Safety analysis and experimental research on the limit state of subsea pipeline by reel-lay method

As the development of offshore oil and gas resources extends into deep sea, the reel-lay method was invented. During the entire process of reeling and unreeling in the reel-lay method, the pipeline undergoes several bends and experiences significant plastic deformation, which may lead to potential failure of the pipeline. To ensure the safety of subsea pipelines laid by the reel-lay method, this paper analyzes failure modes during reel laying and examines the limit states under combined loads. A model is developed using elasto-plastic theory to assess the pipeline’s ultimate bearing and strain capacities during the reeling and unreeling processes. Based on this model, a reel-lay system for 6-in. steel pipes was designed and tested in the Bohai Sea. The results, with average ellipticity at 1.215% and straightness at 0.0014 m, confirm the design’s validity and the accuracy of the proposed analysis method. This study proposes a criterion for ensuring the safety of pipelines during the reel-laying process, which can better guide the design of reel-laying equipment and operational safety.