Study On Several Major Problems In Submarine Pipeline Laying Process By Surface Tow Method

The submarine pipelines are constructed by the lay barge method or pull-and-tow techniques. With the advantage of simple requirement for pipelaying vessels, the towing method is more widely used in shallow water. The towing method can be further divided into bottom tow method, off-bottom tow method and surface tow method. Among them, the surface tow method is commonly used to lay subsea pipelines in Bohai Bay, China. The whole process of surface tow method is simulated and studied. Meanwhile, the profile analysis of the pipeline installed on real seabed is carried out to provide some suggestions for design and construction.The research done in this paper can be summarized into three parts and listed as follows.(1) Numerical simulation and analysis of the pipe during towing process are carried out first. Based on small deformation theory, a mechanical model of the pipe is established. Using FORTRAN language and finite element method, a program to calculate the deformation and stress of the pipe are developed. According to the numerical results, the factors such as the tension, number of tugboats, the ship route and so on, can affect the response of the pipeline.(2) The analysis of the pipe during laying process is performed secondly. Based on the same theory, taking the pipe-soil interaction and the influence of buoys removing into account, a new mechanical model is established. Using FORTRAN language, the laying process of the pipe is programmed. According to the result of the deformation and stress, the laying speed during different stages is proposed. The influences of seabed soil and sea conditions on the stability of the pipe are pointed out. Finally, with the increasing of laying depth, the response of the pipeline in the vertical plane will become more and more important.(3) Simulation and analysis of the pipe on the uneven seabed are carried out finally. Based on nonlinear finite element method, the pipe-soil contact model is established. Using ANSYS program, the influences of the low depression and obstructions on the stress subjected to residual tensions are analyzed. The deformation and stress of the pipeline laid on uneven seabed are calculated and the suggestions on seabed treatment are given.