Hydrodynamic study of a Lander Platform for Subsea Operations

• Autor(a) principal: Gonçalves, José Jerónimo Silva
• Data de Publicação: 2019
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.6/10389
• topic: Carenagem.; Dinâmica de Fluidos Computacional (Cfd); Frequência; Openfoam; Sismómetros de Fundo do Oceano; Domínio/Área Científica::Engenharia e Tecnologia::Engenharia Aeronáutica

The seismography study in the oceans is extremely important for tsunami prevention. It is estimated that annually are detected over 500000 and, in most cases, the epicenter is at the ocean. Thus, its premature detection would be an asset to avoid both human and material losses. Nowadays, Ocean Bottom Seismometers (OBS) are used as instruments of seismic detection. The price of these instruments greatly influences their accuracy, which may cause different frequency ranges to be detected. One of the main issues of these instruments is the noise generated by the flow. This noise, depending on its frequency, can corrupt the signal transmitted. For this reason, it is crucial to study, hydrodynamically, the geometry that generates less noise, in order to avoid undesired alarms. This project has the objective of analysing, through OpenFOAM (Open-source Field Operation And Manipulation) software, the flow behavior around a cylinder, to identify, filter and/or eliminate the undesired frequencies induced by the flow. The maximum velocity considered for this study is 0.08 [m/s], which combined to the flow characteristics and the geometry intended to study, resulted in a Reynolds number of 80000. For this study, the equations utilized were Unsteady Reynolds-Averaged Navier-Stokes (URANS) with the turbulence model ?-? Shear Stress Transport (SST). In order to understand which was the flow-induced frequency variation, different OBS aspect ratios were analysed. It was observed that the induced frequency would decrease with the reduction of the aspect ratio. Following this, it was analysed with a scale 1:1 the conceptual configuration, which the result indicated a maximum frequency of 0.0057 [Hz]. Therefore, in order to minimize the noise generated by the flow, two alternative geometries were suggested: a cylinder with a spherical hubcap and a cylinder with three helical strakes. In the end, it was possible to achieve a 60% reduction in the induced frequency, with the cylinder with a spherical hubcap configuration. Along with the suggested modification, it is possible to obtain results that do not compromise the correct functioning of the seismograph. Failing to eliminate completely the noise, an electronic filter should be developed, so that these unwanted frequencies could be identified and excluded from the sample.