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Abstract

Pipeline abandonment and recovery (A&R) is of great importance in offshore pipeline installation. A mathematical model of the A&R process is proposed based on the large deflection beam theory. To solve the model, an iteration process is adopted. The initial guess of the solution is obtained through the two-catenary approach, which significantly accelerates the iteration. A moving boundary technique is used to solve the governing equation of the suspended pipeline. With the proposed approach, the effects of the dominant parameters in the A&R process are studied. The cable length increases, and the maximum bending moment decreases, as the vessel moves forward when maintaining the top tension. The vessel slightly moves forward and the maximum bending moment decreases with the top tension when maintaining the cable length. The top tension decreases and the maximum bending moment increases as the cable length increases, when the vessel stays static. The maximum bending moment dramatically decreases when the pull-head approaches the seabed at all events. In all the cases, the proposed model shows great advantages over the simplified two-catenary method which tends to overestimate the pipeline bending moment. Three different A&R strategies are compared. The third A&R strategy that repeats the process of static vessel - constant tension can effectively control the maximum bending moment within the designed range. The proposed approach of the A&R analysis and studies conducted should be a valuable foundation for future A&R procedure design.

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