Optimization techniques applied to compliant vertical-access riser

Author(s): 
Michele Agra de Lemos Martins, Eduardo Setton Sampaio da Silveira, Eduardo Nobre Lages (Universidade Federal de Alagoas)

In offshore oil production activities, special pipes, called risers, are employed to connect the wellheads at the sea-bottom to a floating platform unit. The design of risers is a challenging task, due to the continuous increase in exploitation depth, as at pre-salt fields. Conventionally, risers are used in a free-hanging catenary configuration, however at deep and ultra-deep waters the use of such a configuration can become infeasible, due to the excessive tension at top connection with the platform. The design of risers is a challenging task, due to the continuous increase in exploitation depth, as at pre-salt fields. Conventionally, risers are used in a free-hanging catenary configuration, however at deep and ultra-deep waters the use of such a configuration can become infeasible, due to the excessive tension at top connection with the platform.

The compliant vertical-access riser (CVAR) is a relatively new alternative that has been in study for oil exploitation at ultra-deep water fields. It has a different geometrical configuration that can improve the structural performance of the production system by reducing the dynamic effect of the riser and providing several facilities such as the  use of dry completion and the possibility of directaccess to the wellbore for well intervention through the production riser. The geometrical configuration of the CVAR is obtained by the use of a heavy weight coating over part length of the riser pipe, buoyancy modules attached over part length of the riser pipe and a stress joint at bottom and top of the riser.  The CVAR design involves the  definition of the geometrical characteristics of these components, thus adding several more variables to the design. Additionally, a riser must be designed to fulfill engineering criteria under several environmental conditions, requiring time consuming numerical simulations. With the increasing complexity of the risers design and the constant challenge of minimizing costs and improving the performance of structures, the optimization methodology comes out as an efficient alternative. Based on this motivation, this paper presents the optimum design approach of compliant vertical-access risers. Thus, the importance and effectiveness of the use of optimization techniques in the preliminary design of risers is discussed and the applicability of this concept in the design of CVAR is emphasized.
 

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