Doctoral Thesis
DOI
https://doi.org/10.11606/T.3.2010.tde-19082010-112620
Document
Author
Full name
Fernanda Cristina de Moraes Takafuji
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 2010
Supervisor
Committee
Martins, Clóvis de Arruda (President)
Mazzilli, Carlos Eduardo Nigro
Morooka, Celso Kazuyuki
Pesce, Celso Pupo
Santos, Melquisedec Francisco dos
Title in Portuguese
Dinâmica tridimensional de risers.
Keywords in Portuguese
Análise dinâmica global
Engenharia offshore
Risers
Abstract in Portuguese
Title in English
Tridimensional dynamics of risers.
Keywords in English
Global dynamic analysis
Offshore engineering
Risers
Abstract in English
Used in offshore oil exploitation risers are elements that connect the floating unit to the oil well on the seabed. They are exposed to the environmental conditions of the site where they are installed. Normally, the risers global behavior is divided in static and dynamic analyses. The static loads acting on the pipe are: weight, buoyancy and current load. The loads considered in the dynamic analysis are caused by the floating units motion and by the waves. The riser dynamics is essentially non-linear mainly due to the viscous damping, which is quadratic in velocity and due to the unilateral contact between the riser and the seabed. The dynamic simulation can be performed in time domain and in frequency domain. The time domain analysis usually takes longer to perform because the solution is obtained at each time-step. However, the non-linearities of the problem can be considered. The frequency domain analysis is normally faster than the time domain analysis, once the stationary state is directly obtained, nevertheless, the non-linearities must be removed. The objective of this work is to study the risers global dynamics focusing on the modeling of the non-linearities in time domain, as well as removing them for the frequency domain analysis. The static configuration is the initial configuration of the dynamic analysis and the frequency domain dynamic is modeled to be a perturbation of it. The dynamic model was obtained through the Finite Element Method and the riser was represented by a beam element. The only non-linearities covered in this work are caused by the viscous damping and the soil-structure contact. For the frequency domain analysis it was developed, in this work, a way to linearize the viscous damping based on a methodology already in use for two-dimensional analysis. Besides that, the contact with the seabed was represented by linear springs and the curvature was corrected afterwards through a boundary layer technique. Considering that the results compared very well with the ones obtained with a non linear model, one can say that these linearizations had the desired effect. In the time domain analysis the viscous damping could be represented in the non linear form as well as the soil-structure contact, which was represented through the unilateral contact, friction, suction force and trench. Whenever possible, the results were compared with the ones obtained with Orcaflex 9.2a.