Abstract

Drilling systems have become an integral part of oil and gas exploration and production particularly in ultra-deep waters. With increasing drilling depth, heavy weight of the riser system and high-top tension requirement become the potential concern. The study mainly focusses on the optimization of buoyancy settings to achieve better operating performance and lighter weight. The multi-objective optimization of operability envelope is a complicated problem due to the presence of discrete design variables and complex analysis process (emergency disconnection, recoil analysis, drift-off) with different FEA models. In this work, an efficient approach for the multi-objective optimization of operability is proposed for the drilling riser in ultra-deep water. The two main contradictory objective functions include minimize the dry weight of drilling riser system and maximize the area of operability envelopes. Each of these conditions or operational scenarios imposes varying design limits for the riser stack-up. The main riser characteristics related to operational and environmental conditions will be considered. According to suitable criteria and requirements, the operational working envelopes will be defined. The economic cost (weight) and drilling riser system performance (operability window) are considered to obtain the better operational performance with Non-dominated sorting genetic algorithm II (NSGA-II). The use of a RBF metamodel approach could solve two critical problems in multi-objective optimization design, including timeconsuming computation and non-convergent problem during iterative loops.

1 INTRODUCTION

Drilling systems are an integral part of oil and gas exploration and production particularly in deep water. In drilling systems, drilling risers play an important functional role and have to be designed to maximize their operability and cost-effectiveness (Yang et al., 2017). For greater drilling depths, the heavier weight of the riser system and higher top tension requirement impose greater challenges in addition to more demanding environmental conditions. Hence optimization methods are necessary to achieve safe and cost-effective design and operation of drilling risers. Appropriate analysis methodology is essential to maximize the reliability of riser design in terms of operability (Zhang et al., 2017). Furthermore, moving into ultra-deep water would face more challenging problems such as more restrictive operating window due to longer and heavier risers. Increase in riser sizes, buoyancy structure, tensioning system, hook load capacity and storage space will lead to increase in the overall vessel size. To improve the safety and performance of drilling risers in ultra-deep water, multi-objective optimization technologies should be conducted to reduce the weight of riser system and to maximize its operability.

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