This paper applies design of experiments (DOE) methodology to the design of Compliant Vertical Access Risers (CVAR). This relatively new riser configuration is characterized by its differentiated geometry, achieved by the use of syntactic buoyancy at the lower section of the riser and additional weight at its upper section. The characteristic compliance of the CVAR system is obtained by providing an excessive length of pipe and a horizontal offset between the riser top and end connections. Thus, this system provides vertical access to dispersed subsea wells and its compliance can also compensate for vessel motion. CVAR, being vertical access to the wells, brings the advantage of using dry trees, and also allows the completion and workover operations to be performed from the FPU, offering significantly economic and operational benefits to deepwater oil field development. To guarantee such benefits, some operational and structural constraints must be satisfied. The design of the CVAR system is dependent upon several parameters. This study can provide a better understanding about the behavior of the CVAR in terms of its design parameters by the use of the DOE methodology. DOE is a statistical technique that provides an objective measure of how design parameters are correlated and the effective contribution of each at the riser performance. Consideration of the main effects as well as interaction effects coupled with sensitivity analysis is essential for insightful interpretation of model results and effective decision-making. Thus, this study contributes with the design of Compliant Vertical Access Risers as well as with a methodology that can lead to efficient riser design, being a first step in the optimization design process.
Environmental loads are the main source of dynamic excitation of offshore structures. Due to the random nature of these loads, one should consider statistical properties when designing such structures. Recent codes for the design of steel risers address the use of environmental contours or full long-term analysis to take into account the statistics of load effects. Moreover, sea states for fatigue design should also be carefully selected so as to include the region of the scatter diagram which most contributes to the total damage. This paper looks into fatigue and ultimate limit states of steel risers. The first part shows a comparative study of three design criteria for extreme statistical analysis. These criteria are based on design storm, environmental contour, and full long-term statistics. A joint probability distribution found in the literature for a Brazilian location was used to describe the correlation between the significant wave height and the zero crossing wave period. The second part deals with the analysis of the coefficient of contribution for fatigue and long-term extreme statistics. Several full time domain analyses were performed and the most important sea state region for each limit state is shown in two examples of steel catenary risers. A discussion about sea state selection is then presented.
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