Compliant vertical access riser assessment- DOE analysis and dynamic response optimization

Author(s): 
Michele A.L. Martins, Eduardo N. Lages, Eduardo S.S. Silveira (Federal University of Alagoas)

The impact of ultra-deepwaters on the risers design leads to the need of the development of new alternatives for riser configuration, enabling to overcome the barriers imposed by deepwater fields for oil exploitation in an efficient, safe and economic way. An example of a non-classical riser configuration is the compliant vertical access riser (CVAR): a lower-tension riser configuration fixed to a floating platform while still providing dry tree vertical access to the well bore. Its characteristic geometrical shape is achieved by the addition of special elements, such as syntactic buoyancy and additional weight, properly positioned along the riser. The main feature of CVARs is a nearly neutrally buoyant central region near an inflection point that tends to isolate the motions of the upper region from those of the lower region of the riser. The aim of the global analysis methodology is to establish the important effects caused by the CVAR global behavior. These effects are used as the basis for the riser design playing an important role in the optimization design problem definition. We made use of two computer programs: Anflex, developed by Petrobras, for the static and dynamic simulations of the riser, and modeFRONTIER.

In order to determine whether any change in design variables affects the results of the CVAR system, the most intuitive approach would be to vary the factors of interest in a full factorial design, that is, to try all possible combinations of settings. However, being this approach too time-consuming and computationally expensive, a fractional factorial design method has been used for conducting the sensitivity analysis. A multi-objective problem seeking to minimize the volume factor and maximize the minimum stress utilization factor is considered. By this approach we seek to achieve the maximum material used capacity simultaneously with the minimum material cost, equivalent to the minimum amount of material needed to obtain a feasible CVAR configuration, i.e., that properly satisfy previously established structural and operational constraints. Based on successful applications with the use of the method NSGA-II for riser optimization, this work has made use of this specific method for the CVAR optimization. The optimization approach produced a gain of approximately 35% for the stress utilization factor response and a reduction of 46% for the volume factor response.

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