The flow around a circular section has many applications in engineering, specifically in the offshore area, such as platforms, risers, pipelines, etc. These structures, which are submerged in water, are susceptible to flow-induced vibrations (FIV) and might oscillate at unwanted large amplitudes due to currents and waves. The Computational Fluid Dynamics (CFD) is a tool that allows the investigation of different parameters related to the flow and/or to the body. A CFD analysis is important in the initial design stage of riser arrangements, being a way to predict the best relative distance, since the flow around the upstream cylinder may strongly influence the downstream cylinder forces and movements responses, characteristic of wake-induced vibrations (WIV). In this context, this work carried out simulations to initiate numerical investigations about the flow around tandem arrangements and then properly identify characteristics of the fluid model that generate the results obtained in experiments. Two similar rigid circular cylinders in a tandem arrangement were analysed, in the stationary case and in the case allowed to move in two degrees of freedom (2DOF) — transverse and in-flow directions, with mass ratio m* = 10 and no structural damping, ζ = 0%. The computations were performed using OpenFOAM, a CFD open source software, in a two-dimensional flow and, numerical uncertainties studies were conducted to well define the results, presenting the value plus its uncertainty. There were variations of the tandem distances (T = 2D, 3D, 4D, 5.5D and 6D), at Reynolds number Re = 100. These two-dimensional flow simulations at low Re are important at the initial stage of a project, in order to filter the best results and save time for more advanced analysis. A single cylinder in a turbulent flow was also analysed, with m* = 2.52, varying reduced velocity. Analysis were done comparing forces coefficients and amplitudes with the literature, for both upstream and downstream cylinders. The vorticity was investigated, properly identifying vortex shedding pattern. For stationary cylinders, a critical distance of 4D was found, where the forces coefficients increase. For the 2DOF cylinders in laminar flow, the highest amplitude was 1.04D at V R = 7 for the downstream cylinder. When the cylinders are free to move, the initial tandem distance does not interfere the amplitude results of the upstream cylinder when V R > 2, results also found in experiments at high Re, and, that justify the two-dimensional studies in low Reynolds as being the basis of a project.

The vortex-induced motions (VIM) of offshore platforms stand as an intriguing and challenging engineering problem, drawing attention from industry, universities and research institutes. Field observations, model tests and calculations have extensively showed that the complex fluid-structure interaction can result in appreciable motions and increased fatigue of mooring and risers. It is thus a very relevant issue from the engineering standpoint. A large volume of experimental research has been carried out, mainly to verify designs and characterize the occurrence of VIM. Conversely, the numerical investigations applying CFD tools have shown to be a more flexible approach enabling better understanding of the physics at play due to the possibility of investigating the effects of different parameters upon the vortex induced motions of floating platforms. Moreover, the CFD calculations enable investigation of the full-scale behavior of the platforms under VIM, a very controversial issue presently. Bearing upon these issues, the VIM Joint Industry Project aims at increasing physical insight of this phenomenon by means of investigating the influence of geometric design variations, flow conditions and scale effects with the objective of improving practical knowledge that can be applied in the design stage of floating platforms. In this paper, we present some of the CFD studies, results and observations carried out within the JIP, regarding the VIM of a semi-submersible with circular columns in 0 and 45 degrees and over a wide range of reduced velocities. It is confirmed that the 0 degree incidence results in larger motions than the 45 degrees-incidence case, in contrast to the VIM behavior of a semi-submersible with square columns. The tests campaign carried out at the University of São Paulo for the same platform agree with these results. Within the lock-in range, the frequency synchronization of the lift forces on columns and pontoons cause large net transverse forces. Appreciable sway motions thus result. For larger reduced velocities, synchronization of the flow around the columns cease, but the forces on the pontoons then largely contribute to the total force. In this high-reduced velocity range, the phasing between total force and motion is such that energy transfer from the fluid to the body occurs, causing the amplification of the motions.

Floating offshore structures, such as production semi-submersibles and spars, can exhibit significant in-line and transverse oscillatory motions under current conditions. When caused by vortex shedding from the floater, such motions are generally called Vortex-Induced Motions (VIM). For semi-submersibles these motions could have a strong impact on the fatigue life of mooring and riser systems. Some field development studies indicate that the VIM induced fatigue damage for larger diameter Steel Catenary Risers (SCRs) can have a magnitude equal to or larger than the wave-induced fatigue damage. The VIM phenomenon for multi-column floaters is characterized by complex interactions between the flow and the motions of the floater. Presently, model tests are the preferred method to predict the VIM response of a multi-column floater. However, several studies indicate that the observed VIM response in the field is less than what is observed in model test campaigns: typical model test results are very conservative. Using such test results in the development of mooring and riser design can easily result in very conservative designs which can have a significant impact on mooring and riser cost, or even affect SCR selection and/or feasibility. The primary objective of the VIM JIP was to increase the physical insight into the VIM phenomenon. This knowledge is then used to address possible areas that could explain the differences between the results from model tests and field observations. To address these objectives, the JIP focused on model testing and CFD studies. A key segment of the JIP was the use of identical semi-submersible hull geometries for the numerical and experimental studies thereby facilitating the interpretation of the various response comparisons. The JIP identified that a CFD model, at model-scale Reynolds number, can reasonably well reproduce the VIM response observed in model tests. However, to have confidence in the CFD results extensive numerical verification studies have to be carried out. The effect of external damping was investigated in model tests and in CFD calculations. Both the numerical and experimental results show that external damping significantly reduces the VIM response. Comparisons between CFD results at model- and full-scale Reynolds number indicate that Froude scaling is applicable, with minor scale effects identified on the amplitudes of the VIM motions. Changing the mass ratio of the floater has a small influence on the VIM response. Experimentally it was found that VIM response under inline or transverse waves is slightly smaller than without the presence of waves and is wave heading and wave height dependent. The presence of waves does not explain the observed differences between model test results and field observations. The effect of unsteady current on the VIM response is minimal. Based on the results from the JIP it is concluded that increased external damping reduces the VIM response. The questions that remain are if the increased external damping is actually present in full-scale conditions and if the mooring and riser systems provide the required damping to reduce the VIM amplitudes.

The cylinder flow is a canonical problem for Computational Fluid Dynamics (CFD), as it can display several of the most relevant issues for a wide class of flows, such as boundary layer separation, vortex shedding, flow instabilities, laminar-turbulent transition and others. Several applications also display these features justifying the amount of energy invested in studying this problem in a wide range of Reynolds numbers. The Unsteady Reynolds Averaged Navier Stokes (URANS) equations combined with simplifying assumptions for turbulence have been shown inappropriate for the captive cylinder flow in an important range of Reynolds numbers. For that reason, recent improvements in turbulence modeling has been one of the most important lines of research within that issue, aiming at better prediction of flow and loads, mainly targeting the three-dimensional effects and laminar-turbulent transition, which are so important for blunt bodies. In contrast, a much smaller amount of work is observed concerning the investigation of turbulent effects when the cylinder moves with driven or free motions. Evidently, larger understanding of the contribution of turbulence in those situations can lead to more precise mathematical and numerical modeling of the flow around a moving cylinder. In this paper, we present CFD calculations in a range of moderate Reynolds numbers with different turbulence models and considering a cylinder in captive condition, in driven and in free motions. The results corroborate an intuitive notion that the inertial effects indeed play very important role in determining loads and motions. The flow also seems to adapt to the motions in such a way that vortices are more correlated and less influenced by turbulence effects. Due to good comparison of the numerical and experimental results for the moving-cylinder cases, it is observed that the choice of turbulence model for driven and free motions calculations is markedly less decisive than for the captive cylinder case.

Several aspects of risers’ dynamics still demand extensive researches as to improve the understanding about physical phenomena involved, so check the adequacy among analytical and numerical results in offshore applications. Dynamic compression of risers is one of these subjects, which has extreme practical relevance in the sense of both design and structural analysis. In this work a riser is anchored at its top end and the other one remains resting on the seabed. Consider that the riser has quasi-vertical configuration. Thus, the angle between the tangent line at the top end and the vertical is small, and its tension on the vicinity of the touchdown point (TDP) is also small. The riser is statically subject to its own submerged weight and on it can act a horizontal ocean current. Furthermore, it supposed a harmonic motion imposed at the top in the tangent direction to the line, caused by local waves on the floating unit. Since the dynamic tension τ(s) changes cyclically in time and the tension T(s) in the TDP is small, it is possible that in this region, the first exceeds the second one in part of cycle’s wave. In these situations occurs a phenomenon known as dynamic compression, widely discussed in Aranha; Pinto and Silva (2001), Aranha; Pinto (2001), Ramos; Pesce (2003) and Fujarra; Simos (2006). During the dynamic compression occurs riser buckling-like. In a strict sense, the behavior of a riser is similar to a cable, although its bending stiffness is not null. Hence, risers have some resistance to buckling-like and can support a certain level of compression. The maximum loading of compression that can be measured in a riser segment is known as critical load (Pcr). Aranha; Pinto and Silva (2001) proposed a simple analytical formulation for the critical load. The aim of the present paper is to propose a simple analytical equation for the speed of compression waves that propagate from the TDP to the riser’s top end. This formulation depends on physical characteristics of the riser and the levels of tension to which it is submitted. As a result, it is possible to compare the theoretical results obtained with numerical simulations and physical experiments. The outcomes provide more understanding to the phenomenon of dynamic compression of risers.