José Augusto Penteado Aranha

On the dynamic compression of risers: an analytic expression for the critical load

Abstract A riser is anchored at the floating system in a quasi-vertical configuration, the angle between the tangent and the vertical line at the top end being, in general, small. As a consequence, the static tension at the touchdown point is also small and the riser usually becomes dynamically compressed when excited by a moderate sea state. In this paper, a physical argument, coupled with a simple model for the quasi-steady buckling of a infinitely long curved beam , allows one to obtain a simple estimative for the critical load , namely, the maximum value of the compression permitted in a given situation. In this context, the total tension should follow nearly the harmonic result predicted by the algebraic expression derived in Aranha and Pinto [Dynamic tension in risers and mooring lines: an algebraic approximation for harmonic excitation (2001), submitted] but saturated , in the compressed part, at this critical load, a conclusion suggested by experimental results due to Andrade [EPSUP (1993)]. Comparison with numerical results, obtained from nonlinear time domain programs, indicate a fairly good agreement, in the sense that the numerically determined tensions tend, indeed, to ‘saturate’ in compression around the estimated critical load.

2nd Order Hydrodynamic Effects on Resonant Heave, Pitch and Roll Motions of a Large-Volume Semi-submersible Platform

During the last decades, as oil production offshore Brazil moved to deeper waters, technical and economical constraints led to a new generation of floating platforms. Nowadays, in the Brazilian offshore scenario, design trends concerning hull form, size and mooring configurations bring novel characteristics of wave-induced dynamics, including non-linear resonant effects. As part of an extensive study on new semi-submersible configurations for Campos basin, recent model tests have shown that their hulls may be subjected to second-order slow motions in heave, pitch and roll. These resonant motions are directly related to the large dimensions and relatively low natural frequencies of the floating systems. The unexpected effects caused great concern, since, in some cases, the low-frequency motions presented amplitudes comparable to those of the first-order response. This paper discusses the evaluation of the 2nd order wave-induced motions of a large-volume semi-submersible platform using WAMIT® second-order module. It is shown that the hydrodynamic forces induced by the 2nd -order potential represent the prevailing effect in the resonant response. Important aspects concerning the numerical model are addressed, such as the parameters involved in the hull and free-surface panelization. Numerical predictions are directly compared with experimental results obtained with a 1:40 model of the platform. A very good agreement is obtained both for heave and angular (pitch or roll) motions, attesting that the numerical code is able to predict the 2nd order forces accurately. Finally, a simplified procedure for dealing with the slow vertical motions is evaluated, aiming to reduce the substantial computational effort required by the 2nd order calculations. Such procedure takes advantage from the fact that the resonant response spectra of the vertical motions are usually narrow-banded (due to the low damping levels) to propose a “white-noise” approach. According to this approach, 2nd order forces need to be calculated only for one frequency difference, corresponding to the natural frequency of the particular motion. Computational time is, therefore, greatly reduced. It is shown that resonant motions calculated through the simplified approach match those predicted through the “full” analysis perfectly, making it an interesting choice for the evaluation of 2nd order effects, especially in the early stages of the design.Copyright

Structural Behavior of Flexible Pipe Carcass During Launching

The interlocked carcass of a flexible pipe, besides being designed to withstand hydrostatic and squeezing loads applied by the tensioned armor layers, must resist to localized loads applied by caterpillars during the launching operation. Load distribution is dependent on the number of tensioners and on their geometry, as well as on the properties of external plastic layers and cross section design. Being the winding pitch of an interlocked carcass small enough and considering that the plastic layers extrusion process provides some confinement to this structure, one may consider, at least as a first approach, an equivalent pipe to model the structural behavior for radial loads. Any alternative approach, as a finite element model, would depend on a difficult assessment of parameters, as those related to the internal contact problem. Recovering classical results from the theory of elasticity, the structural behavior is formulated, for a general radial load distribution. The instability problem is also addressed. Despite the simplicity of such a model, results agree quite well with experiments conducted under controlled conditions. Applied to a typical carcass, the model predicts a safe behavior regarding instability and recovers elastic deformation as a function of a general dimensionless load parameter.Copyright

EXPERIMENTAL INVESTIGATION OF FLOW-INDUCED VIBRATIONS INTERFERENCE BETWEEN TWO CIRCULAR CYLINDERS IN TANDEM ARRANGEMENTS

This paper presents experimental results concerning flow-induced oscillations of rigid-circular cylinders in tandem. Preliminary results are presented: new measurements on the dynamic response oscillations of an isolated cylinder and flow interference of two cylinders in tandem are shown. The oscillations are due to vortex-induced vibrations (VIV). Models are mounted on an elastic base fitted with flexor blades and instrumented with strain gages. The base is fixed on the test section of a water channel facility. The flexor blades possess a low damping characteristic [ζ ≈ 0.008 and less] and they are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments is from 3,000 to 13,000 and reduced velocities, based on natural frequency in still water, range up to 12. The interference phenomenon on flow-induced vibrations can be investigated by conducting experiments in two ways: first, the upstream cylinder is maintained fixed and the downstream one is mounted on the elastic base; subsequently, an investigation will be carried out letting both cylinders oscillate transversally. The results for an isolated cylinder are in accordance with other measurements in the literature for m* ≈ 2 and m* ≈ 8. For the tandem arrangement (m* ≈ 2), the trailing cylinder oscillation presents what previous researchers have termed interference galloping behaviour for a centre-to-centre gap spacing ranging from 3·0D to 5·6D. These initial results validate the experimental set up and lead the way for future work; including tandem, staggered and side-by-side arrangements with the two cylinders free to move.Copyright

Riser Technology and Industry-University Cooperation: Part I — Riser Mechanics

This is the first 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 covers Riser Mechanics and design issues. Part II is dedicated to Riser Loading and Dynamic Positioning. The work starts with the first SCR (Steel Catenary Riser) concept and engineering analysis that paved the road toward a mature and reliable design which has been applied with success in floating production in Campos Basin, offshore Brazil. Several computer codes, numerical methods and analytical tools were developed and verified based on experimental results. In Part I, SCR dynamics, touch down point dynamic interaction loading, catenary riser dynamic compression under twist, as well as risers and umbilical cables structural mechanics, are treated and modeled through analytical and numerical methods. SCR design issues and fatigue assessment of flexible pipes are also addressed. In Part II [1], VIV (Vortex-Induced Vibration), extreme dynamic loading, nonlinear dynamics of FPSOs and turret positioning are presented. Nonlinear control techniques are also 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

Adaptive Meshes to Improve the Linear Stability Analysis of the Flow Past a Circular Cylinder

Adaptive mesh schemes have been employed to simplify lots of numerical problems, from hard to capture multi-scale to complex transient phenomena. This study integrates a mesh adaptation scheme to the linear stability analysis of the flow past a circular cylinder. Preliminary results for Re = 100 are presented to illustrate the benefits.

On the asymptotic solution of the flow around a circular cylinder

This study presents new advances concerning the asymptotic solution derived for the viscous flow around a circular cylinder. It provides arguments, based on numerical and experimental (DPIV) results, to suport the validity of the solution for Reynolds numbers far beyond the critical value of the first Hopf bifurcation (Re cr ≈ 46). Results are then related to recent studies regarding the construction of reduced models employing POD techniques.

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