Study On Dynamics Model Of Pipe-laying Operation

Deepwater pipe-laying operation is an important part of deepwater oil and gas exploitation.Computer simulation technology is an effective approach of ensuring the security and rapidity of pipe-laying operation.The mathematical model must be run accurately and rapidly in the simulation of pipe laying.This paper aims to solve the dynamic mathematical model problems of the pipe-laying operation.Considering the real-time pipe-laying vessel motion and the coupling effect between pipeline and nonlinear soil stiffness seabed,the research of static and dynamic mathematical model of S-laying and J-laying are investigated.The main contents are as the followings:The research of real-time radiation calculation method is carried out based on the theory of time domain vessel motion.This paper employs the impulse response equation and investigates radiation identification using a least-squares fitting process.A radiation identification model is built and adopts the analysis and calculation for frequency-domain identification and time-domain identification to the radiation force,respectively.It is the basis and support for a real-time pipe-laying dynamic mathematical model.The wind force and ocean current are calculated from test data obtained for a wind-tunnel model.The wave force is calculated employing multi-dimensional interpolation based on off-line computing results.The thruster force is calculated employing a regression method based on an azimuth thruster-pod open-water test.A time-domain motion model that six degrees of freedom is then built and validated and analyzed in a model experiment and sea test.According to the pipeline configuration and force analysis of S-laying,the pipeline is divided into five parts:a stinger segment,middle segment,span segment,boundary segment and touchdown segment.In middle segment and boundary segment’s bending moment and the coupled effect between pipeline and deformable seabed is considered,and the bending moment of the span segment is ignored.The model is solved employing the Newton-Raphson method based on the continuity of the displacement boundary condition,angle boundary condition,force boundary condition and bending-moment boundary condition.Furthermore,a parametric study is carried out to investigate the effects of the thickness of the wall and the outer diameter of the pipeline on the pipe-laying solution.The coupling between J-laying and the seabed with nonlinear stiffness is investigated by dividing the pipeline into two parts:one part suspended in water and the other part lying on the seabed.The mathematical model of the pipeline suspended in water is built using a numerical iterative method.The mathematical model of the pipeline lying on the seabed is built using a finite difference method while considering the coupling between the pipeline and nonlinear seabed.The calculation is carried out based on the boundary conditions of displacement boundary condition,angle boundary condition,force boundary condition and bending moment boundary condition is required at the touchdown point that connects the two parts of the pipeline.Furthermore,a parametric study is carried out to investigate the effects of the mudline shear strength,the gradient of the soil shear strength,and the outer diameter of the pipeline on the pipe-laying solution.To satisfy the real-time requirement of a J-laying dynamic mathematical model,the pipeline is divided into nodes for the masses and forces.Considering the effect of gravity and the buoyancy force and the dynamic forces of the internal tension force,internal damping force and current force acting on the nodes,a real-time mathematical model of J-laying is built.Furthermore,the effects of nonuniform current,layered current flow and vessel motion on the pipeline are analyzed.