The Analysis Of Dynamic Characteristics And Dynamic Responses Of Free Spanning Submarine Pipeline

Submarine pipelines, as the important establishment of oil-gas transmission, have been more widely used with offshore oil-gas exploitation and application. Free span will inevitably form in commission, the frequency lock-in under VIV is the key factor to lead to the destruction of pipelines.To provide theoretical foundation and scientific basis for disaster process, damage mechanism and safe assessment of submarine pipelines in an offshore environmental condition, investigations have been conducted on dynamic response of submarine suspended pipeline, based on formula deduction, model tests and numerical simulation.Free spanning submarine pipelines are complicated systems with soil-pipe-fluid interaction under dead load, internal fluid, external environment and dynamic load, and the span length is the major factor to decide the static and dynamic destruction. Taking to avoid vortex induced vibration (VIV) as the control condition, this paper gives the formula of allowable span length under in-line motion, deduces the formula of natural frequency of pipeline on different boundary conditions, and calculates the allowable span length under cross-flow motion. The results show that boundary conditions have large affect on allowable span length, which of fixed support is larger than that of simply support; the paper introduces construction principals and the measures of fixing support under water for the free spanning pipeline.Due to the complication of effect factors of free spanning submarine pipelines, the dynamic response in earthquake is studied by using an underwater shaking table. Experiments on many groups of different combinations of factors including seismic wave direction, transmitting materials, distance from seabed, water depth and the kind of wave are carried out. The results show that the dynamic response under horizontal seismic input is larger than that under vertical seismic input at the same amplitude; under sine wave, the dynamic response of pipeline of mid span declines as the sediment concentration increases, but becomes stable when the pipeline is in deep water and longer distance from seabed; under horizontal El Centro wave, the strain of mid span raised as water depth and distance from seabed increases.Numerical analysis of free terrestrial spanning pipeline is carried out using finite element software ABAQUS. The relative results show that calculation error of the natural frequency is little; stress nephogram is in good agreement with experimental condition, the variance of magnification of acceleration along the axial direction conforms to experimental condition, under seismic wave, the numerical value of resonance frequency accords with experimental value. So ABAQUS can simulate the dynamic response of pipeline under seismic wave accurately.