Study On Key Mechanical Problems Of Overbend Section In Ultra Deepwater S-lay Operations

S-lay is the primary method of pipe laying operation. During the pipelay process, the pipeline section is transported, welded, inspected, released by the tensioner and then passes through the stinger to the seabed. The pipeline bears the combination loadings of bending, tension and vessel motion. In some operation conditions, the plastic deformation, strain concentration and cross-section ovality are generated. When the pipeline leaves the stinger and moves to the seabed, it may buckle below its capacity under the external pressure. These problems are not proposed in the traditional shallow water S-lay operations. Because in the shallow water pipe installations, the pipe weight is small, the installation loadings are not large enough to destroy the pipe or cause the damage. But in deepwater operations, the pipe configuration and loading conditions change. The overbend pipe-stinger dynamic integration is more complicated. Some new problems need to be studied. The present research is supported by the National High Technology Research and Development Program of China (863program) Deepwater subsea pipe installation technology’ research and the National Basic Research Program of China (973program) Reliability research of the deepwater oil/gas transportation system. The main contents are as follows:(1) Dynamic pipe-stinger integration under the pipelay vessel motion.In deepwater S-lay operations, large installation loadings cause the pipe deform plastically. The vessel motion resulting from the wind, wave and current drives the pipe and stinger impact. The roller force is amplified. Research on the dynamic response of the pipe and stinger interaction under the vessel motion is the bases of the pipelay operation. In the present research, the simulation and large-scale model test are earried out to study this phenomenon. The dynamic pipe sliding down process is simulated. Pipe plastic deformation and cross-section stress profile are obtained. The roller force distribution and dynamic effects are calculated. The dynamic response of the stinger during the laying process is studied.(2) The possible damage results from the pipelay process are investigated. The capacity of the pipe after installation is evaluated.Pipe cross-section deforms after the deepwater S-lay operations. As the previous pipe models are simplified to beam element, the pipe cross-section local deformation and ovality are impossible to calculate. A pipe finite element model built with shell element is used to study the pipe residual deformation and non-uniform ovality. The nonlinear buckling analysis is carried out to calculate the buckling capacity of the deformed pipe with modified Riks method. The research results could offer reference during the deepwater pipelay operations in the large strain installation conditions.(3) An ultra deepwater stinger design method is proposed based on the pipe limit capacity analysis.Stinger is the key equipment of the S-lay vessel. It supports the pipe and guides the pipe to slide into the water during the laying process. The pipelay vessel’s performance depends on the stinger length and curvature radius at a certain extent, which are difficult to confirm in the design phase. In the stinger design procedures, the stinger is designed to meet the pipe limit capacity requirement, structural safety requirement and to keep the balance of the parameters, such as tensioner force, vessel size and the thruster. The current research proposes the stinger’s basic design and detailed design method. An easy-to-use stinger design software DUT-Pipelay is developed. Based on the design tool, the pipelay capacity analysis of HaiYangShiYou201is carried out, a new up-graduated stinger is designed.