Multi-Objective Optimization Based On TTR Interference Analysis

Marine riser clusters are widely used in offshore engineering. A cluster of top tensioned risers (TTRs) link the base well-heads to top platform connectors.Riser interference has become a critical issue in riser design with the progression of oil production into deep water, that is because the static deflections of the risers due current may be proportional to the square of the length The generally accepted philosophy for riser interference or clashing is to prevent occurrence in extreme current conditions. For such a riser system, one of the key design issues is the requirement-of lateral clearance between individual risers to avoid possible clashing. As the water depth increases, larger clearance between risers and/or higher riser top tension is required.Interference between TTRs is a complicated hydrodynamic phenomenon, shielding effects contributes to the deficit of the current velocity of downstream riser, which leads to the difference in hydrodynamic forces between upstream riser and downstream riser. Based on DNV-RP F-203,the interference between two adjacent TTRs is performed in this thesis, including some parameter sensitivity analysis.One of the key problems in riser design is to keep enough clearance to avoid colliding. For riser system, there are two major factors which influence the interference of TTRs, they are top tension and riser spacing, each impactive factor of interference could make a different effect (cost and safety), which is often incompatible. Increasing the riser spacing means increasing the size of the wellbay, which has influence on the global platform parameters such as deck space and load carrying capacity; the other option is increasing the top tension to a high and constant level, which will increase the requirement of the tensioners to provide more tension. A Multi-Objective Optimization method is proposed to harmonize the two incompatible objectives:cost and safety. Therefore, it greatly facilitates to adapt the present method to riser interference optimization. Example is given to demonstrate the effectiveness and robustness of the proposed method.