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.

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.

Different configurations for riser systems have been emerged as suitable technological and economical solutions for offshore petroleum fields in ultra deep water. The study and development of methodologies for riser project considering the uncertainties involved in the manufacture process of offshore risers and effects of meteocean conditions, such as waves, current and platform motions for the fatigue failure is fundamental to increase the reliability of riser structure design. The present work presents a procedure to evaluate the probability of fatigue failure in offshore Steel Catenary Risers (SCR) identifying the influence of the main parameters in the failure process due to fatigue. Meteocean conditions and uncertainties due to accumulated fatigue damage given by Miner-Palmgren’s rule are here considered. The probabilistic methodology is taken into account, and the analytical approach based on the First Order Reliability Method (FORM) is applied considering two practical cases. In the first case, calculations are performed to compare with the results from the literature, and the second one calculation is carried out considering a typical Brazilian offshore field operation. In the calculations, results for the probability of failure, as well as, the riser service life are observed in four different locations along the SCR length. Finally, conclusions are shown to the probability of failure due to fatigue such as influence of each parameter in the process.

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.

The study presents a closed-form solution for the vibration of a simply-supported beam due to vortex shedding, assuming linear elasticity and considering fluid damping. The in-line and cross-flow fluid forces are coupled to the beam equation as harmonic nonhomogeneous terms. Experimental results of 2 DOF VIV of a flexible small scale pipe in a uniform stream are presented for perpendicular an oblique (at 60 degrees of the translation direction) pipe. The range of relative velocity is from 1 to 10. The performance of two fluid damping models (Venugopal, 1996; Blevins – modified, 1990) is evaluated by comparing their predictions to the measurements of the in-line and cross-flow oscillations. Finally, ranges for in-line and cross-flow force coefficients are proposed and compared to the literature.