Estudo paramétrico sobre vida à fadiga de dutos em vãos livres submetidos à VIV

Abstract

Most of the Brazilian oil reserves are located under the seafloor, which presents several factors that hinder their production, such as water depth, fluid-structure interaction, and uneven bathymetry. In this sense, pipelines are the most efficient way of transporting fluids between offshore facilities and have been used for a long time on a large scale in the oil industry. Ducts are designed according to the kind of activity they will perform and environmental conditions, such as current and wave conditions. Due to seabed unevenness, part of a subsea pipeline is suspended between two points, which is referred to as a free span pipeline, which should be considered in the design in order to avoid structural problems as a result of excessive deformation. Structures are subjected to several types of loading, usually grouped in normative references, such as dead loads, accidental loads, and special loads. Since environmental load vary its intensity significantly over the design life of the structure, they are classified as accidental loads and have a great influence on the analysis of pipeline behavior, as they generate cyclic loadings. Among the environmental loads, we find the wind, current, wave, tidal loads, among others. Combined wave and current loads in free spanning pipelines give rise to Vortex-induced vibrations (VIV), a phenomenon that is widely studied in many engineering areas and one of the main challenges for offshore structures. VIV results in periodic stresses and deformations and may lead to fatigue failure. To prevent this, those oscillations should be minimized, with the vibration frequencies of the vortex shedding being as far as possible from the natural frequencies of the structure. The natural frequency of a free spanning pipeline depends on parameters such as pipeline stiffness, free-span length, pipeline mass, including internal fluid mass and additional mass. A study of the problem of free spanning pipeline subjected under VIV was carried out with the development of a finite element model (FEM), which was validated according to the specifications of the Recommended Practice DNVGL-RP-F105. A parametric study was performed to obtain the factors that most influence the fatigue life of the structure, using the technique of Design of Experiments (DOE), using the Isight software. Static and modal analyzes were implemented at Abaqus, with the main results being natural frequencies of the structure. The fatigue life study was carried out, according to the Recommended Practice, using the FatFree spreadsheet. Five case studies were fulfilled, with responses that the factors that most influence the fatigue life of a pipeline are the pipe thickness, which influences positively, and the span length, which influences negatively. A parametric analysis is extremely important to design the pipeline safely, offering subsidies from the design of the structure to its installation and future monitoring.