Abstract
Steel catenary risers (SCR) are a good candidate for deepwater exploration due to their relatively low cost, less demand for subsea intervention, and compliance with the floating structures motions. However, SCR deployment in deepwater faces tremendous challenges such as increased top tension and concern over fatigue performance. This paper aims to search for the global optimal solution of a riser system through the application of genetic algorithm (GA) and finite element analysis. The process method will, in turn, automate the design process, with the implementation of an optimisation routing as a critical resource for initiating and finalising the acceptability of a riser system. The method involves the deployment of GA as the optimiser because of its capability in handling a variety of complex non-linear optimisation problems to ascertain the global optimum solution and the capacity to self-moderate the number of iterations. The total structural weight is the objective function, while bursting criteria, buckling (collapse) criteria, buckling propagation criteria, yielding criteria, and fatigue limit state (FLS) criteria are non-linear constraints. To accurately assess the strength requirements, time-domain approach through OrcaFlex is used to predict the dynamic responses and fatigue life/damage of the SCR. An interface between GA in MATLAB and ORCAFLEX has been programmed to exchange data. The wall thickness, length and declination angle at the hanging-off point are chosen as design variables due to their strong influence on the configuration and dynamic responses of a SCR.
This method was illustrated using a prospective 10inch SCR installed in a 2000m deepwater offshore field off the coast of the oil-rich Niger Delta region, Nigeria. The obtained results have shown a significant reduction in the riser weight.
