Dynamic analysis and multi-objective optimization of an offshore drilling tube system with pipe-in-pipe structure

A complete dynamic model of an offshore drilling tube system with pipe-in-pipe structure is developed in this paper. Specifically, the riser and well are connected at well head to constitute an outer pipe, within which, the drillstring stretching from the drilling platform to downhole is viewed as an inner pipe. The interactions between the inner and outer pipes are described by a series of spring-friction units along the pipe-in-pipe structure. Comparing with the previously published models which mainly focus on the drilling riser, the pipe-in-pipe structure is applied in this new model; moreover, the tube system under the mud line is also considered as an extension of the tube system submerged in the sea. The developed dynamic model is simulated using the finite element (FE) method in Abaqus. Under the same ocean environmental loads, the maximal lateral deflection for the pipe-in-pipe structure is less than that only considering the drilling riser. This finding indicates that, for an actual offshore drilling tube system with the pipe-in-pipe structure, it has stronger capacity of maintaining reliability under heavy ocean environmental loads. Based on the newly developed dynamic model, multi-objective optimization design of the offshore drilling tube system is conducted in Isight. A new flow path of the optimization is designed. Specifically, six-sigma method is adopted to drive genetic algorithm to run the multi-objective optimization, and simultaneously drive Monte Carlo method to analyze the reliability of the obtained optimal solution. Comparing with a series of single-objective optimization designs, the global optimization degree of the obtained multi-objective optimal design is verified as the best.