Basin Central Cluster region at 2200 m water depth. It is observed that there is common sudden change phenomenon on the current direction in Santos Basin area. The effect of bidirectional current is analyzed, and the comparison with unidirectional current is discussed thoroughly. The analyses are focused on the global strength design performance under extreme environmental load and global fatigue design performance of the riser due to wave induced and VIV induced.The results clearly indicate that COBRA riser concept has a robust design and it is feasible for 2200 m water depth, in particular for Santos Basin Cluster Region area. It is also shows that COBRA riser has sufficient strength performance even for extreme bidirectional current.Copyright

Analysis of PIV Tests Results of the Vortex Self Induced Vibration (VSIV) of a Cylinder

The Vortex Self Induced Vibration (VSIV) of cylinders is a phenomenon which occurs in the nature with devices such as Steel Catenary Risers (SRCs) in the deep offshore connected to floating platforms. The devices oscillate transversely due the own vertical motion. This is a currentless (therefore different form VIV) problem. To gain more insight into the nature of VSIV problem, an experimental apparatus was constructed which measure the transversal and response associated with vortex-induced vibration of a totally submerged horizontal cylinder hang on horizontally elastic support under the influence of vertical harmonic oscillating motion. The experiments were conducted in a tank at the LOC/COPPE/UFRJ (Laboratory of Waves and Current of COPPE, Federal University of Rio de Janeiro). The VSIV phenomena of the cylinder in different exciting amplitudes and exciting frequencies could generate different vertical trajectories of VSIV motion such as oval-shape, eight-shape and etc. This paper aims to describe the behavior of the flow around the cylinder during the VSIV motion based on the PIV tests on VSIV motion for different Keulegan-Carpenter numbers and for several exciting frequencies.Copyright