Investigation On Interaction Between Ovebend Pipeline And Stinger Of Deep S-Lay

At present,the oil and gas resources in the land and shallow sea can no longer meet people's increasing demand for energy,which makes deep-sea oil and gas development become the main task confronted by the offshore oil and gas industry.In recent years,many deep-sea oil and gas fields have been explored in the East China Sea,South China Sea and Bohai Sea.However,China currently does not have the ability to develop these deep-sea oil and gas fields,especially the lack of offshore construction experience and pipe-laying equipments for the deepwater installation.S-lay method as a most widely used submarine pipe-laying technique in engineering,employs a stinger device to support and guide the pipeline into water with the designed curvature and angle,and the pipeline finally forms into an S shape.Compared with other pipe-laying methods,S-lay method has the advantages of fast laying speed,high working efficiency and strong applicability,which makes it suitable for solving the pipe installation problems of deep-sea oil and gas fields,becoming one of the key technologies urgently demanded for deep-sea oil and gas development in China.The S-lay process involves the nonlinear interaction of many structures such as the pipeline,the stinger and the lay barge.The main challenges in the analysis and design of deep S-lay include the following three aspects:(1)It is difficult to consider the interaction between the overbend section and the stinger when analyzing or designing the pipeline configuration,which is particularly disadvantageous to the operation safety of deep S-lay;(2)The S-lay process involves the coupling effects of many nonlinear factors,such as the large geometric deformation,the elastic-plastic behavior and contact interactions,which makes it difficult to accurately solve the roller force and pipe strain response by numerical simulation(3)The influences of dynamic factors such as the ocean environment and vessel motions can affect the interaction between the pipeline and the stinger,which makes the dynamic installation problem of deep S-lay more difficult to deal with.It is difficult to accurately predict the dynamic roller forces between the pipeline and the stinger.As a result,the structural safety of the pipeline and the stinger in the S-lay process can not be guaranteed.The aforementioned installation problems of deep S-lay are also the research hotspots in the field of ocean engineering and academia.In this dissertation,these S-lay problems are discussed in an integrated research framework.Based on analytical analysis,numerical simulation and model experiment methods,the key issues of pipeline configuration design,nonlinear coupling effects in the overbend section and dynamic roller forces are studied.The details are as follows:(1)A piecewise analytical method considering the influence of discrete distributed rollers on the overbend section is proposed for the analysis of pipe configuration of S-lay.The governing differential equations of the pipe in the overbend,straight and sagbend sections are established respectively,and solved by numerical iteration method.The effects of design parameters such as the water depth,pipe size,stinger radius and roller distance on the configuration and internal force distribution of the pipeline are analyzed.The results indicate with the increase of water depth and pipe size,the tension and bending moment of the overbend section become much larger;the stinger radius and roller space have little influence on the pipe configuration and tension distribution,reducing the stinger radius or increasing the roller space will increase the bending moment of the overbend section.(2)The nonlinear contact problems between the pipeline and many rollers on the roller box are studied by using a surface-to-surface contact model.Three dimensional finite element models of the pipeline,roller boxes and rollers are established,and the nonlinear coupling effects of large deformation,elastic-plastic deformation,and surface contacts are taken into consideration during the S-lay analysis.The effects of tension on the contact states between the pipeline and roller boxes,and the pipe strain of the overbend section are studied.The results indicate only two wire rollers of the roller box can contact the pipeline,and the pipe rollers do not provide any support forces for the pipeline;the magnitude of tension can affect the contact states between the pipeline and roller boxes,and designing a reasonable tension can evenly distribute the pipe weight on each roller box to the greatest extent.(3)A dynamic substructure experiment is designed for the 3000 meters deep S-lay condition to study the effects of vessel motions on the dynamic interaction between the pipeline and the stinger.The experiment platform of the moderate-or large-scale stinger and pipe model is built in the laboratory.Influences of coupled vessel motions on the interaction between the pipeline and stinger are simulated by the 6-DOF motion platform.The results indicate vessel motions induced by head sea have significant dynamic amplification effects on roller forces,and there is dynamic impact between individual roller and the pipeline,which results in a high dynamic magnification factor.(4)A decoupling analysis method is proposed for the stinger,which can accurately verify the structural design and operational safety of the stinger in dynamic pipe-laying process.The dynamic roller forces measured by the model experiment are transformed into prototype,and loaded to the finite element model of the stinger in the form of external force.The vessel motions are transmitted to the stinger by using the MPC rigid beam connection.The transient responses of stinger are analyzed for pipe-laying operations in different directions of incoming waves.The results indicate the maximum dynamic stress is concentrated on the connecting structures of the stinger,including the pup-pieces,the main hinges and the suspension hinges.By contrast,main hinge is the connecting structure with the most severe stress concentration.(5)Based on Miner's fatigue cumulative damage theory,the fatigue problem of the stinger connection components caused by dynamic pipe-laying process is studied.According to the formulas of stress concentration factor for different types of tubular joints,the hot spot stress of the mian hinge and the suspension hinge is calculated.Based on Palmgren-Miner linear cumulative damage criterion,the fatigue damage of the pup-pieces,the main hinges and the suspension hinges is analyzed for different incoming wave directions.The results indicate the largest fatigue damage on the connecting structures of the stinger is caused by pipe-laying operation in head sea,and main hinge is the connecting structure with the most severe fatigue damage in the same S-lay condition.